WO2004044055A2

WO2004044055A2 - Silicone rubber composition

Info

Publication number: WO2004044055A2
Application number: PCT/JP2003/014551
Authority: WO
Inventors: Katsuya Baba; Kazuo Hirai; Hiroshi Honma; Akito Nakamura
Original assignee: Dow Corning Toray Silicone Co., Ltd.
Priority date: 2002-11-14
Filing date: 2003-11-14
Publication date: 2004-05-27
Also published as: WO2004044055A3; AU2003279575A8; AU2003279575A1; JP2004161930A

Abstract

A silicone rubber composition that demonstrates excellent flowability during molding, especially during extrusion molding comprising (A) 100 parts by weight of a diorganopolysiloxane gum; (B) 10 to 100 parts by weight of reinforcing silica filler, (C) expanded perlite powder in an amount of 5 to 200 % of the weight of component (B); and (D) a curing agent in an amount sufficient to cure the composition. (D) is preferably selected from the group consisting of (D1) an organic peroxide and a combination of (D2) an organohydrogenpolysiloxane and (D3) a platinum catalyst.

Description

DESCRIPTION

SILICONE RUBBER COMPOSITION

Technical Field

[0001] The present invention relates to a silicone rubber composition, in particular, to a silicone rubber composition, which during forming, especially during extrusion, demonstrates excellent flowability and extrudability.

Background Art

[0002] Silicone rubber is characterized by excellent heat-resistant, frost-resistant, and weather-proof properties, along with good softness and electrical insulation characteristics. It is used for manufacturing elongated parts such as silicone-rabber tubes, tapes, round and rectangular rods, electrical insulation coatings, etc., which are normally produced by extrasion. Japanese Unexamined Patent Application Publication (hereinafter referred to as Kokai) H4-59873 discloses a silicone rubber composition for extrusion molding comprising an alkenyl-containing organopolysiloxane gum, fine silica powder, an organohydrogenpolysiloxane and a platinum catalyst. Kokai 2001-181505 describes a silicone composition suitable for extrasion comprising an organopolysiloxane gum, fine silica powder, a boric acid or a metaboric acid, and a curing agent. Kokai 2001-342346 describes another silicone rabber composition for extrasion molding that comprises an organopolysiloxane gum, reinforcing silica filler, diatomaceous earth or quartz powder, and a specific organic peroxide. [0003] None of the above described prior art sought to improve the flowability of the composition during extrusion. Summary of the Invention [0004] Based on the results of studies aimed at the development of silicone rubber compositions with improved flowability, the inventors arrived at the present invention by finding that the flowability of the aforementioned compositions may be improved by using an expanded perlite powder as an extending filler. More specifically, it is an object of the present invention to provide a silicon rubber composition, which during forming, [0005] In accordance with the present invention there are provided a silicone rabber composition comprising

(A) 100 parts by weight of a diorganopolysiloxane gum; (B) 10 to 100 parts by weight of reinforcing fine silica powder;

(C) expanded perlite powder in an amount of 5 to 200 weight.% of the weight of component (B); and

(D) a curing agent in an amount sufficient to cure the composition, use of the aforementioned silicone rabber composition for forming articles by extrasion, a cured silicone rubber article made by curing the aforementioned silicone rubber, and use of expanded perlite powder to improve the flowability of a silicone rabber composition comprising

(A) 100 parts by weight of a diorganopolysiloxane gum;

(B) 10 to 100 parts by weight of reinforcing fine silica powder; and (D) a curing agent in an amount sufficient to cure the composition; in which expanded perlite powder is provided in an amount of from 5 to 200 weight.% of the weight of component (B).

Best Mode for Carrying Out the Invention [0006] Component (A) is a main component of the silicone rubber composition of the present invention. Although a linear molecular structure is preferable, a partially-branched linear structure is allowable. Preferably component (A) has an average degree of polymerization (dp) within the range of from 3,000 to 20,000 and a Williams plasticity, as measured in accordance with Japanese Industrial Standard (JIS) K6249-1997, of from 50 to

250. [0007] The polymeric backbone of component (A) comprises siloxane units, of the formula,

R₂SiO _2/2 and in the case of partial branching includes additional siloxane units of the formula

