Edge-sealed slot liner

Abstract

To provide a slot liner that can favorably resist high-temperature environment, being folded and being roughly handled during assembly, and impacts and vibrational stress during use.SOLUTION: A multilayer slot liner 100 for a rotary electric system includes an electrical insulating base material layer 102, a first thermal activation adhesive layer 104 containing thermally expansible particles disposed on a first surface of the base material layer, and a first adhesive transmissive cover layer 106 disposed on the first thermal activation adhesive layer. At least two of the layers extend together, and terminate at an edge 108. The edge is covered with a seal 109.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a slot liner, and more particularly to a slot liner including thermally expandable particles. [Background technology]

[0002] Slot liners are used in electric motors and generators to provide electrical insulation between the conductive windings and the slots of the stator core that house them. More recently, multi-layer composite laminate slot liners have been developed to exacting specifications such as thickness, flexibility, dielectric strength, temperature rating, low surface friction, etc. to improve performance, reliability, and ease of manufacture to meet necessary performance requirements in electric vehicles.

[0003] The automated construction of electrical machines has led to the use of novel materials for slot liners, which can aid in the automated assembly of stators for motors. For example, US Pat. No. 5,399,433 describes a heat-activated slot liner that expands when heated. However, laminated composites can be affected by operational exposure to heat, solvents, and mechanical shocks, resulting in deformation and delamination. There is a continuing need for more durable slot liners that can withstand these environmental factors to achieve improved reliability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] US Patent Application Publication No. 2021 / 0207010

[0005] Patent Document 1 describes a slot liner having a thermally expandable adhesive layer formed on the surface of a substrate and an adhesive permeable layer provided on the surface of the adhesive layer. The adhesive permeable layer allows the heated adhesive to permeate when the adhesive expands thermally. Thus, by heating the adhesive sheet, the adhesive from the heat-activated adhesive layer permeates the adhesive permeable layer, emerges on the outermost surface of the adhesive sheet, and bonds to the surface adjacent to the adhesive permeable layer, thereby simplifying the deposition of the adhesive. Summary of the Invention

[0006] Problem to be solved by the invention A problem presented by slot liners having a multi-layer structure is that one or more layers may experience deformation or even delamination during curing and after multiple heating and cooling cycles, especially for devices operating in high heat environments. This problem is due to the different thermal expansion coefficients of the materials in each layer and their contraction behavior due to shrinkage upon cooling. Uneven expansion and contraction of adjacent layers results in interlaminar stresses that may not resolve over time, resulting in residual stresses that build up and eventually deform the slot liner. If the interlaminar stresses are large enough, one or more delaminations in the slot liner may occur. Delamination leads to reduced insulation reliability, which in turn leads to premature failure of the motor or generator.

[0007] Means for solving the problem To solve the above problems, in one aspect, the present invention provides a multi-layer slot liner for a rotating electrical system, comprising an electrically insulating substrate layer, a first heat-activated adhesive layer disposed on a first surface of the substrate layer, and a first adhesive-permeable cover layer disposed on the first heat-activated adhesive layer, at least two of the layers extending together and terminating at edges, the edges being covered by a seal. The multi-layer slot liner may further comprise a second heat-activated adhesive layer disposed on a second surface of the substrate layer, the second surface being located opposite the first surface of the substrate layer, and a second adhesive-permeable cover layer disposed on the second heat-activated adhesive layer.

[0008] The present invention, which is configured to seal the edges of the slot liner, can reduce the initiation of delamination at the edges of the slot liner. The seal at the edges of the slot liner mechanically holds the individual layers of the slot liner together. It also prevents the edges of the individual layers of the slot liner from being directly exposed to environmental factors that weaken the cohesion between the layers. In addition, the seal may also include a material with heat resistance, elasticity, and mechanical strength, thereby imparting beneficial properties to the edges of the slot liner and allowing them to better withstand high heat environments, folding and rough handling during assembly, and shock and vibration stresses during use.

