Patent Application
20080066942
The invention is explained in more detail by way of example with reference to the following drawings:
The present invention provides for a versatile insulating tape that comprises a highly thermally conductive dielectric layer sandwiched between two glass carrier layers. In the prior art, mica tapes are fragile and difficult to handle, they are thermally insulating, and not applicable to a wide range of industries. The present invention provides for an electrical insulation tape that uses the electrical resistivity properties of mica, the thermal conductivity of special thermally conductive, electrically insulative fillers in the dielectric layer, and the strength and flexibility of glass carriers.
The tape includes two glass carriers that sandwich a dielectric layer, with mica particles and thermally conductive, electrically insulative particles in a polymeric binder. The dielectric thermally conductive, electrically insulative filler layer is a combination of mica particles/flakelets and thermally conductive filler particles. The filler particles are highly thermally conducting, and relatively not electrically conducting. They can be of a variety of sizes, but in particular, are from 1-1000 nm in their longest dimension, and are particularly formed from nitrides, carbides and oxides. The ratio of filler particles to mica flakelets in the filler layer should be no greater than 1:1 by volume; that is, the solid phase in the filler layer should be at least 50% mica. The inorganic filler particles can be of a variety of shapes, which preferably are complimentary to the mica particles, eg platelets and discoids, to maximize the surface area that would contact adjacent filler particles.
The dielectric filler layer will also have a resin that is uniformly mixed with the mica flakelets and filler particles. The amount of resin in the filler layer can vary, but should be approximately 25-50% by volume of the filler layer to minimize organic content. The glass carrier layers will also be impregnated with a thermoset polymeric resin, although it may be of a different type than that used in the filler layer. For ease of handling, the binder resin in the dielectric layer and the carrier resin can be partially cured or fully cured prior to application of the tape.
The thickness of the filler layer can be varied depending on the application, but in general, is from approximately 3-5 mils (0.076-0.127 mm) thick. The glass carrier layers are from approximately 1-3 mils (0.025-0.050 mm) thick each.
In assembling the tape, the resinous dielectric filler layer is coated on a first layer of resin treated glass carrier. Then, the second layer of resin treated glass carrier is placed on top of the dielectric filler layer. A thermoset polymeric resin can later be impregnated into the wound tape in order to eliminate voids in the taped conductor assembly.
For example, when coating the first glass layer with the filler layer, a sol-gel ceramic/glass modified polymeric formulation can be used as a binder with a high loading of the inorganic thermally conductive, electrically insulative particle fillers added. The dielectric layer is then covered by a thermoset resin treated glass carrier. A top layer of organic resin treated glass carrier is then added for increased tensile strength.
Referring to
Mica, a group of silicates, such as KAl2 AlSi3 O10 (OH)2 (Muscovite) or KMg3 AlSi3 O10 (OH)2 (phlogopite), has long been a key component of high voltage electrical insulation in electrical machines over 440 V, because of its particularly high dielectric strength, low dielectric loss, high resistivity, excellent thermal stability and excellent corona resistance. Presently, mica is used in the form of flakes on a glass carrier backing, which provides mechanical integrity required for machine wrapping of coils, as shown for example in U.S. Pat. Nos. 4,112,183 and 4,254,351 (Smith and Smith et al.), respectively. In many cases, mica tape is wrapped around the coil and then impregnated with low viscosity liquid insulation resin by vacuum-pressure impregnation (“VPI”). That process consists of evacuating a chamber containing the coil in order to remove air and moisture trapped in the mica tape, then introducing the insulation resin under pressure to impregnate the mica tape completely with resin, thus eliminating voids, and producing resinous insulation in a mica matrix. This resin is subsequently cured by a prolonged heating cycle.
