WO2025257635A1

WO2025257635A1 - Adhesive sheet, method for forming an adhesive sheet, electric motor and a method of insulation

Info

Publication number: WO2025257635A1
Application number: PCT/IB2025/055139
Authority: WO
Inventors: Haruna MIZUMACHI; Kengo Imamura; Daiki KOSEKI; Eisuke Fujiwara
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2024-06-14
Filing date: 2025-05-16
Publication date: 2025-12-18
Anticipated expiration: 2026-12-14
Also published as: JP2025187498A

Abstract

An adhesive sheet comprises an electrically insulating substrate, a first adhesive layer disposed on one side of the substrate, and a second adhesive layer disposed on another side of the substrate opposite the first adhesive layer. One or both of the first and the second adhesive layers comprises a curable adhesive composition and a thermally conductive filler distributed therein. The curable adhesive composition comprises a multi-functional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent.

Description

ADHESIVE SHEET, METHOD FOR FORMING AN ADHESIVE SHEET, ELECTRIC MOTOR AND A METHOD OF INSULATION

Technical Field

[0001] The present invention relates to an adhesive sheet, a method for forming an adhesive sheet, an electric motor including the adhesive sheet, and a method of insulation using the adhesive sheet.

Background

[0002] Expandable adhesive sheets are useful for adhering surfaces of two objects that are spaced apart by a small gap that is either physically difficult to reach or difficult for conventional adhesives to be dispensed. An expandable adhesive sheet can be conveniently inserted into the small gap, and then expanded to fdl the gap. Good adhesive contact between the two surfaces can help to improve adhesion as well as thermal conductivity between the two objects.

Patent Literature

[0003] US Patent Number 9,419,489 B2 discloses a polyimide resin which is mixed with heat conductive fillers and a mesh layer to form a composite slot liner.

[0004] US Patent Number 9,537,364 B2 discloses a dual sided epoxy adhesive slot liner that expands upon heating.

Summary

Problem to be Solved by the Invention

[0005] Adhesive sheets based on curable expandable adhesive materials provide ease of processing over traditional methods of adhesive dispensing. The use of expandable adhesive sheets presents new technical challenges in areas such as adhesive flow control during expansion and heat transfer in high heat applications. Resinous long chained polymeric adhesives used in conjunction with particulate filler materials, such as thermally conductive particles, provide thermal stability and thermal conductivity but result in a rather viscous adhesive composition which may sometimes impede the uniform expansion of the adhesive composition and particulate material during curing. It is desirable for an expandable adhesive layer to be comprised of an adhesive composition having sufficient fluidity and flowability during expansion to achieve a desired distribution of adhesive during expansion.

Means to Solve the Problem

[0006] In one aspect, the present disclosure provides an adhesive sheet, comprising an electrically insulating substrate, a first adhesive layer disposed on one side of the electrically insulating substrate, and a second adhesive layer disposed on the other side of the electrically insulating substrate, a first adhesive layer disposed on one side of the substrate, and a second adhesive layer disposed on another side of the substrate opposite the first adhesive layer, wherein one or both of the first and the second adhesive layers comprise a thermally conductive filler distributed in a curable adhesive composition comprising: a multi- functional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent.

[0007] In another aspect, the present disclosure provides a method for forming an adhesive sheet, comprising: providing an electrically insulating substrate, mixing a thermally conductive filler with a curable adhesive composition comprising: a multi-functional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent to form a prepolymer composition; disposing the prepolymer composition on one side of the substrate to form a first adhesive layer; and disposing the prepolymer composition on another side of the substrate opposite the first adhesive layer to form a second adhesive layer.

[0008] In further aspect, the present disclosure provides an electric motor comprising a stator, comprising: a stator core having at least one slot; a winding accommodated within the slot and a slot liner comprising an adhesive sheet arranged between the winding and the stator.

[0009] In a yet further aspect, the present disclosure provides a method of electrically insulating an electric motor comprising a stator, comprising providing a stator core having at least one slot, arranging a slot liner comprising an adhesive according to the present disclosure into the at least one slot, arranging an electrical winding within the at least one slot, and curing the adhesive sheet.

[0010] The adhesive sheet of the present disclosure comprises an expandable adhesive layer that has excellent thermal conductivity after foaming and curing. When cured, a multifunctional epoxy adhesive offers good long term heat resistance and electrical insulation. In order to improve its thermal conductivity, heat conductive particulate filler materials are added to it. However, due to the inherent flow characteristics of multifunctional epoxies, as well as added filler materials having the effect of increasing the overall viscosity, the flow characteristics of the adhesive composition during the adhesive expansion process may not be optimal for achieving consistent flow and expansion. Therefore, the flow properties of the curable adhesive composition in the first and second adhesive layers need to be improved. It has been presently found that a reactive diluent comprising at least one epoxide moiety is able to solve the above-mentioned problems. By including a suitable reactive diluent, rheological characteristics of the adhesive composition can be appropriately tweaked to improve flowability. Additionally, the terminal epoxide moiety present on the reactive diluent eventually reacts with the multifunctional epoxy adhesive. The reactive diluent thus serves to enable a wider range of filler types, a wider range of filler sizes, and higher filler loading, to be achieved in the adhesive composition. This in turn enables a wider range of specifications such as thermal conductivity to be achieved in the adhesive sheet.

Effect of the Invention

[0011] Thus the present invention provides an expandable adhesive sheet with improved flow characteristics during the curing process. This in turn allows a higher loading of thermally conductive filler into the adhesive composition of the adhesive sheet. This in turn means that the expandable adhesive sheet has an improved thermal conductivity performance that has not been achieved before. [0012] These and other advantages of the invention are more fully shown and described in the drawings and detailed description of this invention, where like reference numerals are used to represent similar parts. It is to be understood, however, that the drawings and description are for the purposes of illustration only and should not be read in a manner that would unduly limit the scope of this invention.

[0013] The figures and the detailed description, which follow, more particularly exemplify illustrative embodiments of the invention.

Brief Description of the Drawings

[0014] Preferred embodiments of the present invention will be described with reference to the drawings. In addition, in the description of the drawings, the same elements are given the same reference numerals, and redundant description will be omitted. In addition, some parts of the drawings may be exaggerated for ease of understanding, and the dimensional ratios and the like are not limited to those shown in the drawings.

[0015] FIG.1 is a cross sectional view of an expandable adhesive sheet before expansion of the first and second adhesive layers.

[0016] FIG. 2A is a cross sectional view of the adhesive sheet with an embodiment of the adhesive permeable layer being also permeable to thermally conductive fillers and expandable microspheres of the foaming agent during the expansion of the first and second adhesive layers.

[0017] FIG. 2B a cross sectional view of the adhesive sheet with another embodiment of the adhesive permeable layer being impermeable to thermally conductive fillers and expandable microspheres of the foaming agent during the expansion of the first and second adhesive layers.

Detailed Description

[0018] The adhesive sheet as provided comprises an electrically insulating substrate, a first adhesive layer placed on one side of the electrically insulating substrate, and a second adhesive layer placed on the other side of the electrically insulating substrate. One or both of the first and the second adhesive layers comprise a curable adhesive composition and a thermally conductive fdler distributed in the adhesive composition. The curable adhesive composition comprises a multi-functional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent. The adhesive sheet expands when heated after being placed between the objects to be adhered. Due to its expandability, the adhesive sheet is advantageously placed within gaps between two objects to be adhered together. The adhesive sheet may further comprise an adhesive permeable layer disposed on the first adhesive layer. Moreover, the adhesive sheet may further comprise a second adhesive permeable layer disposed on the second adhesive layer. The first adhesive permeable layer is a layer that allows the adhesive to pass through when the adhesive composition in the first adhesive layer is heated causing the foaming agent therein to expand, and the second adhesive permeable layer is a layer that allows the adhesive to pass through when the adhesive composition in the first adhesive layer is heated causing the foaming agent therein to expand. [0019] When the adhesive sheet is heated, curable adhesive composition in the first adhesive layer and the second adhesive layer becomes flowable. At the same time, it also expands due to the expansion of the foaming agent. Adhesive and the particulate material including heat conductive filler thereby contacts the surfaces of the two objects to be adhered, and the adhesive is cured to establish stable adhesion between the two objects. In some embodiments, the adhesive layers are covered by a protected adhesive permeable layer for protection. The adhesive permeable layer may be designed to be permeable to the expanding adhesive, allowing the adhesive to be transported through the adhesive permeable layer. An advantage of having the first adhesive permeable layer and the second adhesive permeable layer arranged on the outermost layer of the adhesive sheet is that the positioning of the adhesive sheet with respect to the object to be adhered is easier due to the adhesive permeable layers rendering the adhesive sheet non tacky so that the adhesive sheet is prevented from unintentionally adhering to the object to be adhered. The cover layer protects the adhesive surface from potential physical damage by the coil tips during the insertion of the coils into the stator slot and do not cause scraping or dusting issues during the assembly process. The lower friction of the cover layer surface also provides excellent workability.

[0020] Hereinafter, each structure of the adhesive sheet of this embodiment will be explained in detail.

Substrate

[0021] (Electrically insulating substrate) The electrically insulating substrate is a support for forming the first adhesive layer and the second adhesive layer and is a member that substantially defines the size of the adhesive surface of the adhesive sheet. Thus the electrically insulating substrate functions as a base layer for forming the first adhesive layer and the second adhesive layer during the manufacturing stage of the adhesive sheet. The material constituting the electrically insulating substrate should have enough strength to support the first adhesive layer and the second adhesive layer, and the material that makes up the electrically insulating substrate has sufficient strength to support the first adhesive layer and the second adhesive layer. Any material may be used as long as it does not reduce its mechanical strength when heated. Furthermore, when the adhesive sheet of this embodiment is used for applications requiring electrical insulation, electrical insulation can be easily imparted to the adhesive sheet by selecting an electrically insulating substrate.

