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.
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.
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 RaSiO(4xe2x88x92a)/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.
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 RaSiO(4xe2x88x92a)/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: RaSiO(4xe2x88x92a)/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 xcex2-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 20xc3x97104 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 200xc2x0 C. and preferably 60 to 180xc2x0 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 xcexcm, and more preferably 1 to 50 xcexcm. Component (C) may be prepared, for example, by spraying a dispersion of water with a thermoplastic resin dissolved in a solvent from a spray nozzle into a heated air stream and evaporating the organic solvent while granulating the thermoplastic resin. Component (C) is included in the present composition in an amount of 0.01 to 50 parts by weight and preferably 0.1 to 40 parts by weight per 100 parts by weight of component (A).
Component (D) is a characterizing feature of the present composition and is essential in terms of creating fine, uniform cells. Component (a) of component (D) can be any type of water-soluble silicone. Examples of water-soluble silicones are polyoxyalkylene-modified silicone oils, aminoalkyl group-containing silicone oils, amide group-containing silicone oils, carbinol group-containing silicone oils, and the like. Polyoxyalkylene-modified silicone oils are especially preferred. Examples of such polyoxyalkylene-modified silicone oils are organopolysiloxanes of the following average molecular formulas having polyoxyalkylene groups on side chains or terminals. 
In the formula x and y are integers of 1 or greater, z is 0 or an integer of 1 or greater, A is an organic group described by general formula xe2x80x94(CH2)axe2x80x94Oxe2x80x94(C2H4O)p(C3H6O)qR, where a is an integer from 1 to 3, p is an integer of 1 or greater; q is 0 or an integer of 1 or greater, and R is hydrogen or C1-4 alkyl such as methyl, ethyl, or propyl; and B is an organic group described by general formula xe2x80x94(CH2)nxe2x80x94CH3, where n is an integer of 0 or greater. 
(In the formula, x and A are defined as above.) 
(In the formula, x, y, and A are defined as above.)
To achieve good water solubility, the polyoxyalkylene-modified silicone oil preferably has polyoxyethylene or a polyoxyethylene/polyoxypropylene copolymer as the polyoxyalkylene moiety, with the content thereof being 50% by weight or more of the molecule.
The water of component (a) of component (D) can be any type of xe2x80x9chighly purexe2x80x9d water, such as distilled water, purified water, or deionized water.
Component (D) may be readily prepared by mixing component (a) and component (b). To improve the ease of handing and ease of incorporation it is permissible to add a thickener such a powdered silica, provided that the objects of the invention are not impaired thereby.
Component (D) is used in an amount in the present composition such that the amount of component (b) is equivalent to 0.01 to 10 parts by weight, preferably 0.05 to 5 parts by weight per 100 parts by weight of component (A). When the amount of component (b) is less than 0.05, the ability to form fine, uniform cells suffers, while amounts exceeding 5 parts by weight can result in problems such as nonuniform cell formation.
Component (E), the curing agent, is an organic peroxide or a platinum catalyst and an organopolysiloxane containing silicon bonded hydrogen. Examples of the former type curing agent, namely organic peroxides, include benzoyl peroxide, t-butyl perbenzoate, o-methyl benzoyl peroxide, p-methyl benzoyl peroxide, m-methyl benzoyl peroxide, dicumyl peroxide, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane. The amount of the organic peroxide is preferably 0.1 to 10 parts by weight per 100 parts by weight of component (A).
Of the latter type curing agent, namely a platinum catalyst and an organopolysiloxane containing silicon bonded hydrogen, examples of the platinum catalyst are finely divided platinum, platinum black, chloroplatinic acid, alcohol-modified chloroplatinic acid, chloroplatinic acid olefin complexes, chloroplatinic acid/alkenylsiloxane complexes, and chloroplatinic acid/divinyltetramethyldisiloxane complexes. In preferred practice, the amount of the platinum catalyst, expressed as metallic platinum, is from 0.1 to 500 ppm (weight basis) of the total composition. Organopolysiloxanes containing silicon bonded hydrogen are crosslinking agents, and in the presence of platinum catalysts react with the alkenyl groups in component (A) to cure the present compositions. Examples of organopolysiloxanes containing silicon bonded hydrogen are methylhydriopolysiloxane endblocked at both terminals with trimethylsiloxy groups, a copolymer of methylhydriosiloxane and dimethylsiloxane endblocked at both terminals with trimethylsiloxy groups, a copolymer of methylhydriosiloxane and dimethylsiloxane endblocked at both terminals with dimethylhydriosiloxy groups, and tetramethyltetrahydriocyclotetrasiloxane. In preferred practice when the curing agent is a platinum catalyst and an organopolysiloxane containing silicon bonded hydrogen, the amount of the curing agent will be such that the molar ratio of silicon bonded hydrogen in the curing agent to alkenyl groups in component (A) is 0.5:1 to 10:1. Compounds known in the art as agents for regulating the catalytic activity of platinum catalysts, such as 1-ethynyl-cyclohexanol, 3-methyl-1-penten-3-ol, or benzotriazole, may be added as well.
The present composition comprises components (A) to (E) as described above. In addition, additives known in the art for inclusion in silicone rubber sponge composition may be included as well provided that the objects of the invention are not impaired thereby. Examples of such additives include heat stability agents such as iron oxide, cerium oxide, and fatty acid cerium salts; flame retardants such as manganese carbonate, zinc carbonate, and fumed titanium dioxide; pigments such as red iron oxide and titanium dioxide; and silicone oils such as dimethylsilicone oil and methylphenylsilicone oil.
The present composition can be easily prepared by mixing components (A) to (E) plus any other ingredients that may be required. In preferred practice, component (A) is premixed with component (B) to produce a silicone rubber base compound, to which are then added components (C), (D), and (E). Examples of production equipment are kneader mixers, continuous kneader extruders, and other mixing or blending units.
Silicone rubber sponges may be produced from the present composition by heating to a temperature above the softening point of the thermoplastic resin of the hollow thermoplastic resin particles of component (B) and curing. Silicone rubber sponges are formed by blowing and curing of the composition of the invention. Silicone rubber sponges produced in this way have fine, uniform cells and excellent mechanical strength, making them useful as construction materials, airtight retaining gaskets, fire resistant gaskets, sealing materials, O-rings, and cushioning materials, as well as sheath materials for rollers in copiers and the like.