Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-08-24
2001-06-26
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S227000, C524S437000, C524S588000, C524S728000, C524S731000, C524S786000, C528S031000, C528S015000
Reexamination Certificate
active
06251990
ABSTRACT:
This invention relates to a silicone rubber composition which cures at room temperature or elevated temperature into a silicone rubber serving as a high-voltage electrical insulator and which has a sufficient thixotropy to serve as a sealant or repairing compound for use in the repair, fabrication and assembly of silicone rubber or EPDM-coated ceramics and glass insulators or bushings. It also relates to sealing and repairing compounds for use with polymeric insulators or bushings.
BACKGROUND OF THE INVENTION
In general, high-voltage electrical insulating materials for use as insulators and bushings for power transmission lines are of porcelain (ceramics) or glass. Since these insulators are heavy and liable to breakage due to a lack of impact resistance, they require careful handling and impose a burden to workers. In a polluted environment as in seaside areas and industrial areas, there is a tendency that dust, salts and mist attach to the surface of high-voltage electrical insulators, causing leakage currents and dry band discharge leading to flashover failure.
In order to eliminate the drawbacks of ceramic and glass insulators, a number of proposals have been made. For example, U.S. Pat. No. 3,511,698 discloses a weathering resistant high-voltage electrical insulator comprising a member of a thermosetting resin and a platinum catalyst-containing organopolysiloxane elastomer. JP-A 59-198604 corresponding to U.S. Pat. No. 4,476,155 proposes a one-part room temperature curable organopolysiloxane composition which is applied to the outer surface of an electrical insulator of glass or porcelain so that the electrical insulator may maintain its high insulating properties even in the presence of moisture, polluted air, ultraviolet radiation and other outdoor stresses.
JP-A 53-35982 corresponding to U.S. Pat. No. 3,965,065 discloses that a silicone rubber composition with improved electrical insulation is obtained by heating a mixture of an organopolysiloxane capable of heat curing into silicone rubber and aluminum hydrate at temperatures above 100° C. for more than 30 minutes. JP-A 7-57574 corresponding to U.S. Pat. No. 5,519,080 describes that the blending of a methylalkylsiloxane fluid in silicone rubber is effective for providing contact angle recovery with time and preventing flashover failure.
However, the silicone rubber materials used in the prior art techniques mentioned above are not yet fully satisfactory in high-voltage electrical insulation. They must be loaded with large amounts of aluminum hydroxide in order to improve the electrical insulation. This raises a new problem that a high loading of aluminum hydroxide reduces the strength of silicone rubber and renders the silicone rubber brittle. As a consequence, molded parts are often damaged during molding, during attachment and by external factors (weather conditions, human attacks as by gunshots, and bird and animal attacks) after molding.
SUMMARY OF THE INVENTION
An object of the invention is to provide a silicone rubber composition which cures at room temperature or elevated temperature into a silicone rubber having sufficiently high strength and improved electrical insulating properties to serve as a high-voltage electrical insulator and which has a sufficient thixotropy to serve as a sealant and repairing compound for use with polymeric insulators. Another object of the invention is to provide a sealant and a repairing compound for use with polymeric insulators.
The inventors have found that a silicone rubber composition comprising as main components, (A) an organopolysiloxane represented by the average compositional formula (1) defined below and having on the average at least two alkenyl groups in a molecule and a degree of polymerization of up to 1,200, (B) an organohydrogenpolysiloxane having at least two hydrogen atoms each attached to a silicon atom represented by the average compositional formula (2) defined below and being liquid at room temperature, (C) aluminum hydroxide, (D) an addition reaction catalyst, and (E) a thixotropic agent, and exhibiting a thixotropy corresponding to a sag of up to 1 inch after 1 minute as measured by the Boeing flow jig test cures at room temperature or elevated temperature into a silicone rubber having sufficiently high strength and improved electrical insulating properties to serve as a high-voltage electrical insulator. The silicone rubber composition has a sufficient thixotropy to serve as a sealant and repairing compound for use in the repair, fabrication and assembly of silicone rubber or EPDM-coated ceramics and glass insulators or bushings.
More particularly, since the silicone rubber composition for use as a high-voltage electrical insulator according to the invention has a high strength, improved high-voltage electrical insulating properties, and thixotropy at the same time, the composition can be used to fill up defects in insulator caps or disks without detrimental influence on the electrical insulating properties of the insulator itself. Curing of the composition can be finished within a short time by allowing it to stand at room temperature or by heating it by means of a dryer and so, the repair is simple and brief. By way of example, silicone rubber-coated insulator bushings generally use a core of fiber-reinforced plastic (FRP) and a support of metallic material because of the insufficient strength of rubber itself. If a conventional sealant is applied between these materials, the insulator can be damaged first at the seal. The use of the inventive composition as an insulator sealant has the advantages that it can be easily applied due to its thixotropy and it does not adversely affect the electrical insulating properties of the insulator bushing.
According to the invention, there is provided a silicone rubber composition for use as a high-voltage electrical insulator, comprising
(A) 100 parts by weight of an organopolysiloxane represented by the following average compositional formula (1):
R
1
a
SiO
(4-a)/2
(1)
wherein R
1
is a substituted or unsubstituted monovalent hydrocarbon group, 0.01 to 10 mol % of R
1
is alkenyl, at least 90 mol % of R
1
is methyl, and a is a positive number of 1.9 to 2.4, said organopolysiloxane having on the average at least two alkenyl groups in a molecule and a degree of polymerization of up to 1,200,
(B) 0.1 to 50 parts by weight of an organohydrogenpolysiloxane having at least two hydrogen atoms each attached to a silicon atom of the following average compositional formula (2):
R
2
b
H
c
SiO
(4-b-c)/2
(2)
wherein R
2
is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms, b is a positive number of 0.7 to 2.1, c is a positive number of 0.001 to 1.0, and the sum of b+c is from 0.8 to 3.0, said organohydrogenpolysiloxane being liquid at room temperature,
(C) 40 to 400 parts by weight of aluminum hydroxide,
(D) a catalytic amount of an addition reaction catalyst,
(E) 0.1 to 50 parts by weight of a thixotropic agent, and
(F) 0 to 30 parts by weight of a tackifier.
The composition exhibits a thixotropy corresponding to a sag of up to 1 inch after 1 minute as measured by the Boeing flow jig test.
In another aspect, the invention provides a sealant and a repairing compound, each for use with a polymeric insulator and comprising the silicone rubber composition defined above.
REFERENCES:
patent: 3511698 (1970-05-01), Talcott
patent: 3965065 (1976-06-01), Elliot
patent: 4476155 (1984-10-01), Niemi
patent: 4539367 (1985-09-01), Beers
patent: 4618646 (1986-10-01), Takago et al.
patent: 5019419 (1991-05-01), Matsumoto et al.
patent: 5264603 (1993-11-01), Altes et al.
patent: 5519080 (1996-05-01), Matsuhita et al.
patent: 5674936 (1997-10-01), Lucas
patent: 5691407 (1997-11-01), Azechi et al.
patent: 5973047 (1999-10-01), Ernst et al.
patent: 59-198604 (1984-11-01), None
patent: 7-57574 (1995-03-01), None
Japanese abstract JP-A 53-35982; Apr. 3, 1978.
Azechi Syuuichi
Meguriya Noriyuki
Sekiguchi Susumu
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
Zimmer Marc S.
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