[0022] The material constituting the electrically insulating substrate is not particularly limited, examples include polyester resins (for example, polyethylene naphthalate (PEN ), polyethylene terephthalate (PET), etc.), polycarbonate resin, polyimide resin (e.g., polyetherimide (PEI), polyamideimide, etc.), polyamide resin (e.g., polyetheramide, polyaramid, nylon, etc.), acrylic resin, polysulfone Preferred are resins (polysulfone, polyethersulfone, etc.), polyetherketone resins (polyetherketone, polyetheretherketone, etc.), modified polyphenylene oxide, and the like. The electrically insulating substrate may contain only one kind among these, and may contain two or more kinds. [0023] The thickness of the electrically insulating substrate may be adjusted as appropriate depending on the distance between the objects to be adhered. When the gap between the objects to be adhered is large, increasing the thickness of the electrically insulating substrate makes it easier to fill the gap. The thickness of the electrically insulating substrate may be, for example, 2 pm or more, and from the viewpoint of easily improving the dielectric breakdown voltage, it may be 3 pm or more, 5 pm or more, 7 pm or more, 9 pm or more, or 11 pm or more. Further, from the viewpoint of flexibility of the adhesive sheet, the thickness of the electrically insulating substrate may be, for example, 200 pm or less, 150 pm or less, 100 pm or less, or 90 pm or less. The substrate layer, together with the adhesive layers, and optionally the adhesive permeable layers, described below contributed to a total sheet thickness, which may range from, for example, 10 to 2000 pm, or 50 to 500 pm, or more preferably 80 to 300 pm.

Adhesive layers

[0024] The first and/or the second adhesive layers may comprise the same or different materials and dimension. Each adhesive layer is arranged on one side of the electrically insulating substrate, and includes a curable adhesive composition comprising a multifunctional epoxy adhesive, a reactive diluent and a foaming agent. A thermally conductive filler is included in the adhesive composition. Since the first adhesive layer contains the foaming agent, it expands when the objects to be adhered are bonded together and can fill the gaps between the objects to be adhered.

[0025] The minimum thickness of the adhesive layers is not particularly limited, and may be, for example, 5 pm or more, 10 pm or more, or 15 pm or more. The maximum thickness of the first adhesive layer is not particularly limited, and may be, for example, 200 pm or less, and from the viewpoint of improving workability, it may be 100 pm or less, 80 pm or less, or 60 pm or less.

[0026] The multifunctional epoxy adhesive is preferably one that is in a substantially solid state at room temperature, becomes fluid when heated, and can be cured by continued heating. That is, the resin may be a thermosetting adhesive. The adhesive may be chosen from one with low dielectric constant to achieve good electrical insulation and prevent partial discharge. Examples of thermosetting adhesive include epoxy adhesives and the like.

[0027] In an embodiment, the multifunctional epoxy adhesive comprises epoxy resins having Tg greater than 150°C or more preferably greater than 200°C. These may be used in applications involving elevated operating temperatures. Examples of heat resistant epoxy resins include phenolic epoxy resins. These resins combine the heat resistance of phenolic compounds with the versatility of epoxy resins. Examples include phenol novolac epoxy resin and cresol novolac epoxy resin. Another example is Silicon-modified epoxy resins. These resins incorporate silicon groups into the epoxy backbone, providing enhanced heat resistance and improved electrical properties. Examples include epoxy resins modified with siloxane or silsesquioxane moieties. In another example, the heat resistant epoxy resin comprises phthalonitrile epoxy resins. Phthalonitrile-based epoxy resins offer exceptional heat resistance, flame retardancy, and low smoke generation. In yet a further example, the heat resistant epoxy resin comprises bismaleimide (BMI) epoxy resins. In another example, the heat resistant epoxy resin comprises cyanate ester epoxy resins. Cyanate ester-based epoxy resins offer high-temperature stability, low dielectric constant, and low moisture absorption. In yet a further example, the heat resistant epoxy resin comprises polyimide epoxy resins, which combine the heat resistance of polyimides with the processability of epoxy resins, and offer excellent thermal stability and mechanical properties.

[0028] Examples of thermosetting epoxy resins include bisphenol type epoxy resins (e.g., bisphenol A type epoxy resin, bisphenol F type epoxy resin, etc.), aliphatic epoxy resins (e.g., hexanediol diglycidyl ether, etc.), and glycidylamine type epoxy resins, resins (e.g., triglycidylaminophenol, etc.), novolac-type epoxy resins (e.g., phenol novolac epoxy resin, cresol novolac epoxy resin, etc.), alicyclic epoxy resins (e.g., 3, 4-epoxy cyclohexylmethyl, 3,4- epoxycyclohexane carboxylate, bis(3,4-epoxy cyclohexylmethyl adipate), etc.), brominated epoxy resins (e.g., tetrabromobisphenol A diglycidyl ether, etc.), polyfunctional epoxy resins (e.g., tris(hydroxyphenyl)methanetris), Examples include glycidyl ether, sorbitol polyglycidyl ether, tetraglycidyldiaminodiphenylmethane, etc.), crystalline epoxy resins (e.g. tetramethylbisphenol F diglycidyl ether, tetramethylbiphenol diglycidyl ether, etc.), and the like. These may be used alone or in combination of two or more.

[0029] In an embodiment, the multifunctional epoxy adhesive is a phenol novolac epoxy resin having the following stmcture, where n is any integer greater than 10 or 100:

[0030] In a further embodiment, the multifunctional epoxy adhesive is a cresol novolac epoxy resin having the following structure, where n is any integer greater than 10 or 100.

[0031] The multifunctional epoxy adhesive does not have to be an oligomeric or polymeric resin. It may be a non-polymer molecule with two or more epoxy functional groups. To achieve quick cure and for good heat resistance, a trifunctional epoxy adhesive may be preferable, in particular trifunctional aromatic epoxy adhesives with 1, 2 or 3 aromatic rings. Trifunctional heterocyclic epoxy adhesives may also be usable. Examples include:

Reactive diluent

[0032] In the present disclosure, the reactive diluent has at least one terminal epoxide moiety. It is added to the curable adhesive composition to modify viscosity and improve its rheological characteristics. It is preferably a low molecular weight, low-viscosity material. By reducing the viscosity of the adhesive, it becomes easier to expand and penetrate under heating. It may also be added to enhance flexibility. Some reactive diluents can improve the flexibility and toughness of cured epoxy adhesives, making them more resistant to cracking or breaking under stress.

[0033] The epoxide moiety refers to the oxirane ring structure. The presence of at least one epoxide moiety enables the reactive diluent to react with the multifunctional epoxy component. It is thus a reactive compound that reacts with the epoxy resin during the curing process, resulting in a modified polymer after curing. The reactive diluent may also control cure speed by influencing the curing time of the epoxy adhesive, allowing for adjustments to match specific application requirements. The epoxide moiety is a reactive functional group present in epoxy resins. When an epoxy adhesive is mixed and applied, the epoxide moieties undergo a chemical reaction known as epoxy curing or crosslinking. This reaction is typically triggered by heat in the presence of a curing agent or hardener.

[0034] The reaction between the epoxide moiety and the curing agent involves the opening of the oxirane ring and the formation of covalent bonds. This process is commonly referred to as epoxy resin curing or epoxy resin crosslinking. The specific mechanism of the reaction can vary depending on the type of curing agent used, but the general steps involved are, firstly, ring opening in which the curing agent, which is a compound containing amine or hydroxyl groups, reacts with the epoxide moiety by nucleophilic attack. This leads to the opening of the oxirane ring, resulting in the formation of a reactive intermediate. Secondly, crosslinking occurs and the reactive intermediate formed from the ring-opening reaction can undergo further reactions, such as condensation or addition reactions, with other epoxy molecules or curing agent molecules. This leads to the formation of covalent bonds between the epoxy resin molecules, resulting in a three-dimensional network structure. Finally, curing occurs in which the crosslinking reactions progress to cause the epoxy adhesive to transition from a liquid or semi-liquid state to a solid state. Various types of molecules comprise a terminal epoxide moiety, including glycidyl acrylates, glycidyl esters, glycidyl ethers and glycidols, for example.

[0035] In one embodiment, the reactive diluent is a glycidyl ether. Examples of glycidyl ethers include Butyl glycidyl ether (1,2-Epoxybutane), Phenyl glycidyl ether (1,2-Epoxyphenyl ethyl ether), Allyl glycidyl ether (2,3-Epoxypropyl prop-2 -enyl ether), Octyl glycidyl ether (1,2-Epoxyoctane), Isobutyl glycidyl ether (1,2-Epoxy -2 -methylpropyl ether), Benzyl glycidyl ether (l,2-Epoxy-3- phenoxypropane), Ethyl glycidyl ether (1,2-Epoxy ethyl ethyl ether), Methyl glycidyl ether (l,2-Epoxy-3- methoxypropane), Isopropyl glycidyl ether (1,2-Epoxy -2 -methylpropyl ether), Propyl glycidyl ether (1,2- Epoxypropyl propyl ether), Cyclohexyl glycidyl ether (l,2-Epoxy-3-cyclohexoxypropane), Tetrahydrofurfuryl glycidyl ether (l,2-Epoxy-3-(oxolan-2-yl)propane), and 2-Ethylhexyl glycidyl ether ( 1 ,2-Epoxy-3 -(2-ethylhexoxy)propane).

