Adhesive sheet, method for forming adhesive sheet, electric motor and method of insulation

Abstract

To provide an expandable adhesive sheet having improved flow characteristics in a curing process, and a method for forming the adhesive sheet.SOLUTION: Provided is 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 comprises a curable adhesive composition and a thermally conductive filler dispersed therein, and the curable adhesive composition comprises a multi-functional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent.SELECTED DRAWING: Figure 1

Description

[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 an insulation method using the adhesive sheet. [Background technology]

[0002] Expandable adhesive sheets are useful for bonding the surfaces of two objects separated by a narrow gap that is physically difficult to access or apply with conventional adhesives. Expandable adhesive sheets can be easily inserted into such narrow gaps and then expanded to fill the gap. Obtaining good adhesive contact between the two surfaces improves adhesion and also helps improve thermal conductivity between the two objects.

[0003] [Patent documents] US Pat. No. 9,419,489 B2 discloses a polyimide resin mixed with a thermally conductive filler and a mesh layer to form a composite slot liner.

[0004] US Patent No. 9,537,364 B2 discloses a double-sided epoxy adhesive slot liner that expands upon heating. Summary of the Invention [Problem to be solved by the invention]

[0005] Adhesive sheets based on curable expandable adhesive materials offer ease of processing compared to traditional adhesive application methods. However, the use of expandable adhesive sheets presents new technical challenges related to controlling the flow of the adhesive during expansion and heat conduction in high-temperature applications. Resinous long-chain polymer adhesives used in combination with particulate filler materials, such as thermally conductive particles, provide thermal stability and thermal conductivity, but the viscosity of the adhesive composition increases, which can hinder uniform expansion of the adhesive composition and the particulate material during curing. Expandable adhesive layers are desirably composed of adhesive compositions that have sufficient fluidity and flowability upon expansion to achieve the desired adhesive distribution upon expansion. [Means for solving the problem]

[0006] In one embodiment, 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, i.e., the first adhesive layer disposed on one side of the substrate and the second adhesive layer disposed on another side of the substrate opposite the first adhesive layer, wherein one or both of the first and second adhesive layers comprises a thermally conductive filler dispersed in a curable adhesive composition comprising a multifunctional 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 of forming an adhesive sheet, the method comprising the steps of providing an electrically insulating substrate; mixing a curable adhesive composition comprising a multifunctional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a blowing agent with a thermally conductive filler 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 yet another aspect, the present disclosure provides an electric motor including a stator, the electric motor including: a stator core having at least one slot; a winding housed in the slot; and a slot liner including an adhesive sheet disposed between the winding and the stator.

[0009] In yet another aspect, the present disclosure provides a method of electrically insulating an electric motor having a stator, the method including the steps of providing a stator core having at least one slot, disposing a slot liner in the at least one slot including an adhesive according to the present disclosure, disposing an electrical winding in the at least one slot, and curing the adhesive sheet.

[0010] The adhesive sheet according to the present disclosure includes an expandable adhesive layer that exhibits excellent thermal conductivity after foaming and curing. When cured, the multifunctional epoxy adhesive provides good long-term heat resistance and electrical insulation. To improve its thermal conductivity, a thermally conductive particulate filler is added. However, due to the inherent flow properties of the multifunctional epoxy and the effect of added filler materials increasing the overall viscosity, the flow properties of the adhesive composition during the adhesive expansion process may not be optimal for achieving stable flow and expansion. Therefore, it is necessary to improve the flow properties of the curable adhesive composition in the first and second adhesive layers. It has now been discovered that a reactive diluent containing at least one epoxide moiety can solve the above-mentioned problems. The inclusion of an appropriate reactive diluent can appropriately adjust the rheological properties of the adhesive composition and improve its flow. Furthermore, the terminal epoxide moiety contained in the reactive diluent ultimately reacts with the multifunctional epoxy adhesive. Therefore, reactive diluents allow for a wider variety of fillers, a wider range of filler sizes, and higher filler loadings in the adhesive composition. This allows the adhesive sheet to achieve a wider range of specifications, such as thermal conductivity. [Effects of the Invention]

[0011] Therefore, the present invention provides an expandable adhesive sheet with improved flow properties during the curing process, which allows a larger amount of thermally conductive filler to be loaded into the adhesive composition of the adhesive sheet, meaning that the expandable adhesive sheet has improved thermal conductivity performance that has not been achieved before.

[0012] These and other advantages of the present invention are more fully shown and described in the drawings and detailed description of the invention, in which like parts are designated with like reference numerals, which drawings and description are for purposes of illustration only and should not be construed to unduly limit the scope of the invention.

[0013] Illustrative embodiments of the invention are more particularly described in the drawings and detailed description that follow. [Brief explanation of the drawings]

[0014] A preferred embodiment of the present invention will be described with reference to the drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted. Furthermore, some of the drawings are exaggerated for ease of understanding, and the dimensional ratios and the like are not limited to those shown in the drawings.

[0015] [Figure 1] FIG. 2 is a cross-sectional view of an expandable adhesive sheet before the first and second adhesive layers expand.

[0016] [Figure 2A] FIG. 10 is a cross-sectional view of an adhesive sheet of an embodiment in which the adhesive permeable layer is also permeable to the thermally conductive filler and expandable microspheres of the blowing agent upon expansion of the first and second adhesive layers.

[0017] [Figure 2B]FIG. 10 is a cross-sectional view of an adhesive sheet of another embodiment in which the adhesive permeable layer is impermeable to the thermally conductive filler and expandable microspheres of the blowing agent upon expansion of the first and second adhesive layers.

[0018] [Figure 3] A table containing a list of materials is labeled Table 1.

[0019] [Figure 4] 1 is a table labeled Table 2 containing the weight percentage of each component in the sample and calculations on a weight percentage basis of the reactive diluent added.

[0020] [Figure 5] 1 is a table labeled Table 3 containing measured dimensions of thermally conductive fillers.

[0021] [Figure 6] 1 is a table labeled Table 4 containing criteria for evaluating measured physical properties of adhesive sheets.

[0022] [Figure 7] Table 5 is a table containing the experimental results for Samples 1-1 (control), IS-1, IS-2, and 1-2 through 1-7.

[0023] [Figure 8] 1 is a table labeled Table 6 containing the experimental results for Samples 1-9 to 1-13.

[0024] [Figure 9] 1 is a table labeled Table 7 containing the experimental results for samples IS-6, IS-7, IS-12 and IS-13. DETAILED DESCRIPTION OF THE INVENTION

[0025] The adhesive sheet provided includes 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 substrate. One or both of the first and second adhesive layers comprises a curable adhesive composition and a thermally conductive filler dispersed therein. The curable adhesive composition includes a multifunctional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a foaming agent. The adhesive sheet expands upon heating after being placed between objects to be bonded. Because of its expandability, the adhesive sheet can be advantageously placed in the gap between two objects to be bonded. The adhesive sheet may further include an adhesive-permeable layer disposed on the first adhesive layer. Furthermore, the adhesive sheet may further include a second adhesive-permeable layer disposed on the second adhesive layer. The first adhesive-permeable layer is a layer through which the adhesive can pass when the adhesive composition in the first adhesive layer is heated and the blowing agent in the composition expands, and the second adhesive-permeable layer is a layer through which the adhesive can pass when the adhesive composition in the second adhesive layer is heated and the blowing agent in the composition expands.

[0026] When the adhesive sheet is heated, the curable adhesive composition in the first and second adhesive layers becomes flowable. At the same time, the expansion of the foaming agent causes the adhesive composition to expand. This brings the particulate material, including the adhesive and thermally conductive filler, into contact with the surfaces of the two objects to be bonded, where the adhesive hardens and establishes a stable bond between the two objects. In one embodiment, the adhesive layer is covered with a protective adhesive-permeable layer for protection. The adhesive-permeable layer is designed to be permeable to the expanding adhesive, allowing the adhesive to migrate through the layer. The advantage of having the first and second adhesive-permeable layers on the outermost layers of the adhesive sheet is that it facilitates positioning the adhesive sheet relative to the object to be bonded, as the adhesive-permeable layer renders the adhesive sheet non-tacky and prevents it from accidentally adhering to the object. The cover layer protects the adhesive surface from physical damage caused by the coil tip when inserting the coil into the stator slot, preventing chipping and dust generation during the assembly process. The cover layer has low surface friction, making it easy to work with.

[0027] Each component of the adhesive sheet of this embodiment will be described in detail below.

