Impregnation of coating layers for insulating sheets

Abstract

The present invention relates to an insulating sheet of a nonwoven sheet, for example a sheet containing aramid fibers, containing 15-80 weight percent of a crystalline silicate mineral powder, for example mica powder, in which the nonwoven sheet is impregnated with a coating of an impregnating resin based on an acrylate derivative, which coating is suitable for increasing the compactness of the insulating sheet and preventing the silicate mineral powder from detaching from the nonwoven sheet during use. The impregnating resin also contains an organic solvent, which can reduce the viscosity of the impregnating resin and promote its deep penetration into the sheet. In addition, the impregnating resin contains a UV polymerization photoinitiator suitable for curing the resin only after the resin has been applied to the sheet and has penetrated deep into the sheet. The present invention also describes a method for impregnating an insulating sheet.

Description

[Technical field]

[0001] The present invention relates to the field of manufacturing impregnating insulating coating layers. In particular, the present invention relates to impregnating coating layers comprising nonwoven sheets containing crystalline silicate mineral powders, such as aramid / mica electrical insulators. Even more particularly, the present invention relates to impregnating coating layers made of acrylate resins for aramid / mica electrical insulators. [Background technology]

[0002] In recent years, with the rapid development of electric machines, materials with excellent insulation properties and high mechanical properties under harsh working conditions have attracted more and more attention. For example, aramid / mica sheets are used for electric insulation motors.

[0003] Mica has excellent physical, electrical and thermal properties, and has high stability and adaptability even at high temperatures. However, it has weak mechanical strength and therefore cannot be directly used to make insulating sheets. Advantageously, mica can be added to aramid sheets to give them higher resistance to partial discharge.

[0004] One of the main challenges in the manufacture of aramid / mica insulating sheets is that when mica is introduced into the aramid paper structure, it has little cohesion and tends to fall off, so the surface of the aramid / mica paper is powdery and difficult to laminate and impregnate with epoxy and polyester resins or to use as an insulator in rotating or stationary electrical machines.

[0005] To improve the properties of aramid / mica papers, some works known in the prior art propose the introduction of an adhesive layer to increase the surface bond between the aramid fibers and the mica.

[0006] For example, US Patent Application Publication No. 2014 / 0028140 describes an aramid / mica tape wrapped around a rotating coil with the purpose of providing the rotating coil with electrical insulation from the outside. The aramid / mica tape has a layer of adhesive resin containing an acryloyl compound, which is used to improve the adhesion between the mica and the aramid. However, the adhesive resin layer is applied to the already formed aramid / mica paper and therefore does not penetrate deeply into the aramid / mica paper. Therefore, the compactness of the aramid / mica sheet is only partially increased.

[0007] US 2021 / 0062429 proposes an alternative method to improve the mechanical properties of aramid / mica sheets based on incorporating and using aramid nanofibers instead of aramid microfibers in insulating sheets containing mica. In this way, the surface area of ​​interaction between aramid and mica is increased, promoting the compactness of the aramid / mica sheet. However, the process of making aramid nanofibers described in US 2021 / 0062429 is complex and laborious, and therefore expensive. Summary of the Invention [Problem to be solved by the invention]

[0008] In view of the above-mentioned problems and shortcomings associated with the manufacture of insulating sheets, it is an object of the present invention to provide an insulating sheet comprising a nonwoven sheet, such as aramid paper, containing a crystalline silicate mineral powder, such as mica, characterized by high surface bonding strength and a densified surface. For example, it is an object of the present invention to provide an aramid / mica insulating sheet in which the mica particles are stably bonded to the aramid fibers, and therefore the mica particles do not detach during use. [Means for solving the problem]

[0009] According to one embodiment of the present invention, there is provided an insulating sheet comprising a nonwoven sheet containing a crystalline silicate mineral powder, the nonwoven sheet containing 15 to 80 weight percent of the crystalline silicate mineral powder, and the insulating sheet also including a coating of an impregnating resin based on an acrylate derivative.

[0010] This configuration is particularly advantageous in that the insulation sheet comprising the nonwoven sheet containing the crystalline silicate mineral powder is impregnated with a layer of coating resin, which can penetrate deep into the insulation sheet and thus improve the denseness of the insulation sheet.

[0011] Preferably, the nonwoven sheet comprises aramid paper, which is advantageously used because aramid is a material characterized by excellent mechanical capabilities, such as high dielectric strength, flexibility and elasticity, and high thermal stability.

[0012] Preferably, the crystalline silicate mineral powder comprises mica.