R-SiO _3/2. Each R group may be the same or different and may be represented by a substituted or unsubstituted monovalent hydrocarbon, for example, an alkyl group such as methyl, ethyl, propyl, butyl, octyl, or isopropyl group; an aryl group such as, phenyl or tolyl; an alkenyl group such as vinyl, allyl, butenyl , hexenyl or heptenyl group of which vinyl and/or hexenyl groups are most preferred; a halogenated alkyl group such as 3-chloropropyl or 3,3,3-trifluoropropyl group. R groups other than alkenyl groups may be the same or different and preferably comprise alkyl groups, typically methyl groups, alone or in combination with phenyl groups. [0008] Hence, each siloxane unit, of the formula R₂SiO 2/2 , may be the same or different and may for example comprise dialkylsiloxane units such as dimethylsiloxane units, methylvinylsiloxane units, methylhexenylsiloxane units, methylphenylsiloxane units, diphenylsiloxane units and/or methyl-(3,3,3-trifluoroproρyl)-siloxane units. [0009] The siloxane units that form molecular terminals of component (A) have the general formula R^SiO wherein each R¹ may be the same or different and may be R, an hydroxy group or an alkoxy group such as, for example, methoxy, ethoxy, propoxy, butoxy, t-butoxy or i-propyloxy group. Hence, R^] ₃SiO -_# may, for example comprise trimethylsiloxy units, dimethylhydroxysiloxy units, dimethylalkenylsiloxy units and/or dimethylmethoxysiloxy units. [0010] There are no special limitations with regard to the bonding positions of alkenyl groups in the molecule, and these groups may be located at the molecular terminals, in side molecular chains, or in both positions simultaneously. However, when, as shall be further explained below, the curing agent, component (D), is a non-acyl type organic peroxide or is the combination of an organohydrogenpolysiloxane (hereafter referred to as component E) and a platinum-type catalyst (hereafter referred to as component F), each molecule of component (A) must comprise at least 2 alkenyl groups.

[0011] Component (B), a reinforcing fine silica powder makes the silicone rubber composition of the present invention suitable for milling and imparts excellent mechanical strength to silicone rabber obtained by curing the composition in accordance with the present invention. Preferably the specific surface area of component (B) exceeds 50 m²/g.

Component (B) may comprise one or more of fumed silica (dry-process silica), precipitated silica (wet-process silica), silica aerogel or a mixture of any two or more thereof. [0012] In order to facilitate mixing with component (A) and to improve flowability of the silicone composition during extrasion, component (B) may be hydrophobically pre-treated prior to mixing with the other components in the composition or may be added to the composition in an untreated form together with hydrophobic treatment agents, such as a silanol-capped dimethylsiloxane oligomer, silanol-capped methylvinylsiloxane oligomer, silanol-capped methylphenylsiloxane oligomer, diphenylsilane diol, hexamethyldisilazane, cyclic dimethylsiloxane oligomer (e.g., octamethylcyclotetrasiloxane), trimethylchlorosilane and/or dimethyldichlorosilane. The hydrophobic treatment agent should be added in an amount sufficient for imparting hydrophobicity to component (B). It is difficult to give a common recommendation for the molecular weight and/or the amount of silanol groups in the hydrophobic treatment agent, since these characteristics will depend on the content of silanol groups and on the specific surface area of component (B), but in general the content of the hydrophobic treatment agent should be within the range of 1 to 40 wt.%, preferably 2 to 25 wt.%, based on the weight of component (B). [0013] Component (B) is used in the composition in an amount of 10 to 100 parts by weight, preferably 15 to 60 parts by weight, for each 100 parts by weight of component (A).

This is because, if component (B) is used in an amount less than 10 parts by weight, the silicone rabber will have insufficient mechanical strength. If, on the other hand, the amount of component (B) exceeds 100 parts by weight, it would be more difficult to mix it with component (A).

[0014] The introduction of Component (C), the expanded perlite powder is the main reason for the flowability properties observed in accordance with the present invention, in particular the introduction of component (C) causes a significant improvement in flowability of the composition during forming, especially by extrasion.

[00l5] Component (C) is preferably obtained by heating the perlite raw material, crashing it after expansion in order to break the resulting hollow structures, and sorting the powder particles by dimensions. The particles are irregularly shaped, but it was found that when their average dimension was too big, the silicone rubber lost its strength. Preferably, therefore the average dimension of the particles does not exceed 50 μm, preferably 30 μm and even more preferably 15 μm. The inventors noted that whilst the use of perlite resulted in improved flowability, other physical properties had poorer other physical properties. Whilst such compositions may be prevented from use for some applications they may, for example be used for fabricating silicone rabber moldings which do not require such good physical properties for their respective use. It is recommended to use component (C) in an amount of from 5 to 200 wt.%, preferably 10 to 100 wt.%, and even more preferably 15 to 60 wt.% based on the weight of component (B).