[0009] In another aspect, the present invention provides a method of sealing an edge of a slot liner in a stator of a rotating electrical system, the method comprising: providing a continuous multi-layer sheet comprising an electrically insulating substrate layer, a first heat-activated adhesive layer disposed on a first surface of the substrate layer, and a first adhesive-permeable cover layer disposed on the first heat-activated adhesive layer, at least two of the layers extending together and terminating at an edge; cutting a portion of the sheet to form a slot liner, repeatedly placing the slot liner in the slots of the stator until all slots are filled; applying a sealing composition to the edge of the slot liner; and curing the sealing composition to form a seal over the edge of the slot liner. An assembly of a conductive winding into the slot of the stator can then be performed. After the conductive winding is assembled, the stator can be heated to activate and cure the adhesive present in the heat-activated adhesive layer in the slot liner.

[0010] With the invention configured in this way, after the slots have been filled according to conventional processes, the slot liner edges can be batch sealed in situ within the stator by using any suitably designed applicator or coating process to deposit a sealing composition on the slot liner edges and applying heat to cure the sealing composition. Alternatively, the slot liners may be pre-manufactured with sealed edges and then inserted into the stator slots. In either case, sealing of the slot liner edges is compatible with existing slot liner assembly processes without the need to substantially modify existing equipment or process steps.

[0011] If the seal is formed during assembly of the slot liner to the stator, the seal preferably comprises an adhesive having a high viscosity in an uncured state. An adhesive having viscous properties in the form of a viscous fluid or gel facilitates controlled deposition onto the edge of the slot liner. The seal may also preferably comprise a waterproof, heat resistant, and mechanically strong material, such as those available from cyanoacrylate or urethane adhesives.

[0012] The heat-activated adhesive layer preferably comprises heat-expandable particles having an expansion start temperature Te. To ensure good adhesion between the layers in the slot liner, the heat-expandable particles are preferably activated gradually and completely. For this purpose, a two-stage continuous heating process is preferably performed. In the first heating step, the slot liner is heated to a first temperature T1 ≦ T2 to start the expansion of the adhesive layer. S and in a second step, the slot liner is heated to a second temperature T2>T S is heated until

[0013] Effect of the Invention The present invention provides a multi-layer slot liner containing thermally expandable particles that has improved durability over existing slot liners due to sealing the edges of the slot liner to prevent initiation of delamination of the layers at the edges. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a cross-sectional view of a single sided slot liner according to one embodiment.

[0015] [Diagram 2] FIG. 2 is a cross-sectional view of a double-sided slot liner according to one embodiment.

[0016] [Diagram 3] FIG. 2 is a cross-sectional view of a double-sided slot liner in a stator slot after heat treatment.

[0017] [Figure 4] FIG. 1 is a perspective view of a slot liner disposed in a stator with either a long edge or a short edge sealed.

[0018] [Diagram 5] FIG. 2 is a perspective view of a slot liner having conductive windings and both its long and short edges sealed;

[0019] [Figure 6] Table 1 is provided which lists the components of the heat-activatable adhesive layer.

[0020] [Figure 7] Table 2 listing edge sealing compositions A, B, and C is provided.

[0021] [Figure 8] Table 3 listing the test results for Example 1 is provided.

[0022] [Figure 9] Table 4 listing the test results for Example 2 is provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The present invention provides an edge-sealed slot liner with improved reliability due to the encapsulation of the slot liner edges with a seal. FIG. 1 is a cross-sectional view showing a multi-layer slot liner configuration according to an embodiment of the present invention. As shown in FIG. 1, the slot liner 100 includes a substrate layer 102 having a surface 1021 and a surface 1022 on either side. A heat-activated adhesive layer 104 is disposed on the substrate layer 102 surface 1021. An adhesive-permeable cover layer 106 is disposed on the heat-activated adhesive layer 104. At least two of the layers 102, 104, 106 extend together across the slot liner and terminate to form an edge 108. A seal 109 is formed at the edge 108, contacting the ends of the layers forming the edge 108 and completely covering the ends of the individual layers within the slot liner, thereby encapsulating the edge 108. In this embodiment, all three layers 102, 104, 106 are coextensive and terminate to form an edge. The seal 109 completely covers the ends of each of the three layers in the slot liner, thereby enclosing the edge 108. In another embodiment, only the heat-activated adhesive layer 104 and the adhesive-permeable cover layer 106 terminate to form an edge, and the ends of the substrate layer 102 terminate before or extend beyond the edge. Thus, in this embodiment, the seal covers the ends of the heat-activated adhesive layer 104 and the adhesive-permeable cover layer 106. In this embodiment, the slot liner provides adhesion on only one side. The other side of the slot liner is the substrate layer surface, which can be optionally coated with other materials, such as a low-friction material to facilitate insertion into the slot, e.g., polytetrafluoroethylene (PTFE), or a thermally conductive and electrically insulating material, e.g., boron nitride, to improve heat transfer away from the conductive windings and into the stator body.