[0013] According to a preferred configuration, an aramid / mica insulating sheet is provided, in which the surface of the sheet is coated with a low viscosity acrylate resin, for example a resin having a viscosity of 5 cps to 150 cps, preferably equal to 10 cps. Due to the low viscosity, the penetration of the resin into the sheet is promoted, so that the impregnating resin layer has a thickness of at least 5 μm, preferably at least 10 μm, even more preferably at least 15 μm, for example for an aramid sheet having a thickness of 0.08 mm to 0.15 mm, and at least 15 μm, preferably at least 40 μm, for an aramid sheet having a thickness of 0.2 mm to 0.25 mm.

[0014] According to a further embodiment of the present invention there is provided an insulating sheet, wherein the crystalline silicate mineral powder comprises mica.

[0015] This configuration is advantageous since mica can be used as an additive to increase partial discharge resistance since it is characterized by unique physical, thermal and electrical properties as well as high stability, elasticity, resilience and adaptability.

[0016] According to a preferred configuration, an insulating sheet is provided that comprises aramid and mica. Indeed, aramid paper that contains a certain amount of mica in its structure is characterized by a higher partial discharge resistance than aramid paper that does not contain the additive.

[0017] According to one embodiment, the insulating sheet comprises a nonwoven sheet having a first surface and a second surface, the nonwoven sheet comprising 15-80 weight percent of crystalline silicate mineral powder, 5-25 weight percent of heat resistant flock, and 20-60 weight percent of binder based on the total amount of the crystalline silicate mineral powder, flock, and binder in the nonwoven sheet, each of the first and second surfaces of the nonwoven sheet being coated with an impregnating resin, and each of the first and second surfaces being stabilized by a region of the impregnating resin covering and extending into each of the first and second surfaces. The region of impregnating resin extends a distance of 10-50 micrometers into the sheet from each of the first and second surfaces, and further, the region of impregnating resin has a thickness of impregnating resin on each of the first and second surfaces of 5-50 micrometers. In some embodiments, the region of impregnating resin has a thickness of impregnating resin on each of the first and second surfaces of 5-25 micrometers. Preferably, the crystalline silicate mineral powder is mica.

[0018] In some preferred embodiments, the insulating sheet has a thermal conductivity of greater than 0.18 watts per meter Kelvin when measured in accordance with ASTM D5470.

[0019] According to one embodiment, the insulating sheet has a total thickness of 0.10 to 0.4 millimeters, and in another embodiment, the insulating sheet has a total thickness of 0.15 to 0.35 millimeters.

[0020] In some embodiments, the nonwoven sheet of the insulation sheet comprises 20 to 70 weight percent mica, hi some other embodiments, the nonwoven sheet of the insulation sheet comprises 30 to 60 weight percent mica.

[0021] According to a preferred embodiment, the insulating sheet comprises a heat-resistant flock, preferably an aramid flock, the preferred aramid flock being poly(metaphenylene isophthalamide) flock. In some preferred embodiments, the insulating sheet comprises a binder, preferably an aramid fibrid binder, the preferred aramid fibrid being poly(metaphenylene isophthalamide) fibrid.

[0022] According to a further embodiment of the present invention, an insulating sheet is provided, wherein the coating comprises a combination of an acrylate monomer and an acrylate oligomer.

[0023] The advantage of this configuration is that the acrylate monomer and acrylate oligomer compounds are characterized by high mobility and low molecular weight, for example, a molecular weight of 200 Daltons to 500 Daltons, and therefore can penetrate through the porous parts of the insulating sheet.

[0024] In accordance with a preferred configuration, an insulating sheet is provided that includes aramid and mica and is characterized by a coating of a resin that includes acrylate monomers and acrylate oligomers. The acrylate compound permeates the aramid / mica paper, densifying the mica powder and permanently fixing it into the aramid paper structure.

[0025] According to a further embodiment of the present invention, an insulating sheet is provided, wherein the acrylate monomer is a di- or trifunctional methacrylate, such as 1,6-hexanediol diacrylate (HDDA), dipropylene glycol diacrylate (DPGDA), butanediol dimethacrylate (BDMA), neopentyl glycol diacrylate (NPGDA), isobornyl acrylate (IBOA), hydroxyethyl methacrylate (HEMA), hydroxyethyl acrylate (HEA), trimethylolpropane triacrylate (TMPTA), pentaerythrityl triacrylate (PETIA), tetramethylene dimethacrylate, hexanediol dimethacrylate, propoxylates and / or ethoxylates.

[0026] Mono-, di- or trifunctional acrylate monomers are particularly advantageous for increasing the bond density of the polymerization product.

[0027] According to a further embodiment of the present invention, there is provided an insulating sheet, wherein the acrylate oligomer is a urethane acrylate, an epoxy acrylate, and / or a polyester acrylate.

[0028] In accordance with a further embodiment of the present invention, an insulating sheet is provided, wherein the impregnating resin coating comprises an unsaturated polyesterimide material.

