Enhanced dielectric strength mica tapes

Details Enhanced dielectric strength mica tapes Enhanced dielectric strength mica tapes

2000-02-24

2001-02-20

Krynski, William (Department: 1771)

Stock material or miscellaneous articles

Composite

Of epoxy ether

C428S324000, C428S363000, C428S377000, C428S413000

Reexamination Certificate

active

06190775

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to enhanced dielectric strength capability mica tapes which utilize impregnated epoxy resins, and epoxy chromium, epoxy tin and epoxy zinc ionic bonding within chromium, tin and zinc intercalated potassium-based mica flake substrates, to provide high voltage mica tapes. These mica tape substrates can be used for a wide variety of insulation applications for generator stators and rotors. The high dielectric strength will allow its use as a very thin insulation substrate.
BACKGROUND INFORMATION
Mica, commercially considered as a group of hydroxyl containing potassium aluminum silicates, such as KAl
2
AlSi
3
O
10
(OH)
2
(muscovite) or KMg
3
AlSi
3
O
10
(OH)
2
(phlogopite),
The Condensed Chemical Dictionary
, 6
th
Ed., Rheinhold Publishing, 1961, p. 752, has been long been a key component of high voltage electrical ground insulation in electrical machines over 7 Kv, 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 large splittings or flakes bonded to a flexible, pliable polyethylene glycol terephthalate mat or a glass fabric to provide an insulating substrate in itself, having the 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 flake 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 flake tape, then introducing the resin under pressure to impregnate the mica flake tape completely with resin eliminating voids, producing resinous insulation in a mica flake matrix. This resin is subsequently cured during the heating cycle. In practice, complete elimination of voids is difficult, and even though the mica flake tape is thick, bulky, and difficult to apply to the coils, it has remained the ultimate standard in the industry.
Problems with mica as presently used in ground insulation occur in two areas: (1) microscopically, at the interface between the mica and polymeric insulation, and (2) in the VPI process required to fill the mica tape layers completely with polymeric insulation. The mica surface is a problem area because it is not “wet” very well by the insulation resin. Thus, there is a tendency for voids to form at the mica surface that are not completely eliminated during evacuation of the coil prior to impregnation with the insulation resin. Surface treatments of the mica or addition of wetting agents to the resin have not completely eliminated this problem to date. These voids can have significant consequences for both the electrical performance of the coil and its mechanical integrity. Electrically the voids can act as locations for partial discharges. Mechanically the voids can be places where delamination can begin, causing potential disintegration of the coil.
Recently, the requirement of even using mica for high voltage insulation has been questioned. Bjorklund et al., of ABB, in “A New Mica-Free Turn Insulation For Rotating HV Machines,” the
Conference Record of the
1994
IEEE International Symposium on Electrical Insulation
, Jun. 5-8, 1994 pp. 482-484, taught use of a chromium oxide protective layer for a resin enamel as copper turn insulation, which was thin and easily manufactured, as a substitute for aramid paper containing 50% mica flake. The nonlinearity of the chromium oxide apparently has a large impact on the absorption of free electron charges.
Others had previously experimented with highly positive charged materials having good thermal stability. Drljaca et al. in “Intercalation of Montmorillonite with Individual Chromium (III) Hydrolytic Oligomers”,
Inorganic Chemistry
, vol. 31, no. 23, 1992, pp. 4894-4897, taught chromium inserted/intercalated pillared clays as having sorptive and catalytic properties and possible substitutes for zeolites, that is, sodium or calcium aluminosilicates used for ion exchange water softening. Drljaca et al. further described, in “A New Method for Generating Chromium (III) Intercalated Clays,”
Inorganica Chimica Acta
, 256, 1997, pp. 151-154, Cr (III) dimer reaction with other dimer units to form planar sheets for intercalation into montmorillonite clays, Al
2
O
3
.4SiO
2
.H
2
O
In a different area, though still related to clays, Miller, in “Tiny Clay Particles Pack Patent Properties Punch,”
Plastics World
, Fillers, October 1997, pp. 36-38, described mineral filled plastic nanocomposites having excellent mechanical strength, heat resistance, flame retardancy and gas-barrier properties. These composites originally used nylon materials containing bundles of small platelets of montmorillonite clay, about 0.5 micrometer to 2 micrometers wide and 1 nanometer (nm) thick, that is, 0.001 micrometer thick, for automobile timing belts. More recently, attempts have been made to incorporate such platelets into other resins. Miller further describes the platelets as having a high “aspect ratio,” that is, high width compared to thickness, where molecular bonds are formed between the platelets and a polymer during compounding. The clay producers, such as Nancor Inc. and AMCOL Intl., chemically stretch, that is, “open” the spacing between the platelets from about 4 Angstrom Units, about 0.0004 micrometer, to a thickness such that organic resin molecules can directly ionically or covalently attach to the platelet surface, allowing the platelet to directly react into the polymer structure during subsequent polymerization/compounding. The platelet bundles are also exfoliated into individual platelets by the clay producers to aid in polymerization/compounding. The molecular “tail”, Miller states, has the chemical functionality to overcome the incompatibility between the hydrophilic (having an affinity for water) clay and the hydrophobic (water-repelling) organic polymer and enable them to directly form a molecular bond, that is, directly intercalate the polymer into the nanoclay. Besides timing belts, additional uses appear to be thermoplastic resin gas barrier packaging, microwavable containers, and epoxy resin circuit boards.
These processes are also generally described by Usuki et al., of Toyota Chou, in U.S. Pat. No. 4,889,885. There, onium ions, from materials such as ammonium salts, sulfonium salts and phosphonium salts, were used to expand the interlayer distance of a clay such as montmorillonite through ion exchange with inorganic ions in the clay mineral. This permits the clay mineral to take a polymer into the interlayer space and connects the layers of clay mineral and polymer directly to each other through ionic bonds. The onium salt has a molecular skeleton which becomes the polymerization initiator. In cases where the onium salt has a molecular skeleton which becomes the basic constituting unit of the resin, the salt will have a phenol group (for phenolic resin), an epoxy group (for epoxy resin) and a polybutadiene group (for acrylonitrilebutadiene rubber). Yano and Usuki et al. of Toyota R&D, in “Synthesis and Properties of Polyamide—Clay Hybrid”,
Journal of Polymer Science
, Part A, Polymer Chemistry, vol. 31, 1993, pp. 2493-2498, describe use of montmorillonite clay intercalated with an ammonium salt of dodecylamine as an aligned filler in a polyamide resin hybrid, for use as a gas barrier film. There, it appears a sodium type montmorillonite was mixed with hot water to disperse the sodium, which was then replaced with the ammonium salt of dodecylamine which then interacted with dimethylacetamide (“DMAC”) to “open” the platelets of montmorillonite. The intercalated montmorillonite was then simply dispersed into a polyamide matrix and cast as a film, where the montmorillonite oriented parallel to the film surface to provide barriers to gas permeation.
The exfoliation and p

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