[0024] The heat-activated adhesive layer and the adhesive permeable cover layer may be provided on only one side of the substrate layer as shown in FIG. 1, or on both sides of the substrate. In a preferred embodiment, the second heat-activated adhesive layer is disposed on the second surface of the substrate, the second surface being located opposite the first surface of the substrate, and the second adhesive permeable cover layer is disposed on the second heat-activated adhesive layer. In other words, in this embodiment, both sides of the substrate layer each comprise an adhesive layer and an adhesive permeable cover layer. Referring to FIG. 2, the slot liner 200 includes a substrate layer 202 having a surface 2021 and a surface 2022 on both sides. The heat-activated adhesive layer 204 is disposed on the surface 2021, and the adhesive permeable cover layer 106 is disposed on the heat-activated adhesive layer 204. Similarly, on the other side, the heat-activated adhesive layer 205 is disposed on the surface 2022, and the adhesive permeable cover layer 107 is disposed on the heat-activated adhesive layer 205. In this embodiment, the slot liner may be bonded to the conductive windings and to the stator wall adjacent the slot liner on either side of the slot liner.

[0025] At least two of the layers 202, 204, 205, 206, and 207 extend together across the slot liner and terminate to form an edge 208. A seal 209 is formed at edge 208 and contacts and covers the ends of each of the layers that form edge 208, thereby enclosing edge 208. In the embodiment shown in Figure 2, all five layers 202, 204, 205, 206, and 207 are coextensive and terminate to form edge 208. In another embodiment, only some of the layers terminate to form an edge. For example, heat-activated adhesive layer 204 and adhesive permeable cover layer 206 may terminate on surface 2021 to form a first edge, and heat-activated adhesive layer 205 and adhesive permeable cover layer 207 may terminate on surface 2022 to form a second edge, while the ends of substrate layer 202 terminate before or extend beyond the first and second edges. In this embodiment, seal 209 may be formed separately to cover the first and second edges.

[0026] The substrate serves as a support for carrying all the other layers of the slot liner. Therefore, any material with sufficient mechanical strength can be used. Since the slot liner is folded before assembly into the slot, the material used should be elastic and flexible, not rigid. In addition, the substrate is preferably an electrically insulating material. An exemplary substrate should be a heat-resistant film material that can withstand exposure to heat, but does not necessarily have to be thermally insulating. Exemplary substrates include, for example, polyaramid film, polycarbonate film, polyimide (PI) film, polyester (PET) film, polyethylene naphthalate (PEN) film, polyetheretherketone (PEEK) film, nylon, or PEN / polymethylmethacrylate multilayer film. Considering several factors such as strength, heat resistance, and other factors, it is preferable to use, for example, polyimide or polyethylene naphthalate film as the substrate layer 202.

[0027] The heat-activated adhesive layer 204, 205 includes any suitable adhesive that is thermosetting. One example is a thermosetting adhesive that cures at high temperatures. In addition, the adhesive preferably has thermoplastic properties, being initially in a non-fluid state, but becomes fluid when heated. In a preferred embodiment, an epoxy is used in the adhesive layer. Difunctional epoxies, trifunctional epoxies, tetrafunctional epoxies, and generally epoxies with multifunctionality are preferred for curing into hard, durable, and heat-resistant materials. In particular, a thermosetting epoxy resin can be used as the adhesive. Examples of resins that can be used as the thermosetting epoxy resin include, but are not limited to, bisphenol epoxy resins, such as bisphenol A epoxy resin and bisphenol F epoxy resin, epoxy resins with aliphatic backbones, such as hexanediol diglycidyl ether, glycidylamine epoxy resins, such as triglycidylaminophenol, novolac epoxy resins, such as phenol novolac epoxy resin and cresol novolac epoxy resin, brominated epoxy resins, cycloaliphatic epoxy resins, and mixtures thereof. The thermosetting epoxy resin may further contain a phenoxy resin (a polyhydroxy polyether synthesized from bisphenol and epichlorohydrin) as a thermoplastic component. In a preferred embodiment, the adhesive in the heat-activatable adhesive layer is derived from a triphenylmethane-based phenolic resin and its binary or ternary resin system incorporating other resins.