[0029] This configuration is particularly advantageous because the polyesterimide resin can penetrate deep into the matrix of the insulating sheet, avoiding any possibility of dust falling and falling off, thereby improving the processability of the insulating material. Moreover, this polyesterimide resin, which is commonly used to impregnate electric motors, can be converted from a heat-curable coating to a UV-polymerizable coating simply by adding a photoinitiator to the resin mixture.

[0030] According to a further embodiment of the present invention, an insulating sheet is provided, wherein the impregnating resin also comprises a polymerization photoinitiator, in particular a UV polymerization photoinitiator.

[0031] Non-limiting examples of photoinitiators for use in the present invention include Omnirad TPO-L, 2-hydroxy-2-methyl-1-phenylpropanone, diphenyl-2,4,6-trimethylbenzoylphosphine oxide (TPO), phosphine oxide (BAPO), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), 1-hydroxycyclohexylphenylketone (HCPK) or 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP) or photoinitiators belonging to the class of bi-acyl phosphate oxide polymers (TPO) or photoinitiators belonging to the class of α-hydroxyketones.

[0032] The advantage of this configuration is that polymerization of the impregnating resin occurs after application of the resin to the insulating sheet and exposure to UV light, thus allowing the impregnating resin to penetrate deep into the insulating paper and densify the insulating paper, preventing, for example, detachment and detachment of mica in subsequent processing steps.

[0033] According to a further embodiment of the present invention, an insulating paper is provided, wherein the impregnating resin also includes an organic solvent, for example methyl ethyl ketone.

[0034] The advantage of this configuration is that the viscosity of the impregnating resin is reduced. According to a preferred configuration, the impregnating resin comprises methyl ethyl ketone (MEK). MEK is an organic solvent characterized by a high evaporation rate and is widely used in acrylic and polyurethane paints and coating thinners to reduce their viscosity.

[0035] According to a further embodiment of the invention there is provided an insulating sheet comprising an impregnating resin based on an acrylate derivative in an amount comprised between 1 g and 10 g per square meter of insulating sheet.

[0036] According to a further embodiment of the present invention there is provided a method of manufacturing an insulating sheet, the method comprising the steps of: a) providing a nonwoven backing sheet comprising 15 to 80 weight percent of a crystalline silicate mineral powder, such as mica; b) depositing a coating layer of an impregnating resin based on an acrylate derivative onto the backing sheet to obtain a resin-impregnated sheet; Includes.

[0037] This configuration is particularly advantageous because the backing sheet, made of aramid paper containing mica, is impregnated with a coating resin layer that can penetrate deep into the matrix. In this way, the mica is prevented from detaching from the aramid paper after exposure to various mechanical stresses during the mounting process, while the electrical properties of the aramid / mica paper are not affected. In particular, the aramid / mica sheet is characterized by a higher partial discharge resistance than the aramid sheet.

[0038] The method of the invention therefore makes it possible to simplify the subsequent processing and layering of the aramid / mica insulating sheet and to facilitate its use as an insulating material, for example for insertion into the stator slots of electric motors.

[0039] Preferably, the aramid / mica backing sheet is supplied on a reel and the impregnating resin is applied to the sheet using a "roll-to-roll" process.

[0040] According to one embodiment, the method includes a nonwoven backing sheet having a first surface and a second surface, the nonwoven backing sheet comprising 15-80 weight percent crystalline silicate mineral powder, 5-25 weight percent heat resistant flock, and 20-60 weight percent binder based on the total amount of said crystalline silicate mineral powder, flock, and binder in the nonwoven backing sheet, and a coating layer of impregnating resin is deposited on each of the first and second surfaces of the nonwoven backing sheet. The coating layer provides a region of said impregnating resin covering and extending into each of said first and second surfaces. The region of impregnating resin extends into the nonwoven backing sheet from each of said first and second surfaces for a distance of 10-50 micrometers, and further, the region of impregnating resin has a thickness of impregnating resin on each of said first and second surfaces of 5-50 micrometers. In some embodiments, the region of impregnating resin has a thickness of impregnating resin on each of said first and second surfaces of 5-25 micrometers. Preferably, the crystalline silicate mineral powder is mica.

[0041] In some embodiments of the method, the nonwoven sheet of the insulation sheet comprises 20 to 70 weight percent mica, hi some other embodiments, the nonwoven sheet of the insulation sheet comprises 30 to 60 weight percent mica.

[0042] According to preferred embodiments of the method, the insulating sheet comprises a heat-resistant flock, preferably aramid flock, the preferred aramid flock being poly(metaphenylene isophthalamide) flock. In some preferred embodiments, the insulating sheet comprises a binder, preferably an aramid fibrid binder, the preferred aramid fibrid being poly(metaphenylene isophthalamide) fibrid.

