Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-05-23
2002-04-02
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S065000, C521S134000, C521S154000
Reexamination Certificate
active
06365640
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a silicone rubber sponge composition, to a silicone rubber sponge, and to a process for production of a silicone rubber sponge. More particularly, it relates to a silicone rubber sponge composition that does not give rise to harmful decomposition products during sponge production and that with curing gives a silicone rubber sponge having fine, uniform cells, to a silicone rubber sponge, and to a process for production of a silicone rubber sponge.
BACKGROUND OF THE INVENTION
Due to their outstanding heat and weather resistance and light weight, silicone rubber sponges are used for automotive parts, such as packings, gaskets, and O-rings; as sheath materials for rollers in copiers, and as sealing materials of various kinds. A number of silicone rubber sponge compositions have been proposed to date. Patent Publication 44-461 and Unexamined Patent Application 7-247436, for example, teach silicone rubber sponge compositions containing thermally decomposing organic blowing agents such as azobisisobutyronitrile. However, these compositions give rise to harmful decomposition products during sponge production and thus pose an environmental problem. Patent Publication 7-122000 proposes a silicone rubber sponge composition consisting of an oil-in water type emulsion of dimethylpolysiloxane, an emulsifier, water, and a thickener. However, this composition does not readily give silicone rubber sponges with uniform cells. Further, since oil-in water type emulsions and silicone rubber compositions are basically immiscible, the process of evenly dispersing the silicone rubber composition throughout the composition is time consuming, and throughput is poor.
It is an object of the present invention to provide a silicone rubber sponge composition that does not give rise to harmful decomposition products during sponge production and that with curing gives a silicone rubber sponge having fine, uniform cells.
SUMMARY OF THE INVENTION
The present invention is a silicone rubber sponge composition, a silicone rubber sponge, and a process for making a silicone rubber sponge. The silicone rubber sponge composition comprises (A) 100 parts by weight of an organopolysiloxane gum described by average structural unit R
a
SiO
(4−a)/2
, where R is a monovalent hydrocarbon radical or haloalkyl and a is 1.95 to 2.05, (B) 1 to 400 parts by weight of an inorganic filler, (C) 0.01 to 50 parts by weight of hollow thermoplastic resin particles (D) a mixture of (a) water-soluble silicone and (b) water in an amount such that the water in the mixture is equivalent to 0.01 to 10 parts by weight per 100 parts by weight of component (A), and (E) a curing agent in an amount sufficient to cure the composition.
DESCRIPTION OF THE INVENTION
A first embodiment of the present invention is a silicone rubber sponge composition comprising
(A) 100 parts by weight of an organopolysiloxane gum described by average structural unit R
a
SiO
(4−a)/2
, where R is a monovalent hydrocarbon radical or haloalkyl and a is 1.95 to 2.05,
(B) 1 to 400 parts by weight of an inorganic filler,
(C) 0.01 to 50 parts by weight hollow thermoplastic resin particles
(D) a mixture of (a) water-soluble silicone and (b) water in an amount such that the water in the mixture is equivalent to 0.01 to 10 parts by weight per 100 parts by weight of component (A), and
(E) a curing agent in an amount sufficient to cure the composition.
A second embodiment of the present invention is a silicone rubber sponge produced by heat curing of the present silicone rubber sponge composition. A third embodiment of the present invention is a process for production of a silicone rubber sponge comprising the step of curing the present composition by heating to a temperature equal to or above the softening point of the thermoplastic resin component of the hollow thermoplastic resin particles of component (C).
Component (A) is the principal component of the present composition and consists of an organopolysiloxane gum described by average unit formula: R
a
SiO
(4−a)/2
, where R is a monovalent hydrocarbon radical or haloalkyl. Examples of the monovalent hydrocarbon radicals include alkyls such as methyl, ethyl, and propyl; alkenyls such as vinyl and allyl; cycloalkyls such as cyclohexyl; aralkyls such as &bgr;-phenylethyl; aryls such as phenyl and tolyl; and haloalkyl groups such as 3,3,3-trifluoropropyl and 3-chloropropyl. In the average unit formula for component (A), a is 1.95 to 2.05.