[0028] In one embodiment, the heat-activated adhesive layer includes a foaming agent that allows the adhesive to foam and thereby expand. The foaming agent may be any suitable foaming agent and is not specifically limited. 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, azides, and the like; N,N-organic foaming agents such as fluorinated alkanes such as trichloromonofluoromethane, azo compounds such as azobisisobutyronitrile, hydrazine compounds such as paratoluenesulfonylhydrazide, semicarbazide compounds such as p-toluenesulfonylsemicarbazide, triazole compounds such as 5-morpholyl-1,2,3,4-thiatriazole, and N-nitroso compounds such as dinitrosoterephthalamide, and the like. An example of a chemical foaming agent is a combination of aluminum powder, NaOH, and NaHCOs, which can be used in epoxy resins. Polysilazanes can also be used as chemical foaming agents to modify polyamine curing agents for epoxy adhesives.

[0029] Preferably, the blowing agent comprises thermally expandable particles. The thermally expandable particles are microcapsules containing low boiling point liquid hydrocarbons trapped in a polymer shell. An exemplary microcapsule is available under the trade name Matsumoto Microsphere® available from Matsumoto Yushi Seiyaku Co., Ltd. (Japan). When the microcapsule is heated, the thermoplastic resin shell of the microcapsule softens and the hydrocarbon liquid inside the microcapsule turns into gas, resulting in the microcapsule expanding to several tens of times its original volume. When further heated, the gas inside the shell permeates the microcapsule shell wall and diffuses into the air, so that only the shell of the microcapsule remains in the adhesive after full curing.

[0030] The temperature at which the thermally expandable particles begin to expand thermally is known as the expansion initiation temperature (Te). Te can occur over a range of temperatures. Te may be, for example, 90°C, or may be 95°C or higher or 100°C or higher, from the viewpoint that the adhesive becomes sufficiently soft before expansion begins. Furthermore, the expansion initiation temperature Te of the foaming agent may be, for example, 140°C or lower, or may be 135°C or lower or 130°C or lower, from the viewpoint that a sufficient expansion ratio can be easily obtained before the adhesive (A) hardens.

[0031] The heat-activated adhesive may further include other additives. A curing agent may be included to accelerate the curing of adhesives such as epoxies. A high temperature latent curing agent may be used to accelerate the curing of high temperature curing adhesives. In one embodiment, the heat-activated adhesive layer includes an epoxy adhesive and a latent curing agent such as imidazole. Additives such as phenoxy resins can impart flexibility, toughness, adhesive cohesive strength, and chemical, corrosion, and heat resistance. Other additives such as impact modifiers, inorganic filler particles, and coupling agents are commonly employed in the field of epoxy adhesives.

[0032] Prior to heat activation, the adhesive-permeable cover layer functions to protect the heat-activated adhesive layer from damage during handling and to reduce contamination by preventing direct contact with contaminants that may be picked up by the tacky adhesive layer surface. Any suitable material can be selected as the material that constitutes the adhesive-permeable cover layer, as long as it is permeable to the heat-activated adhesive. The material should also withstand the curing temperatures used throughout the process for the slot liner and up to the operating temperature of the stator. Porous woven fabrics, papers, and nonwoven fabrics may be suitable for this purpose. Nonwoven fabrics based on natural fibers, polymer fibers, or mixtures thereof are preferred as materials for the adhesive-permeable cover layer. Examples of nonwoven fabrics include, for example, glass fiber nonwoven fabrics, polyester nonwoven fabrics, aromatic polyamide (aramid) nonwoven fabrics, polyolefin-based nonwoven fabrics, polysulfone-based nonwoven fabrics, polyaramid nonwoven fabrics, polyphenylsulfide-based nonwoven fabrics, polyester-based nonwoven fabrics, and nylon-based nonwoven fabrics. Nonwovens typically have a large fluid-connected interfiber porosity of 50% or more that allows for penetration of the adhesive into the nonwoven material. Upon heating, the adhesive in the heat-activated adhesive layer penetrates through one surface of the adhesive-permeable cover layer. As the adhesive enters the nonwoven material, a wicking effect occurs that draws in more adhesive, saturating the adhesive-permeable cover layer. With increasing hydrostatic pressure from the expanding adhesive below, the adhesive pushed towards the adhesive-permeable cover layer is free to ooze out the top surface of the adhesive-permeable cover layer.

