IMPREGNATING COATING LAYER FOR INSULATING SHEETS

Abstract

The present invention relates to an insulating sheet of a nonwoven sheet, for example a sheet comprising aramid fiber, comprising 15 to 80 percent by weight of a crystalline silicate mineral powder, for example a mica powder, the nonwoven sheet impregnated with a coating of an impregnating resin based on acrylate derivatives, which is suitable for increasing the compactness of the insulating sheet and for preventing the silicate mineral powder from detaching from the nonwoven sheet during use. The impregnating resin also comprises an organic solvent, which is capable of reducing the viscosity thereof and of promoting its deep penetration into the sheet. In addition, the impregnating resin comprises a UV polymerization photoinitiator which is suitable for curing the resin, only after said resin has been applied to the sheet and has penetrated deeply therein. The invention also describes the method for impregnating the insulating sheets.

Claims

1. An insulating sheet comprising a nonwoven sheet containing a crystalline silicate mineral powder, characterized in that: said insulating sheet also comprises a coating of an impregnating resin based on acrylate derivatives.

2. The insulating sheet according to claim 1, wherein said nonwoven sheet contains 15 to 80 percent by weight of the crystalline silicate mineral powder.

3. The insulating sheet according to claim 1, in which said crystalline silicate mineral powder comprises mica.

4. The insulating sheet according to claim 1, wherein the nonwoven sheet has a first surface and a second surface, the nonwoven sheet comprising 15 to 80 weight percent crystalline silicate mineral powder, 5 to 25 weight percent heat-resistant floc, and 20 to 60 weight percent binder, based on the total amount of said crystalline silicate mineral powder, floc, and binder in the nonwoven sheet, wherein each of the first and second surfaces of the nonwoven sheet are coated with the impregnating resin, and each of the first and second surfaces are stabilized by a region of the said impregnating resin that both covers and extends into each of said first and second surfaces, the region of impregnating resin extending into the sheet a distance of 10 to 50 micrometers from each of said first and second surfaces, and further, the region of impregnating resin having a thickness of impregnating resin of from 5 to 50 micrometers on each of said first and second surfaces.

5. The insulating sheet according to claim 3 wherein the nonwoven sheet comprises 20 to 70 weight percent mica.

6. The insulating sheet according to claim 4 wherein the heat-resistant floc is aramid floc.

7. The insulating sheet according to claim 4 wherein the binder includes aramid fibrids.

8. The insulating sheet as claimed in claim 1, in which said coating of impregnating resin comprises an unsaturated polyester imide material.

9. The insulating sheet as claimed in claim 1, in which said impregnating resin also comprises a polymerization photoinitiator.

10. A method for producing an insulating sheet, comprising the following steps: a) providing a nonwoven backing sheet comprising a crystalline silicate mineral powder; characterized in that said method also comprises the following step: b) depositing a coating layer of an impregnating resin based on acrylate derivatives onto said nonwoven backing sheet so as to obtain a resin-impregnated sheet.

11. The method according to claim 10, wherein said nonwoven backing sheet comprises 15 to 80 percent by weight of the crystalline silicate mineral powder.

12. The method as claimed in claim 10, wherein the nonwoven backing sheet has a first surface and a second surface, the nonwoven backing sheet comprising 15 to 80 weight percent crystalline silicate mineral powder, 5 to 25 weight percent heat-resistant floc, and 20 to 60 weight percent binder, based on the total amount of said crystalline silicate mineral powder, floc, and binder in the nonwoven backing sheet, and wherein the coating layer of impregnating resin is deposited on each of the first and second surfaces of the nonwoven backing sheet, the coating layer providing a region of the said impregnating resin that both covers and extends into each of said first and second surfaces, the region of impregnating resin extending into the nonwoven backing sheet a distance of 10 to 50 micrometers from each of said first and second surfaces, and further, the region of impregnating resin having a thickness of impregnating resin of from 5 to 50 micrometers on each of said first and second surfaces.

13. The method as claimed in claim 10 wherein the nonwoven backing sheet comprises 20 to 70 weight percent mica.

14. The method as claimed in claim 12 wherein the heat-resistant floc is aramid floc.

15. The method as claimed in claim 12 wherein the binder includes aramid fibrids.

16. The method as claimed in claim 10, in which said coating layer comprises an unsaturated polyester imide material.

17. The method as claimed in claim 10, in which said coating layer comprises an organic solvent and said method also comprises the following step: c) evaporation of said organic solvent by drying said resin-impregnated sheet.