[0016] Component (D), the curing agent may comprise component (Dl) an organic peroxide which causes curing of the silicone composition under heating by cross-linking component (A). Appropriate organic peroxides may be represented by benzoyl peroxide, di- t-butylperoxide, 2,5-dimethyl-2,5-di(t-butylperoxy) hexane, bis(o-methylbenzoyl) peroxide, bis (m-methylbenzoyl) peroxide, bis(p-methylbenzoyl) peroxide, or a similar monomethylbenzoyl peroxide, bis(2,4-dimethylbenzoyl) peroxide, or a similar dimethylbenzoyl peroxide, and bis(2,4,6-trimethylbenzoyl) peroxide. Component (Dl) is preferably used in an amount of from 0.1 to 10 parts by weight, preferably 0.8 to 6 parts by weight for each 100 parts by weight of component (A).

[0017] When component (A) is an alkenyl-containing diorganopolysiloxane with 2 or more alkenyl groups per molecule, curing of the composition by cross-linking may be carried out using a hydro silylation process, wherein component (D) comprises Component

(D2) an organohydrogenpolysiloxane containing at least two and preferably three or more silicon-bonded hydrogen atoms and component (D3) is a platinum based catalyst.

[0018] Component (D2) may be exemplified by the following compounds: a trimethylsiloxy-terminated methylhydrogenpolysiloxane; a trimethylsiloxy-terminated copolymer of methylhydrogensiloxane and dimethylsiloxane units; a dimethylhydrogensiloxy-terminated copolymer of methylhydrogensiloxane and dimethylsiloxane units; a cyclic copolymer of methylhydrogensiloxane and dimethylsiloxane; a cyclic methylhydrogensiloxane oligomer; a methylhydrogenpolysiloxane composed of (CH₃)₃SiO-_/2 siloxane units, (CH₃)₂HSiO_1/2 siloxane units, and SiO₄/₂ siloxane units; a methylhydrogenpolysiloxane composed of (CH₃)₂HSiO-/₂ siloxane units and (CH₃SiO _/2 siloxane units; a methylhydrogenpolysiloxane composed of (CH₃)₂HSiO-/₂ siloxane units, (CH₃)₂Siθ_2/2 siloxane units, and CH₃SiO₃/₂ siloxane units; a dimethylhydrogensiloxy-terminated dimethylpolysiloxane; a dimethylhydrogensiloxy-terminated copolymer of methylphenylsiloxane and dimethylsiloxane units; a dimethylhydrogensiloxy-terminated copolymer of methyl (3,3,3-trifluoropropyl) siloxane and dimethylsiloxane units, and/or a mixture of two or more of the above.

[0019] Although there are no special restrictions with regard to viscosity of component

(D2), it is preferred that the viscosity of this component be in the range of from 1 to 1000 mPa-s at 25°C. Component (D2) should be used in such an amount that the ratio of the total mole number of the silicon-bonded hydrogen atoms in Component (D2) to the total mole number of alkenyl groups in component (A) is in the range of from 0.6 to 10, and preferably of from 0.7 to 5.0.

[0020] Component (D3) the platinum-type catalyst may be exemplified by a fine platinum powder, chloroplatinic acid, an alcoholic denaturate of chloroplatinic acid, a complex of chloroplatinic acid with dialkenyltetramethyldisiloxane, or the aforementioned catalysts on alumina, silica, or carbon-black carriers. The most preferable among the above catalysts is a complex of chloroplatinic acid with 1,3-divinyltetramethyldisiloxane, which is characterized by high hydrosilylation activity. The catalyst may also be used in the form of spherical microparticles of a thermoplastic resin with more than 0.1 wt.% of atoms of metallic platinum. Platinum catalyst (D3) should be used in a so-called catalytic quantity, preferably in an amount of 0.01 to 500 parts by weight, in particular, 0.1 to 100 parts by weight for each 10 parts of component (A).