[0033] FIG. 3 is a cross-sectional view of a slot liner 200 disposed within a slot of a stator, in a final state where the slot liner has been thermally activated and cured, with the slot liner disposed between the wall 210 of the slot and the copper winding 212 in the stator. Upon heating, the heat-activated adhesive layer 204, 205 undergoes two changes. First, it turns into a flowable gel or fluid, and second, the thermally expansive particles present therein expand with a foaming effect that can be vigorous or slow depending on the temperature. These two changes cause the adhesive to expand outward on both sides in the direction of the black arrows shown in FIG. 3, toward the adhesive permeable cover layers on each side, thereby soaking and saturating the adhesive permeable cover layers with adhesive. As the adhesive continues to expand, it eventually seeps out from the outer surface of the adhesive permeable cover layers to form adhesive layers 2041, 2051. As a result, the adhesive layers 2041, 2051 are interposed between the adhesive permeable cover layer and the stator slot walls 210 and copper windings 212, respectively. Now, with the heat-activated adhesive layer fully expanded, the adhesive permeable cover layer is essentially embedded within the expanded adhesive. Now, by curing the expanded adhesive, a bond is established between the slot liner 200 and the stator slot walls as well as the copper windings, thereby providing mechanical stability to the mechanism. It also facilitates some degree of heat conduction, so that heat generated in the conductive windings can be conducted through the adhesive away from the conductive windings and towards the stator.

[0034] The thickness of the heat-activatable adhesive layer is not specifically limited and may be, for example, 200 μm or less, or from the viewpoint of good workability, 100 μm or less, 80 μm or less, or 60 μm or less.

[0035] Compared to the substrate, the adhesive permeable cover layer may experience different levels of shrinkage after heating and cooling cycles. This results in residual stresses in the slot liner, which in turn tend to curl the edges of the slot liner as the adhesive permeable cover layer separates from the substrate, resulting in delamination. At the edges of the adhesive sheet, foaming is more vigorous due to free foaming without an interface, compared to the inner portion of the slot liner, where the heat-activated adhesive layer is restricted between the substrate and the adhesive permeable cover layer. As a result, there is a higher tendency for delamination to occur at the edges of the slot liner. Thus, the inventors have found that the edges of the slot liner may be advantageously sealed. The seal 209 provides an impermeable seal that holds the edges of the slot liner together and prevents the expanding adhesive from expanding laterally toward and seeping out of the edges of the slot liner.

[0036] The material of the seal is not specifically limited. Due to the high temperature environment in which the stator operates, the seal is preferably constructed of a heat-resistant material that should remain unaffected by heat and be durable over many heating and cooling cycles through years of operation. The seal may include a sealing adhesive tape laminated onto the edge of the slot liner. This may be done before the insertion of the slot liner, i.e., an edge-sealed slot liner is pre-fabricated for insertion. The slot liner edge can also be sealed by suitable modifications to the slot liner slicing and insertion process after the slot liner is inserted into the stator slot.

[0037] More preferably, the seal is formed by applying a sealing composition to the slot liner edge during the process of assembling the slot liner to the stator. In a preferred embodiment, the seal and sealing composition includes an adhesive selected from urethane adhesives, epoxy adhesives, and acrylic adhesives. For example, a heat curing adhesive by 3M™ Scotch-Weld™ urethane adhesive and 3M™ Scotch-Weld™ epoxy adhesive can be used. In another embodiment, the seal includes an instant adhesive. An exemplary instant adhesive includes the cyanoacrylate adhesive 3M™ Scotch-Weld™ instant adhesive. In another embodiment, the seal includes a silicone adhesive. A seal including high temperature silicone may be advantageous due to the silicone's ability to withstand a wide range of temperatures, such as when allowing the equipment housing the stator to operate in different climatic conditions. The sealing composition applied to the slot liner edge is preferably formulated to be an adhesive in the form of a high viscosity fluid. An adhesive that is thixotropic and viscous in the form of a viscous liquid, or more preferably a gel or jelly, or a paste, may help achieve a non-dripping quality for accurate dispensing. The viscosity is not specifically limited. For example, liquids and gels with a viscosity of more than 500 centipoise (0.5 Pa.s) and up to 50,000 centipoise (50 Pa.s) can be used. For direct dispensing of the sealing composition, jellies or pastes with a viscosity of more than 50,000 centipoise (50 Pa.s), or preferably more than 80,000 centipoise (80 Pa.s), can be used. When dip-coating of the slot liner edge with the sealing composition is performed, the sealing composition used can be formulated as a liquid or gel. For dispensing on vertically arranged edges, the sealing composition is preferably a viscous non-sagging jelly or paste.