[0028] [Base material] (Electrically Insulating Substrate) The electrically insulating substrate is a support for forming the first and second adhesive layers and is a component that essentially determines the size of the adhesive surface of the adhesive sheet. Therefore, the electrically insulating substrate functions as a base layer for forming the first and second adhesive layers in the adhesive sheet manufacturing process. The material constituting the electrically insulating substrate must have sufficient strength to support the first and second adhesive layers, and the material constituting the electrically insulating substrate must have sufficient strength to support the first and second adhesive layers. Any material can be used as long as its mechanical strength is not reduced when heated. Furthermore, when the adhesive sheet of this embodiment is used in an application requiring electrical insulation, electrical insulation can be easily imparted to the adhesive sheet by selecting an electrically insulating substrate.

[0029] The material constituting the electrically insulating substrate is not particularly limited, and examples include polyester resins (e.g., polyethylene naphthalate (PEN), polyethylene terephthalate (PET), etc.), polycarbonate resins, polyimide resins (e.g., polyetherimide (PEI), polyamideimide, etc.), polyamide resins (e.g., polyetheramide, polyaramid, nylon, etc.), acrylic resins, polysulfone resins (polysulfone, polyethersulfone, etc.), polyetherketone resins (polyetherketone, polyetheretherketone, etc.), modified polyphenylene oxide, etc. The electrically insulating substrate may contain only one of these materials, or two or more of them.

[0030] The thickness of the electrically insulating substrate can be adjusted appropriately depending on the distance between the objects to be bonded. When the gap between the objects to be bonded 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 μm or more, and from the viewpoint of easily improving the breakdown voltage, it may be 3 μm or more, 5 μm or more, 7 μm or more, 9 μm or more, or 11 μm or more. Furthermore, from the viewpoint of flexibility of the adhesive sheet, the thickness of the electrically insulating substrate may be, for example, 200 μm or less, 150 μm or less, 100 μm or less, or 90 μm or less. The substrate layer, together with the adhesive layer and, optionally, the adhesive permeable layer described below, contributes to the thickness of the entire sheet, and the thickness of the entire sheet may be, for example, in the range of 10 to 2000 μm, or 50 to 500 μm, more preferably 80 to 300 μm.

[0031] [Adhesive layer] The first and / or second adhesive layers may be made of the same or different materials and have different dimensions. Each adhesive layer is disposed on one side of the electrically insulating substrate and includes a curable adhesive composition containing a multifunctional epoxy adhesive, a reactive diluent, and a foaming agent. The adhesive composition also includes a thermally conductive filler. The first adhesive layer contains a foaming agent, which allows it to expand when bonding objects together and fill gaps between the objects.

[0032] The minimum thickness of the adhesive layer is not particularly limited and may be, for example, 5 μm or more, 10 μm or more, or 15 μm or more. The maximum thickness of the first adhesive layer is not particularly limited and may be, for example, 200 μm or less, and from the viewpoint of improving workability, may be 100 μm or less, 80 μm or less, or 60 μm or less.

[0033] The multifunctional epoxy adhesive is preferably substantially solid at room temperature, fluidized by heating, and hardened by continued heating. That is, the resin may be a thermosetting adhesive. The adhesive may be selected to have a low dielectric constant in order to provide good electrical insulation and prevent partial discharge. Examples of thermosetting adhesives include epoxy adhesives.

[0034] In one embodiment, the multifunctional epoxy adhesive includes an epoxy resin with a Tg greater than 150°C, or more preferably greater than 200°C. These can be used in applications involving high 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. Specific examples include phenolic novolac epoxy resins and cresol novolac epoxy resins. Another example includes silicon-modified epoxy resins. These resins provide improved heat resistance and electrical properties by introducing silicon groups into the epoxy backbone. Specific examples include epoxy resins modified with siloxane or silsesquioxane moieties. Another example includes a phthalonitrile-based epoxy resin. Phthalonitrile-based epoxy resins have excellent heat resistance, flame retardancy, and low smoke generation. In yet another example, the heat-resistant epoxy resin includes a bismaleimide (BMI)-based epoxy resin. In another example, the heat-resistant epoxy resin includes a cyanate ester-based epoxy resin. Cyanate ester-based epoxy resins offer high-temperature stability, low dielectric constant, and low moisture absorption. In yet another example, heat-resistant epoxy resins include polyimide-based epoxy resins, which combine the heat resistance of polyimides with the processability of epoxy resins, resulting in excellent thermal stability and mechanical properties.

[0035] Examples of thermosetting epoxy resins include bisphenol-based epoxy resins (e.g., bisphenol A-type epoxy resins, bisphenol F-type epoxy resins, etc.), aliphatic epoxy resins (e.g., hexanediol diglycidyl ether, etc.), glycidylamine-based epoxy resins (e.g., triglycidylaminophenol, etc.), novolac-based epoxy resins (e.g., phenol novolac epoxy resins, cresol novolac epoxy resins, etc.), alicyclic epoxy resins (e.g., 3,4-epoxycyclohexylmethyl, 3,4-epoxycyclohexanecarboxylate, bis(3,4-epoxycyclohexylmethyl)adipate, etc.), brominated epoxy resins (e.g., tetrabromobisphenol A diglycidyl ether, etc.), multifunctional epoxy resins (e.g., triglycidyl ether of tris(hydroxyphenyl)methane, sorbitol polyglycidyl ether, tetraglycidyldiaminodiphenylmethane, etc.), and crystalline epoxy resins (e.g., tetramethylbisphenol F diglycidyl ether, tetramethylbiphenol diglycidyl ether, etc.). These may be used alone or in combination of two or more.

[0036] In one embodiment, the multifunctional epoxy adhesive is a phenolic novolac epoxy resin having the following structure, where n is 10 or any integer greater than 100: [ka]

[0037] In yet another embodiment, the multifunctional epoxy adhesive is a cresol novolac epoxy resin having the following structure, where n is 10 or any integer greater than 100: [ka]

[0038] Multifunctional epoxy adhesives do not have to be oligomeric or polymeric resins. They can be non-polymeric molecules with two or more epoxy functional groups. Trifunctional epoxy adhesives may be preferred for rapid cure and excellent heat resistance, especially trifunctional aromatic epoxy adhesives with one, two, or three aromatic rings. Trifunctional heterocyclic epoxy adhesives can also be used. Specific examples include the following compounds: [ka]

[0039] [Reactive diluent] In the present disclosure, reactive diluents have at least one terminal epoxide moiety. They are added to curable adhesive compositions to adjust viscosity and improve their rheological properties. Reactive diluents are preferably low-molecular-weight, low-viscosity materials. Lowering the adhesive's viscosity facilitates expansion of the adhesive upon heating and improves its permeability. They may also be added to enhance flexibility. Certain reactive diluents can improve the flexibility and toughness of cured epoxy adhesives, improving their resistance to fracture and cracking under stress.

[0040] The epoxide moiety refers to an oxirane ring structure. Having at least one epoxide moiety allows the reactive diluent to react with the multifunctional epoxy component. That is, the reactive diluent is a reactive compound that reacts with the epoxy resin during the curing process to form a modified polymer after curing. The reactive diluent also influences the curing time of the epoxy adhesive, thereby controlling the cure rate and tailoring it to specific application requirements. The epoxide moiety is a reactive functional group present in the epoxy resin. Once the epoxy adhesive is mixed and applied, the epoxide moiety initiates a chemical reaction known as epoxy curing or crosslinking, which is typically triggered by heating in the presence of a curing agent or curing accelerator.

[0041] The reaction between the epoxide moiety and the curing agent involves the opening of the oxirane ring and the formation of a covalent bond. This process is commonly referred to as curing or crosslinking of the epoxy resin. While the detailed mechanism of the reaction varies depending on the type of curing agent used, it generally involves a ring-opening reaction in which the curing agent, a compound containing an amino or hydroxyl group, reacts with the epoxide moiety via nucleophilic attack, opening the oxirane ring and generating a reactive intermediate. Second, a crosslinking reaction occurs, in which the reactive intermediate formed by the ring-opening reaction undergoes condensation or addition reactions with other epoxy or curing agent molecules, forming covalent bonds between epoxy resin molecules and forming a three-dimensional network structure. Finally, curing occurs, and the crosslinking reaction progresses, transforming the epoxy adhesive from a liquid or semi-liquid state to a solid state. Various types of molecules contain terminal epoxide moieties, including glycidyl acrylates, glycidyl esters, glycidyl ethers, and glycidiols.