[0036] For example, the glycidyl ether is a Butyl Glycidyl Ether (BGE): BGE is a low-viscosity reactive diluent that improves the flow and wetting properties of epoxy adhesives. It may be used to reduce the viscosity of high-viscosity epoxy resins and enhance their handling characteristics.

[0037] In another example, the glycidyl ether is a Phenyl Glycidyl Ether (PGE): PGE is a reactive diluent that offers improved flexibility and impact resistance to epoxy adhesives. It may be used in applications where increased toughness and durability are required.

[0038] In another example, the glycidyl ether is Cresyl Glycidyl Ether (CGE): CGE is a reactive diluent that provides excellent chemical resistance and thermal stability to epoxy adhesives. It may be used in applications where resistance to harsh chemicals or elevated temperatures is necessary.

[0039] In another example, the glycidyl ether is Octyl Glycidyl Ether (OGE): OGE is a reactive diluent that offers good flexibility and low viscosity to epoxy adhesives. It may be used in applications where improved flexibility and impact resistance are desired. [0040] In another example, the glycidyl ether is Isobomyl Glycidyl Ether (IBGE): IBGE is a reactive diluent that provides excellent adhesion and chemical resistance to epoxy adhesives. It may be used in applications where high bond strength and resistance to chemicals are required.

[0041] In another example, the glycidyl ether is Allyl Glycidyl Ether (AGE): AGE is a reactive diluent that imparts good flexibility and toughness to epoxy adhesives. It may be used in applications where improved impact resistance and elongation properties are desired.

[0042] In a preferred embodiment, the reactive diluent is selected from a diglycidyl ether. The diglycidyl ether comprises two terminal epoxide moieties. Examples of usable diglycidyl ethers include diglycidyl ether of bisphenol A (DGEBA), Diglycidyl ether of bisphenol F (DGEBF), Diglycidyl ether of bisphenol S (DGEBS), Diglycidyl ether of resorcinol (DGER), Diglycidyl ether of hydroquinone (DGEH), Diglycidyl ether of 1,4-butanediol (DGE-1,4-BD), Diglycidyl ether of neopentyl glycol (DGE- NPG), Diglycidyl ether of polyethylene glycol (DGE-PEG), Diglycidyl ether of polypropylene glycol (DGE-PPG), Diglycidyl ether of polytetrahydrofuran (DGE-PTHF), Diglycidyl ether of glycerol (DGE- G), Diglycidyl ether of trimethylolpropane (DGE-TMP), Diglycidyl ether of pentaerythritol (DGE-PE), Diglycidyl ether of diethylene glycol (DGE-DEG), Diglycidyl ether of dipropylene glycol (DGE-DPG). [0043] In a preferred embodiment, the reactive diluent is a diglycidyl ether having formula (I): wherein

Rl, Rl’, R2 and R2’ are each independently selected from a Cl -CIO straight chain, branch chain, saturated or unsaturated or aromatic hydrocarbon moiety wherein hydrogen bonded to a carbon atom is optionally substituted by a halogen; and

R3 and R3' are each independently selected from -H, or Cl -CIO alkyl, alkenyl, phenyl, alkynyl, carbonyl moiety wherein hydrogen bonded to a carbon atom is optionally substituted by a halogen. [0044] In illustrative embodiments, the diglycidyl ether is selected from Ethyleneglycol diglyddyl ether 1,4-Butanediol diglyddyl ether 1,6-Hexanediol diglyddyl ether

Diethylene glycol diglyddyl ether Neopentyl glycol diglyddyl ether 1,4-Cyclohexanedimethanol diglyddyl ether 1,2-Cyclohexanediol diglyddyl ether Rezorcinol diglyddyl ether

[0045] In another embodiment, the reactive diluent comprises a triglycidyl ether. Examples of usable triglycidyl ethers include Triglycidyl isocyanurate (TGIC) Triglycidyl ether of trimellitic anhydride (TGEMA), Triglycidyl ether of glycerol (TGE-G), Triglycidyl ether of pentaerythritol (TGE- PE), Triglycidyl ether of tris(hydroxyethyl) isocyanurate (TGETHIC), Triglycidyl ether of triethylene glycol (TGE-TEG), Triglycidyl ether of tripropylene glycol (TGE-TPG), Triglycidyl ether of triethanolamine (TGE-TEA), Triglycidyl ether of triethylene diamine (TGE-TEDA), Triglycidyl ether of tris(2-aminoethyl)amine (TGE-TAEA), Triglycidyl ether of tris(2-hydroxyethyl)amine (TGE-THEA), Triglycidyl ether of tris(2-aminoethyl)amine (TGE-TAEA), Triglycidyl ether of tris(2- hydroxypropyl)amine (TGE-THPA), Triglycidyl ether of tris(2-hydroxyethyl)isocyanurate (TGE- THEIC), Triglycidyl ether of tris(2-hydroxyethyl)phosphate (TGE-THEP).

[0046] In a preferred embodiment, the triglycidyl ether has a formula (II):

Wherein R4 and R5 are each independently selected from a Cl -CIO straight chain, branch chain, saturated or unsaturated or aromatic hydrocarbon moiety wherein hydrogen bonded to a carbon atom is optionally substituted by a halogen; and R6 is selected from -H, or C1-C10 alkyl, alkenyl, phenyl, alkynyl, carbonyl moiety wherein hydrogen bonded to a carbon atom is optionally substituted by a halogen.

[0047] In preferred embodiments, the triglycidyl ether is selected from any one of the following: (g ycero r g yc y e er).

[0048] In an embodiment, the reactive diluent may comprises an aromatic monoglycidyl ether.

Examples include Phenyl glycidyl ether (PGE), Benzyl glycidyl ether (BGE), 4-Methoxyphenyl glycidyl ether, 4-Ethoxyphenyl glycidyl ether, 4-Chlorophenyl glycidyl ether, 4-Bromophenyl glycidyl ether, 4- Fluorophenyl glycidyl ether, 4-Nitrophenyl glycidyl ether, 4-Hydroxyphenyl glycidyl ether, 2-Naphthyl glycidyl ether, 4-Methylphenyl glycidyl ether, 4-Ethylphenyl glycidyl ether, 4-Isopropylphenyl glycidyl ether, 4-tert-Butylphenyl glycidyl ether, 4-Phenoxyphenyl glycidyl ether. In a preferred embodiment, the aromatic monoglycidyl ether has the formula (III):

(III) : wherein R7 is a Ci to Ce straight chain or branched alkyl or alkenyl hydrocarbon moiety. In preferred embodiments, R7 is selected from 1,1 dimethyl ethyl, 1,2 dimethyl propyl, 1 -methylpropyl, or 1 -methylbutyl moieties.

[0049] By selecting an appropriate low molecular weight reactive diluent, several objectives can be achieved. Low molecular weight reactive diluents have a low viscosity, which helps in reducing the overall viscosity of the epoxy adhesive. This makes the adhesive easier to handle, mix, and apply. It also achieves improved wetting and penetration, allowing it to spread more easily and uniformly over the substrate. This results in better adhesion and bonding. Additionally, by varying the amount of low molecular weight reactive diluent added, the cure time can be adjusted to match specific application requirements. Low molecular weight reactive diluents can accelerate the curing process of epoxy adhesives for faster bonding. They promote the crosslinking reaction between the epoxy resin and curing agent, leading to faster curing times. Some low molecular weight reactive diluents can enhance the flexibility and toughness of the cured epoxy adhesive. This helps to improve the adhesive's resistance to cracking or breaking under stress. Certain low molecular weight reactive diluents can improve the chemical resistance of the epoxy adhesive, the adhesive's ability to withstand exposure to certain chemicals, solvents, and environmental conditions. In the context of this disclosure, “low molecular weight” of the reactive diluent is relative and presently defined to be lower than the molecular weight of the multifunctional epoxy adhesive. In various embodiments, the reactive diluent has a molecular weight of less than 1000, or less than 800, or preferably less than 500, or more preferably less than 300, or less than 250, or most preferably less than 210 g per epoxy group (g/ep).

[0050] The weight percentage range of reactive diluent in epoxy can vary depending on the specific application and desired properties of the adhesive. However, as a general guideline, the weight percentage of reactive diluent in epoxy adhesive formulations typically falls within the range of 10% to 50%. The specific percentage within this range will depend on factors such as the desired viscosity, cure speed, flexibility, and other performance requirements of the adhesive. It's important to note that the addition of reactive diluent can significantly impact the properties of the epoxy adhesive, so the percentage should be carefully determined based on the specific needs of the application. In a preferred embodiment, the reactive diluent is present in an amount of between 1 to 12% by weight of the curable adhesive composition excluding the weight of filler.

[0051] The reactive diluent may in certain cases comprise a monoglycidyl ether. In an embodiment, the monoglycidyl ether is an aliphatic monoglycidyl ether having formula (IV):

(IV): R8 wherein R8 is selected from a C2-C14 straight chain or branched or cyclic alkyl, alkenyl, phenylalkyl moiety. Examples include the following:

[0052] It is possible to use a single glycidyl ether type as mentioned above, or combine different types of glycidyl ethers, for example, combining diglycidyl ethers with triglycidyl ethers, or combining monoglycidyl ethers, diglycidyl ethers and triglycidyl ethers.