18. The method as claimed in claim 10, in which said impregnating resin also comprises a polymerization photoinitiator, and in which said method also comprises the following step: d) exposing said resin-impregnated sheet to UV radiation so as to bring about the polymerization of said coating layer.

19. The method according to claim 10, wherein said impregnating resin based on acrylate derivatives has a viscosity comprised between 5 cps and 150 cps.

20. A laminate comprising at least one insulating sheet as claimed in claim 1 and at least one layer of plastic film, in which said insulating sheet is laminated on one side of said layer of plastic film, preferably so as to form the outer side of said laminate.

21. The laminate as claimed in claim 20, comprising a plurality of said insulating sheets and a plurality of said layers of plastic film, in which said insulating sheets are alternated with said layers of plastic film, preferably so as to form the outer side of said laminate.

22. An electric motor comprising one or more insulating sheets as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0071] The present invention will be described with reference to the attached figures in which the same numbers and/or reference marks indicate the same and/or similar and/or corresponding parts of the system.

[0072] FIG. 1 schematically shows an aramid/mica insulating sheet used as an insulating coating for the stator slots of an electric motor.

[0073] FIG. 2 schematically shows the process for producing aramid/mica insulating sheets also comprising a coating resin layer, according to one embodiment of the present invention.

DETAILED DESCRIPTION

[0074] In the text hereinbelow, the present invention is described with reference to particular embodiments as illustrated in the attached figures. However, the present invention is not limited to the particular embodiments described in the following detailed description and represented in the figures, but rather the described embodiments simply exemplify the various aspects of the present invention, the scope of which is defined by the claims. Further modifications and variations of the present invention will emerge clearly to a person skilled in the art.

[0075] The present invention relates to an insulating sheet of a nonwoven sheet 10 comprising a crystalline silicate mineral powder, in particular mica, impregnated with a coating of a resin based on acrylate derivatives which penetrates deep into the sheet itself. By nonwoven sheet, it is meant a sheet structure produced by bonding and/or interlocking of fibrous material into a random web or mat by mechanical, chemical, or thermal methods, and combinations thereof. One preferred type of nonwoven sheet is a paper.

[0076] 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 along with aramid fiber or other thermostable floc or fiber or mixtures of such flocs or fibers. By thermostable it is meant that the given floc or fiber can withstand a long exposure to the end-use temperature without significant degradation (usually, to remain at least 50% of its initial properties after exposure for 100,000 hours to the necessary temperature, which in some instances is greater than 180? C.).

[0077] The term aramid, as used herein, means aromatic polyamide, wherein at least 85% of the amide (CONH) linkages are attached directly to two aromatic rings. Optionally, additives can be used with the aramid and may be dispersed throughout the polymer structure. It has been found that up to as much as about 10 percent by weight of other polymeric material can be blended with the aramid. It has also been found that copolymers can be used having as much as about 10 percent of other diamines substituted for the diamine of the aramid or as much as about 10 percent of other diacid chlorides substituted for the diacid chloride of the aramid.

[0078] The preferred aramid is a meta-aramid. The aramid polymer is considered a meta-aramid when the two rings or radicals are meta oriented with respect to each other along the molecular chain. The 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 are illustrative of useful methods for making aramid fibers that could be used to make aramid floc.

[0079] The preferred aramid floc is a meta-aramid floc, and especially preferred is floc made from the meta-aramid poly(meta-phenylene isophthalamide)(MPD-I). The term floc, as used herein, means fibers that are cut to a short length and that are customarily used in the preparation of papers. Typically, floc has a length of from about 3 to about 20 millimeters. A preferred length is from about 3 to about 7 millimeters. Floc is normally produced by cutting continuous fibers into the required lengths using well-known methods in the art.

[0080] The term fibrids, as used herein, means very small, nongranular, fibrous or film-like particles with at least one of their three dimensions being of minor magnitude relative to the largest dimension. These particles are prepared by precipitation of a solution of polymeric material using a non-solvent under high shear. Aramid fibrids are non-granular film-like particles of aromatic polyamide having a melting point or decomposition point above 320? C. The preferred aramid fibrid is a meta-aramid fibrid, and especially preferred are fibrids made from the meta-aramid poly(meta-phenylene isophthalamide)(MPD-I). Fibrids generally have a largest dimension length in the range of 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 micron, for example, about 0.1 microns to about 1.0 micron. While not required, it is preferred to incorporate aramid fibrids into the layers while the fibrids are in a never-dried state.

[0081] By aramid paper, as used herein, it is meant a planar sheet made from one or more plies or layers of aramid materials that are prepared by paper-making processes. Representative devices and machinery that can be used to make plies or layers include continuous-processing equipment such as, for example without limitation to, a Fourdrinier or inclined wire machine, or batch-processing equipment such as those that make paper by hand in a hand-sheet mold containing a forming screen.