[0042] 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-epoxyphenylethyl ether), allyl glycidyl ether (2,3-epoxypropylprop-2-enyl ether), octyl glycidyl ether (1,2-epoxyoctane), isobutyl glycidyl ether (1,2-epoxy-2-methylpropyl ether), benzyl glycidyl ether (1,2-epoxy-3-phenoxypropane), ethyl glycidyl ether (1,2-epoxyethyl ethyl ether), methyl glycidyl ether (1,2-epoxy ... glycidyl ether (1,2-epoxy-3-methoxypropane), isopropyl glycidyl ether (1,2-epoxy-2-methylpropyl ether), propyl glycidyl ether (1,2-epoxypropylpropyl ether), cyclohexyl glycidyl ether (1,2-epoxy-3-cyclohexyloxypropane), tetrahydrofurfuryl glycidyl ether (1,2-epoxy-3-(oxolan-2-yl)propane), and 2-ethylhexyl glycidyl ether (1,2-epoxy-3-(2-ethylhexyloxy)propane).

[0043] For example, glycidyl ethers include butyl glycidyl ether (BGE), a low-viscosity reactive diluent that improves the flow and wettability of epoxy adhesives. It can be used to reduce the viscosity of high-viscosity epoxy resins and improve their handling properties.

[0044] In another example, the glycidyl ether is phenyl glycidyl ether (PGE), which is a reactive diluent that provides improved flexibility and impact resistance to epoxy adhesives and can be used in applications requiring improved toughness and durability.

[0045] In yet another example, the glycidyl ether is cresyl glycidyl ether (CGE), a reactive diluent that imparts excellent chemical resistance and thermal stability to epoxy adhesives, making them suitable for applications requiring resistance to harsh chemical and high-temperature environments.

[0046] In yet another example, the glycidyl ether is octyl glycidyl ether (OGE), a reactive diluent that provides good flexibility and low viscosity to epoxy adhesives, and can be used in applications requiring improved flexibility and impact resistance.

[0047] In yet another example, the glycidyl ether is isobornyl glycidyl ether (IBGE), a reactive diluent that imparts excellent adhesion and chemical resistance to epoxy adhesives. It can be used in applications where high adhesive strength and chemical resistance are required.

[0048] In yet another example, the glycidyl ether is allyl glycidyl ether (AGE), a reactive diluent that imparts good flexibility and toughness to epoxy adhesives, and can be used in applications requiring improved impact resistance and elongation.

[0049] In a preferred embodiment, the reactive diluent is selected from diglycidyl ethers, which have two terminal epoxide moieties. Examples of diglycidyl ethers that can be used 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 (PEG), and the like. Diglycidyl ethers of diethylene glycol (DGE-DEG), diglycidyl ethers of dipropylene glycol (DGE-PEG), diglycidyl ethers of polypropylene glycol (DGE-PPG), diglycidyl ethers of polytetrahydrofuran (DGE-PTHF), diglycidyl ethers of glycerol (DGE-G), diglycidyl ethers of trimethylolpropane (DGE-TMP), diglycidyl ethers of pentaerythritol (DGE-PE), diglycidyl ethers of diethylene glycol (DGE-DEG), and diglycidyl ethers of dipropylene glycol (DGE-DPG).

[0050] In a preferred embodiment, the reactive diluent has the following formula (I): [ka] [In the formula, R1, R1', R2 and R2' are each independently selected from a C1-C10 linear, branched, saturated or unsaturated, or aromatic hydrocarbon moiety, in which the hydrogens attached to the carbon atoms are optionally replaced by halogens; R3 and R3' are each independently selected from -H or a C1-C10 alkyl, alkenyl, phenyl, alkynyl, or carbonyl moiety, wherein the hydrogen attached to the carbon atom is optionally replaced by a halogen. is a diglycidyl ether having the formula:

[0051] In an exemplary embodiment, the diglycidyl ether is selected from: [ka]

[0052] In another embodiment, the reactive diluent comprises a triglycidyl ether. Examples of triglycidyl ethers that can be used 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), triethylenediamine (TGE-DI), triglycidyl ether of propylene ... triglycidyl ether of tris(2-hydroxyethyl)amine (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-hydroxypropyl)amine (TGE-THPA), triglycidyl ether of tris(2-aminoethyl)amine (TGE-TAEA), triglycidyl ether of tris(2-hydroxyethyl)isocyanurate (TGE-THEIC), and triglycidyl ether of tris(2-hydroxyethyl)phosphate (TGE-THEP).

[0053] In a preferred embodiment, the triglycidyl ether has the following formula (II): [ka] [In the formula, R4 and R5 are each independently selected from a C1-C10 linear, branched, saturated or unsaturated, or aromatic hydrocarbon moiety, in which the hydrogen attached to the carbon atom is optionally replaced by a halogen; R6 is selected from -H or a C1-C10 alkyl, alkenyl, phenyl, alkynyl, carbonyl moiety, wherein the hydrogen attached to the carbon atom is optionally replaced by a halogen.

[0054] In a preferred embodiment, the triglycidyl ether is selected from any of the following: [ka]

[0055] In one embodiment, the reactive diluent may comprise an aromatic monoglycidyl ether, examples of which include phenyl glycidyl ether (PGE), benzyl glycidyl ether (BGE), 4-methoxyphenyl glycidyl ether, 4-ethoxyphenyl glycidyl ether, and 4-chlorophenyl glycidyl ether.

[0056] Examples of suitable aromatic monoglycidyl ethers include 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, and 4-phenoxyphenyl glycidyl ether. In a preferred embodiment, the aromatic monoglycidyl ether has the following formula (III): [ka] wherein R7 is a C1-C6 straight or branched chain alkyl or alkenyl hydrocarbon moiety. In preferred embodiments, R7 is selected from a 1,1-dimethylethyl, 1,2-dimethylpropyl, 1-methylpropyl, or 1-methylbutyl moiety.

[0057] Selecting the appropriate low-molecular-weight reactive diluent can achieve multiple goals. Low-molecular-weight reactive diluents have low viscosity, which helps reduce the overall viscosity of the epoxy adhesive. This makes the adhesive easier to handle, mix, and apply. They also improve wetting and penetration, allowing the adhesive to spread more easily and evenly on substrates, improving adhesion and bonding. Furthermore, adjusting the amount of low-molecular-weight reactive diluent added allows the cure time to be tailored to specific application requirements. Low-molecular-weight reactive diluents can accelerate the curing process of epoxy adhesives, achieving faster bonds. They promote the crosslinking reaction between the epoxy resin and the curing agent, shortening the cure time. Some low-molecular-weight reactive diluents can increase the flexibility and toughness of cured epoxy adhesives, improving the adhesive's resistance to cracking and fracture under stress. Furthermore, certain low-molecular-weight reactive diluents can improve the chemical resistance of epoxy adhesives, improving their resistance to exposure to certain chemicals, solvents, and environmental conditions. In this disclosure, the term "low molecular weight" of a reactive diluent is a relative term and is defined herein as a molecular weight lower than that of multifunctional epoxy adhesives. In various embodiments, the reactive diluent has a molecular weight per epoxy group less than 1000, less than 800, preferably less than 500, more preferably less than 300, less than 250, and most preferably less than 210 g / ep.

[0058] The weight percentage range of the reactive diluent in an epoxy adhesive 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 an epoxy adhesive ranges from 10 to 50%. The specific value within this range is determined by the adhesive's viscosity, cure speed, flexibility, and other performance requirements. Because the addition of reactive diluent significantly affects the properties of the epoxy adhesive, its proportion should be carefully determined based on the specific needs of the application. In a preferred embodiment, the reactive diluent is present in the range of 1 to 12 weight percent of the curable adhesive composition, excluding the weight of the filler.

[0059] The reactive diluent may in some cases comprise a monoglycidyl ether. In one embodiment, the monoglycidyl ether is an aliphatic monoglycidyl ether having the following formula (IV): [ka] wherein R8 is selected from C2-C14 linear or branched or cyclic alkyl, alkenyl, phenyl-alkyl moieties. Specific examples include the following compounds: [ka]

[0060] The above-mentioned various glycidyl ether types may be used alone or in combination of different types of glycidyl ethers, for example, a combination of a diglycidyl ether and a triglycidyl ether, or a combination of a monoglycidyl ether, a diglycidyl ether and a triglycidyl ether.