Foaming agent

[0053] The foaming agent may be any foaming agent that can foam and expand the adhesive layer when bonded to the adherend. The foaming agent is preferably a temperature-sensitive blowing agent. Examples of the foaming agent include inorganic blowing agents such as ammonium carbonate, ammonium hydrogen carbonate, ammonium nitrite, ammonium borohydride, and azides; fluorinated alkanes such as trichloromonofluoromethane, and azobisisobutyronitrile. Azo compounds such as, hydrazine compounds such as para-toluene sulfonyl hydrazide, semi carbazide compounds such as p- toluene sulfonyl semi carbazide, triazole compounds such as 5-morpholyl-l,2,3,4-thiatriazole, N,N - Organic blowing agents such as N-nitroso compounds such as dinitroso terephthalamide; thermally expandable microcapsules in which a thermally expandable agent (e.g. a hydrocarbon compound, etc.) is microencapsulated; and the like. Among these, thermally expandable particles are preferred from the viewpoint of not easily inhibiting the curing of the adhesive.

[0054] The thermally expandable particles may include, for example, a thermoplastic resin shell and an expanding agent (e.g. liquid hydrocarbon) encapsulated in the shell. Examples of the thermally expandable particles include Matsumoto Microsphere (registered trademark) series (manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.). [0055] The foaming start temperature (Tc) of the foaming agent may be, for example, 90°C or higher, and from the viewpoint that foaming is likely to start after the curable adhesive composition is sufficiently softened, the foaming start temperature (Tc) is 95°C or higher or 100°C or more. The foaming start temperature (Tc) of the foaming agent may be, for example, 140°C or lower, and from the viewpoint of easily obtaining a sufficient expansion ratio before the curable adhesive composition hardens, it is 135°C or lower. Alternatively, the temperature may be 130°C or lower.

[0056] The content of the foaming agent may be such that the expansion ratio described below can be achieved. The content of the foaming agent may be, for example, 0.5 parts by mass or more, 1 part by mass or more, 3 parts by mass or more, or 5 parts by mass or more with respect to 100 parts by mass of the thermosetting resin. Further, the content of the foaming agent may be, for example, 30 parts by mass or less, 25 parts by mass or less, or 20 parts by mass or less.

[0057] The first adhesive layer is foamed and cured by heating. The expansion ratio of the first adhesive layer is not particularly limited and may be determined as appropriate depending on the thermal conductivity and adhesive strength required of the foamed cured product, or the distance between the objects to be adhered, and the like. The expansion ratio of the first adhesive layer may be, for example, 1.5 times or more, 2 times or more, 2.5 times or more, or 3 times or more. Further, the expansion ratio of the first adhesive layer may be, for example, 10 times or less, 9 times or less, 8 times or less, or 7 times or less. Note that the larger the expansion ratio, the more voids may be generated by foaming, and the resulting thermal conductivity after foaming and curing tends to be lower. Hence an optimal expansion ratio may be determined by experimentation. In this specification, the expansion ratio of the first adhesive layer is a value determined as the ratio of the thickness of the first adhesive layer before and after foaming and curing.

[0058] The expansion ratio of the first adhesive layer can be adjusted as appropriate by, for example, the content of the foaming agent.

Curing agent

[0059] The curable adhesive composition may further contain a curing agent. The curing agent may be any curing agent capable of curing the thermosetting resin, and may be appropriately selected from known curing agents. As the curing agent, a latent curing agent is preferable from the viewpoint of avoiding curing of the thermosetting resin before foaming.

[0060] For epoxy adhesives, examples of curing agents include dicyandiamide (DICY): DICY is a widely used epoxy curing agent known for its excellent heat resistance and electrical properties. It is commonly used in applications such as adhesives, coatings, and composites. Polyamides are a class of curing agents derived from the reaction of polyamines with dimerized fatty acids. They offer good chemical resistance and flexibility, making them suitable for applications such as coatings, adhesives, and encapsulants.

[0061] Aromatic Amines: Aromatic amines, such as diaminodiphenylmethane (DDM) and diaminodiphenylsulfone (DDS), are commonly used as epoxy curing agents. They provide excellent chemical resistance and high-temperature performance, making them suitable for applications in aerospace, automotive, and electrical industries.

[0062] Anhydrides: Anhydrides, such as methylhexahydrophthalic anhydride (MHHPA) and nadic methyl anhydride (NMA), are widely used as epoxy curing agents. They offer good heat resistance and electrical properties, making them suitable for applications in electrical insulation, laminates, and composites.

[0063] Phenalkamines: Phenalkamines are a class of epoxy curing agents derived from the reaction of phenolic compounds with polyamines. They provide fast cure, good chemical resistance, and excellent adhesion to various substrates. Phenalkamines are commonly used in marine coatings, flooring, and concrete repair applications.

[0064] Cycloaliphatic Amines: Cycloaliphatic amines, such as isophoronediamine (IPDA) and diaminocyclohexane (DACH), are epoxy curing agents known for their excellent chemical resistance and low viscosity. They are often used in applications requiring high chemical resistance, such as tank linings, chemical storage, and corrosion protection coatings.

[0065] Imidazoles: Imidazoles, such as 2-methylimidazole (2 -MI) and 2-ethyl-4-methylimidazole (2-E4MI), are commonly used as epoxy curing agents. They provide fast cure at room temperature and are often used in applications such as adhesives, potting compounds, and encapsulants.

[0066] Latent Curing Agents: Latent curing agents, such as dicyandiamide (DICY) and imidazole derivatives, are designed to provide delayed or controlled curing of epoxy resins. They are often used in applications where extended shelf life is required. Examples include dicyandiamide, 2,4-diamino-6-[2'- methimidazolyl-(r)]-ethyl-s-triazine isocyanur acid adducts, etc. The content of the curing agent may be adjusted as appropriate depending on the type of thermosetting resin and the type of curing agent. The content of the curing agent may be, for example, 1 part by mass or more, 2 parts by mass or more, 3 parts by mass or more, 4 parts by mass or more, or 5 parts by mass or more, based on 100 parts by mass of the thermosetting resin. Further, the content of the curing agent may be, for example, 20 parts by mass or less, 18 parts by mass or less, 16 parts by mass or less, 14 parts by mass or less, 12 parts by mass or less, based on 100 parts by mass of the thermosetting resin. It may be 10 parts by mass or less.

[0067] The adhesive(A) may further contain a curing accelerator. The curing accelerator may be any one that can accelerate the curing of the thermosetting resin by the curing agent, and may be appropriately selected from known curing accelerators. When the curable adhesive composition is an epoxy adhesive, examples of the curing accelerator include imidazole curing accelerators (for example, 2,4-diamino-6-[2'-methylimidazolyl-(l')]) -ethyl-s-triazine, etc.), urea-based curing accelerators (for example, 4,4'-methylenebisphenyldimethylurea, 3-(3,4-dichlorophenyl)-l,l-dimethylurea, etc.), etc.

[0068] The content of the curing accelerator may be adjusted as appropriate depending on the type of thermosetting resin and the type of curing agent. The content of the curing accelerator may be, for example, 0.1 parts by mass or more, 0.2 parts by mass or more, 0.3 parts by mass or more, 0.4 parts by mass, based on 100 parts by mass of the thermosetting resin, or more or 0.5 part by mass or more. Further, the content of the curing accelerator may be, for example, 10 parts by mass or less, 8 parts by mass or less, 6 parts by mass or less, 4 parts by mass or less, or 2 parts by mass or less, based on 100 parts by mass of the thermosetting resin.

Other components

[0069] The first adhesive layer may further contain components other than the curable adhesive composition, the thermally conductive filler, and the foaming agent. Other components include, for example, thickeners, impact modifiers, dispersants for inorganic thermally conductive fillers, and the like. [0070] As used herein, the term “dispersant” refers to a substance that may be added to the composition in order to improve the separation of the thermally conductive filler particles by wetting the particles and breaking apart agglomerates. The dispersant, if present, may be present in the composition in an amount of at least 0.05% by weight based on total weight of the composition, such as 0.1% by weight, such as 0.3% by weight, and may be preferably present in an amount of 0.5% by weight. Suitable dispersants for use in the composition include fatty acid, phosphoric acid esters, polyurethanes, polyamines, polyacrylates, polyalkoxylates, sulfonates, polyethers, and polyesters, or any combination thereof. Non-limiting examples of commercially available dispersants include ANTI-TERRA-U100, DISPERBYK-102, DISPERBYK-103, DISPERBYK-111, DISPERBYK-145, DISPERBYK-171, DISPERBYK-2151, DISPERBYK-2059, DISPERBYK-2000, DISPERBYK-2117, and DISPERBYK- 2118 available from BYK Company; and SOLSPERSE 24000SC, SOLSPERSE 16000 and SOLSPERSE 8000 hyperdispersants available from The Lubrizol Corporation.

[0071] The content of the thickener is not particularly limited. The first adhesive layer may be formed by applying and drying a coating liquid containing an curable adhesive composition, a thermally conductive filler, a foaming agent, and a solvent. The thickener may be blended so that the viscosity of the coating liquid becomes a viscosity suitable for coating, and the content of the thickener is determined so that the viscosity of the coating liquid becomes a viscosity suitable for coating. The amount may be adjusted so that the content of the thickener may be, for example, 5% by mass or less, 3% by mass or less, 2.5% by mass, based on the total amount of components other than the thermally conductive filler in the first adhesive layer. It may be less than or equal to 2% by mass. Further, the content of the thickener may be, for example, 0.1% by mass or more, and 0.3% by mass or more, based on the total amount of components other than the thermally conductive filler in the first adhesive layer, or 0.5% by mass or more, 0.7% by mass or more, or 1% by mass or more.

