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
1999-05-17
2001-05-15
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...
C524S262000, C524S264000, C524S265000, C524S266000, C524S493000, C524S588000, C523S203000, C523S209000, C523S212000
Reexamination Certificate
active
06232387
ABSTRACT:
This invention relates to a silicone rubber composition which on heat curing provides silicone rubber having sufficiently improved properties to serve as high-voltage electrical insulators.
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. 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 porcelain 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 198604/1984 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-B 35982/1978 corresponding to U.S. Pat. No. 3,965,065 and JP-A 209655/1992 corresponding to U.S. Pat. No. 5,369,161 disclose 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.
However, the silicone rubber compositions mentioned above are not yet fully satisfactory in high-voltage electrical insulation under rigorous conditions. Silicone rubber compositions loaded with large amounts of aluminum hydrate have a higher moisture pickup than unloaded silicone rubber since aluminum hydrate itself is hygroscopic. Thus the loaded compositions lose electrical properties in humid or wet conditions. The moisture pickup gives rise to another problem that the corona resistance required for high-voltage electrical insulators is lost. This problem cannot be solved simply by surface treating aluminum hydrate with chemical agents. There is a desire to solve this and other problems.
SUMMARY OF THE INVENTION
An object of the invention is to provide a silicone rubber composition which cures into silicone rubber having sufficiently improved properties to serve as high-voltage electrical insulators, such as weather, stain, voltage, tracking, arc and erosion resistance even under air polluted conditions or rigorous weather conditions, especially under humid conditions.
It has been found that when a mixture of at least two aluminum hydroxides each surface treated with silicon-containing compound and having different mean particle sizes, especially a mixture of a first aluminum hydroxide surface treated with a silicon-containing compound and having a mean particle size of 5 to 20 &mgr;m and a second aluminum hydroxide surface treated with a silicon-containing compound and having a mean particle size of 0.1 to 2.5 &mgr;m is blended in a silicone rubber composition comprising an organopolysiloxane of the following average compositional formula (1), finely divided silica, and an organic peroxide, the aluminum hydroxide is prevented from absorbing moisture, and the problem of corona resistance which is difficult to solve simply by surface treating aluminum hydroxide with a chemical agent can be satisfactorily solved. The resulting silicone rubber composition cures into silicone rubber which exhibits sufficiently improved high-voltage electrical insulating properties, such as weather, stain, voltage, tracking, arc and erosion resistance even when exposed under air polluted conditions or rigorous weather conditions, especially under humid conditions, for a long period of time.
The invention provides a silicone rubber composition for use as a high-voltage electrical insulator, comprising
(A) 100 parts by weight of an organopolysiloxane of the following average compositional formula (1):
R
1
n
SiO
(4−n)/2
(1)
wherein R
1
, which may be the same or different, is a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.98 to 2.02,
(B) 1 to 100 parts by weight of finely divided silica,
(C) 50 to 300 parts by weight of a mixture of at least two aluminum hydroxides each surface treated with silicon-containing compound and having different mean particle sizes, and
(D) 0.01 to 10 parts by weight of an organic peroxide.
DETAILED DESCRIPTION OF THE INVENTION
A first essential composition of the silicone rubber composition for use as high-voltage electrical insulators according to the invention is an organopolysiloxane of the following average compositional formula (1):
R
1
n
SiO
(4−n)/2
(1)
wherein R
1
, which may be the same or different, is a substituted or unsubstituted monovalent hydrocarbon group and n is a positive number of 1.98 to 2.02.
In formula (1), R
1
represents substituted or unsubstituted monovalent hydrocarbon groups bonded to silicon atoms, preferably of 1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms. Included are unsubstituted monovalent hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, hexyl, and octyl; cycloalkyl groups such as cyclohexyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, and hexenyl; aryl groups such as phenyl and tolyl; and aralkyl groups such as benzyl, phenylethyl, and phenylpropyl; as well as substituted monovalent hydrocarbon groups in which some or all of the hydrogen atoms attached to carbon atoms in the foregoing groups are substituted with halogen atoms, cyano groups, etc., for example, halogen- and cyano-substituted alkyl groups such as chloromethyl, bromoethyl, trifluoropropyl, and cyanoethyl. The substituents represented by R
1
may be identical or different.
It is preferred that 0.001 to 5 mol %, especially 0.01 to 1 mol % of all the R
1
groups in a molecule is an alkenyl group. The remaining is a methyl or phenyl group. In this case, at least 95 mol %, especially at least 99 mol % of all the R
1
groups is preferably a methyl group.
No particular limits are imposed on the molecular structure of the organopolysiloxane of formula (1) although those blocked with triorganosilyl groups such as trimethylsilyl group, dimethylvinylsilyl group, divinylmethylsilyl group and trivinylsilyl group at the end of their molecular chain are preferred. Basically, linear organopolysiloxanes in which the main chain of the molecule consists essentially of the recurrence of diorganosiloxane units are preferable although the linear organopolysiloxanes may contain a small amount of mono-organosiloxane units and branched siloxane units such as SiO
2
units in a molecule, and a mixture of two or more organopolysiloxanes having different molecular structures is acceptable.
The organopolysiloxane preferably has an average degree of polymerization (or the number of silicon atoms in a molecule) of about 100 to about 100,000, especially about 4,000 to about 20,000, and a viscosity of at least 100 centistokes at 25° C., especially 100,000 to 10,000,000 centistokes at 25° C.
A second component (B) of the silicone rubber composition is finely divided silica which is essential to produce silicone rubber having improved mechanical strength. To this end, silica should preferably have a specific surface area of at least about 50 m
2
/g, more preferably about 50 to 500 m
2
/g, especially about 100 to 300 m
2
/g as measured by the BET method. When silica with a specific surface area of less than 50 m
2
/g is used, some cured parts would have poor mechanical strength.
Examples of such reinforcing silica include fumed silica and precipitated silica, which may be surface treated to be hydrophobic with such chemical agent
Azechi Syuuichi
Meguriya Noriyuki
Sekiguchi Susumu
Yoshida Takeo
Dawson Robert
Robertson Jeffrey B.
Shin-Etsu Chemical Co. , Ltd.
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