In the present composition, when the curing agent consists of either an alkyl peroxide or a platinum catalyst used concomitantly with an organopolysiloxane containing silicon bonded hydrogen atoms, the organopolysiloxane gum molecule of component (A) must have at least two silicon-bonded alkenyls. Alkenyl here refers, for example, to vinyl, allyl, propenyl, and hexenyl groups. Silicon-bonded organic groups other than alkenyl include alkyls such as methyl, ethyl, and propyl; aryls such as phenyl and tolyl; and haloalkyls such as 3,3,3-trifluoropropyl and 3-chloropropyl. The molecular structure of component (A) may be linear or linear containing branches. The degree of polymerization of component (A) is typically 3,000 to 20,000, with the weight-average molecular weight being 20×10
4
or above. Component (A) has a Williams plasticity of 50 or greater and preferably 100 or greater, and has the properties of a gum. Component (A) may be a homopolymer, copolymer, or a blend of polymers. Specific examples of the siloxane unit of component (A) are dimethylsiloxane, methylvinylsiloxane, methylphenylsiloxane, and (3,3,3-trifluoropropyl)methylsiloxane units. The molecular chain terminals of component (A) are preferably endblocked by triorganosiloxy or hydroxyl groups. Examples of molecular chain terminal endgroups are trimethylsiloxy, dimethylvinylsiloxy, methylvinylhydroxysiloxy, and dimethylhydroxysiloxy groups. Examples of such organopolysiloxane gums include a copolymer gum of methylvinylsiloxane and dimethylsiloxane that is endblocked at both terminals with dimethylvinylsiloxy groups, dimethylpolysiloxane gum that is endblocked at both terminals with dimethylvinylsiloxy groups, a copolymer gum of methylvinylsiloxane and dimethylsiloxane that is endblocked at both terminals with dimethylhydroxysiloxy groups, and a copolymer gum of methylvinylsiloxane and dimethylsiloxane that is endblocked at both terminals with methylvinylhydroxysiloxy groups.
Examples of the inorganic filler of component (B) are reinforcing fillers such as finely divided silica (e.g. dry process silica or wet process silica) and finely divided silica whose surfaces have been treated with an organochlorosilane, organoalkoxysilane, hexaorganodisilazane, organosiloxane oligomer, or the like; and semi-reinforcing or extending fillers such as powdered quartz, diatomaceous earth, heavy calcium carbonate, light calcium carbonate, magnesium oxide, calcium silicate, mica, aluminum oxide, aluminum hydroxide, and carbon black. In excessively large amounts component (B) is difficult to incorporate in component (A) and accordingly the range 1 to 400 parts by weight per 100 parts by weight of component (A) is preferred. Even more preferred is an amount of component (B) of 1 to 150 parts by weight per 100 parts by weight of component (A).
The hollow thermoplastic resin particles used as component (C) in the present composition serve as nuclei for cell formation and also make cell distribution uniform. An exemplary component (C) is a material consisting of thermoplastic resin shells having an inert gas enclosed therein. Thermoplastic resins include silicone resins, acrylic resins, and polycarbonate resins. In the preferred practice, the thermoplastic resin has a softening point of from 40 to 200° C. and preferably 60 to 180° C. Inert gases include air, nitrogen gas, helium gas, and the like. Component (C) average particle size is preferably within the range of 0.1 to 500 &mgr;m, and more preferably 1 to 50 &mgr;m. Component (C) may be prepared, for example, by spraying a di
Baba Katsuya
Honma Hiroshi
Nakamura Akito
Suto Manabu
Cesare James L. De
Dow Corning Toray Silicone Company, Ltd.
Foelak Morton
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