[0072] Silane coupling agents may be added to the adhesive layers. Examples of silane coupling agents include amino silanes such as Aminopropyltriethoxysilane (APTES), Aminopropyltrimethoxy silane (APTMS), N-(2-Aminoethyl)-3 -aminopropyltrimethoxy silane (AEAPTMS), N-(2-Aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPMDMS); Epoxy Silanes such as Glycidyloxypropyltrimethoxysilane (GPTMS), Glycidyloxypropylmethyldimethoxysilane (GPMDS), 3-Glycidoxypropyltriethoxysilane (GPTES); Vinyl Silanes such as Vinyltrimethoxysilane (VTMS), Vinyltriethoxysilane (VTES) Vinyltris(2-methoxyethoxy)silane (VTMOS); Methacryloxy Silanes such as Methacryloxypropyltrimethoxysilane (MPTMS), Methacryloxypropylmethyldimethoxysilane (MPMDS); Mercapto Silanes such as Mercaptopropyltrimethoxysilane (MPTMS) Mercaptopropylmethyldimethoxysilane (MPMDS); Chloro Silanes such as Chloromethyltrimethoxysilane (CMTMS), Chloromethyltriethoxysilane (CMES) Dichlorodimethylsilane (DCDMS); Alkoxy Silanes such as Methoxytrimethylsilane (MTMS), Ethoxytrimethylsilane (ETMS), Propoxytrimethylsilane (PTMS), Isobutoxytrimethylsilane (IBTMS); and mixtures thereof containing a combination of different functional groups, such as amino, epoxy, vinyl, or methacryloxy, to provide multifunctionality.

[0073] Examples of impact resistance modifiers include core-shell type impact resistance modifiers. Examples of core-shell type impact modifiers include core-shell rubber.

[0074] Core-shell rubber contains different materials in the inner core part and the outer shell part. The glass transition temperature (Tg) of the shell portion is preferably higher than the Tg of the core portion. The Tg of the core portion may be, for example, from -110°C to -30°C, and the Tg of the shell portion may be, for example, from 0°C to 200°C. In this specification, Tg of the core portion and the shell portion is defined as the temperature of the peak value of tan 5 in dynamic viscoelasticity measurement. The core portion of the core-shell rubber acts as a stress concentration point, improving impact resistance, and the shell portion also suppresses undesirable agglomeration of the core-shell rubber, resulting in a uniform distribution of the core-shell mbber.

[0075] Core-shell rubbers include, for example, polymers of conjugated dienes such as butadiene, isoprene, 1,3 -pentadiene, cyclopentadiene, and dicyclopentadiene; polymers of non-conjugated dienes such as 1,4-hexadiene and ethylidene norbomene; conjugated dienes or Non-conjugated dienes and monofunctional monomers (e.g. aromatic vinyl compounds such as styrene, vinyltoluene, a- methylstyrene, unsaturated nitrile compounds such as acrylonitrile and methacrylonitrile, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate) (methjacrylate etc.); acrylic rubber such as polybutyl acrylate; silicone rubber; IPN type composite mbber consisting of silicone and polyalkyl acrylate; It may be a core-shell type graft copolymer having a shell portion formed by copolymerizing a (methjacrylic acid ester around the core. For the core part, polybutadiene, butadiene-styrene copolymer, or acrylic - butadiene rubber-styrene copolymer can be advantageously used, and for the shell part, one formed by copolymerizing methyl (methjacrylate is advantageously used. Can be used. The shell portion may be layered and may consist of one or more layers. As the core-shell rubber, two or more kinds of core-shell rubbers may be used in combination.

[0076] Examples of the core-shell rubber include methyl methacrylate-butadiene copolymer, methyl methacrylate-butadiene-styrene copolymer, methyl methacrylate-acrylonitrile-butadiene-styrene copolymer, methyl methacrylate-acrylic rubber copolymer, and methyl methacrylate-acrylic. Examples include rubber-styrene copolymer, methyl methacrylate-acrylic/butadiene rubber copolymer, methyl methacrylate-acrylic/butadiene rubber-styrene copolymer, methyl methacrylate-(acrylic/silicone IPN rubber) copolymer, and the like. Among these, methyl methacrylate-butadiene copolymer, methyl methacrylate-butadiene-styrene copolymer, and methyl methacrylate-acrylic butadiene rubber-styrene copolymer can be advantageously used as the core-shell rubber.

[0077] The average primary particle size (mass average particle size) of the core-shell rubber may be, for example, 0.05 pm or more, or 0.1 pm or more. Further, the average primary particle diameter (mass average particle diameter) of the core-shell rubber may be, for example, 5 pm or less, 3 pm or less, or 1 pm or less. Note that the average value of the primary particle diameter of the core-shell rubber is calculated from the value obtained by zeta potential particle size distribution measurement. The content of the impact resistance modifier is not particularly limited, and may be, for example, 20% by mass or less, and 15% by mass or less, based on the total amount of components other than the thermally conductive filler in the first adhesive layer. Alternatively, it may be 10% by mass or less. Further, the content of the impact resistance modifier may be, for example, 1% by mass or more, 2% by mass or more, or 3% or more by mass based on the total amount of components other than the thermally conductive filler in the first adhesive layer.

[0078] The curable adhesive composition may also comprise polymers that provide performance enhancements for epoxy systems. For example, phenoxy resins may be included to promote adhesion and as a strengthening enhancer in epoxy composites, as well as promote flexibility and boost chemical resistance. Examples include polyhydroxy ether phenoxy resins in colloidal dispersions or in organic solvents such as methyl ether ketone. In embodiments, the phenoxy resin has formula (V) and a molecular weight (Mw) of between 60K to 80K g/mol: wherein n is an integer required for the resin to be above 50k, or within the 60k to 80k g/mol molecular weight range, e.g. n = 800 to 3000.

Compositional ranges of curable adhesive composition components

[0079] As noted above, the reactive diluent may be present in an amount of between 10 to 50% by weight (wt%) of the curable adhesive composition, excluding the weight of filler. The curable adhesive composition may further comprise, based on the total amount of the curable adhesive composition, multifunctional epoxy resin ranging from 40 to 80 wt% of the curable adhesive composition. The foaming agent may be present in the range of 5 to 25 wt%. Apart from reactive diluent, multifunctional epoxy resin and faming agent, other components may be added to the curable adhesive composition as needed. In an embodiment, the curable adhesive composition comprises 5 to 35 wt% of an additional component selected from one or more of a curing agent, coupling agent, toughening agent, and accelerator. In a further embodiment, the curable adhesive composition further comprises a phenoxy resin. The phenoxy resin amount is not limited, and in some embodiments herein, it may be present in an amount of between 1 to 15% by weight. Filler

[0080] The thermally conductive filler is a not particularly limited in shape or size provided it is compatible for mixing with the curable adhesive composition. In some examples, it is a filler with an average short side length of 1 pm or more. A filler with an average short side length of less than 1 pm has a low contribution to thermal conductivity and is difficult to function as a thermally conductive filler. The average short side length of the thermally conductive filler is 1 pm or more, and may be 1.5 pm or more or 2 pm or more from the viewpoint of increasing the contribution to thermal conductivity. Further, the average short side length of the thermally conductive filler may be, for example, 100 pm or less, 90 pm or less, 80 pm or less, or 70 pm or less.

[0081] As the reactive diluent improves the characteristics of the adhesive composition, the content of the thermally conductive filler may be increased. Based on the total volume of the first or second adhesive layer, the content of the thermally conductive filler present is at least 10% by volume. In some embodiment, it is 37% by volume or less based on the total volume of the first adhesive layer, and from the viewpoint of further improving the adhesive strength with the adhered object, it is 35% by volume or less. Alternatively, it may be 33% by volume or less.

[0082] In addition, when the adhesive sheet has a first adhesive permeable layer, the adhesive easily permeates the first adhesive permeable layer, and from the viewpoint that high adhesive strength with the object to be adhered is easily obtained. The content of the thermally conductive filler is based on the total volume of the first adhesive layer, 30% by volume or less, 27% by volume or less, 25% by volume or less, 23% by volume or less, 21% by volume or less, 19% by volume or less, less than 17% by volume, or less than 15% by volume.

[0083] The material of the thermally conductive fdler is not particularly limited and can be appropriately selected from known thermally conductive fillers. The thermally conductive filler may contain, for example, at least one member selected from the group consisting of boron nitride, aluminum nitride, alumina, magnesium oxide, anhydrous magnesium carbonate, magnesium hydroxide, silicon oxide, and silicon nitride.

[0084] The thermally conductive filler may include a filler having an aspect ratio of 1.3 or more. Since such fdler has a high aspect ratio, it forms a heat conduction path from one side of the adhesive layer to the other side even if the gap between the bubbles generated during expansion is narrow. It's easy to do. Therefore, according to the filler, even when foaming and curing is performed at a high expansion ratio (for example, 1.4 times or more), a foamed and cured product having high thermal conductivity can be obtained.

[0085] The shape of the filler is not particularly limited as long as it satisfies the above aspect ratio, and may be, for example, platelet-like, whisker-like, agglomerate-like, or the like.

[0086] The aspect ratio of the filler may be, for example, 1.4 or more, 1.5 or more, or 1.6 or more, from the viewpoint of achieving the above effects more significandy.

[0087] The aspect ratio of the filler may be, for example, 200 or less, 150 or less, or 100 or less. [0088] When the filler is in the form of platelets or agglomerates, the aspect ratio of the filler may be, for example, 50 or less, 30 or less, 10 or less, 5 or less, or 3 or less. When the filler is whisker-like, the aspect ratio of the filler may be, for example, more than 50, 60 or more, 70 or more, 80 or more, or 90 or more.

[0089] The average short side length of the filler may be the same as the average short side length of the thermally conductive filler.

[0090] In this embodiment, the filler with a high aspect ratio efficiently forms a heat conduction path between the bubbles. The thermally conductive filler may be at least partially filler or may be entirely filler.