[0021] Hence, preferred compositions in accordance with the present invention may comprise

100 parts by weight of component (A); 10 to 100 parts by weight of component (B); component (C) in an amount of 5 to 200 weight per cent of the weight of component (B), and component (Dl) an organic peroxide in an amount of from 0.1 to 10 wt.% of the weight of component (A); or 100 parts by weight of component (A);

10 to 100 parts by weight of component (B);

Component (C) in an amount of from 5 to 200 weight % of the weight of component (B);

Component (D2), an organohydrogenpolysiloxane in an amount such that the mole ratio of the silicon-bonded hydrogen atoms in component (D2) to silicon-bonded alkenyl groups contained in component (A) is within the range of from 0.6 to 10.0; and Component (D3) comprising a catalytic quantity of a platinum catalyst. [0022] If necessary, the silicone rubber composition of the present invention may be combined with conventional additives such as pigments, heat-resistant agents, flame retardants, mold-release agents, foaming agents, bactericides, electroconductive fillers, or the like, which should be added in amounts not conflicting with the objects of the invention.

[0023] Examples of pigments which may be utilized include carbon black, iron oxide red, and titanium oxide. The heat-resistant agent may, for example, comprise one or more rare- earth-element oxides, rare-earth-element hydroxides, cerium silanolate, and/or cerium fatty acid salts. [0024] Foaming agents may be exemplified by inorganic foaming agents such as sodium hydrogencarbonate, sodium bicarbonate and/or potassium azide, azo compounds such as, azo-bis-isobutylonitrile, 2,2'-azo-bis (2,4-dimethylvaleronitrile), dimethyl-2,2-azo-bis- isobutylate and/or azo-dicarbonamine; nitroso compounds such as N,N'- dinitrosopentamethylenetetramine and/or N,N' -dimethyl-N,N' -dinitrosoterephthalamide; sulfonylhydrazide compounds such as p-toluenesulfonylhydrazide, 4,4'- oxybisbenzenesulfonylhydrazide and/or diphenylsulfone-3,3 '-disulfonylhydrazide; carbonylhydrazine compounds such as 2-propenic acid hydrazide, acetylhydrazine; diazido compounds such as 4,4'-diazidodiphenyl, 4,4'-diazidobenzophenone, 2,5-diazidotoluene.

[0025] Electroconductive fillers may comprise acetylene black, furnace black, thermal black, or a similar carbon black. It is recommended that the oil absorption of the electroconductive filler be within the range of from 50 to 150.

[0026] When component (D) comprises (D2) the organohydrogenpolysiloxane and (D3) the platinum-type catalyst as the curing agent, then, for improving workability and storage stability, the silicone rubber composition of the present invention may further comprise one or more hydrosilylation reaction inhibitors. These inhibitors may comprise acetylene-type compounds such as, for example, 2-methyl-3-butyn-2-ol, 2-phenyl-3-butyn-2-ol, 3,5- dimethyl-l-hexyn-3-ol, 1-ethynyl-l-cyclohexanol, 1,5-hexadi-yne and/or 1,6-heptadi-yne; an enyl-type compound such as, for example, 3,5-dimethyl-l-hexen-l-yl, 3-ethyl-3-buten-l-yl and/or 3-phenyl-3-buten-l-yl; an alkenylsiloxane oligomer such as, for example, 1,3- divinyltetramethyldisiloxane, 1,3,5,7-tetravinyltetramethylcyclotetrasiloxane and/or 1,3- divinyl-l,3-diphenyldimethyldisiloxane; an ethenyl-containing silicon compound such as for example methyl-tris(3 -methyl- 1 -buten-3 -oxy) silane; a nitrogen-containing compound such as for example tributylamine, tetramethylethylenediamine and/or benzotriazole; a phosphorus-containing compound such as for example, triphenylphosphine. Other hydrosilylation reaction inhibitors include sulfur- containing compounds, hydroperoxides and maleic-acid derivatives.

