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
1999-12-15
2001-05-29
Moore, Margaret G. (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...
C525S430000, C525S532000
Reexamination Certificate
active
06239205
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to a silicone rubber composition superior in heat resistance, and, in particular, relates to a silicone rubber composition superior in storage stability during the silicone rubber base compound stage and, upon curing, is capable of producing silicone rubber that has a low degree of thermal degradation and is superior in heat resistance.
The technique of admixing cerium oxide powder into silicone rubber as a heat stabilizer is known in the art (for example, see Japanese Kokai (Unexamined) Patent Publication No. Sho 52(1977)-14654, Japanese Kokai (Unexamined) Patent Publication No. Sho 60(1985)-173050, and Japanese Kokai (Unexamined) Patent Publication No. Hei 07(1995)-11010). Incidentally, it is commonly believed that it is difficult to compound cerium oxide powder with silicone rubber compositions in a uniform manner. The term “silicone rubber compositions” here means silicone rubber compositions prepared in such a manner that a silicone rubber base compound is first made by mixing diorganopolysiloxane polymer, reinforcing fillers, and, if necessary, various additives used for improving the physical properties of the silicone rubber composition, and the silicone rubber base compound is then stored for future use, allowing for curing by admixing a curing agent at the time of molding articles from the silicone rubber. Compounding the cerium oxide powder with the silicone rubber composition in a uniform manner during the stage of silicone rubber base compound preparation presents certain difficulties, however. For example, the problem that arises when a cerium oxide powder with a large specific surface area and a small particle size is added and kneaded during the stage of silicone rubber base compound preparation is that heat is released and the viscosity of the silicone rubber base compound itself increases, resulting in gelling in severe cases. Furthermore, the problem is that silicone rubber base compounds with cerium oxide powder added thereto have inferior storage properties, and when stored over an extended period of time, their viscosity increases while their rollability characteristics decrease, with gelling occurring in severe cases.
As a result of in-depth investigations aimed at eliminating the above described problems the inventors arrived at the present invention by discovering that a silicone rubber base compound obtained by compounding a special cerium oxide powder is superior in storage stability and that, moreover, silicone rubber obtained by curing a silicone rubber composition produced by compounding a curing agent with the silicone rubber base compound having such a special cerium oxide powder compounded therewith exhibits a low degree of thermal degradation under high temperature conditions exceeding 200° C.
Namely, it is an object of the present invention to provide a silicone rubber composition that is capable of producing silicone rubber superior in storage stability during the silicone rubber base compound stage, and, upon curing, exhibiting a low degree of thermal degradation under high temperature conditions and is superior in heat resistance.
SUMMARY OF INVENTION
A silicone rubber composition characterized by comprising about 0.05 to 10 wt % cerium oxide powder having a BET specific surface area of not less than 80 m
2
/g and a pH value measured of a 3 wt % aqueous dispersion thereof of about pH 2 to 5.5.
DESCRIPTION OF INVENTION
The silicone rubber composition of the present invention is characterized by comprising about 0.05 to 10 wt % cerium oxide powder having a BET specific surface area of not less than 80 m
2
/g and a pH value for a 3 wt % aqueous dispersion thereof of about pH 2 to 5.5. Here, the term “cerium oxide powder” means powder made from a compound represented by the formula: CeO
&agr;
where &agr; is about 0.2 to 2.1, including the so-called hydrated compounds with a small amount of crystallization water bound therein. The cerium oxide powder used in the present invention is such that its BET specific surface area is not less than 80 m
2
/g, preferably, not less than 100 m
2
/g, and even more preferably, not less than 140 m
2
/g. In addition, the cerium oxide powder is such that the pH value of a 3 wt % aqueous dispersion thereof is within the range of from about 2 to 5.5, preferably, within the range of about 2 to 5.0, and even more preferably, within the range of about 3 to 5.0. The pH value of a 3 wt % aqueous dispersion of the cerium oxide powder is obtained by measuring with a pH meter the pH value of the supernatant obtained by subjecting 3 parts by weight of cerium oxide powder and 97 parts by weight of water to agitation for 15 minutes using a stirrer and then allowing the mixture to stand for 1 hour.