[0091] From the viewpoint of forming more heat conduction paths, the content of the filler may be, for example, 0.01% by volume or more, and 0.05% by volume based on the total volume of the first adhesive layer, 0.1 volume % or more, or 0.15 volume % or more.

[0092] The thermally conductive filler may further include a filler having an aspect ratio of less than 1.3. That is, the thermally conductive filler may include a fdler with an aspect ratio of 1.3 or more and a filler with an aspect ratio of less than 1.3.

[0093] The proportion of the filler in the thermally conductive filler may be, for example, 0.1% by volume or more based on the total volume of the thermally conductive filler, making the heat conduction path more efficient. From the viewpoint of good formation, the content may be 0.3% by volume or more, 0.5% by volume or more, 0.7% by volume or more, or 1% by volume or more.

[0094] When the aspect ratio of the filler exceeds 50 (for example, when the filler has a whisker shape), the proportion of the filler in the thermally conductive filler, based on the total volume of the conductive filler, may be, for example, 100 volume% or less, 50 volume% or less, 30 volume% or less, 10 volume% or less, 5 volume% or less, or 3 volume% or less.

[0095] When the aspect ratio of the filler is 50 or less (for example, when the filler has platelet-like or lump-like shape), the proportion of the filler in the thermally conductive filler is, for example, 10 volume% or more, 30 volume% or more, 50 volume% or more, 70 volume% or more, or 90 volume% or more, and 100 volume% or more, based on the total volume of the thermally conductive filler.

Viscosity

[0096] The curable adhesive composition is generally in an unflowable solid or semi-solid state at ambient temperature and pressure. Upon heating, it starts to soften and gradually turns into a flowable liquid or semi-liquid state. In an embodiment, the curable adhesive composition has a softening point above 60°C [Ring and Ball apparatus according to the ASTM D-2398 Test Method]. When fully converted into liquid state, the curable adhesive composition has a melt viscosity of less than 800,000, or less than 500,000, or preferably less than 300,000 Pa.s at 110°C [ASTM D3835-16 , Standard Test Method for Determination of Properties of Polymeric Materials by Means of a Capillary Rheometer], The curable adhesive composition may have a melt viscosity of more than 10 to 100 Pa.s as a lower limit. Flowable liquids can have a wide range of viscosities. For example, water has a relatively low viscosity of about 1 cP at room temperature (0.001 Pa.s), while honey has a relatively higher viscosity of around 10,000 cP (10 Pa.s). The curable adhesive composition of the present application may have a melt viscosity value between 100 to 400 Pa.S when heated.

Adhesive permeable layers

[0097] The adhesive sheet may further comprise an adhesive permeable layer disposed on one or both of the first and second adhesive layers, said adhesive permeable layer being permeable to the curable adhesive composition when heated until one or both of the first and second adhesive layer foams. The adhesive permeable layer is permeable in that it allows the curable adhesive composition to flow from one main surface side of the first adhesive permeable layer to the other main surface side when the first adhesive layer expands (foams). In order to be permeable, the adhesive permeable layer is porous, i.e. it has voids present across the layer from one main surface to the other main surface. In this case, when the first adhesive layer that is in contact with only one main surface of the first adhesive permeable layer expands (foams), the curable adhesive composition passes through the first adhesive layer through the voids when it is heated and flowable.

[0098] The material constituting the adhesive permeable layer is not particularly limited, and any material may be used as long as it can maintain a shape that allows the curable adhesive composition to permeate at the curing start temperature of the curable adhesive composition.

[0099] The adhesive permeable layer may be, for example, a nonwoven fabric made of natural fibers, chemical fibers, or a mixture thereof. Since the nonwoven fabric has a large number of through voids inside, the above effect can be significantly obtained.

[00100] The basis weight of the first adhesive permeable layer may be, for example, 10 g/m² or more, or 11 g/m² or more. The upper limit of the basis weight of the first adhesive permeable layer is not particularly limited, and may be within a range that satisfies the thickness range of the first adhesive permeable layer, which will be described later.

[00101] The thickness of the first adhesive permeable layer may be, for example, 55 pm or less, and when the first adhesive layer expands, the amount of curable adhesive composition that oozes out to the surface increases, resulting in higher adhesive strength. From the viewpoint of this, it may be 50 pm or less or 47 pm or less. The lower limit of the thickness of the first adhesive permeable layer is not particularly limited, and may be within a range that satisfies the above-mentioned range of basis weight of the first adhesive permeable layer, for example.

Method of preparation

[00102] In another aspect, the present disclosure provides a method for forming an adhesive sheet, comprising: providing an electrically insulating substrate, mixing a thermally conductive filler with a curable adhesive composition comprising: a multi-functional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent to form a prepolymer composition; disposing the prepolymer composition on one side of the substrate to form a first adhesive layer; and disposing the prepolymer composition on another side of the substrate opposite the first adhesive layer to form a second adhesive layer.

[00103] In an embodiment, the prepolymer composition is a hotmelt composition, wherein the prepolymer composition is heated to around, or above, its glass transition temperature so that it becomes flowable and capable of deposition on the substrate. After deposition, the prepolymer composition is left to cool and solidify.

[00104] In another embodiment, the prepolymer composition further comprises a solvent, such as methyl ethyl ketone (MEK), so that it is rendered flowable and suitable for deposition on the substrate. After coating the substrate, the solvent is allowed to evaporate naturally or by heat, to form the adhesive layer.

[00105] The processing temperature for liquefying the prepolymer composition, either as a hotmelt, or to remove the solvent, is not particularly limited. Preferably it is below 150°C, or below 100°C.

[00106] If it is desired to apply an adhesive permeable layer on each of the adhesive layers, heat lamination may be carried out at slightly elevated temperatures without initiating heat cure of the adhesive layer, for example at a temperature of above 40°C to 100°C, or more preferably between 50°C to 90°C, or between 55°C to 65°C.

Figures

[00107] FIG. 1 is a cross-sectional view showing one embodiment of an adhesive sheet before heating and curing. The adhesive sheet 10 shown in FIG. 1 comprises an electrically insulating substate 20, first and second adhesive layers 30a and 30b, and adhesive permeable layers 40a and 40b. Foaming agent comprising heat expandable microspheres 91 are shown as small circles present in the first and second adhesive layers. Thermally conductive filler 92 are denoted by dots present in the first and second adhesive layers.

[00108] FIG. 2A is a cross-sectional view showing one embodiment of an adhesive sheet 11 after heating. Foaming agent comprising heat expandable microspheres 91 expand to form expanded microspheres 93 as a result of heat, pushing the liquefied curable adhesive composition and thermally conductive filler 92 towards the adhesive permeable layers 40a and 40b. Gradually, the curable adhesive composition saturate the adhesive permeable layers 40a and 40b, and thereafter the curable adhesive composition emerge on the outer surface of the adhesive permeable layers, forming an outer adhesive layer 50a and 50b. Upon further heating, the curable adhesive composition cures and forms an outer cured adhesive layer on the adhesive sheet 11. In this embodiment, the adhesive permeable layer is also permeable to the thermally conductive filler and expandable microsphere, so that they also penetrate the adhesive permeable layer.

[00109] FIG. 2B is a cross-sectional view showing another embodiment of an adhesive sheet 12 after heating. By selecting the adhesive permeable layer to be one that is impermeable to the thermally conductive filler and expandable microsphere, so that they are retained within the initial adhesive layers, it is possible to cause only the curable adhesive composition to permeate the adhesive permeable layer during expansion to form outer adhesive layers 50c, 50d over the adhesive permeable layers.

[00110] The adhesive sheet 10 shown in FIG. 1 can be said to be in a tack-free state because the first adhesive permeable layer 40a, 40b are the outermost layers. The adhesive sheet 10 thus efficiently bonds to surfaces to be adhered through a single heating step.

[00111] The foamed cured product of this embodiment includes a electrically insulating substrate, a first foamed layer formed by foaming and curing of the first adhesive layer, and a second foamed layer formed by foaming and curing of the second adhesive layer.

[00112] In the foamed cured product of this embodiment, the first adhesive permeable layer may be embedded within the first foamed layer. Further, in the foamed cured body of the embodiment, a second adhesive permeable layer may be embedded within the second foamed layer.

[00113] The expansion ratio of the adhesive sheet of this embodiment is not particularly limited and may be determined as appropriate depending on the thermal conductivity and adhesive strength required of the foamed cured product, the distance between the objects to be adhered, and the like. The expansion ratio of the adhesive sheet may be, for example, 1.3 times or more, 1.35 times or more, or 1.4 times or more. Further, the expansion ratio of the adhesive sheet may be, for example, 6.5 times or less, 5.8 times or less, 5.3 times or less, or 4.6 times or less. In the specification, the expansion ratio of an adhesive sheet is a value determined as the ratio of the thickness of the adhesive sheet to the thickness of a foamed and cured product obtained by foaming and hardening the adhesive sheet.

Uses of the adhesive sheet

[00114] In a further aspect, the present disclosure is directed to an electric motor comprising a stator, comprising: a stator core having at least one slot; a winding accommodated within the slot and an insulating slot liner comprising an adhesive sheet as presently defined in this application arranged between the stator core and the winding. In one embodiment, the adhesive sheet is cured.

[00115] In yet a further aspect, the present disclosure is direct to a method of electrically insulating an electric motor comprising a stator, comprising providing a stator core having at least one slot, arranging an insulating slot liner comprising an adhesive sheet as presently defined in the at least one slot, arranging an electrical winding within the slot, and curing the adhesive sheet.