[0043] According to a further embodiment of the present invention, there is provided a method of manufacturing an insulating sheet, wherein the coating layer comprises a combination of an acrylate monomer and an acrylate oligomer.

[0044] The advantage of this composition is that the compound of acrylate monomer and acrylate oligomer is characterized by high mobility and low molecular weight, for example, a molecular weight of 200 Daltons to 500 Daltons, and therefore can penetrate the porous parts of the insulating sheet.

[0045] Specifically, the acrylate compound is able to penetrate deeply into the aramid / mica paper, densifying the mica powder and permanently locking it into the structure of the aramid paper.

[0046] According to a further embodiment of the present invention, there is provided a method for the manufacture of an insulating sheet, wherein the acrylate monomer is a di- or trifunctional methacrylate, such as 1,6-hexanediol diacrylate (HDDA), dipropylene glycol diacrylate (DPGDA), butanediol dimethacrylate (BDMA), neopentyl glycol diacrylate (NPGDA), isobornyl acrylate (IBOA), hydroxyethyl methacrylate (HEMA), hydroxyethyl acrylate (HEA), trimethylolpropane triacrylate (TMPTA), pentaerythrityl triacrylate (PETIA), tetramethylene dimethacrylate, hexanediol dimethacrylate, propoxylates and / or ethoxylates.

[0047] According to a further embodiment of the present invention, there is provided a method of manufacturing an insulating sheet, wherein the acrylate oligomer is a urethane acrylate, an epoxy acrylate and / or a polyester acrylate.

[0048] According to a further embodiment of the present invention, a method of manufacturing an insulating sheet is provided, wherein the coating layer comprises an unsaturated polyesterimide material.

[0049] This configuration is particularly advantageous because the polyesterimide resin can penetrate deep into the matrix of the insulating sheet, avoiding any possibility of dust falling and falling off, thereby improving the processability of the insulating material. In addition, this polyesterimide resin, which is commonly used to impregnate electric motors, can be converted into a UV-polymerizable coating by adding a photoinitiator to the resin mixture.

[0050] According to a further embodiment of the present invention, there is provided a method of producing an insulating sheet, wherein the coating layer comprises an organic solvent, and the method comprises: c) drying the resin-impregnated sheet to evaporate the organic solvent Also includes.

[0051] The advantage of this configuration is that an organic solvent, such as methyl ethyl ketone, is added to the coating formulation to improve the deep penetration of the acrylate oligomers and monomers. The organic solvent is used as a non-reactive diluent to reduce the viscosity of the coating formulation, allowing for the application of a wet coating by rotogravure, thus providing a uniform and homogenous surface. Once the coating resin has penetrated into the backing sheet, the organic solvent is evaporated.

[0052] According to a further embodiment of the present invention, there is provided a method for producing an insulating sheet, wherein the impregnating resin also comprises a polymerization photoinitiator, in particular a UV polymerization photoinitiator, such as Omnirad TPO-L or 2-hydroxy-2-methyl-1-phenylpropanone, and the method comprises the steps of: d) exposing the resin-impregnated sheet to ultraviolet light to effect polymerization of the coating layer. Also includes.

[0053] The advantage of this configuration is that polymerization of the impregnating resin occurs after application of the resin to the insulating sheet and exposure to UV light, thus allowing the impregnating resin to penetrate deep into the insulating paper and densify the insulating paper, preventing, for example, detachment and detachment of mica in subsequent processing steps.

[0054] Preferably, the UV polymerization process is triggered by exposure to an ultraviolet lamp, preferably having a wavelength in the range of 200 nm to 400 nm, whose emission is focused on a substrate coated with a liquid formulation containing a photoinitiator. Following UV irradiation, radicals are generated, which interact with the impregnating resin, causing polymerization and crosslinking of the formulation, which changes from a liquid to a solid.

[0055] The aramid / mica paper thus impregnated can be advantageously used in the construction of electric motors, for example as secondary insulation (slot insulation).

[0056] According to a further embodiment of the present invention there is provided a laminate comprising at least one insulating sheet as described above and at least one plastic film layer, wherein the insulating sheet is preferably laminated onto one side of the plastic film layer so as to form an outer surface of the laminate.

[0057] This configuration is particularly advantageous because the combination of at least one aramid paper insulation sheet with at least one plastic film layer makes it possible to obtain an insulation laminate having physical, dielectric, thermal and mechanical properties that can be adapted to the needs of the user.

[0058] Preferably, the various layers of aramid sheets and plastic film are adhered to one another by a suitable adhesive.

[0059] In a preferred configuration, a laminate is provided that includes an insulating sheet of aramid paper firmly adhered to one or both sides of a layer of plastic film, whereby the insulating sheet forms an outer layer of the laminate, the outer layer being the layer in contact with the external environment.