The cerium oxide powder of the present composition can be produced by contacting, for example, carbon dioxide, carbonic acid, acetic acid, formic acid, nitric acid, sulfuric acid, and phosphoric acid with ordinary calcined cerium oxide powder. Carbon dioxide, carbonic acid, and acetic acid are preferable as the acids used for acid treatment.
The composition of the present invention is a silicone rubber composition containing about 0.05 to 20 wt % of the above described treated cerium oxide powder. Well-known conventional silicone rubber compositions can be utilized as the silicone rubber composition used here. There are no particular limitations concerning its curing mechanism, which may be exemplified by organic peroxide curing, hydrosilylation reaction curing, and condensation reaction curing. Among them, the silicone rubber composition shown hereinbelow is preferable:
(A) 100 parts by weight of alkenyl-containing diorganopolysiloxane,
(B) about 0.1 to 10 parts by weight cerium oxide powder having a BET specific surface area of not less than 80 m
2
/g and a pH value of a 3 wt % aqueous dispersion thereof in a range of about pH 2 to 5.5,
(C) about 5 to 150 parts by weight of a reinforcing filler, and
(D) a curing agent in an amount sufficient to effect curing of the composition.
To explain the composition in greater detail, the diorganopolysiloxane of component (A) is the main component of the composition of the present invention, and usually has at least 2 silicon-bonded alkenyl groups in one molecule. The alkenyl groups are exemplified by vinyl groups, alkyl groups and hexenyl groups. The molecular structure of component (A) is substantially linear, but part of the molecular chain may have some branching. Other organic groups which can be substituted on the silicon atoms of the diorganopolysiloxane include methyl, ethyl, propyl, butyl, octyl and other alkyl groups; phenyl, tolyl, and other aryl groups; 3,3,3-trifluoropropyl, 3-chloropropyl, and other halogenated alkyl groups; and 3-cyanoalkyl groups. The diorganopolysiloxane is exemplified by dimethylpolysiloxane endblocked with dimethylvinylsiloxy groups; a copolymer of methylvinylsiloxane and dimethylsiloxane endblocked with dimethylvinylsiloxy groups; a copolymer of methylphenylsiloxane and dimethylsiloxane endblocked with dimethylvinylsiloxy groups; a copolymer of methylphenylsiloxane, methylvinylsiloxane, and dimethylsiloxane endblocked with dimethylvinylsiloxy groups; a copolymer of diphenylsiloxane and dimethylsiloxane endblocked with dimethylvinylsiloxy groups; a copolymer of diphenylsiloxane, methylvinylsiloxane, and dimethylsiloxane endblocked with dimethylvinylsiloxy groups; a copolymer of methyl(3,3,3-trifluoropropyl)siloxane and dimethylsiloxane endblocked with dimethylvinylsiloxy groups; a copolymer of methyl(3,3,3-trifluoropropyl)siloxane, methylvinylsiloxane, and dimethylsiloxane endblocked with dimethylvinylsiloxy groups; methylvinylpolysiloxane endblocked with trimethylsiloxy groups; a copolymer of methylvinylsiloxane and dimethylsiloxane endblocked with trimethylsiloxy groups; a copolymer of methylphenylsiloxane and methylvinylsiloxane endblocked with trimethylsiloxy groups; a copolymer of methylphenylsiloxane, methylvinylsiloxane, and dimethylsiloxane endblocked with trimethylsiloxy groups; a copolymer o
Fujiwara Kazuhito
Hasegawa Chiichiro
Tokimatsu Junsuke
Boley William F.
Dow Corning Toray Silicone Co.
Moore Margaret G.
Warren Jennifer S.
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