[0061] [Foaming agent] The foaming agent may be any foaming agent capable of foaming and expanding the adhesive layer when adhered to an adherend. The foaming agent is preferably a temperature-sensitive foaming agent. Examples of foaming agents include inorganic foaming agents such as ammonium carbonate, ammonium bicarbonate, ammonium nitrite, ammonium borohydride, and azides; fluorinated alkanes such as trichloromonofluoromethane; and azobisisobutyronitrile. Other examples include organic foaming agents such as azo compounds, hydrazine compounds such as p-toluenesulfonylhydrazide, semicarbazide compounds such as p-toluenesulfonylsemicarbazide, triazole compounds such as 5-morpholyl-1,2,3,4-thiatriazole, and N-nitroso compounds such as terephthalamidodinitroso; and thermally expandable microcapsules containing a thermally expandable agent (e.g., a hydrocarbon compound). Among these, thermally expandable particles are preferred because they are less likely to interfere with the curing of the adhesive.

[0062] The thermally expandable particles may include, for example, a thermoplastic resin shell and an expanding agent (e.g., a liquid hydrocarbon) encapsulated therein. Examples of thermally expandable particles include the Matsumoto Microsphere (registered trademark) series manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.

[0063] The foaming agent may have a foaming initiation temperature (Tc) of, for example, 90°C or higher, and from the viewpoint that foaming is likely to begin after the curable adhesive composition has sufficiently softened, the foaming initiation temperature (Tc) is preferably 95°C or higher or 100°C or higher. The foaming initiation temperature (Tc) may be, for example, 140°C or lower, and from the viewpoint that a sufficient expansion ratio is likely to be obtained before the curable adhesive composition is cured, the foaming initiation temperature (Tc) is preferably 135°C or lower. Alternatively, the temperature may be 130°C or lower.

[0064] The content of the foaming agent may be any amount that can achieve the expansion ratio described below. 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 per 100 parts by mass of the thermosetting resin. 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.

[0065] The first adhesive layer is foamed and cured by heating. The expansion ratio of the first adhesive layer is not particularly limited and is determined appropriately depending on the thermal conductivity and adhesive strength required for the foamed and cured product, the distance between the objects to be bonded, 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. The expansion ratio may also 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 are generated by foaming, and the thermal conductivity after foaming and curing tends to decrease. Therefore, the optimal expansion ratio can be determined experimentally. In this specification, the expansion ratio of the first adhesive layer is a value calculated as the ratio of the thickness of the first adhesive layer before and after foaming and curing.

[0066] The expansion ratio of the first adhesive layer can be adjusted appropriately by, for example, adjusting the content of the foaming agent.

[0067] [Hardening agent] 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 preferred from the viewpoint of preventing the thermosetting resin from curing before foaming.

[0068] An example of a curing agent in epoxy adhesives is dicyandiamide (DICY). DICY is a widely used epoxy curing agent due to its excellent heat resistance and electrical properties. It is commonly used in applications such as adhesives, coatings, and composite materials. Polyamide is a type of curing agent obtained by the reaction of polyamine with dimerized fatty acid. It exhibits excellent chemical resistance and flexibility, making it suitable for applications such as coatings, adhesives, and sealants.

[0069] Aromatic amines: Aromatic amines, such as diaminodiphenylmethane (DDM) and diaminodiphenylsulfone (DDS), are commonly used as epoxy curing agents. They have excellent chemical resistance and high-temperature performance, making them suitable for aerospace, automotive, and electrical industries.

[0070] Anhydrides: Anhydrides such as methylhexahydrophthalic anhydride (MHHPA) and nadic methyl anhydride (NMA) are widely used as epoxy curing agents. They have excellent heat resistance and electrical properties, making them suitable for applications such as electrical insulation materials, laminates, and composites.

[0071] Phenalkamines: Phenalkamines are a type of epoxy hardener obtained by the reaction of phenolic compounds with polyamines. They offer fast cure, excellent chemical resistance, and excellent adhesion to a variety of substrates. Phenalkamines are commonly used in marine coatings, flooring, and concrete repair applications.

[0072] Cycloaliphatic amines: Cycloaliphatic amines, such as isophoronediamine (IPDA) and diaminocyclohexane (DACH), are epoxy curing agents characterized by excellent chemical resistance and low viscosity. They are used in applications requiring high chemical resistance, such as tank interiors, chemical storage, and corrosion-resistant coatings.

[0073] Imidazoles: Imidazoles, such as 2-methylimidazole (2-MI) and 2-ethyl-4-methylimidazole (2-E4MI), are commonly used as epoxy curing agents. They exhibit fast curing properties at room temperature and are used in adhesives, potting materials, sealants, and other applications.

[0074] Latent curing agents: Latent curing agents, such as dicyandiamide (DICY) and imidazole derivatives, are designed to delay or control the curing of epoxy resins. These are often used in applications requiring extended pot life or low-temperature curing. Examples include dicyandiamide, 2,4-diamino-6-[2'-methylimidazolyl(1')]ethyl-s-triazine isocyanurate, and acid adducts. The content of the curing agent may be adjusted appropriately 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 per 100 parts by mass of the thermosetting resin. The amount of the curing agent may be 20 parts by mass or less, 18 parts by mass or less, 16 parts by mass or less, 14 parts by mass or less, or 12 parts by mass or less per 100 parts by mass of the thermosetting resin. The amount may be 10 parts by mass or less.

[0075] The adhesive (A) may further contain a curing accelerator. The curing accelerator may be any that can accelerate the curing of the thermosetting resin by the curing agent, and can be appropriately selected from known curing accelerators. When the curable adhesive composition is an epoxy adhesive, examples of the curing accelerator include imidazole-based curing accelerators (e.g., 2,4-diamino-6-[2'-methylimidazolyl(1')]ethyl-s-triazine, etc.) and urea-based curing accelerators (e.g., 4,4'-methylenebisphenyldimethylurea, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, etc.).

[0076] The content of the curing accelerator may be adjusted appropriately 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 or more, or 0.5 parts by mass or more per 100 parts by mass of the thermosetting resin. The content of the curing accelerator may be 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 per 100 parts by mass of the thermosetting resin.

[0077] [Other ingredients] The first adhesive layer may further contain components other than the curable adhesive composition, the thermally conductive filler, and the foaming agent, such as a thickener, an impact modifier, or a dispersant for the inorganic thermally conductive filler.

[0078] As used herein, the term "dispersant" refers to a substance that can be added to a composition to wet the thermally conductive filler particles, break up agglomerates, and improve particle separation. When a dispersant is included, it may be present in an amount of 0.05% by weight or more, for example, 0.1% by weight or more, 0.3% by weight or more, and preferably 0.5% by weight, based on the weight of the total composition. Dispersants that can be used in the composition include fatty acids, phosphate esters, polyurethanes, polyamines, polyacrylates, polyalkoxylates, sulfonates, polyethers, polyesters, and 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, DISPERBYK-2118 manufactured by BYK, and SOLSPERSE 24000SC, SOLSPERSE 16000, SOLSPERSE 8000 hyperdispersants manufactured by Lubrizol.

[0079] The content of the thickener is not particularly limited. The first adhesive layer may be formed by applying a coating liquid containing a curable adhesive composition, a thermally conductive filler, a foaming agent, and a solvent, followed by drying. The thickener is blended so that the viscosity of the coating liquid is suitable for application, and the content of the thickener is determined so that the viscosity of the coating liquid is suitable for application. The content of the thickener may be, for example, 5% by mass or less, 3% by mass or less, or 2.5% by mass or less, based on the total amount of components in the first adhesive layer other than the thermally conductive filler. This amount may also be 2% by mass or less. Furthermore, the content of the thickener may be, for example, 0.1% by mass or more, 0.3% by mass or more, 0.5% by mass or more, 0.7% by mass or more, or 1% by mass or more, based on all components in the first adhesive layer other than the thermally conductive filler.

[0080] Silane coupling agents may be added to the adhesive layer. Examples of silane coupling agents include aminosilanes, such as aminopropyltriethoxysilane (APTES), aminopropyltrimethoxysilane (APTMS), N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPTMS), N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPMDMS), epoxysilanes, such as glycidyloxypropyltrimethoxysilane (GPTMS), glycidyloxypropylmethyldimethoxysilane (GPMDS), 3-glycidyloxypropyltriethoxysilane (GPTES), vinylsilanes, such as vinyltrimethoxysilane (VTMS), vinyltriethoxysilane (VTES), vinyltris(2-methoxyethoxy)silane (VTMOS), and methacryloxysilanes, such as methacryloxy. propyltrimethoxysilane (MPTMS), methacryloxypropylmethyldimethoxysilane (MPMDS), mercaptosilanes such as mercaptopropyltrimethoxysilane (MPTMS), mercaptopropylmethyldimethoxysilane (MPMDS), chlorosilanes such as chloromethyltrimethoxysilane (CMTMS), chloromethyltriethoxysilane (CMES), dichlorodimethylsilane (DCDMS), alkoxysilanes such as methoxytrimethylsilane (MTMS), ethoxytrimethylsilane (ETMS), propoxytrimethylsilane (PTMS), isobutoxytrimethylsilane (IBTMS), and mixtures thereof that contain a combination of different functional groups such as amino, epoxy, vinyl, or methacryloxy groups to provide multifunctionality.