[0060] According to a further preferred configuration, a laminate is provided which comprises several layers of plastic film interleaved with aramid paper insulating sheets, said insulating sheets forming at least one outer layer of the laminate. For example, the laminate may comprise alternating layers of plastic film and insulating paper sheets and may have one outer surface formed by aramid paper sheets and one outer surface formed by a plastic film layer, or may have both outer surfaces formed by aramid paper sheets.

[0061] According to a further preferred configuration, there is provided a laminate comprising one or more layers of plastic film and one or more insulating sheets of aramid paper laminated together in any order, said insulating sheets forming at least one outer layer of the laminate. For example, the laminate may comprise one or more layers of plastic film and insulating paper sheets laminated together in any order, may have one outer surface formed by an aramid paper sheet and one outer surface formed by a plastic film layer, or may have both outer surfaces formed by aramid paper sheets.

[0062] According to a further embodiment of the present invention there is provided a laminate comprising a plurality of insulating sheets and a plurality of plastic film layers as described above, wherein the insulating sheets are preferably interleaved with the plastic film layers to form an outer surface of the laminate.

[0063] This configuration is particularly advantageous because the combination of multiple insulating sheets of aramid paper with multiple layers of plastic film makes it possible to obtain an insulating laminate with physical, dielectric, thermal and mechanical characteristics that can be adapted to the needs of the user.

[0064] Preferably, the various layers of aramid sheets and plastic film are adhered to one another by a suitable adhesive.

[0065] According to a further embodiment of the present invention there is provided an electric motor comprising one or more insulating sheets as described above or one or more laminated insulating sheets as described above.

[0066] This configuration is particularly advantageous since it is possible to insulate the phases from the stator of the electric motor with an insulating sheet which is characterized by high partial discharge resistance and has excellent mechanical properties even under harsh operating conditions, such as at high voltages or under exposure to mechanical stress.

[0067] The present invention will be described with reference to the accompanying figures, in which like numerals and / or reference characters indicate same and / or similar and / or corresponding parts of the system. [Brief description of the drawings]

[0068] [Figure 1] 1 illustrates diagrammatically an aramid / mica insulating sheet used as an insulating coating for the stator slots of an electric motor; [Diagram 2] 1 illustrates a schematic of a process for making an aramid / mica insulating sheet that also includes a coating resin layer, according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0069] In the text that follows, the present invention will be described with reference to specific embodiments as shown in the accompanying drawings. However, the present invention is not limited to the specific embodiments described in the following detailed description and depicted in the drawings; rather, the described embodiments are merely illustrative of various aspects of the invention, the scope of which is defined by the claims. Further modifications and variations of the present invention will be apparent to those skilled in the art.

[0070] The present invention relates to an insulating sheet of a nonwoven sheet 10 comprising crystalline silicate mineral powder, specifically mica, impregnated with a coating of resin based on acrylate derivatives that penetrates deep into the sheet itself. By nonwoven sheet is meant a sheet structure produced by bonding and / or interlocking fibrous materials into a random web or mat by mechanical, chemical and thermal methods and combinations thereof. One preferred type of nonwoven sheet is paper.

[0071] Preferably, the nonwoven sheet is an aramid paper comprising aramid floc, aramid fibrids or mixtures thereof. The aramid paper preferably contains meta-aramid fibrids as a binder, together with aramid fibers or other heat stable floc or fibers or mixtures of such flocs or fibers. Heat stability means that a given floc or fiber can withstand long-term exposure to the temperatures of the end use without showing significant degradation (usually retaining at least 50% of its initial properties after 100,000 hours of exposure to the required temperatures, sometimes above 180°C).

[0072] As used herein, the term "aramid" refers to an aromatic polyamide in which at least 85% of the amide (-CONH-) bonds are directly attached to two aromatic rings. Optionally, additives may be used with the aramid and dispersed throughout the polymer structure. It has been found that up to about 10 weight percent of other polymeric materials can be blended with the aramid. It has also been found that copolymers having up to about 10 percent of other diamines replacing the diamines of the aramid or up to about 10 percent of other diacid chlorides replacing the diacid chlorides of the aramid can be used.

[0073] A preferred aramid is a meta-aramid. An aramid polymer is considered to be a meta-aramid when two rings or radicals are oriented meta to each other along the molecular chain. A preferred meta-aramid is poly(meta-phenylene isophthalamide) (MPD-I). U.S. Pat. Nos. 3,063,966, 3,227,793, 3,287,324, 3,414,645, and 5,667,743 describe useful methods for producing aramid fibers that can be used to produce aramid floc.