[0038] The thickness of the cured seal is not specifically limited and may be thicker, thinner or equal to the thickness of the individual layers of the slot liner, for example, equal to the thickness of the heat-activated adhesive layer, and may be 50 μm or more, 100 μm or more, or 500 μm or more.

[0039] FIG. 4 is a perspective view of a stator 310 with slot liners assembled in some of the slots. The slot liner 300 is shown as having its long edges 309 sealed. The long edges 309 refer to the edges that run parallel to the longitudinal axis of the stator 310 as indicated by arrows 331. The slot liner 400 is shown as having its short edges 409 sealed. The short edges 409 refer to the edges that run parallel to the radial axis of the stator 310 as indicated by arrows 332. FIG. 5 is a perspective view of a slot liner 500 with copper windings 512 disposed between the folds. In this view, all edges 509, including the long and short edges, are sealed.

[0040] According to the above figures, the present invention provides a convenient method for sealing the edges of a slot liner, comprising: providing a continuous multi-layer sheet comprising an electrically insulating substrate layer, a first heat-activated adhesive layer disposed on a first surface of the substrate layer, and a first adhesive-permeable cover layer disposed on the first heat-activated adhesive layer, at least two of the layers extending together and terminating at an edge; repeatedly cutting portions of the sheet to form slot liners and placing the slot liners in slots of a stator until all slots are filled; applying a sealing composition to the edges of the slot liner; and curing the sealing composition to form a seal over the edges of the slot liner. The sealing composition can be applied to short edges by dip-coating the edges in the adhesive or by using a dispenser to dispense the adhesive directly onto the edges. For long edges, the sealing composition can be applied using an applicator designed to be inserted into the central borehole of the stator. The application of the sealing composition to the edges of the slot liner can be done individually and continuously, or more preferably, in a batch manner using an applicator designed to distribute the sealing composition on the edges in a batch manner. A suitable applicator, such as a multi-nozzle dispenser, can be used to apply the sealing composition to several slot liner edges simultaneously. Depending on the adhesive used, the sealing composition can be thermally cured by heating to the required curing temperature, by curing under ultraviolet light, or by drying.

[0041] The conductive windings can then be assembled into the slots of the stator. After the conductive windings are assembled, the stator is heated to activate and cure the adhesive present in the heat-activated adhesive layers in the slot liners.

[0042] The thermally expandable particles and adhesives in the thermally activated adhesive and the sealing composition can be selected such that the thermal activation of the thermally expandable particles and the curing temperature of the adhesive are compatible with each other. In one embodiment, the slot liner is cured in a first step by preheating to a first temperature T1 to initiate expansion and foaming of the thermally expandable particles in the adhesive layer, and in a second step by further heating the slot liner to a second temperature T2, where T2 > T1. This selected temperature T1 is preferably low enough to ensure that the expansion of the adhesive is not overly active, to allow time for the release of gas from the thermally expandable particles, and to avoid trapping gases that could result in large voids in the cured adhesive. T2 is selected to ensure that the thermally expandable particles are fully expanded.

[0043] T1 and T2 are not specifically limited. In one embodiment, T1 is 50 - 150 °C, more preferably 80 - 110 °C. T2 is higher than T1 and is between 100 - 200 °C, more preferably between 130 - 190 °C. The foaming start temperature Te of the thermally expandable particles preferably falls within the range T1 ≤ Te < T2. Further, if it is desired to cure the sealing composition simultaneously, the curing temperature Ts of the sealing composition can be set below T1 to cure at T1 in the first heating step. Alternatively, it can be set higher than T1 but below T2 to cure at T2 in the second heating step. Purely by way of example, if the thermally expandable particles are selected such that Te = 140 °C, the first heating step can be T1 = 100 °C, the second heating step can be T2 = 160 °C, and the sealing composition has a curing temperature of Ts = 160 °C.

[0044] Next, examples of the present invention will be described in detail.

