Silicone rubber sponge composition

Details Silicone rubber sponge composition Silicone rubber sponge composition

2001-04-04

2001-10-09

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...

C521S134000, C521S154000, C521S079000, C521S081000

Reexamination Certificate

active

06300384

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a silicone rubber sponge composition, a silicone rubber sponge, and a process for production thereof. More particularly, it relates to a silicone rubber sponge composition that gives a silicone rubber sponge having fine, uniform cells, a silicone rubber sponge, and a process for production thereof.
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. Japanese Patent Publication No. Sho 44-461 and Japanese Patent Application Laying Open No. 7-247436, for example, teach silicone rubber sponge compositions containing thermally decomposable organic blowing agents such as azobisisobutyronitrile. However silicone rubber sponges produced from these compositions frequently have coarse cells, making it difficult to consistently produce silicone rubber sponges having fine, uniform cells. Accordingly, there is a need for a silicone rubber sponge composition having finer, more uniform cells.
It is an object of the present invention to provide a silicone rubber sponge composition that gives a silicone rubber sponge having fine, uniform cells, a silicone rubber sponge, and a process for production thereof.
SUMMARY OF THE INVENTION
The present invention relates to a silicone rubber sponge composition, a silicone rubber sponge, and a process for production thereof. The silicone rubber sponge composition comprises (A) 100 parts by weight organopolysiloxane gum described by average structural unit R
a
SiO
(4−a)/2
, where R is a monovalent hydrocarbon group or haloalkyl and a is 1.8 to 2.3 and having a viscosity at 25° C. of 1,000,000 mPa·s or above, (B) 1 to 400 parts by weight inorganic filler, (C) 0.01 to 50 parts by weight hollow thermoplastic resin particles, (D) 0.1 to 10 parts by weight thermally decomposable blowing agent, 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 organopolysiloxane gum described by average structural unit R
a
SiO
(4−a)/2
, where R is a monovalent hydrocarbon group or haloalkyl and a is 1.8 to 2.3 and having a viscosity at 25° C. of 1,000,000 mPa·s or above,
(B) 1 to 400 parts by weight inorganic filler,
(C) 0.01 to 50 parts by weight hollow thermoplastic resin particles,
(D) 0.1 to 10 parts by weight thermally decomposable blowing agent, 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 silicone rubber sponge composition. A third embodiment of the present invention is a process for production of a silicone rubber sponge composition comprising the steps of: combining components (A) and (B) to produce a silicone rubber base compound, and incorporating components (C), (D), and (E) into the silicone rubber base compound. A fourth embodiment of the present invention is a process for production of a silicone rubber sponge article comprising the step of curing the present composition by heating to a temperature equal to or above the softening point of the thermoplastic resin of component (C).
Component (A) is the principal component of the present composition. Component (A) must have a viscosity at 25° C. of 1,000,000 mPa·s or above, and preferably 5,000,000 mPa·s or above. Since component (A) is a gum, it will have a Williams plasticity of 50 or greater, preferably 100 or greater, and more preferably 120 or greater. 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. The class of compounds known as organopolysiloxane gums used as main ingredients in organic peroxide-curing millable compositions can be used for component (A). Component (A) consists of an organopolysiloxane gum represented by average unit formula R
a
SiO
(4−a)/2
, where R is a monovalent hydrocarbon group or haloalkyl and a is 1.8 to 2.3. Monovalent hydrocarbon groups represented by R include alkyls such as methyl, ethyl, and propyl; alkenyls such as vinyl and allyl; cycloalkyls such as cyclohexyl; aralkyls such as phenylethyl; and aryls such as phenyl and tolyl. Haloalkyl groups represented by R include 3,3,3-trifluoropropyl and 3-chloropropyl.
Where 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 must have at least two silicon-bonded alkenyls. Alkenyl here refers, for example, to vinyl, ally, propenyl, and hexenyl groups. The molecular structure of component (A) may be linear or linear containing branches. 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. Examples of molecular chain terminal endgroups are trimethylsiloxy, dimethylvinylsiloxy, methylvinylhydroxysiloxy, and dimethylhydroxysiloxy groups. Examples of such organopolysiloxane gums include methylvinylpolysiloxane gum that is endblocked at both terminals with trimethylsiloxy groups, a copolymer gum of methylvinylsiloxane and dimethylsiloxane that is endblocked at both terminals with trimethylsiloxy 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 dimethylvinylsiloxy groups, a copolymer gum of methylvinylsiloxane and dimethylsiloxane that is endblocked at both terminals with dimethylhydroxysiloxy groups, a copolymer gum of methylphenylsiloxane, methylvinylsiloxane, and dimethylsiloxane that is endblocked at both terminals with methylvinylhydroxysiloxy groups, and a copolymer gum of (3,3,3-trifluoropropyl)methylsiloxane, 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 rendered hydrophobic 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, carbon black, and the like. In excessively large amounts, component (B) is difficult to incorporate into component (A), therefore the range of 1 to 400 parts by weight per 100 parts by weight of component (A) is preferred. More preferred is using from 1 to 100 parts by weight for the reinforcing fillers and from 1 to 150 parts by weight for the semi-reinforcing or extending fillers, per 100 parts by weight of component (A).
The hollow thermoplastic resin particles of component (C) 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 preferred practice the thermoplastic resin will have a softening point of from 40 to 200° C., and especially 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 an aque

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