[0027] These inhibitors should be added in amounts that essentially stop a hydrosilylation reaction at room temperature, but promotes rapid hydrosilylation with heating to a temperature of 70 to 200°C. They are normally added in an amount of from 0.1 to 6 parts by weight for each 100 parts by weight of component (A). [0028] The silicone rubber composition of the present invention may be easily prepared by uniformly mixing aforementioned components (A), (B), (C) and (Dl) or (A), (B), (C) (D2) and (D3), or by premixing components (A) (B) and (C), and then adding at room temperature component (Dl) or components (D2) and (D3). If a hydrophobic agent is used, the hydrophobic agent may be preliminarily uniformly mixed with components (A), (B) and (C), and then the mixtures may be combined at room temperature with component (Dl) or with components (D2) and (D3). The mixing of components (A), (B) and (C), may be carried out by means of a high shear mixer such as a kneader mixer. A high shear mixer may also be used for preparation of mixtures with the aforementioned hydrophobic agent, i.e., for mixing the hydrophobic agent with components (A), (B) and (C). Since during mixing the interior of the mixer can reach temperatures of from 80 to 120°C without any external heating, merely due to friction, it is preferred that, prior to addition of the curing agent, the contents of the mixer are positively cooled or allowed to cool naturally. Where required, in order to shorten the mixing cycle and to obtain a stabilized uniform mixture, mixing may be carried out with heating up to a temperature of from 120 to 190°C and with subsequent positive cooling. If necessary, compounding with component (Dl), or with components (D2) and (D3), may be carried in a two-roll milll.

[0029] Improved flowability and extradability of the silicone rubber composition of the present invention makes it possible to increase the linear speed during extrusion of the composition through an extruder or a similar machine. This, in turn, makes it possible to significantly improve productivity of elongated articles, decreases weld defects during formation in metal molds, and enables molded silicone rabber articles of complicated shapes to be made due to improved filling of mold cavities. These factors contribute to improvement in the quality of molded articles and an increased productivity in manufacturing processes. Examples

[0030] The invention will be further described with reference to the following practical and comparative examples. In these examples, the amount of each component is provided in parts by weight unless otherwise indicated, and values of various characteristics were measured at 25°C in accordance with the provisions of JIS K 6249.

[Practical Example 1]

[0031] A kneader-mixer was loaded with the following components: 100 parts of a dimethylvinylsiloxy terminated copolymer gum of methylvinylsiloxane and dimethylsiloxane (99.6 mole % of dimethylsiloxane units and 0.4 mole % of dimethylvinylsiloxane units) (average degree of polymerization: 5000; Williams plasticity according to JIS K 6249-1997: 140); 40 parts of dry-process silica having a specific surface area of 200 m²/g (Aerosil 200, the product of Nippon Aerosil Co., Ltd.); 15 parts of an expanded perlite powder with a 5.0 μm average particle size (Topco # 54; the product of Toko Perlite Kogyo Co., Ltd.; oil absorption: 220 ml/100 g; cake bulk density: 0.35 g/cm³); and 8 parts of a 60 mPa-s viscosity dimethylsiloxane oligomer having both molecular terminals capped with dimethylhydroxysiloxy groups (which acts as the hydrophobing treating agent for the silica filler). The components were kneaded for 60 min. at 175°C and then cooled. After cooling to room temperature a uniform silicone rabber base was produced. A silicone rubber composition in accordance with the present invention was prepared by mixing in a two-roll mill and uniformly kneading a mixture consisting of 100 parts of the aforementioned silicone rubber base and 1.3 parts of a silicone oil paste containing 50 wt.% of bis(p-methylbenzoyl)peroxide. [0032] The production output of the resulting silicone rabber composition was measured on a Laboplast Mill (Toyo Seiki Mfg. Co., Ltd.), equipped with a length/diameter (LID) =10/1 variable-type screw and a 10 mm diameter die. The barrel and head portions were heated to 70°C, and the screw was rotated at 50 rpm and 100 rpm. The obtained silicone rubber composition was measured with regard to specific gravity, tensile strength, elongation at break, volumetric resistivity, and insulating strength, in accordance with JIS K 6249. The resulting measurements are shown in Table 1.

[Practical Example 2]

[0033] A silicone rabber composition was prepared by the same method as in Practical

Example 1, with the exception that 5 parts of expanded perlite powder having a 5.0 μm average particle size was used. Similar to Practical Example 1, the resulting silicone rabber composition was measured with regard to specific gravity, tensile strength, elongation at break, volumetric resistivity, and insulating strength. The results of measurements are shown in Table 1.