[0074] A preferred aramid floc is meta-aramid floc, and especially preferred is floc made from meta-aramid poly(meta-phenylene isophthalamide) (MPD-I). As used herein, the term "floc" refers to fibers cut into short lengths and typically used in the preparation of paper. Typically, floc has a length of about 3 to about 20 millimeters. A preferred length is about 3 to about 7 millimeters. Floc is typically produced by cutting continuous fibers to the required length using methods known in the art.

[0075] As used herein, the term "fibrids" refers to very small non-granular, fibrous or film-like particles with at least one of their three dimensions being small compared to the largest dimension. These particles are prepared by precipitating a solution of a polymeric material with a non-solvent under high shear. Aramid fibrids are non-granular film-like particles of aromatic polyamides having a melting or decomposition point above 320°C. Preferred aramid fibrids are meta-aramid fibrids, and particularly preferred are fibrids made from meta-aramid poly(meta-phenylene isophthalamide) (MPD-I). Fibrids generally have a maximum dimension length ranging from about 0.1 mm to about 1 mm with a length to width aspect ratio of about 5:1 to about 10:1. The thickness dimension is on the order of a fraction of a micrometer, e.g., about 0.1 micrometer to about 1.0 micrometer. Although not required, it is preferred to incorporate the aramid fibrids into the layer while the fibrids are in a wet state.

[0076] As used herein, aramid paper means a flat sheet made from one or more plies or layers of aramid material prepared by a papermaking process. Exemplary devices and machinery that can be used to make the plies or layers include, for example, but are not limited to, continuous processing equipment such as a Fourdrinier or inclined wire machine, or batch processing equipment such as equipment that makes paper manually in a handsheet form containing a forming screen.

[0077] To make mica-containing sheets or papers, in one preferred method, a mixture of mica and fibrous material in desired amounts is mixed with water, and then this mica-containing aqueous dispersion is used to make mica-containing nonwoven sheets by a suitable nonwoven process. As one specific example for making mica-containing aramid paper, the amounts of mica, MPD-I fibrids and MPD-I floc described hereinabove are mixed with water to form a mica-containing aqueous dispersion suitable as a furnish for papermaking. The mica-containing dispersion is then fed to the headbox of a papermaking machine to form a wet-laid web. The speed of the papermaking machine is controlled to provide the desired thickness of the wet-laid web. The wet-laid web is then dried to form an uncured mica-containing aramid formed web or layer, which can be further cured by calendering or combined with other similarly formed webs and then cured by calendering.

[0078] A non-exhaustive list of types of mica that may be used for the present invention includes muscovite, phlogopite, fluorophlogopite, and synthetic mica.

[0079] The acrylate resin is obtained after condensation between hydroxylated resin and acrylic or methacrylic acid. The acrylate resin is preferably diluted with a reactive acrylate solvent and a non-reactive organic solvent, such as methyl ethyl ketone, to reduce its viscosity during application and promote the penetration of the coating into the aramid paper. In this way, the aramid / mica paper is densified in depth to prevent the mica from detaching and falling off during subsequent processing steps.

[0080] After the coating layer is applied onto the aramid / mica sheet and the coating penetrates deep into the aramid / mica sheet, polymerization of the coating resin is induced. Specifically, polymerization of the coating resin is induced by the addition of a photoinitiator and exposure to ultraviolet light, preferably having a wavelength in the range of 200 nm to 400 nm.

[0081] Non-limiting examples of photoinitiators for use in the present invention include Omnirad TPO-L, 2-hydroxy-2-methyl-1-phenylpropanone, diphenyl-2,4,6-trimethylbenzoylphosphine oxide (TPO), phosphine oxide (BAPO), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), 1-hydroxycyclohexylphenylketone (HCPK) or 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP) or photoinitiators belonging to the class of bi-acyl phosphate oxide polymers (TPO) or photoinitiators belonging to the class of α-hydroxyketones.

[0082] In a preferred configuration, the insulating sheet 10 may be used to insulate rotating or stationary components of an electric machine.

[0083] In a further preferred configuration, the insulating sheet 10 can be used as an outer layer on one or both sides of a composite laminate of plastic film layers, which can be used to insulate rotating or stationary components of an electric machine. In a further preferred configuration, one or more insulating sheets 10 can be used to form a multi-layer laminate, preferably including multiple plastic film layers laminated together, interleaved with layers of aramid paper, which can be used to insulate rotating or stationary components of an electric machine.

[0084] The insulating sheets 10 or laminates described above can be used in rotating electric machines for insulating stator slots, for closing the stator slots and also for separating phases at the head of the windings, and in stationary electric machines they can be used as interlayer insulation.

