Glass flake paper

Details Glass flake paper Glass flake paper

2002-05-24

2004-08-10

Kelly, Cynthia (Department: 1774)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

C428S293400, C428S323000, C428S325000, C162S100000, C162S156000

Reexamination Certificate

active

06773793

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a sheet material for electrical insulation, and, more specifically, relates to a sheet material for electrical insulation suitable for an electrical coil to be used where a high dielectric strength insulation is required.
BACKGROUND OF THE INVENTION
Insulated electrical coils for conventional electrical machines have been produced for many years with an electrical insulation tape made of a glass cloth bonded to a laminated mica paper. This tape is wound around a coil conductor and is typically impregnated with a thermosetting resin and cured. In the alternative, the tape may be a semi-cured (e.g. “b-stage”) prepregnated tape which is wound around the coil and then cured.
The dielectric strength of such conventional tapes is mainly derived from the mica, and the mechanical strength of the tape is largely due to the glass cloth. However, because of the dielectric strength of such conventional tapes, they have not always proved satisfactory.
It would be advantageous, therefore, to produce a sheet material for electrical insulation combining similar mechanical strength and improved dielectric strength.
SUMMARY OF INVENTION
In accordance with one aspect, the present invention provides an electrically insulating sheet material composed primarily of D-glass flake. The sheet material includes about 50-99% D-glass flakes and about 1-50% additives such as bonding agents, reinforcing agents, and porosity control agents.
DESCRIPTION OF EXAMPLE EMBODIMENTS
As used herein and in the claims, all percentages are given as weight percentages unless otherwise indicated. As used herein and in the claims, all weight percentages are percentages of the total weight of the electrical sheet material. As used herein, when a preferred range such as 5-25 is given, this means preferably at least 5% and, separately and independently, preferably not more than 25% of that component are used.
D-glass flake has an intrinsic dielectric strength 25 to 35% higher than mica and therefore will provide superior performance as an insulating sheet. Unlike mica, however, glass flake has no self-bonding characteristics. As a result, additive components must be used in the D-glass flake insulating sheet to provide for the physical integrity of the sheet during manufacture and use. The additives contributions include, but are not limited to: 1) improved retention of the glass flake during manufacture, 2) improved wet handling of the glass flake sheet during manufacture, 3) porosity control, 4) bonding of the glass flake into the insulating sheet for improved dry flake retention and sheet strength, and 5) reinforcement of the insulating sheet for improved physical strength.
The present invention embodies two forms which are similar but distinct. The first form of the invention is an insulating sheet in which the majority of the physical strength is provided by a fabric or unidirectional continuous reinforcing yarns. In the second form of the invention, the reinforcement is provided by discrete fibers intermingled with the glass flake, referred to herein as an “un-backed” insulating sheet.
The electrical insulating sheet material of the present invention has the following preferred formulations or tables of components, depending on whether it is the reinforced or the “un-backed” form. In these formulations or tables of components, any preferred or less preferred weight percent or weight percent range of any component can be combined with any preferred or less preferred weight percent or weight percent range of any of the other components; it is not required or necessary that all or any of the weight percents or weight percent range come from the same column. Note however, that the weight percentage of components other than D-glass must equal at least one percent.
TABLE OF COMPONENTS FOR A BACKED INSULATING SHEET
(EXCLUDING CONTINUOUS REINFORCING YARNS)
WEIGHT PERCENTS
Less Preferred
Less Preferred
Preferred
D-Glass
50-99
70-95
80-95
Retention Aid/Porosity
 0-45
 0-10
0-3
Control Agent
Bonding Agent
 0-30
 5-17
 5-12
Discrete Fiber
 0-20
0-7
0-5
TABLE OF COMPONENTS FOR A UN-BACKED
INSULATING SHEET
WEIGHT PERCENTS
Less Preferred
Less Preferred
Preferred
D-Glass
50-99
70-95
80-90
Retention Aid/Porosity
 0-45
 0-10
0-3
Control Agent
Bonding Agent
 0-30
 5-25
 5-15
Discrete Fiber
 0-20
 5-15
 7-12
The additives used in D-glass flake insulating sheet according to the invention constitute the following broad categories:
retention aids
porosity control agents
bonding agents
reinforcements
In some cases, an additive may fit in more than one category.
Retention aids help capture the finer flake particles during the wet-forming of the insulating sheet. They also help retain the flake by mechanical entrapment in the dry, finished sheet. Examples of retention aids are fibrids, flocs or synthetic pulps.
Porosity control agents disrupt the planarity of glass flake particles creating channels for the absorption of saturating resins. It should be understood that the porosity of the insulating sheet needs to be controlled in that too porous of a structure could lead to voids in the finished electrical component while too closed of a structure will be hard to saturate. Porosity control agents need to be thermally stable to the conditions used during manufacture and saturation of the insulating sheets. Examples of porosity control agents are fibrids and cylindrical fine fiber flocs produced from continuous fiber filaments.
Bonding agents are used to bond the glass flake together and to provide improved mechanical strength to the insulating sheet. The bonding agent can be either a thermoplastic or thermoset resin. The thermoplastic can be solid fusible particle, fiber, floc or an aqueous dispersion of a thermoplastic resin. It is important that the thermoplastic bonding agent needs to flow and bond at lower temperatures and pressures than the materials used as porosity control agents and reinforcing agents. Thermoset bonding agents will be aqueous dispersions of the uncured resin or a particulate b-staged form of the resin. Examples of thermoplastic bonding agents are low softening point flocs or binder fibers such as polyolefin floc. Examples of thermoset bonding agents are aqueous dispersions of epoxy or urethane resins.
Reinforcements can be of two types. The first type encompasses fabrics and continuous yarns which can be incorporated into the insulating sheet during sheet manufacture or as a post manufacture lamination step. These fabrics and yarns run continuously along the length of the insulating sheet. As with the porosity control agents, reinforcing agents need to be thermally stable, i.e. not stretch or shrink, under the thermal conditions encountered during manufacture or application of the insulating sheet. Some examples of scrims and continuous yarns that can be are polyaramid, E-glass, D-glass, polyimide, and polyester.
The second type of reinforcement is the use of discrete, dispersible fibers which form a random array within the insulating sheet and are bonded into the insulating sheet by a bonding agent. Examples of suitable discrete fibers are polyaramid, E-glass, D-glass, polyester, and nylon.
An embodiment of the invention is an electrical insulating sheet material (or “glass flake paper”) the primary component of which is D-glass flake. D-glass, as is known in the art, is a boro silicate glass having approximately the following composition in weight percent:
SiO
2
72-75%
CaO
 0-1%
Na
2
O, K
2
O, and Li
2
O
 1-4%
B
2
O
3
21-25%
D-glass has superior dielectric strength compared to other silicate glasses, which is important in an insulating sheet material. D-glass is commercially available in a yarn form from at least one supplier.
The D-glass flakes may be produced in several ways, for example a melt drawing process. In melt drawing, the D-glass raw materials are heated to a temperature of 2650° F., at which point the molten D-glass exhibits a viscosity of approximately 750 poise. The molte

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