Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
1999-02-26
2001-09-25
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C524S493000, C524S847000, C524S860000
Reexamination Certificate
active
06294636
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a silicone rubber composition and to a method for manufacturing products formed from the silicone rubber composition. More specifically, the invention relates to a silicone rubber composition suitable for forming tubes, sheets, electric coating materials, or similar extruded products, as well as to a method for manufacturing products formed from the silicone rubber composition.
BACKGROUND OF THE INVENTION
Silicone rubber is frequently used for the extrusion molding of tube, tape, sheet, wire and cable coatings, and the like because silicone rubber offer an excellent heat resistance, cold resistance, weathering resistance, and electrical characteristics. 2,4-Dichlorobenzoyl peroxide, ortho-chlorobenzoyl peroxide, and similar chlorinated benzoyl peroxides have been used as a curing agents for curing silicone rubber compositions used as the feedstock in these extrusion molding applications. However, various problems are associated with silicone rubber compositions that contain these chlorinated benzoyl peroxides: they evolve an unpleasant odor when cured, their cure yields moldings that have a sticky, or tacky surface, and blooming occurs onto the surface of their silicone rubber moldings. Blooming is the phenomenon of whitening of the surface as a result of gradual deposition of decomposition products of the curing agent on the surface of the molded product.
Japanese Patent Application 59-18758 teaches a solution to the above problems by using bis-(ortho-methylbenzoyl) peroxide as a curing agent. Japanese Application 62-185750 teaches a method in which a curing agent is bis-(para-methylbenzoyl) peroxide. However, in the course of curing, silicone rubber compositions which contain the aforementioned methyl-substituted benzoyl peroxides as curing agents, are subject to the formation of voids. This often produces a product formed from such silicone rubber that is has non-uniform physical properties, or causes spark-outs of insulated wires or a decrease in voltage required for insulation breakdown.
It is an object of the present invention to provide a silicone rubber composition which has significantly reduced deviations in the physical properties caused by voids in the products formed from the silicone rubber, as well by significantly reduced spark-outs of insulated wires and little decrease in voltage required for insulation breakdown.
SUMMARY OF THE INVENTION
The present invention is a curable composition comprising:
(A) 100 parts by weight of a polyorganosiloxane gum;
(B) 10 to 100 parts by weight of microparticulate silica; and
(C) 0.05 to 10 parts by weight of a methyl-substituted benzoyl peroxide having an average diameter of particles less than or equal to about 30 &mgr;m and a maximum diameter of particles less than or equal to about 60 &mgr;m. When extruded and cured, the resulting silicone rubber has significantly reduced deviations in the physical properties caused by voids in the products formed from the silicone rubber, as well by significantly reduced spark-outs of insulated wires and little decrease in voltage required for insulation breakdown.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a silicone rubber composition comprising:
(A) 100 parts by weight of a polyorganosiloxane gum represented by the following average structural formula: R
a
SiO
(4−a)/2
(where R is a substituted or non-substituted monovalent hydrocarbon, and a is a number between 1.8 and 2.3);
(B) 10 to 100 parts by weight of microparticulate silica; and
(C) 0.05 to 10 parts by weight of a methyl-substituted benzoyl peroxide having an average diameter of particles less than or equal to about 30 &mgr;m and a maximum diameter of particles less than or equal to about 60 &mgr;m.
Component (A), a polyorganosiloxane gum, is the main component of the present composition and is represented by the following average structural formula: R
a
SiO
(4−a)/2
, where R is a substituted or non-substituted monovalent hydrocarbon, which is represented by the following specific examples: a methyl group, an ethyl group, a propyl group, or a similar alkyl group, a cyclohexyl group such as cycloalkyl group; a vinyl group, an allyl group, a butynyl group, a hexenyl group, or a similar alkenyl group; a phenyl group, a tolyl group, or a similar aryl group; a 3,3,3-trifluoropropyl group, a 2-phenylethyl group, a 2-cyanoethyl group, or a similar substituted hydrocarbon group. In the above formula, a is a number between 1.8 and 2.3. It is preferable that Component (A) have a linear molecular structure, however, a partially-branched structure is acceptable. Component (A) may have a degree of polymerization within the range suitable for use in connection with conventional polyorganosiloxane gums known in the art. Normally, Component (A) has a viscosity exceeding 1×10
6
mPa·s at 25° C. and a number average molecular weight exceeding 25×10
4
.
Component (B), a microparticulate silica, is used for imparting improved mechanical properties to a silicone rubber obtained by curing the composition. Such a microparticulate silica may be a fumed silica or a similar dry-process silica, or a precipitation silica or a similar wet-process silica. The microparticulate silica may have a surface which has been subjected to hydrophobic treatment with organochlorosilane, hexaorganodisilazane, diorganocyclopolysiloxane, or a similar organic silicon compound. Preferably, Component (B) has a specific surface area exceeding 50 m
2
/g. Component (B) is used in an amount of 10 to 100 parts by weight for every 100 parts by weight of Component (A). If Component (B) is used in an amount of less than 10 parts by weight, the product will be obtained with a reduced mechanical strength, and if the amount exceeds 100 parts by weight, it is difficult to compound Component (B) with Component (A).
Component (C) of the composition is a methyl-substituted benzoyl peroxide, used as a curing agent for thermal curing of the composition. Component (C) is a distinguishing feature of the present composition. In the compositions of this invention, Component (C) comprises grains with a maximum diameter of less than or equal to about 60 &mgr;m, preferably less than or equal to about 50 &mgr;m, and even more preferably, less than or equal to about 40 &mgr;m. The average diameter of the grains in Component (C) is less than or equal to about 30 &mgr;m, and preferably, is less than or equal to about 20 &mgr;m, and even more preferably, is less than or equal to 15 &mgr;m. If the maximum diameter of the grains exceeds about 60 &mgr;m, this will cause either the appearance of voids leading to deviations in physical characteristics of the product formed from the silicone rubber, or will increase the frequency of spark-outs in the insulated electric wires. If the average diameter of particles exceeds about 30 &mgr;m, this will reduce the voltage required for insulation breakdown in the insulated wires. The aforementioned maximum and average diameters of grains are determined in accordance with a method for testing grains as specified by item 4.4 of JIS K 5400 (Japanese Industrial Standard) for coating materials. The following are examples of compounds suitable for use as Component (C): bis(ortho-methylbenzoyl) peroxide, bis(meta-methylbenzoyl) peroxide, bis(para-methylbenzoyl) peroxide, or a similar monomethylbenzoyl peroxide, bis(2,4-dimethylbenzoyl) peroxide, or a similar dimethylbenzoyl peroxide, bis(2,4,6-trimethylbenzoyl) peroxide, or a similar trimethylbenzoyl peroxide. Component (C) is added in an amount of 0.05 to 10 parts by weight, preferably 0.1 to 5 parts by weight for each 100 parts by weight of Component (A).
Although the present composition is a silicone rubber composition consisting of Components (A) through (C), it may be combined with various conventional additives used in conjunction with known silicone rubber compositions, provided that these additives are used in quantities which do not interfere with achieving objectives of the present invention. Such additives may in
Baba Katsuya
Hirai Kazuo
Matsushita Takao
Boley William F.
Dawson Robert
Dow Corning Toray Silicone Company, Ltd.
Robertson Jeffrey B.
Warren Jennifer S.
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