Fillers may be metal oxides, metal nitrides, and metal carbides, as well as some non-metal oxides, nitrides and carbides. For example, alumina, magnesia, zirconia and other metal oxides as well as boron nitride, aluminum nitride, other metal nitrides, metal carbides and diamond of natural or synthetic origin. The filler particles can be of the various physical forms of each type listed, and the particles may be hybrids of the materials mentioned and have stoichiometric and non-stoichiometric mixed oxides, nitrides and carbides. More specific examples of these include Al2O3, AIN, MgO, ZnO, BeO, BN, Si3N4, SiC, SiO, and SiO2 with mixed stoichiometric and non-stoichiometric combinations. Also, non-oxide ceramics with high thermal conductivity like silicides or nitrides. Further, these particles will be surface treated to introduce a variety of surface functional groups which are capable of participating in reactions with the host organic polymer resin binder of the dielectric filler layer. It is also possible to coat non-High Thermal Conductivity (HTC) materials, such as silica and other filler particle materials, with an HTC material. In particular embodiments, filler particles are from 5-100,000 nm in major axis length and have an aspect ratio of between 3-100. These inorganic nano-particles can also contain reactive surfaces to form intimate covalently bonded hybrid organic-inorganic homogeneous materials.
One type of resinous composition that can be used can be obtained by reacting epichlorohydrin with a dihydric phenol in an alkaline medium at about 50° C., using 1 to 2 or more moles of epichlorohydrin per mole of dihydric phenol. The heating is continued for several hours to effect the reaction, and the product is then washed free of salt and base. The product, instead of being a single simple compound, is generally a complex mixture of glycidyl polyethers, providing a diglycidyl ether of bisphenol A type epoxide or a diglycidyl ether of bisphenol F type epoxide. The bisphenol epoxides have a 1,2-epoxy equivalency greater than one and will generally be diepoxides.
Other glycidylether resins that are useful include polyglycidyl ethers of a novolac prepared by reacting an epihalohydrin with an aldehyde, for example, a phenol formaldehyde condensate. Cycloalyphatic type epoxides are also useful, as are glycidyl ester epoxy resins, both being non-glycidyl ether epoxides, all of which are well known in the art and described in detail by Smith et al, in U.S. Pat. No. 4,254,351, where epoxidized polybutadiene, also useful in this invention, is described. These resinous compositions are referred to as polyepoxide resins. Example of thermoset resins include epoxies, polyesters, phenolics, cyanate esters, polyimides, silicone resins, and styrenated resins. A special version epoxy resins based on Liquid Crystal Thermoset (LCT) structures provide improved thermal conductivity versus amorphous epoxy polymeric resins.
Other useful resins include polyesters, and 1-2, polybutadienes, all of which are well known in the art. Generally, polyester resins are a large group of synthetic resins, almost all produced by reaction of dibasic acids with dihydric alcohols. In a few cases, trifunctional monomers such as glycerol or citric acid are used. The term polyester resin applies especially to the products made from unsaturated dibasic acids such as maleic acid. Unsaturated polyester resins can be further polymerized through cross linking. Often, another unsaturated monomer such as styrene is added during this second stage of the polymerization, which can occur at ordinary temperatures with suitable peroxide catalysts. Maleic anhydride and fumaric acid are the usual unsaturated acid components, while phthalic anhydride, or adipic or azelaic acid are the corresponding saturated materials. Commonly used glycols include ethylene, propylene, diethylene, dipropylene, and certain butylene glycols. The added polymerizable monomer includes styrene, vinyltoluene, diallyl phthalate or methyl methacrylate. In addition to the unsaturated polyester resins, there are other important types. One large group are the alkyd resins, which are made from saturated acid and alcohol monomers with many types of modifications, usually the inclusion of an unsaturated fatty acid.
Commonly used epoxy resins are bisphenol A and bisphenol F resins, which are readily commercially available from Dow Chemical Co and other resin suppliers. The bisphenol F is more fluid and therefore may more readily penetrate the damaged areas in many circumstances. Though the patching resin of the present invention may have a great range of viscosities depending on use, in a preferred embodiment, the viscosity is 100-300 centipoise (cps), with a particular viscosity of 120-175 cps.
In one embodiment, the present invention provides for an electrical insulation tape that comprises a first and a second carrier layer, and a dielectric thermally conductive, electrically insulative filler layer that comprises mica particles, thermally conductive, electrically insulative filler particles and a binder resin, disposed between the first and the second carrier layers. In one aspect the thermally conductive, electrically insulative filler particles are discoids and platelets, and in another aspect, the binder resin includes epoxy, polyimide epoxy, liquid crystal epoxy and cyanate-ester.