[0082] To make the mica-containing sheets or papers, in one preferred method, a mixture of the desired amounts of mica and fibrous material is combined in water to make an aqueous dispersion; and then this mica-containing aqueous dispersion is then used to make the mica-containing nonwoven sheets by a suitable nonwoven process. For one specific example, to make a mica-containing aramid paper, mica, MPD-I fibrids, and MPD-I floc in the amounts previously mentioned herein, are combined in water to form a mica-containing aqueous dispersion suitable as a furnish for papermaking. The mica-containing dispersion is then supplied to the headbox of a papermaking machine and a wet laid web is formed. The speed of the paper machine is controlled to provide the desired thickness of the wet laid web. The wet laid web is then dried to form an unconsolidated mica-containing aramid formed web or layer, which can be further consolidated by calendering or can be combined with other like formed web(s) and then consolidated by calendering.

[0083] A nonexhaustive list of mica types that may be used for the present invention includes: muscovite, phlogopite, fluorophlogopite, and synthetic mica.

[0084] The acrylate resin is obtained following condensation between a hydroxylated resin and an acrylic or methacrylic acid. The acrylate resin is preferably diluted in a reactive acrylate solvent and in an unreactive organic solvent, for example methyl ethyl ketone, in order to lower its viscosity in application and to promote the penetration of the coating into the aramid paper. In this manner, the aramid/mica paper is compacted deep down and the detachment and shedding of the mica in the subsequent processing steps are hindered.

[0085] The polymerization of the coating resin is induced after the coating layer has been applied to the aramid/mica sheet and after the coating has penetrated into the aramid/mica sheet. In particular, polymerization of the coating resin is induced by the addition of a photoinitiator and exposure to UV light, preferably with a wavelength in the range between 200 nm and 400 nm.

[0086] Nonlimiting examples of photoinitiators used 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-hydroxycyclohexyl phenyl ketone (HCPK), 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP), or the photoinitiators belonging to the class of bi-acylphosphate oxide polymers (TPO), or the photoinitiators belonging to the class of ?-hydroxy ketones.

[0087] In a preferred configuration, the insulating sheet 10 may be used for insulating rotating or static components of electrical machines.

[0088] In a further preferred configuration, the insulating sheet 10 may be used as an outer layer on one or both sides of a composite laminate of plastic film layers, and said laminate may be used for insulating rotating or static components of electrical machines. In a further preferred configuration, one or more insulating sheets 10 may be used to form a multilayer laminate, in which the laminate comprises a plurality of plastic film layers laminated together, preferably alternating with layers of aramid paper, and the laminate may be used for insulating rotating or static components of electrical machines.

[0089] The insulating sheet 10 or the laminates described above may be used, in rotating electrical machines, for insulation of the stator slots, for closing same, and also for separating phases on the head of windings; in static electrical machines, they may be used as interlayer insulators.

[0090] For example, FIG. 1 shows a suitably sized insulating sheet 11 used for insulating the stator 20 slot of an electric motor. Preferably, aramid/mica insulating sheets 11, impregnated with resin, are cut into segments essentially equal in length to the depth of the stator 20 and equal in width to the development of the slot to be insulated. The insulating sheet 11 may advantageously be inserted into the grooves of stator 20 of an electric motor by pushing it 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 toward separating the magnetic sheet metal from the copper winding 30.

[0091] The process for producing aramid/mica insulating sheets impregnated with a coating layer is described with reference to FIG. 2.

[0092] Preformed aramid/mica backing sheets 7 are preferably supplied in reels, having, for example, a width equal to 1 m and a length of between 100 m and 2000 m. The backing sheets 7 may be, for example, DuPont? Nomex? 818 sheets, such as those described in the document DuPont? Nomex? 818, Technical Data Sheet, @ Copyright 2016 DuPont. The backing sheets 7 may include, for example, an amount of mica equal to 50% of the total composition.

[0093] The acrylate resin coating is applied onto the preformed aramid/mica backing sheets 7 by means of a roll to roll process schematically illustrated in FIG. 2.

[0094] An unwinder 101 supplies the production line 100 with an aramid/mica backing reel 7. A liquid formulation is prepared comprising a resin based on acrylate monomers and acrylate oligomers, for example an unsaturated polyester imide resin, to which a UV polymerization photoinitiator is added, for example Omnirad TPO-L or 2-hydroxy-2-methyl-1-phenylpropanone, and an unreactive organic solvent, for example methyl ethyl ketone (MEK), to lower the viscosity of the resin itself and to allow better penetration of the resin into the aramid paper matrix. Said liquid formulation is applied onto one of the surfaces of the aramid/mica backing sheet 7 at the application station 102, 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, for example it is first applied to one side and then to the opposite side.