[00116] The use of the adhesive sheet of this embodiment is not particularly limited, and it can be used for various purposes of adhering objects to be adhered to each other. Since the adhesive sheet of this embodiment has excellent thermal conductivity after foaming and curing, it can be suitably used for applications requiring characteristics such as thermal conductivity and heat dissipation. Moreover, since the adhesive sheet of this embodiment is expandable, it can be adapted to the surface shape of the object to be adhered and can fill in the gaps between the objects to be adhered. Moreover, even if unintended irregularities occur on the surface of the objects to be adhered, the objects to be adhered can be bonded to each other suitably. Therefore, the adhesive sheet of this embodiment can be suitably used in cases where objects to be bonded have irregularities on their surfaces or in cases where gaps between objects to be bonded need to be filled.

[00117] The adhesive sheet of this embodiment can be suitably used, for example, as a slot liner for a stator core in an electric motor. That is, the adhesive sheet of this embodiment may be used, for example, to be placed between the stator core and the winding to bond the stator core and the winding. The electric motor of this embodiment may include a stator, which includes a stator core having at least one slot, a winding at least partially housed in the slot, and an adhesive layer bonding the stator core and the winding.

[00118] The adhesive layer may be a layer formed by foaming and curing the adhesive sheet. That is, the adhesive layer may be a layer containing a foamed and cured adhesive sheet. In the electric motor of this embodiment, each structure other than the adhesive layer is not particularly limited, and may be the same as each structure in a known electric motor. Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments.

Examples

[00119] Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

[00120] Ingredients used are shown in Table 1. The amount of each ingredient used is shown in Table 2.

[00121] Note that Table 2 describes the calculation basis for each of the examples below. To establish the “Control” curable adhesive composition without any reactive diluent, 0.0% reactive diluent was added (sample 1-1). To form an adhesive composition according to the present invention, Table 2 shows a sample with 5 wt% of ED523T added to a control composition i.e. the reactive diluent added is 5% of the weight of the control. When normalized against the total weight of the new adhesive composition with reactive diluent added, the amount of ED523T present is 4.8%.

Table 1

Table 2

Thermally conductive filler size measurement

[00122] To determine the size of the fillers used, filler was sprinkled onto the conductive doublesided tape attached to the sample stage, and excess filler was removed with a blower. Next, the filler was coated with osmium using an osmium plasma coater (Japan Laser Electronics Co., Ltd. OPC80N), and the sample was subjected to conductive treatment.

[00123] A secondary electron image of the filler was obtained using a scanning electron microscope (Hitachi High-Tech Corporation S3400N) at an accelerating voltage of 10 kV, a working distance of 10 mm, and an observation magnification in the range of 100 times to 3000 times. From the obtained image, the minimum rectangle (minimum circumscribed rectangle) that can cover the filler was confirmed visually and by image analysis, and the long sides and short sides of the rectangle were determined as the long sides and short sides of the filler.

[00124] At least 3 images for each type of filler was acquired, and measurements were made of the long and short sides of 10 to 30 fillers per image, and 50 or more fillers in total, and calculate the average value as the average long side of the filler length and average short side length. Further, the aspect ratio of the filler was determined by dividing the average long side length by the average short side length.

The filler dimension measurements are shown in Table 3. Criteria for evaluating melt viscosity, thermal conductivity, overlap shear (OLS) strength, and Tg of each sample is found in Table 4.

Table 3

Table 4

Samples (A) for thermal conductivity & overlap shear strength

[00125] As a base material, a 75 pm thick PEN film (trade name: Teonex Q51, manufactured by

Toyobo Co., Ltd.) was obtained. Curable adhesive composition was obtained by mixing BTA731 and NPPN442, then adding other materials shown in Table 1, and mixing with a mixer. Each of the samples IS- 1 to IS-4, and 1-1 to 1-7, were prepared by mixing 100 parts by mass of the curable adhesive composition prepared by the above method and 24.9 parts by mass of the thermally conductive filler with its content in the adhesive layer set as 13.9% by volume of the total volume of the curable adhesive composition. ED523T was used as the reactive diluent. Examples 1-9 to 1-13 were prepared in a similar manner except that the amount of filler BN-CFP-012 was varied from 0 Vol.% to 30 Vol.%. Examples IS-6 and IS-7 were prepared in a similar manner with thermally conductive filler set at 13.9% by volume except that the reactive diluent type was varied.

[00126] A coating liquid containing curable adhesive composition and a thermally conductive filler distributed therein for forming an adhesive layer was obtained. This coating liquid was applied to one side of the base material and dried at 65°C for 3 minutes and at 90°C for 3 minutes to form a first adhesive layer. The thickness of the first adhesive layer was about 50 pm. The thickness of the first adhesive layer is determined by measuring the thickness at three arbitrary points on the A4 size area of the sample after forming the first adhesive layer using a desktop micrometer and finding the average thickness of the sample. The average thickness was subtracted from the thickness of the base material. [00127] Next, nonwoven sheets (PET, basis weight 13 g/m²) are laminated and heated and pressed using a roll laminating machine at a roll temperature of 60°C on top of the first adhesive layer.

[00128] Next, the coating liquid was applied to the other side of the base material and dried at 65°C for 3 minutes and at 90°C for 3 minutes to form a second adhesive layer. The thickness of the second adhesive layer was about 49 pm. The thickness of the second adhesive layer is determined by measuring the thickness of the A4 size area of the sample before and after forming the second adhesive layer at three arbitrary points using a desktop micrometer, and then measuring the thickness of the second adhesive layer. The average value of the measured values before the formation of the second adhesive layer was subtracted from the average value of the measured values after the formation of the adhesive layer.

[00129] Thereafter, a nonwoven sheet (PET, basis weight 13 g/m²) was laminated on the second adhesive layer, and heated and pressed using a roll laminating machine at a roll temperature of 60°C to obtain a layer with a thickness (Tl) of 190 pm.

[00130] In this manner, a complete adhesive sheet comprising five layers, as exemplified by FIG. 1, was obtained.

Thermal conductivity measurements

[00131] Sample (A) adhesive sheets were cut into 50 mm x 50 mm. Two fluororesin sheets (Alam Co., Ltd., thickness 0.2 mm) were prepared, and an adhesive sheet and a 400 pm thick spacer surrounding the entire circumference of the adhesive sheet were placed on top of one fluororesin sheet, and then the other fluororesin sheet was placed on the adhesive sheet and spacer to sandwich it, and hot pressed at 160° C for 10 minutes to obtain a cured adhesive having a thickness (T2) of 360 pm. This cured adhesive was used as a test body for measuring thermal conductivity. In addition, the thickness (T2) of the cured adhesive was measured using the thickness measurement function of the thermal conductivity measuring device at the thermal conductivity measurement point when measuring the thermal conductivity described later. Using a thermal conductivity measurement device (manufactured by Analysis Tech Inc., Thermal Interface Material Tester TIM Tester Model 1300, the thermal conductivity of the test specimen for thermal conductivity measurement was measured in accordance with ASTM D5470.

Measurement of shear strength

[00132] Sample (A) adhesive sheets were cut to 12.5 mm x 25 mm. Two SPCC boards (100 mm x 25 mm x 1.6 mm (according to JIS G 3141)) were prepared and surfaces were cleaned with methyl ethyl ketone. An adhesive sheet with a thickness of 400 pm was placed on top of one SPCC board in order from the end side. A spacer was on the adhesive sheet, while another SPCC board was placed on top of the adhesive sheet and spacer to sandwich it, and heat pressed at 160°C for 10 minutes to obtain a shear test specimen (T3) containing a cured foam with a thickness (T3) of 380 pm (JIS K 6850 compliant) was obtained. The thickness (T3) of the foamed cured product was determined by measuring the entire thickness of the shear test specimen and subtracting the thickness of two SPCC plates (3200 pm) from the measured value.

[00133] A material testing machine (RTC-1325A manufactured by ORIENTEC) equipped with a constant temperature testing device was used. The measurement sample (shear test specimen) was left standing in a material testing machine heated to 230° C for 10 minutes to sufficiently heat it. Thereafter, the shear strength was measured at 230° C at a shear tension rate of 5 mm/min.

Samples (B) for measuring melt viscosity and glass transition temperature after curing

[00134] Thermally conductive fillers (Boron Nitride, BN-CFP-012) were added to each adhesive composition without expandable microspheres, accelerator and curing agents (FN-100SSD, 2MZA-PW, and DICY 1400F) so that curing and expansion are not triggered during the evaluation. An adhesive composition was applied to the water-repellent surface of a 50 pm silicone coating sheet and dried at 65°C for 3 minutes and at 90°C for 3 minutes to form an adhesive layer.

Viscosity measurements

[00135] Sample (B) adhesive sheets were peeled off from the silicone sheet and five similar sheets were laminated to obtain a sheet with a thickness of about 1500 pm. The sheet was molded into a cylindrical shape with a diameter of 8 mm to obtain a sample for melt viscosity measurement.

[00136] Dynamic viscoelastic properties of the molded sample were evaluated using ARES-G2(TA Instruments) under conditions of a heating rate of 5°C/min, an oscillation strain of 0.01%, and a frequency of 1.0Hz. Complex viscosity at 110°C was calculated as melt viscosity (Pa.S).

Tg measurements

[00137] Sample (B) adhesive sheets were cut into 50 mm x 50 mm. Two fluororesin sheets (Alam Co., Ltd., thickness 0.2 mm) were prepared, and an adhesive sheet and a 250 pm thick spacer surrounding the entire circumference of the adhesive sheet were placed on top of one fluororesin sheet, and then the other fluororesin sheet was placed on the adhesive sheet and spacer to sandwich it, and hot pressed at 160°C for 10 minutes to obtain a cured adhesive with a thickness of about 200 pm. The prepared sample was cut into a suitable size, and its dynamic viscoelastic properties were evaluated using RSA-G2 manufactured by TA Instruments under conditions of a heating rate of 5°C/min, an oscillation strain of 0.01%, and a frequency of 1 to 1 Hz. The temperature at tan§ peak was defined as Tg after curing.