Examples

[0045] Example 1 In this example, a heat-activatable adhesive layer was prepared using the ingredients listed in Table 1 of Figure 6. The ingredients were mixed to form an adhesive composition, which was then coated onto one side of a 75 μm PEN film (Teonex™ Q5175 from Toyobo Film Solutions Co., Ltd.). The coating weight of the adhesive composition was 35 g / m2 per side. 2 After drying in an oven at 100°C, a polyester sheet (Daioh Paper Co., Ltd., basis weight 23 g / m) was applied to the coated surface as an adhesive permeable cover layer. 2 After lamination of the adhesive permeable cover layer, the multi-layer sheet was again heated at 87° C. The same adhesive composition was coated on the other side of the PEN film, which was then dried and again laminated with a polyester sheet to form a 5-layer multi-layer sheet.

[0046] Then, a portion from the multi-layer sheet was cut to obtain four slot liner specimens (Reference Example 1, Comparative Example 1, Example 1, Example 2) with dimensions of 15 mm x 40 mm. Three different adhesive sealing compositions (A, B, C) were prepared for sealing the slot liner edges, as listed in Table 2 of FIG. 7. No sealing composition was applied to Reference Example 1. In Comparative Example 1, sealing composition A, which is a low viscosity liquid, was applied to its edges. Sealing compositions B and C, which are high viscosity jelly and resin paste, respectively, were potted on the edges of the specimens of Example 1 and Example 2. After placing the specimens for one day, each specimen was set with a gap of 1.65 mm between two SPCC specimens and cured in a one-step heat treatment at 160°C for 5 minutes in a heat press. The cured specimens were then analyzed.

[0047] The shrinkage of each specimen was measured using an optical microscope (VHZ20, Keyence Corporation) and the presence or absence of delamination was visually inspected. Applying Equation 1:

number

[0048] Example 2 In this example, a two-step heat treatment to activate and cure the heat-activated adhesive layer of a slot liner was evaluated for its effect on the expansion and contraction of the heat-activated adhesive layer without sealing the edges of the slot liner. The first heating step is performed at a lower temperature below the foaming onset temperature Te of the thermally expandable particles.

[0049] A heat-activatable adhesive layer was similarly prepared using the ingredients listed in Table 1 of FIG. 6. Methyl Ethyl Ketone (MEK) was used as a solvent to aid in mixing the ingredients. The formulation information in Table 1 is provided on a dry weight percent basis (i.e., without solvent). The ingredients were mixed to form a heat-activatable adhesive composition, which was then coated onto one side of a 75 μm PEN film (Teonex™ Q5175 from Toyobo Film Solutions Co., Ltd.). After drying in an oven, a polyester sheet (Daioh Paper Co., Ltd., basis weight 23 g / m2) was applied to the coated surface as an adhesive permeable cover layer. 2 The coating weight of the adhesive composition was 35 g / m per side. 2 After lamination of the adhesive permeable cover layer, the multi-layer sheet was heated at 87° C. The same adhesive composition was coated on the other side of the PEN film, which was then dried and laminated again with a polyester sheet to form a 5-ply multi-layer sheet.

[0050] Then, a portion from the multilayer sheet was cut to obtain seven slot liner specimens (Reference Example 1, Comparative Example 1, Example 1, Example 2, Example 3, Example 4, Example 5, and Example 6) with dimensions of 15 mm x 40 mm. The duration of the first heating step was varied for each specimen. For Reference Example 1, the duration of the first heating step was 0 minutes. For Comparative Example 1, the first heating step was 10 minutes. For Examples 1 to 6, the duration of the first heating step was increased from 20 minutes (Example 1) to 60 minutes (Example 6). The temperature of the first heating step was set to 100°C, which is lower than the Te of the thermally expandable particles, which is about 140°C. Then, each specimen was set in a gap of 1.65 mm between two SPCC specimens and cured in a two-stage heating step treatment at 160°C for 5 minutes. The cured specimens were then analyzed.

[0051] The initial thickness of each specimen, the thickness after the first heating step, and the thickness after the second heating step were initially measured using a thickness gauge (Mitutoyo Corporation). From the thickness measurements, the expansion ratio was calculated using Equation 2:

number

[0052] The shrinkage of each specimen was measured using an optical microscope (VHZ20, Keyence Corporation), and the shrinkage of each specimen was calculated according to Equation 1. The specimens were also examined for delamination. Using the collected data, the foaming ratio and shrinkage were calculated and summarized in Table 4 in Figure 9.