[0081] Examples of impact modifiers include core-shell type impact modifiers, and examples of core-shell type impact modifiers include core-shell rubbers.

[0082] Core-shell rubbers contain different materials for the internal core portion and the external shell portion. The glass transition temperature (Tg) of the shell portion is preferably higher than that of the core portion. The Tg of the core portion may be, for example, -110°C to -30°C, and the Tg of the shell portion may be, for example, 0°C to 200°C. In this specification, the Tg of the core portion and the shell portion are defined as the temperatures at which tan δ peaks in dynamic viscoelasticity measurements. The core portion of the core-shell rubber functions as a stress concentration point, improving impact resistance, while the shell portion suppresses non-uniform aggregation of the core-shell rubber, resulting in uniform dispersion of the core-shell rubber.

[0083] 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 norbornene; polymers of conjugated or non-conjugated dienes and monofunctional monomers (e.g., aromatic vinyl compounds such as styrene, vinyltoluene, and α-methylstyrene; unsaturated nitrile compounds such as acrylonitrile and methacrylonitrile; 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, and (meth)acrylates); acrylic rubbers such as polybutyl acrylate; silicone rubbers; and IPN composite rubbers made of silicone and polyalkyl acrylate. Core-shell graft copolymers with a shell formed by copolymerizing the periphery of a core with a (meth)acrylic acid ester may also be used. The core is preferably made of polybutadiene, butadiene-styrene copolymer, or acrylic-butadiene-styrene copolymer, while the shell is preferably made of a material obtained by copolymerizing methyl (meth)acrylate. The shell may have a laminated structure and may be composed of one or more layers. Two or more types of core-shell rubber may be used in combination.

[0084] 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, methyl methacrylate-acrylic rubber-styrene copolymer, methyl methacrylate-acrylic / butadiene rubber copolymer, methyl methacrylate-acrylic / butadiene rubber-styrene copolymer, methyl methacrylate-(acrylic / silicone IPN rubber) copolymer, etc. Among these, methyl methacrylate-butadiene copolymer, methyl methacrylate-butadiene-styrene copolymer, and methyl methacrylate-acrylic butadiene rubber-styrene copolymer are preferably used as the core-shell rubber.

[0085] The average particle size (mass average particle size) of the primary particles of the core-shell rubber may be, for example, 0.05 μm or more, or 0.1 μm or more. The average particle size (mass average particle size) of the primary particles may be, for example, 5 μm or less, 3 μm or less, or 1 μm or less. The average value of the primary particles of the core-shell rubber is calculated from the value obtained by zeta potential particle size distribution measurement. The content of the impact modifier is not particularly limited and may be, for example, 20% by mass or less, 15% by mass or less, or 10% by mass or less, based on the total amount of the components in the first adhesive layer other than the thermally conductive filler. The content of the impact modifier may be, for example, 1% by mass or more, 2% by mass or more, or 3% by mass or more, based on the total amount of the components in the first adhesive layer other than the thermally conductive filler.

[0086] The curable adhesive composition may further comprise a polymer that improves the performance of the epoxy system. For example, phenoxy resins can be added to improve adhesion, as toughening promoters, and to provide flexibility and chemical resistance. Examples include polyhydroxyether phenoxy resins in colloidal dispersions or organic solvents such as methyl ether ketone. In embodiments, the phenoxy resin has the following formula (V) and a molecular weight (Mw) in the range of 60K to 80Kg / mol: [ka] [In the formula, n is an integer necessary for the molecular weight of the resin to exceed 50 K or fall within the range of 60 K to 80 Kg / mol, for example, n=800 to 3000].

[0087] [Content range of components of curable adhesive composition] As described above, the reactive diluent may be present in a range of 10 to 50 wt % based on the curable adhesive composition excluding the weight of the filler. The curable adhesive composition may contain 40 to 80 wt % of a multifunctional epoxy resin based on the total amount of the curable adhesive composition. The blowing agent may be present in a range of 5 to 25 wt %. In addition to the reactive diluent, multifunctional epoxy resin, and blowing agent, other components may be added to the curable adhesive composition as needed. In some embodiments, the curable adhesive composition may contain 5 to 35 wt % of an additional component selected from one or more of a curing agent, a coupling agent, a toughening agent, and an accelerator. In yet another embodiment, the curable adhesive composition may further contain a phenoxy resin. The amount of phenoxy resin is not particularly limited, and in some embodiments, it may be present in a range of 1 to 15 wt %.

[0088] [Filler] The shape and size of the thermally conductive filler are not particularly limited as long as it can be mixed with the curable adhesive composition. In one embodiment, the filler has an average short side length of 1 μm or more. Fillers with an average short side length of less than 1 μm contribute little to thermal conductivity and are unlikely to function as thermally conductive fillers. The average short side length of the thermally conductive filler is 1 μm or more, and may be 1.5 μm or more or 2 μm or more from the viewpoint of enhancing the contribution to thermal conductivity. The average short side length of the thermally conductive filler may also be, for example, 100 μm or less, 90 μm or less, 80 μm or less, or 70 μm or less.

[0089] The reactive diluent improves the properties of the adhesive composition, making it possible to increase the content of the thermally conductive filler. The content of the thermally conductive filler is at least 10% by volume, based on the total volume of the first or second adhesive layer. In some embodiments, the content is 37% by volume or less relative to the total volume of the first adhesive layer, and from the viewpoint of further improving the adhesive strength with the object to be adhered, it is 35% by volume or less. Alternatively, it may be 33% by volume or less.

[0090] Furthermore, when the adhesive sheet has a first adhesive-permeable layer, from the viewpoint that the adhesive can easily penetrate the first adhesive-permeable layer and high adhesive strength with the object to be adhered can be obtained, the content of the thermally conductive filler relative to the total volume of the first adhesive layer may be 30% by volume or less, 27% by mass or less, 25% by mass or less, 23% by mass or less, 21% by mass or less, 19% by mass or less, or may be less than 17% by mass or less, or less than 15% by mass.

[0091] The thermally conductive filler material is not particularly limited and may be appropriately selected from known thermally conductive fillers. The thermally conductive filler may contain, for example, at least one selected from the group consisting of boron nitride, aluminum nitride, aluminum oxide, magnesium oxide, anhydrous magnesium carbonate, magnesium hydroxide, silicon oxide, and silicon nitride.

[0092] The thermally conductive filler may include a filler with an aspect ratio of 1.3 or more. Fillers with such high aspect ratios tend to form thermal conduction paths from one side of the adhesive layer to the other, even if the gaps between the bubbles generated during foaming are narrow. Therefore, using such fillers makes it possible to obtain a foamed, cured product with high thermal conductivity, even when foaming and curing are carried out at a high expansion ratio (e.g., 1.4 times or more).

[0093] The shape of the filler is not particularly limited as long as it satisfies the above aspect ratio, and may be, for example, a plate-like shape, a whisker-like shape, an aggregate-like shape, or the like.

[0094] From the viewpoint of obtaining the above-mentioned effects more significantly, the aspect ratio of the filler may be, for example, 1.4 or more, 1.5 or more, or 1.6 or more.

[0095] The aspect ratio of the filler may be, for example, 200 or less, 150 or less, or 100 or less.

[0096] When the filler is in the form of plates or aggregates, the aspect ratio may be, for example, 50 or less, 30 or less, 10 or less, 5 or less, or 3 or less. When the filler is in the form of whiskers, the aspect ratio may be, for example, greater than 50, 60 or more, 70 or more, 80 or more, or 90 or more.

[0097] The average short side length of the filler may be the same as the average short side length of the thermally conductive filler.

[0098] In this embodiment, the filler having a high aspect ratio efficiently forms a thermal conduction path between the cells. The thermally conductive filler may be at least partially or entirely composed of the filler.

[0099] In order to form more thermal conduction paths, the filler content may be, for example, 0.01 vol.% or more, 0.05 vol.% or more, 0.1 vol.% or more, or 0.15 vol.% or more, based on the total volume of the first adhesive layer.

[0100] 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 filler having an aspect ratio of 1.3 or more and a filler having an aspect ratio of less than 1.3.

[0101] The proportion of the filler in the thermally conductive filler may be, for example, 0.1% by volume or more relative to the total volume of the thermally conductive filler, which makes the thermal conduction path more efficient. From the viewpoint of increasing the efficiency of forming the thermal conduction path, 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.