Expansion measurements

[00138] Measurement of expansion ratio of cured foam in test specimen for thermal conductivity measurement: The expansion ratio was determined by T2/T1 using the thickness of the adhesive sheet (Tl) and the thickness of the cured foam body (T2).

[00139] Measurement of expansion ratio of cured foam in specimen for shear test: The expansion ratio was determined by T3/T1 using the thickness of the adhesive sheet (Tl) and the thickness of the cured foam in the shear test specimen (T3).

Results

[00140] Examples IS- 1 to IS-4 and 1-1 to 1-7: Adhesive sheet samples were obtained with the amount of ED-523T varied as shown in the table below between 1 wt% to 16.7 wt%, and filler is fixed as CFP-012 set at 13.9% by volume of the curable adhesive composition. The results are shown in Table 5.

Table 5

* wt% per adhesive composition (without thermal conductive filler)

[00141] Examples 1-9 to 1-13: Adhesive sheet samples were obtained in the same manner as IS-1, except that the amount of BN-CFP-012 was varied from 0% to 30% by volume as shown in Table 6. [00142] Examples IS-6 and IS-7: Adhesive sheet samples were obtained in the same manner as IS-1, except that the kind of additives of ED-523T was changed as shown in the table below.

[00143] Examples 1S-12 and IS-13: Adhesive sheet samples were obtained in the same manner as before, except that the type and amount of fillers of BN-CFP-012 was changed as shown in the table below. The reactive diluent was also changed to ED-505 and EP-4005. The results are shown in Table 7. Melt viscosity of the control was originally 762 Pa.S. The addition of the reactive diluent (ED-523T) from 1 wt% to 16.7 wt% caused the viscosity to drop by half or more while thermal conductivity increased from 0.19 to 0.21 to 0.31 in IS-1 to IS-4 and 1-1 to 1-7. However, Tg of the cured adhesive was impacted with increasing reactive diluent quantity from 9.1 wt% to 16.7%, making it preferable not to apply excessive reactive diluent. In samples 1-9 to 1-13, the optimal loading of filler was found to be in the range below 30 Vol%, preferably between 1 to 20 Vol%, because an excess of filler probably resulted in inconsistent expansion of the adhesive sheet leading to poorer thermal conductivity. In IS- 12 and IS- 13, the use of triglycidyl ether (ED-505) and high molecular weight/high viscosity glycidyl ether (EP- 4005) were similarly excellent for thermal conductivity characteristics as with diglycidyl ether (ED- 523T).

Table 6

* wt% per adhesive composition (without thermal conductive filler) Table 7

[00144] From the above results, it can be summarized that it was found that by adding an appropriate amount of selected reactive diluents, it was possible to successfully increase the thermal conductivity by controlling the rheological properties, such as the melt viscosity, of the adhesive composition of the adhesive layers to be within the appropriate range so that the expansion of the adhesive layers can occur effectively in order to deliver the adhesive and heat conductive filler material across the adhesive permeable layers, as may be required in applications requiring good thermal conductivity, such as when it is used as electric motor slot liners.

[00145] Various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

What is claimed is:

1. An adhesive sheet, comprising: an electrically insulating substrate, a first adhesive layer disposed on one side of the substrate, and a second adhesive layer disposed on another side of the substrate opposite the first adhesive layer, wherein one or both of the first and the second adhesive layers comprising a curable adhesive composition and a thermally conductive filler distributed therein; and wherein the curable adhesive composition comprises: a multi-functional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent.

2. The adhesive sheet of claim 1, wherein the reactive diluent is a glycidyl ether.

3. The adhesive sheet of claim 2, wherein the reactive diluent is a diglycidyl ether having formula (I): wherein

Rl, Rl’, R2 and R2’ are each independently selected from a Cl -CIO hydrocarbon wherein hydrogen is optionally substituted by a halogen; and

R3 and R3’ are each independently selected from -H, or Cl -CIO alkyl, alkenyl, phenyl, alkynyl, carbonyl moiety wherein hydrogen is optionally substituted by a halogen.

4. The adhesive sheet of claim 2, wherein the reactive diluent comprises a triglycidyl ether having a formula (II): wherein R4 and R5 are each independently selected from -CH₂-, -C2H5-, -C3H6-, -C4H8- or -GTC-; and R6 is selected from -H, -CH3 or -C2H5.

5. The adhesive sheet of claim 2, wherein the reactive diluent comprises an aromatic monoglycidyl ether having formula (III): wherein R7 is a C3 to G, straight chain or branched saturated hydrocarbon moiety.

6. The adhesive sheet of claim 2, wherein the reactive diluent comprises an aliphatic monoglycidyl ether having formula (IV): wherein R8 is selected from a C2-C14 straight chain or branched or cyclic alkyl, alkenyl, phenylalkyl moiety.

7. The adhesive sheet of any of claims 1 to 6, wherein the reactive diluent has a molecular weight of less than 250g per epoxy group (g/ep).

8. The adhesive sheet of any of claims 1 to 7, wherein the reactive diluent is present in an amount of 1 to 12% by weight of the curable adhesive composition excluding the weight of filler.

9. The adhesive sheet of claim 8, wherein the curable adhesive composition further comprises, based on the total amount of the curable adhesive composition,

40 to 80 wt% of the multi-functional epoxy resin, and

5 to 25 wt% of the foaming agent, and

5 to 35 wt% of an additional component selected from one or more of a curing agent, coupling agent, toughening agent, and accelerator.

10. The adhesive sheet of claim 9, wherein the multi-functional epoxy resin comprises a tri-functional epoxy resin.

11. The adhesive sheet of any of claims 1 to 10, wherein the curable adhesive composition further comprises a phenoxy resin present in an amount of 1 to 15 wt% based on the total amount of the curable adhesive composition.

12. The adhesive sheet of claim 11, wherein the phenoxy resin has a molecular weight (Mw) of between 60K to 80K g/mol.

13. The adhesive sheet of any of claims 1 to 12, wherein the curable adhesive composition is in an unflowable state at ambient temperature and pressure, and has a softening point above 60°C [Ring and Ball apparatus according to the ASTM D-2398 Test Method], and a melt viscosity of less than 300,000 Pa.s at 110°C. [ASTM D3835-16 , Standard Test Method for Determination of Properties of Polymeric Materials by Means of a Capillary Rheometer],

14. The adhesive sheet of any of claims 1 to 13, wherein the thermally conductive filler in the first and the second adhesive layer is present in an amount of at least 10 % of the total volume of the curable adhesive composition and the thermally conductive filler.

15. The adhesive sheet of claim 14, wherein the thermally conductive filler is selected from the group consisting of boron nitride, aluminum oxide and aluminum nitride.

16. The adhesive sheet of claim 14 or 15, wherein the thermally conductive filler has a length to width aspect ratio of 1.3 or more.

17. The adhesive sheet according to claims 14 to 16, wherein the filler is platelet-shaped, whiskershaped, or agglomerated.

18. The adhesive sheet of any of claims 1 to 17, wherein the foaming agent comprises heat expandable microspheres.

19. The adhesive sheet of any of claims 1 to 18, further comprising an adhesive permeable layer disposed on one or both of the first and second adhesive layers, said adhesive permeable layer being permeable to the curable adhesive composition when heated until one or both of the first and second adhesive layer foams.

20. The adhesive sheet of claim 19, wherein the adhesive permeable layer comprises non-woven polyethylene terephthalate fabric having a basis weight of 5 to 30 g/m².

21. The adhesive sheet of any of claims 1 to 20, wherein the adhesive sheet has a total thickness of 80 to 300 pm.

22. The adhesive sheet of any of claims 1 to 21, wherein the curable adhesive composition extends across the adhesive permeable layer and is cured.

23. A method for forming an adhesive sheet, comprising: providing an electrically insulating substrate, mixing a thermally conductive filler with a curable adhesive composition comprising: a multi-functional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent, to form a prepolymer composition, disposing the prepolymer composition on one side of the substrate to form a first adhesive layer, and disposing the prepolymer composition on another side of the substrate opposite the first adhesive layer to form a second adhesive layer.

24. The method of claim 23, wherein the prepolymer composition is a hotmelt composition, and wherein said prepolymer composition is heated to, or above, its glass transition temperature for deposition, and after deposition, the prepolymer composition is allowed to cool to form the first and second adhesive layers.

25. The method of claim 23, wherein the prepolymer composition further comprises a solvent which renders the prepolymer composition a fluid suitable for deposition, and after deposition, the solvent is removed by evaporation or heated to form the first and second adhesive layers.

26. The method of any of Claims 23 to 25, further comprising laminating an adhesive permeable layer on the first adhesive layer and the second adhesive layer at a temperature of 50°C to 65 °C.

27. An electric motor comprising a stator, comprising: a stator core having at least one slot; a winding accommodated within the slot and an insulative slot liner comprising an adhesive sheet as defined in any one of claims 1 to 21 arranged between the stator core and the winding.

28. The electric motor of claim 23, wherein the adhesive sheet is cured.

29. A method of electrically insulating an electric motor comprising a stator, comprising providing a stator core having at least one slot, arranging an insulative slot liner comprising an adhesive sheet as defined in any one of claims 1 to 21 in the at least one slot, arranging an electrical winding within the slot, and curing the adhesive sheet.