In a more particular aspect, the carrier layers are glass fabric. The ratio of the mica particles to the thermally conductive, electrically insulative filler particles is at least 1:1 by volume, the percentage of the binder resin in the dielectric thermally conductive, electrically insulative filler layer is 25-50% by volume, and the first and the second carrier layers are impregnated with a second resin.
In a particular aspect, the second resin is a thermoset polymeric resin like epoxy or other previously mentioned thermoset resin, and in another aspect, the second resin is a b-stage resin.
In one aspect, the inorganic thermally conductive, electrically insulative filler particles have a length of between 1-100,000 nm and an aspect ratio of approximately 5-50. In another aspect, the inorganic thermally conductive, electrically insulative filler particles are selected from at least one of oxides, nitrides, and carbides, or in another aspect, the dielectric thermally conductive, electrically insulative filler layer is at least one of oxides, nitrides, and carbides comprising Al2O3, AlN, MgO, ZnO, BeO, BN, Si3N4, SiC and SiO2 with mixed stoichiometric and non-stoichiometric combinations.
In yet another aspect, the inorganic thermally conductive, electrically insulative filler particles have been surface treated to introduce surface functional groups that allow for the essentially complete co-reactivity with the binder resin, and in a more particular aspect, the functional groups comprise at least one of hydroxyl, carboxylic, amine, epoxide, silane and vinyl groups.
In another embodiment, the present invention provides for an electrical insulation tape that comprises a first and a second glass fabric layer, and a dielectric thermally conductive, electrically insulative filler layer that comprises mica particles, inorganic thermally conductive, electrically resistive thermally conductive, electrically insulative filler particles and a binder resin disposed between the first and the second glass fabric layers. This embodiment provides for a ratio of mica particles to thermally conductive, electrically insulative filler particles of at least 1:1 by volume, and the percentage of the binder resin in the thermally conductive, electrically insulative filler layer is 25-50% by volume. Finally, the first and the second glass fabric layers are pre-impregnated with a second thermoset polymeric resin. In a more particular aspect, the second thermoset polymeric resin is a b-stage resin.
In yet another embodiment, the present invention provides for a method for making an insulation tape that comprises obtaining a first carrier layer and a second carrier layer, such as a glass fabric, impregnating a second resin into the first and second carrier layers. The carrier layers can be pre-impregnated with the second resin. The coating the first carrier layer with a dielectric thermally conductive, electrically insulative filler layer, and adding the second carrier layer to the dielectric thermally conductive, electrically insulative filler layer.
In a particular aspect, the coating of the dielectric thermally conductive, electrically insulative filler layer onto the first carrier layer is done by a sol-gel liquid ceramic/glass modified polymeric formulation binder with a high loading of the inorganic particle thermally conductive, electrically insulative fillers. The dielectric thermally conductive, electrically insulative filler layer is comprised of mica particles, thermally conductive, electrically insulative filler particles, and a binder resin, the percentage of the binder resin in the dielectric thermally conductive, electrically insulative filler layer is 25-50% by volume, the ratio of the mica particles to the thermally conductive, electrically insulative filler particles in the dielectric thermally conductive, electrically insulative filler layer is at least 1:1 by volume, and the dielectric thermally conductive, electrically insulative filler layer is at least one of oxides, nitrides, and carbides. And finally, in one aspect, the insulation tape is fully cured after being applied to an electrical device.
The term carrier as used herein, refers to any type of glass-fiber-based or organic-fiber-based material commonly used in insulating tape. The carrier's physical form can include mats, fabrics and fleeces composed of materials like polyester, E-glass or ceramic metal oxides. Although high thermal conductive glass or ceramic metal oxide carriers are common embodiments of this invention, the present invention may also be used with organic polymer carriers such as Dacron™, polyester terapthalate (PET) or densified aramid paper commonly known as Nomex™.
While specific embodiments of the invention have been described in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only, and not meant to limit the scope of the invention, which is to be given the full breadth of the claims appended and any and all equivalents thereof.