[0095] The liquid formulation is applied to the aramid/mica backing sheets 7 preferably by the gravure or gravure engraved roller coating technique, or the Meyer bar technique, or impregnation using Foulard machines. In the configuration with coating by gravure engraved roller, the liquid formulation is initially collected in the cavities of the engraved roller and is subsequently 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 in the range between 10 g and 20 g per square meter of backing sheet 7, where the liquid formulation comprises a solution containing between 10% by weight and 50% by weight of acrylate resin in an unreactive organic solvent, so as to obtain a dry deposit of acrylate resin of between 1 g and 10 g per square meter of backing sheet 7 and to reach an impregnation depth of between 5 ?m and 50 ?m.

[0096] After application of the liquid formulation, the impregnated sheet 8 reaches the heating station 103 comprising an industrial oven, where the unreactive organic solvent is evaporated off. For example, the impregnated sheet 8 is heated to a temperature in the range between 90? C. and 120?? C. to evaporate off the solvent part and to dry the coating. At the end of the heating process, a sheet 9 is obtained in which the unreactive organic solvent has evaporated off.

[0097] The sheet 9 then reaches the UV irradiation station 104, where it is exposed to radiation emitted by an ultraviolet lamp. In particular, the UV radiation is focused on a substrate onto which a liquid formulation containing the photoinitiator has been applied. As a result of the UV light irradiation, free radicals are generated which interact with the acrylate resin that has penetrated into the sheet and induce polymerization of the formulation, which changes from liquid to solid. At the end of the UV exposure, an insulating sheet 10 is thus obtained in which the acrylate-based impregnating resin is polymerized.

[0098] Preferably, the operations of liquid formulation application, organic solvent evaporation and UV irradiation are repeated for the opposite side of the insulating sheet 10. For example, the insulating sheet 10 comprising a resin-impregnated surface can be returned to the application station 102, where the liquid formulation is applied to the opposite surface, and then it can be passed once again through the heating station 103 and the UV irradiation station 104.

[0099] Finally, the insulation sheet 10 is cut into smaller sheets, from which individual pieces of insulation 11 are made. Preferably, the insulating sheet 10 is cut into reels with a width equal to the development of the stator slot, for example with a width of between 1 cm and 10 cm.

[0100] Although the present invention has been described with reference to the embodiments described above, it is clear to a person skilled in the art that various modifications, variations and improvements of the present invention in light of the teaching described above and within the scope of the appended claims are possible without departing from the subject matter and the scope of protection of the invention.

[0101] For example, although it has been described that the crystalline silicate mineral powder comprises mica, it is clear that there may be other crystalline silicate mineral powders, for example including vermiculite, calcined clay, silica, talc, wollastonite, and/or combinations thereof. For example, each of the listed powders may be used with aramid fibers as a reinforcing element.

[0102] For example, although it has been described that the nonwoven sheet is preferably a paper structure comprising aramid fibers, it is clear that there may be different types of fibers, for example, glass fibers, a mixed aramid paper, or a film of polyamide-imide, polyester, polyimide ether, polyether ketone, polyether sulfone, polysulfide, polyimide, or the like. For example, each of the indicated reinforcing elements may be used to contain mica as a crystalline silicate mineral powder.

[0103] Finally, those areas considered to be known to experts in the field have not been described to avoid unnecessarily overshadowing the described invention.

[0104] For example, the process of gravure coating, or gravure engraved roller coating, Meyer bar, or Foulard machine impregnation have not been described in detail since they are considered to be known to those skilled in the art.

[0105] For example, the process of deposition and polymerization of the impregnating resin on an aramid/mica paper by means of a roll to roll process has not been described in detail, as it is considered to be known to those skilled in the art.

[0106] Accordingly, the invention is not limited to the embodiments described above, but is only limited by the scope of protection of the appended claims.

REFERENCE NUMBERS

[0107] 7: backing sheet [0108] 8: sheet impregnated with resin [0109] 9: sheet from which the unreactive organic solvent is evaporated [0110] 10: insulating sheet [0111] 11: cut insulating sheet [0112] 20: stator slot [0113] 30: copper winding [0114] 100: coating roll-to-roll deposition and polymerization station [0115] 101: unwinder [0116] 102: liquid formulation application station [0117] 103: heating station [0118] 104: UV irradiation station [0119] 105: cutting station