[0085] For example, FIG. 1 shows a suitably dimensioned insulating sheet 11 used to insulate the slots of a stator 20 of an electric motor. Preferably, the aramid / mica insulating sheet 11, impregnated with resin, is cut into segments having a length essentially equal to the depth of the stator 20 and a width equal to the development of the slots to be insulated. The insulating sheet 11 can be advantageously inserted into the grooves of the stator 20 of the electric motor by pressing it in using a presser. The function of the insulating sheet 11 is "secondary insulation". The current passes through the copper winding 30, which already has its own primary insulation, while the slot insulating sheet 11 contributes to separating the magnetic sheet metal from the copper winding 30.

[0086] The process for manufacturing an aramid / mica insulating sheet impregnated with a coating layer will now be described with reference to FIG.

[0087] The preformed aramid / mica backing sheet 7 is preferably supplied on reels having a width, for example, equal to 1 m and a length of 100 m to 2000 m. The backing sheet 7 may for example be a DuPont™ Nomex™ 818 sheet as described in the document "DuPont™ Nomex™ 818, Technical Data Sheet, Copyright 2016 DuPont". The backing sheet 7 may for example comprise mica in an amount equal to 50% of the total composition.

[0088] The acrylate resin coating is applied onto a pre-formed aramid / mica backing sheet 7 using a "roll-to-roll" process as shown diagrammatically in FIG.

[0089] An unwinder 101 feeds an aramid / mica backing reel 7 to the production line 100. A liquid formulation is prepared comprising a resin based on acrylate monomers and acrylate oligomers, e.g., an unsaturated polyesterimide resin, to which a UV polymerization photoinitiator, e.g., Omnirad TPO-L or 2-hydroxy-2-methyl-1-phenylpropanone, and a non-reactive organic solvent, e.g., methyl ethyl ketone (MEK), are added to reduce the viscosity of the resin itself and allow better penetration of the resin into the aramid paper matrix. The liquid formulation is applied at a coating station 102 onto one surface of the aramid / mica backing sheet 7, thus producing an impregnated aramid / mica sheet 8, i.e., a sheet further comprising the liquid formulation. Preferably, the liquid formulation is applied onto both surfaces of the backing sheet 7, e.g., first on one side and then on the opposite side.

[0090] The liquid formulation is preferably applied to the aramid / mica backing sheet 7 by impregnation using the gravure or gravure engraved roller coating technique, or the Mayer bar technique, or a Foulard machine. In the configuration using coating by gravure engraved roller, the liquid formulation is first collected in the cavities of the engraved roller and then transferred onto the aramid / mica backing sheet 7. The amount of liquid formulation applied depends on the depth of the cells of the gravure engraved roller. For example, it is possible to apply an amount of liquid formulation ranging from 10 g to 20 g per square meter of backing sheet 7, the liquid formulation comprising a solution containing 10% to 50% by weight of acrylate resin in a non-reactive organic solvent, thereby obtaining a dry deposit of 1 g to 10 g of acrylate resin per square meter of backing sheet 7, reaching an impregnation depth of 5 μm to 50 μm.

[0091] After application of the liquid formulation, the impregnated sheet 8 reaches a heating station 103 which comprises an industrial oven where the non-reactive organic solvent is evaporated. For example, the impregnated sheet 8 is heated to a temperature in the range of 90°C to 120°C to evaporate the solvent components and dry the coating. At the end of the heating process, a sheet 9 is obtained from which the non-reactive organic solvent has been evaporated.

[0092] The sheet 9 then reaches the UV irradiation station 104 where it is exposed to radiation emitted by UV lamps. Specifically, UV light is focused onto the substrate on which a liquid formulation containing a photoinitiator has been applied. As a result of the UV irradiation, free radicals are generated, which interact with the acrylate resin permeated into the sheet, causing the polymerization of the formulation, which changes from liquid to solid. Thus, at the end of the UV exposure, an insulating sheet 10 is obtained, in which the acrylate-based impregnating resin has been polymerized.

[0093] Preferably, the operations of applying the liquid formulation, evaporating the organic solvent, and exposing to UV light are repeated for the opposite surface of the insulating sheet 10. For example, the insulating sheet 10 including the resin-impregnated surface may be returned to the application station 102 where the liquid formulation may be applied to the opposite surface and then passed through the heating station 103 and the UV light application station 104 again.

[0094] Finally, the insulating sheet 10 is cut into smaller sheets from which the individual insulating pieces 11 are produced. Preferably, the insulating sheet 10 is cut into reels having a width equal to the development of the stator slots, for example a width of 1 cm to 10 cm.

[0095] Although the present invention has been described with reference to the above-mentioned embodiments, it will be apparent to those skilled in the art that various modifications, variations and improvements of the present invention are possible in light of the above teachings and within the scope of the appended claims, without departing from the subject matter and scope of protection of the present invention.

[0096] For example, although the crystalline silicate mineral powder is described as including mica, it is clear that other crystalline silicate mineral powders can be present, such as vermiculite, calcined clay, silica, talc, wollastonite, and / or combinations thereof. For example, each of the listed powders can be used with aramid fibers as the reinforcing element.