[0102] When the aspect ratio of the filler exceeds 50 (for example, when the filler is whisker-like), the proportion of the filler in the thermally conductive filler may be, for example, 100% by volume or less, 50% by volume or less, 30% by volume or less, 10% by volume or less, 5% by volume or less, or 3% by volume or less, based on the total volume of the thermally conductive filler.

[0103] When the aspect ratio of the filler is 50 or less (e.g., when the filler is plate-shaped or chunk-shaped), the proportion of the filler in the thermally conductive filler may be, for example, 10 vol. % or more, 30 vol. % or more, 50 vol. % or more, 70 vol. % or more, 90 vol. % or more, or 100 vol. % based on the total volume of the thermally conductive filler.

[0104] [viscosity] Curable adhesive compositions generally exist in a non-flowable solid or semi-solid state at room temperature and pressure. Upon heating, they begin to soften and gradually transition to a flowable liquid or semi-liquid state. In certain embodiments, the curable adhesive composition has a softening point greater than 60°C, as measured using a ring and ball apparatus in accordance with ASTM D-2398. Upon transitioning completely to a liquid state, the curable adhesive composition has a melt viscosity at 110°C of less than 800,000 Pa·s, less than 500,000 Pa·s, or preferably less than 300,000 Pa·s, as measured in accordance with ASTM D3835-16 (Standard Test Methods for Characterization of Polymeric Materials by Capillary Rheometry). The curable adhesive composition may have a melt viscosity of greater than 10 Pa·s to 100 Pa·s, as a lower limit. The flowable liquid can have a wide range of viscosities. For example, water has a relatively low viscosity of about 1 cP (0.001 Pa·s) at room temperature, while honey has a relatively high viscosity of about 10,000 cP (10 Pa·s). The curable adhesive composition of the present application may have a melt viscosity in the range of 100 to 400 Pa·s when heated.

[0105] [Adhesive permeable layer] The adhesive sheet may further include an adhesive-permeable layer disposed on one or both of the first and second adhesive layers, which is permeable to the curable adhesive composition until one or both of the first and second adhesive layers are foamed by heating. The adhesive-permeable layer is "permeable" in the sense that the curable adhesive composition can pass from one major surface of the first adhesive-permeable layer to the other major surface of the first adhesive-permeable layer as the first adhesive layer expands (foams). To be permeable, the adhesive-permeable layer must be porous, i.e., have a structure in which voids exist from one major surface to the other. In this case, when the first adhesive layer in contact with only one major surface of the first adhesive-permeable layer expands (foams), the curable adhesive composition, which has become fluid upon heating, penetrates the layer through the voids.

[0106] The material constituting the adhesive permeable layer is not particularly limited, and any material can be used as long as it can maintain a permeable shape at the curing initiation temperature of the curable adhesive composition.

[0107] The adhesive-permeable layer may be, for example, a nonwoven fabric made of natural fibers, chemical fibers, or a mixture thereof. The nonwoven fabric has a large number of through-holes therein, which can significantly enhance the above-mentioned effects.

[0108] The basis weight of the first adhesive permeable layer is, for example, 10 g / m 2 or more, or 11 g / m 2 The upper limit of the basis weight of the first adhesive permeable layer is not particularly limited, and may be within the range that satisfies the thickness range of the first adhesive permeable layer described below.

[0109] The thickness of the first adhesive permeable layer may be, for example, 55 μm or less, or may be 50 μm or less or 47 μm or less from the viewpoint that when the first adhesive layer expands, the amount of curable adhesive composition that seeps out to the surface increases, thereby improving adhesive strength. The lower limit of the thickness of the first adhesive permeable layer is not particularly limited, and may be within the range of the basis weight of the first adhesive permeable layer described above, for example.

[0110] [Manufacturing method] In another aspect, the present disclosure provides a method of forming an adhesive sheet, the method including the steps of providing an electrically insulating substrate; mixing a thermally conductive filler with a curable adhesive composition comprising a multifunctional epoxy resin, a reactive diluent having at least one terminal epoxide moiety, and a blowing 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.

[0111] In one embodiment, the prepolymer composition is a hot melt composition, which is heated to near or above its glass transition temperature to render it flowable and ready for deposition onto a substrate, after which the prepolymer composition is cooled and solidified.

[0112] In another embodiment, the prepolymer composition further comprises a solvent, such as methyl ethyl ketone (MEK), which makes the prepolymer composition flowable and suitable for coating a substrate. After coating the substrate, the solvent is evaporated by air drying or heating to form an adhesive layer.

[0113] The processing temperature for liquefying the prepolymer composition (as a hot melt or to remove the solvent) is not particularly limited, and is preferably less than 150°C, or less than 100°C.

[0114] If it is desired to apply an adhesive permeable layer on top of each adhesive layer, the heat lamination process may be carried out at a slightly higher temperature so long as the adhesive layer does not begin to thermally cure, for example, at a temperature above 40°C to 100°C, more preferably 50°C to 90°C, or 55°C to 65°C.

[0115] [drawing] Figure 1 is a cross-sectional view of one embodiment of an adhesive sheet prior to heating and curing. The adhesive sheet 10 shown in Figure 1 includes an electrically insulating substrate 20, first and second adhesive layers 30a and 30b, and adhesive permeable layers 40a and 40b. A foaming agent 91, including thermally expandable microspheres, is shown as small circles present in the first and second adhesive layers. Thermally conductive fillers 92 are shown as dots in the first and second adhesive layers.

[0116] 2A is a cross-sectional view of one embodiment of adhesive sheet 11 after heating. When heated, blowing agent 91, which contains thermally expandable microspheres, expands to form expandable microspheres 93, which extrude the fluidized curable adhesive composition and thermally conductive filler 92 toward adhesive-permeable layers 40a and 40b. Gradually, the curable adhesive composition saturates adhesive-permeable layers 40a and 40b, after which the curable adhesive composition exudes onto the outer surfaces of the adhesive-permeable layers, forming outer adhesive layers 50a and 50b. Further heating cures the curable adhesive composition, forming outer cured adhesive layers on adhesive sheet 11. In this embodiment, the adhesive-permeable layers are permeable to the thermally conductive filler and expandable microspheres, allowing them to penetrate the adhesive-permeable layers.

[0117] 2B is a cross-sectional view of another embodiment of adhesive sheet 12 after heating. The adhesive-permeable layer can be selected to be impermeable to the thermally conductive filler and expandable microspheres, thereby retaining them within the initial adhesive layer, allowing only the curable adhesive composition to penetrate the adhesive-permeable layer upon expansion to form outer adhesive layers 50c and 50d thereon.

[0118] 1, the adhesive sheet 10 is in a tack-free state because the first adhesive-permeable layers 40a and 40b are the outermost layers, allowing the adhesive sheet 10 to be efficiently bonded to the surface to be bonded with a single heating step.

[0119] The foamed and cured product of this embodiment includes an electrically insulating substrate, a first foam layer formed by foaming and curing a first adhesive layer, and a second foam layer formed by foaming and curing a second adhesive layer.

[0120] In the foamed cured product of this embodiment, the first adhesive-permeable layer may be embedded in the first foamed layer, and the second adhesive-permeable layer may be embedded in the second foamed layer.

[0121] The expansion ratio of the adhesive sheet in this embodiment is not particularly limited and is determined appropriately depending on the thermal conductivity and adhesive strength required of the foamed and cured product, the distance between objects to be bonded, 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. The expansion ratio of the adhesive sheet may also 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 this specification, the expansion ratio of the adhesive sheet is defined as the ratio between the thickness of the adhesive sheet and the thickness of the foamed and cured product obtained by foaming and curing it.

[0122] [Applications of adhesive sheets] In yet another aspect, the present disclosure relates to an electric motor including a stator comprising a stator core having at least one slot, a winding housed within the slot, and an insulating slot liner disposed between the stator core and the winding, the adhesive sheet comprising an adhesive sheet as defined herein, in some embodiments, the adhesive sheet being cured.

[0123] In yet another aspect, the present disclosure relates to a method of electrically insulating an electric motor including a stator, the method comprising the steps of providing a stator core having at least one slot, disposing an insulating slot liner including an adhesive sheet as defined herein in the at least one slot, disposing an electrical winding in the slot, and curing the adhesive sheet.