[0053] Examples 1 to 5 had both lower expansion ratios and lower shrinkage ratios than Reference Example 1. Generally, Examples 1 to 5 achieved shrinkages of less than 1.5%, or more specifically, less than 1.3%, and did not experience delamination. Both Reference Example 1 and Comparative Example 1 experienced delamination.

[0054] In summary, in light of these results, a two-step heating process at T1 and then T2 (T1 ≦ Te < T2, and Te = the foaming start temperature of the thermally expandable particles) can relax the expansion rate of the adhesive during the heating and foaming processes, whereby it has been found that the shrinkage of the cured adhesive-permeable cover layer is advantageously reduced. Alternatively, in order to control the foaming momentum of the thermally expandable particles, it is also conceivable that the heating process can be variably adjusted as appropriate by linearly, exponentially, or logarithmically increasing the temperature from T1 to T2.

[0055] Various other modifications and adaptations of the present invention will be apparent to those skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the present invention, and all such modifications and adaptations are intended to fall within the scope of the appended claims.

Claims

1. 1. A multi-layer slot liner for a rotating electrical system, comprising: an electrically insulating substrate layer; a first heat-activated adhesive layer including thermally expandable particles disposed on a first surface of the substrate layer; and a first adhesive-permeable cover layer disposed on the first heat-activated adhesive layer, wherein at least two of the layers extend together and terminate at edges, the edges being covered by a seal.

2. 10. The multi-layer slot liner of claim 1, further comprising: a second heat-activated adhesive layer disposed on a second surface of the base layer, the second surface being located opposite the first surface of the base layer; and a second adhesive-permeable cover layer disposed on the second heat-activated adhesive layer.

3. The multi-layer slot liner of claim 2 wherein said layers all terminate at said edge and said seal contacts an end of each of said layers at said edge.

4. 3. The multi-layer slot liner of claim 1 or 2, wherein the seal comprises an adhesive selected from the group consisting of a cyanoacrylate adhesive and a urethane adhesive.

5. 3. The multi-layer slot liner of claim 1, wherein the heat-activatable adhesive layer comprises an epoxy adhesive and a latent hardener.

6. 6. The multi-layer slot liner of claim 5, wherein the epoxy adhesive is derived from a triphenylmethane-based phenolic resin.

7. 3. The multi-layer slot liner of claim 1 or 2, wherein the adhesive-permeable cover layer comprises nonwoven polyester fibers.

8. 3. The multi-layer slot liner of claim 1, wherein the adhesive-permeable cover layer shrinks in length by less than 1.5% compared to its initial length after curing.

9. A stator assembly for a rotating electrical system, said stator assembly comprising a plurality of slots insulated by a multi-layer slot liner according to claim 1 or 2.

10. The stator assembly of claim 9 , wherein the multi-layer slot liner is configured to have the edge covered by the seal disposed outside the slot when the slot liner is positioned within the slot.

11. 1. A method of sealing a slot in a stator of a rotating electrical system, comprising: providing a continuous multi-layer sheet comprising an electrically insulating substrate layer, a first heat-activatable adhesive layer disposed on a first surface of the substrate layer, and a first adhesive-permeable cover layer disposed on the first heat-activatable adhesive layer, at least two of the layers extending together and terminating at an edge; cutting portions of the sheet to form slot liners and repeatedly placing the slot liners into the slots of the stator until all slots are filled; applying a sealing composition to the edge of the slot liner; allowing the sealing composition to cure to form a seal over the edge of the slot liner; A method comprising:

12. The method of claim 11 , wherein the sealing composition comprises a high viscosity adhesive fluid.

13. The heat-activatable adhesive layer has a foaming initiation temperature T ｅ and in a first step, the slot liner is heated to a first temperature T to initiate expansion of the heat-activated adhesive layer. １ ≦T ｅ and in a second step, the slot liner is heated to a second temperature T to cure the heat-activatable adhesive layer. ２ >T ｅ 12. The method of claim 11 wherein the temperature is increased to

14. 12. The method of claim 11, further comprising: placing a conductive winding in the slot after sealing the edge of the slot liner; and heating the slot liner to activate and cure the heat-activated adhesive layer.

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