[0097] For example, although the nonwoven sheet is described as preferably being a paper structure containing aramid fibers, it will be apparent that it could be a film of different types of fibers, such as fiberglass, mixed aramid papers, or polyamideimide, polyester, polyimideether, polyetherketone, polyethersulfone, polysulfide, polyimide, etc. For example, each of the reinforcing elements shown could be used to include mica as a crystalline silicate mineral powder.

[0098] Finally, areas that would be believed to be known to those skilled in the art have not been described in order to avoid unnecessarily obscuring the invention being described.

[0099] For example, the processes of gravure coating or gravure engraved roller coating, Mayer bar or Foulard machine impregnation have not been detailed as they are deemed to be known to those skilled in the art.

[0100] For example, the deposition and polymerization process of the impregnating resin on the aramid / mica paper using a "roll-to-roll" process has not been detailed as it is deemed to be known to those skilled in the art.

[0101] Therefore, the present invention is not limited to the above-described embodiments, but only by the scope of protection of the attached claims. [Explanation of symbols]

[0102] 7 Backing sheet 8 Resin-impregnated sheet 9 Sheet from which non-reactive organic solvent has evaporated 10 Insulation sheet 11 Cut insulation sheet 20 stator slots 30 copper winding 100 Coating Roll-to-Roll Deposition and Polymerization Station 101 Unwinding machine 102 Liquid Mixture Application Station 103 Heating Station 104 UV irradiation station 105 Cutting Station

Claims

1. An insulating sheet (10, 11) containing a non-woven sheet containing a crystalline silicate mineral powder, for example, an insulating sheet including aramid paper, characterized in that it also includes a coating of an impregnating resin based on an acrylate derivative.

2. The non-woven sheet has a first surface and a second surface, and the non-woven sheet contains 15 to 80 weight percent of the crystalline silicate mineral powder, 5 to 25 weight percent of the heat-resistant flock, and 20 to 60 weight percent of the binder based on the total amount of the crystalline silicate mineral powder, flock, and binder in the non-woven sheet. Each of the first and second surfaces of the non-woven sheet is coated with the impregnating resin, and each of the first and second surfaces is covered by and stabilized by the region of the impregnating resin extending therein. The region of the impregnating resin extends 10 to 50 micrometers into the sheet from each of the first and second surfaces, and further, the region of the impregnating resin has a thickness of 5 to 50 micrometers of the impregnating resin on each of the first and second surfaces. The insulating sheet (10, 11) according to Claim 1.

3. The region of the impregnating resin has a thickness of 5 to 25 micrometers of the impregnating resin on each of the first and second surfaces. The insulating sheet (10, 11) according to Claim 2.

4. A method for manufacturing an insulating sheet (10, 11), a) providing a non-woven backing sheet (7), for example, aramid paper, containing a crystalline silicate mineral powder, for example, mica in a method including b) depositing a coating layer of an impregnating resin based on an acrylate derivative on the non-woven backing sheet (7) to obtain a resin-impregnated sheet (8). A method characterized by also including.

5. The non-woven backing sheet has a first surface and a second surface, and the non-woven backing sheet contains 15 to 80 weight percent of the crystalline silicate mineral powder, 5 to 25 weight percent of the heat-resistant flock, and 20 to 60 weight percent of the binder based on the total amount of the crystalline silicate mineral powder, flock, and binder in the non-woven backing sheet. The coating layer of the impregnating resin is deposited on each of the first and second surfaces of the non-woven backing sheet. The coating layer covers each of the first and second surfaces and provides a region of the resin for impregnation that extends therein. The region of the resin for impregnation extends from each of the first and second surfaces into the nonwoven backing sheet over a distance of 10 to 50 micrometers, and further, the region of the resin for impregnation has a thickness of the resin for impregnation of 5 to 50 micrometers on each of the first and second surfaces. The method according to claim 4.

6. The region of the resin for impregnation has a thickness of the resin for impregnation of 5 to 25 micrometers on each of the first and second surfaces. The method according to claim 5.

7. A laminate comprising at least one insulating sheet (10, 11) according to claim 1 and at least one layer of a plastic film, wherein the insulating sheet (10, 11) is preferably laminated on one surface of the layer of the plastic film so as to form an outer surface of the laminate.

8. Comprising a plurality of the insulating sheets (10, 11) and a plurality of the layers of the plastic film, wherein the insulating sheets (10, 11) are preferably arranged alternately with the layers of the plastic film so as to form the outer surface of the laminate. The laminate according to claim 7.

9. An electric motor comprising one or more insulating sheets (10, 11) according to any one of claims 1 to 3 or one or more laminates according to claim 7 or 8.

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