[0124] The adhesive sheet of this embodiment can be used for a variety of purposes, including bonding objects together. The adhesive sheet of this embodiment has excellent thermal conductivity after foaming and curing, making it suitable for use in applications requiring thermal conductivity and heat dissipation. Furthermore, because the adhesive sheet of this embodiment is expandable, it can conform to the surface shape of the objects to be bonded and fill gaps between the objects. Even if the surfaces of the objects have unintended irregularities, the objects can be properly bonded together. Therefore, the adhesive sheet of this embodiment can be used effectively when the objects to be bonded are uneven or when it is necessary to fill gaps between the objects.

[0125] The adhesive sheet of this embodiment can be suitably used, for example, as a slot liner for a stator core of an electric motor. That is, the adhesive sheet of this embodiment may be disposed, for example, between the stator core and a winding and used to bond the stator core and the winding. The electric motor of this embodiment includes a stator, which may include a stator core having at least one slot, a winding at least partially housed in the slot, and an adhesive layer that bonds the stator core and the winding.

[0126] The adhesive layer may be a layer formed by foaming and curing an adhesive sheet. That is, the adhesive layer may be a layer including a foamed and cured adhesive sheet. In the electric motor of this embodiment, the components other than the adhesive layer are not particularly limited and may be the same as the components in known electric motors. While preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. [Example]

[0127] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0128] The ingredients used are shown in Table 1. The amounts of each ingredient used are shown in Table 2.

[0129] The calculation criteria for each of the following examples are listed in Table 2. To establish a "control" curable adhesive composition that did not contain 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 to which 5% by weight of ED523T was added to the control composition (i.e., the reactive diluent added was 5% of the weight of the control composition). When normalized to the total weight of the new adhesive composition including reactive diluent, the ED523T content is 4.8%.

[0130] [Measurement of thermally conductive filler size] To measure the size of the filler particles used, the filler particles were sprayed onto a sample stage with conductive double-sided tape attached, and excess filler was removed using a blower. Next, the filler particles were coated with osmium using an osmium plasma coater (OPC80N, manufactured by Japan Laser Electronics Co., Ltd.) to provide a conductive coating.

[0131] Secondary electron images of the filler were obtained using a scanning electron microscope (Hitachi High-Tech S3400N) at an accelerating voltage of 10 kV, a working distance of 10 mm, and observation magnifications ranging from 100x to 3000x. From the obtained image, the smallest rectangle (minimum circumscribing rectangle) that could cover the filler was confirmed visually and by image analysis, and the long and short sides of that rectangle were taken as the long and short sides of the filler, respectively.

[0132] Three or more images were taken for each type of filler, and the long and short sides of 10 to 30 fillers per image were measured, for a total of 50 or more fillers. The average values ​​were calculated as the average long and short side lengths of the filler. Furthermore, the aspect ratio of the filler was calculated by dividing the average long side length by the average short side length. The filler dimensional measurement results are shown in Table 3. The criteria for evaluating the melt viscosity, thermal conductivity, overlap shear (OLS) strength, and Tg of each sample are shown in Table 4.

[0133] [Sample (A) for evaluation of thermal conductivity and overlap shear strength] A 75 μm-thick PEN film (trade name: Teonex Q51, manufactured by Toyobo Co., Ltd.) was prepared as the substrate. The curable adhesive composition was obtained by mixing BTA731 and NPPN442, then adding the other materials listed in Table 1 and mixing them in a mixer. Each of Samples IS-1 to IS-4 and 1-1 to 1-7 was prepared by mixing 100 parts by mass of the curable adhesive composition prepared by the above method with 24.9 parts by mass of a thermally conductive filler. The filler content in the adhesive layer was set to 13.9 vol% relative to the total volume of the curable adhesive composition. ED523T was used as the reactive diluent. Examples 1-9 to 1-13 were prepared in the same manner, except that the content of the filler BN-CFP-012 was varied between 0 vol% and 30 vol%. Examples IS-6 and IS-7 were prepared in the same manner, except that the thermally conductive filler was set to 13.9 vol% and the type of reactive diluent was varied.

[0134] To form an adhesive layer, a coating liquid containing a curable adhesive composition and a thermally conductive filler dispersed therein was prepared. The coating liquid was applied to one side of a substrate and dried at 65°C for 3 minutes and then at 90°C for 3 minutes to form a first adhesive layer. The thickness of the first adhesive layer was approximately 50 μm. The thickness of the first adhesive layer was measured at three random points in an A4-sized area after the first adhesive layer was formed using a desktop micrometer, and the average thickness was calculated. The average thickness was calculated by subtracting the thickness of the substrate.

[0135] Next, a nonwoven sheet (PET, basis weight 13 g / m 2 ) was laminated on the first adhesive layer, and heated and pressed at a roll temperature of 60°C using a roll laminator.

[0136] Next, the coating liquid was applied to the other side of the substrate 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 approximately 49 μm. The thickness of the second adhesive layer was measured by measuring the thickness of three arbitrary points in an A4-sized area of ​​the sample before and after the formation of the second adhesive layer with a bench micrometer. The average thickness before the formation of the second adhesive layer was subtracted from the average thickness after the formation of the second adhesive layer.

[0137] Then, a nonwoven fabric sheet (PET, basis weight 13 g / m) was placed on the second adhesive layer. 2 ) were laminated together, and heated and pressed using a roll laminator with a roll temperature of 60°C to obtain a laminate with a thickness (T1) of 190 µm.

[0138] In this way, an adhesive sheet having a five-layer structure as shown in FIG. 1 was obtained.

[0139] [Measurement of thermal conductivity] The adhesive sheet for sample (A) was cut to a 50 mm x 50 mm piece. Two fluororesin sheets (manufactured by Alum Co., Ltd., 0.2 mm thick) were prepared. One adhesive sheet was placed on top of the other, and a 400 μm thick spacer was placed around the entire periphery. The other fluororesin sheet was then placed on top of the adhesive sheet and spacer, sandwiched between them, and hot pressed at 160°C for 10 minutes to obtain a cured adhesive sheet with a thickness (T2) of 360 μm. This cured adhesive sheet was used as a test specimen for thermal conductivity measurement. When measuring thermal conductivity using the method described below, the thickness (T2) of the cured adhesive was measured using the thickness measurement function at the measurement point of the thermal conductivity measurement device. The thermal conductivity of the test specimen was measured using a thermal conductivity measurement device (Thermal Interface Material Tester TIM Tester Model 1300) manufactured by Analysis Tech Inc., in accordance with ASTM D5470.

[0140] [Shear strength measurement] The adhesive sheet for sample (A) was cut to a size of 12.5 mm x 25 mm. Two SPCC plates (100 mm x 25 mm x 1.6 mm, JIS G 3141 compliant) were prepared and their surfaces were cleaned with methyl ethyl ketone. A 400 μm-thick adhesive sheet was placed on one of the SPCC plates, starting from the edge. With a spacer placed on the adhesive sheet, another SPCC plate was placed on top of the adhesive sheet and spacer, sandwiched between them, and hot-pressed at 160°C for 10 minutes to obtain a shear test specimen (JIS K 6850 compliant) containing a foamed, cured product with a thickness (T3) of 380 μm. The thickness (T3) of the foamed, cured product was calculated by measuring the total thickness of the shear test specimen and subtracting the thickness of the two SPCC plates (3200 μm).

[0141] A materials testing machine (Orientec RTC-1325A) equipped with a constant temperature testing device was used. The measurement sample (shear test specimen) was placed in the materials testing machine heated to 230°C for 10 minutes to be sufficiently heated. Then, the shear strength was measured under shear tensile conditions of 230°C and a tensile speed of 5 mm / min.

[0142] [Sample (B) for measuring melt viscosity and glass transition temperature after curing] A thermally conductive filler (boron nitride, BN-CFP-012) was added to each adhesive composition that did not contain expandable microspheres, accelerators, or curing agents (FN-100SSD, 2MZA-PW, and DIC1400F) to prevent curing and expansion during evaluation. The adhesive composition was applied to the water-repellent side of a 50 μm silicone-coated sheet and dried at 65°C for 3 minutes and at 90°C for 3 minutes to form an adhesive layer.

[0143] [Viscosity measurement] The adhesive sheet of sample (B) was peeled off from the silicone sheet, and five similar sheets were stacked to obtain a sheet approximately 1500 μm thick. This sheet was then molded into a cylindrical shape with a diameter of 8 mm to serve as a sample for melt viscosity measurement.

[0144] The dynamic viscoelastic properties of the molded samples were evaluated using an ARES-G2 (TA Instruments) under conditions of a heating rate of 5°C / min, an amplitude strain of 0.01%, and a frequency of 1.0 Hz. The complex viscosity at 110°C was calculated as the melt viscosity (Pa·S).

[0145] [Tg measurement] The adhesive sheet for sample (B) was cut to a 50 mm x 50 mm piece. Two fluororesin sheets (Alum Co., Ltd., 0.2 mm thick) were prepared. One adhesive sheet was placed on top of the other, surrounded by a 250 μm-thick spacer. The other fluororesin sheet was then placed on top of the adhesive sheet and spacer, sandwiched between them, and hot-pressed at 160°C for 10 minutes to obtain a cured adhesive with a thickness of approximately 200 μm. The resulting sample was cut to the appropriate size and its dynamic viscoelastic properties were evaluated using a TA Instruments RSA-G2 at a heating rate of 5°C / min, an amplitude strain of 0.01%, and a frequency of 1 to 1 Hz. The peak temperature of tan δ was defined as the Tg after curing.

[0146] [Expansion ratio measurement] Measurement of the expansion ratio of the cured foam in the thermal conductivity measurement specimen: The expansion ratio was calculated by T2 / T1 using the thickness of the adhesive sheet (T1) and the thickness of the cured foam (T2).

[0147] Measurement of the expansion ratio of the cured foam in the shear test specimen: The expansion ratio was calculated by T3 / T1 using the thickness of the adhesive sheet (T1) and the thickness of the cured foam in the shear test specimen (T3).

[0148] [result] Examples IS-1 to IS-4 and 1-1 to 1-7: As shown in the table below, adhesive sheet samples were obtained by varying the amount of ED-523T in the range of 1 wt% to 16.7 wt%, and fixing the filler CFP-012 at 13.9 vol% of the curable adhesive composition. The results are shown in Table 5.

[0149] Examples 1-9 to 1-13: Adhesive sheet samples were prepared in the same manner as in IS-1, but as shown in Table 6, the amount of BN-CFP-012 was varied from 0 to 30% by volume.

[0150] Examples IS-6 and IS-7: Adhesive sheet samples were prepared in the same manner as IS-1, but with varying additives in ED-523T as shown in the table below.

[0151] Examples IS-12 and IS-13: Adhesive sheet samples were prepared similarly to those described above, except that the filler type and amount of BN-CFP-012 was varied. The reactive diluent was also changed to ED-505 and EP-4005. The results are shown in Table 7. The initial melt viscosity of the control was 762 Pa·s. In IS-1 through IS-4 and 1-1 through 1-7, the addition of reactive diluent (ED-523T) from 1 wt% to 16.7 wt% reduced the viscosity by more than half and improved the thermal conductivity from 0.19 to 0.21 to 0.31. However, increasing the amount of reactive diluent from 9.1 wt% to 16.7 wt% affected the Tg of the cured adhesive, indicating that adding excessive reactive diluent is undesirable. In Samples 1-9 to 1-13, excessive filler likely leads to uneven expansion of the adhesive sheet, resulting in a decrease in the thermal conductivity of the adhesive sheet. Therefore, it was found that the optimum filler loading is less than 30% by volume, preferably in the range of 1 to 20% by volume. In IS-12 and IS-13, the use of triglycidyl ether (ED-505) and high molecular weight / high viscosity glycidyl ether (EP-4005) also showed excellent results in terms of thermal conductivity, similar to diglycidyl ether (ED-523T).

[0152] From the above results, it has become clear that by adding an appropriate amount of a selected reactive diluent, the rheological properties of the adhesive composition in the adhesive layer, such as the melt viscosity, can be controlled within an appropriate range, allowing the adhesive layer to expand effectively, distributing the adhesive and thermally conductive filler through the adhesive permeable layer, and improving thermal conductivity in applications requiring good thermal conductivity, such as electric motor slot liners.

[0153] It goes without saying that various changes and modifications of the present invention will become apparent to those skilled in the art after reading the disclosure herein without departing from the spirit and scope of the present invention, and all such changes and modifications are intended to be included within the scope of the appended claims.

Claims

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 the other side of the substrate opposite the first adhesive layer; Equipped with one or both of the first and second adhesive layers comprises a curable adhesive composition and a thermally conductive filler dispersed in the composition; The curable adhesive composition comprises: multifunctional epoxy resins, a reactive diluent having at least one terminal epoxide moiety, and foaming agent Including, adhesive sheet.

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

3. The reactive diluent has the following formula (I): 【Chemistry 1】 [In the formula, R1, R1', R2 and R2' are each independently selected from C1-C10 hydrocarbons, wherein hydrogen is optionally replaced by halogen; R3 and R3' are each independently selected from -H or C1-C10 alkyl, alkenyl, phenyl, alkynyl, and carbonyl moieties, wherein the hydrogen is optionally substituted with a halogen. The adhesive sheet according to claim 2, wherein the diglycidyl ether has the formula:

4. The reactive diluent has the following formula (II): 【Chemistry 2】 [In the formula, R4 and R5 are each independently —CH ２ -, -C ２ H ５ -, -C ３ H ６ -, -C ４ H ８ -or-C ６ H ４ - is selected from, R6 is —H, —CH ３ or -C ２ H ５ Selected from 3. The adhesive sheet of claim 2, comprising a triglycidyl ether having the formula:

5. The reactive diluent is represented by the following formula (III): 【Transformation 3】 wherein R7 is C ３ ~C ６ is a linear or branched saturated hydrocarbon moiety of the formula The adhesive sheet of claim 2 , comprising an aromatic monoglycidyl ether having the formula:

6. The reactive diluent is represented by the following formula (IV): 【Chemistry 4】 wherein R8 is C ２ ~C １４ and selected from linear, branched or cyclic alkyl, alkenyl, phenyl-alkyl moieties of the formula:

3. The adhesive sheet of claim 2, comprising an aliphatic monoglycidyl ether having the formula:

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

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

9. The curable adhesive composition comprises, based on the total amount of the curable adhesive composition: 40 to 80 wt. % of a multifunctional epoxy resin; 5 to 25 wt. % of a blowing agent, and 5 to 35 weight percent of additional components selected from one or more of curatives, coupling agents, toughening agents, and accelerators The adhesive sheet of claim 8 further comprising:

10. The adhesive sheet of claim 9 , wherein the multifunctional epoxy resin comprises a trifunctional epoxy resin.

11. The adhesive sheet according to any one 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 60K to 80Kg / mol.

13. 13. The adhesive sheet according to any one of claims 1 to 12, wherein the curable adhesive composition is in a non-flowable state at room temperature and pressure, and has a softening point of greater than 60°C measured using a ring and ball apparatus in accordance with ASTM D-2398 test method, and a melt viscosity of less than 300,000 Pa s at 110°C measured using an ASTM D3835-16 Standard Test Method for Characterization of Polymeric Materials by Capillary Rheometer.

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

15. 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. 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 greater.

17. The adhesive sheet according to any one of claims 14 to 16, wherein the filler is in the form of plates, whiskers or aggregates.

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

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

20. The adhesive permeable layer has a thickness of 5 to 30 g / m ２ 20. The adhesive sheet of claim 19, comprising a polyethylene terephthalate nonwoven fabric having a basis weight of

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

22. The adhesive sheet according to any one of claims 1 to 21, wherein the curable adhesive composition is spread throughout the adhesive permeable layer and cured.

23. 1. A method of forming an adhesive sheet, comprising: providing an electrically insulating substrate; The thermally conductive filler multifunctional epoxy resins, a reactive diluent having at least one terminal epoxide moiety, and foaming agent with a curable adhesive composition comprising: disposing the prepolymer composition on one side of the substrate to form a first adhesive layer; and disposing the prepolymer composition on the other side of the substrate opposite the first adhesive layer to form a second adhesive layer; A method comprising:

24. 24. The method of claim 23, wherein the prepolymer composition is a hot melt composition, and the prepolymer composition is heated above its glass transition temperature to be coated, and after coating, the prepolymer composition is cooled to form the first and second adhesive layers.

25. 24. The method of claim 23, wherein the prepolymer composition further comprises a solvent that makes the prepolymer composition fluid and suitable for coating, and after coating, the solvent is removed by evaporation or heating to form the first and second adhesive layers.

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

27. a stator core having at least one slot; a winding housed within the slot; an insulating slot liner disposed between the stator core and the winding, the insulating slot liner comprising the adhesive sheet according to any one of claims 1 to 21; An electric motor including a stator comprising:

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

29. 1. A method of electrically insulating an electric motor including a stator, comprising: providing a stator core having at least one slot; placing an insulating slot liner comprising the adhesive sheet of any one of claims 1 to 21 in said at least one slot; placing an electrical winding within the slot; curing the adhesive sheet A method comprising:

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