[Practical Example 3] [0034] A silicone rabber composition was prepared by the same method as in Practical

Example 1, with the exception that 15 parts of the expanded perlite powder having a 23.0 μm average particle size (Topco # 31; the product of Toko Perlite Kogyo Co., Ltd.; oil absorption: 290 ml/100 g; cake bulk density: 0.22 g/cm³) were used instead of 15 parts of the expanded perlite particles having a 5 μm average particle size. Similar to Practical Example 1, the obtained silicone rabber composition was measured with regard to specific gravity, tensile strength, elongation at break, volumetric resistivity, and insulating strength. The results of measurements are shown in Table 1.

[Comparative Example 1] [0035] A silicone rabber composition was prepared by the same method as in Practical Example 1, with the exception that no expanded perlite powder was added. Similar to Practical Example 1, the obtained silicone rubber composition was measured with regard to specific gravity, tensile strength, elongation at break, volumetric resistivity, and insulating strength. The results of measurements are shown in Table 1.

[Comparative Example 2]

[0036] A silicone rubber composition was prepared by the same method as in Practical Example 1, with the exception that 15 parts of quartz powder having a 5 μm average particle size (Minisil 5 micron, the product of US Silica) were used instead of 15 parts of the expanded perlite powder having a 5 μm average particle size. Similar to Practical Example 1, the obtained silicone rabber composition was measured with regard to specific gravity, tensile strength, elongation at break, volumetric resistivity, and insulating strength. The results of measurements are shown in Table 1.

[Comparative Example 3] [0037] A silicone rabber composition was prepared by the same method as in Practical

Example 1, with the exception that 15 parts of a diatomaceous earth powder having a 10 μm average particle size ("Radiolight Microfme" produced by Showa Chemical Industry Co., Ltd.) were used instead of 15 parts of the expanded perlite powder having a 5 μm average particle size. [0038] Similar to Practical Example 1, the obtained silicone rabber composition was measured with regard to specific gravity, tensile strength, elongation at break, volumetric resistivity, and insulating strength. The results of measurements are shown in Table 1. [Table 1]

[0039] It will be noted that whilst Practical Example 3 provides excellent flowability properties other physical properties are comparatively poor as compared to Practical Example 1, Practical Example 2, and Comparative Examples, however such a composition having excellent flowability properties remains useful for applications where such good values of physical properties are not required for the application concerned.

[Practical Example 4] [0040] The silicone rabber composition obtained in Practical Example 1 was loaded into a 65 mm diameter single-screw extruder, and a 0.7 mm diameter metal core wire placed into the cross head of the extruder was coated with the aforementioned composition. A 7.2 m piece of the wire insulated with the 0.8 mm thick layer of the silicone rubber was produced by unloading the product from a 400°C heating oven at rate of 60 m min. [0041] The resulting wire insulated with the silicon rubber coating was subjected to a spark test at a AC voltage of 5 kV for 0.15 seconds, in accordance with Section G of UL 758 and

UL 1581. Not a single spark-out occurred on the length of 5000 m.

[0042] A silicone rabber rod having a circular cross section with a 10 mm outer diameter was manufactured in the aforementioned extruder and heating oven, at a rate of lOm/min.

No fluctuation of the outer diameter over a length of 5000 m was observed. The results of measurements are shown in Table 2.

[Comparative Example 4] [0043] The silicone rabber composition obtained in Comparative Example 1 was loaded into a 65 mm diameter single-screw extruder, and a 0.7 mm diameter metal core wire placed into the cross head of the extruder was coated with the aforementioned composition. A 7.2 m piece of the wire insulated with the 0.8 mm thick layer of the silicone rubber was produced by unloading the product from a 400°C heating oven at rate of 60 m/min. [0044] The obtained wire insulated with the silicon rabber coating was subjected to the aforementioned spark test at a AC voltage of 5 kV for 0.15 seconds. Twelve spark-outs occurred on the length of 5000 m.

A silicone rabber rod having a circular cross section with a 10 mm outer diameter was manufactured in the aforementioned extruder and heating oven at a rate of lOm min (the outer diameter is measured using calipers). Some sections of the resulting cured rod were found to have a diameter (measured using calipers) of less than 10mm over a length of 5000 m. The results of measurements are shown in Table 2. Table 2

[Practical Example 5]

[0045] A kneader-mixer was loaded with the following components: 100 parts of a copolymer gum of methylvinylsiloxane and dimethylsiloxane capped at both terminals with dimethylvinylsiloxy groups (99.85 mole % of dimethylsiloxane units and 0.15 mole % of methylvinylsiloxane units) (average degree of polymerization: 5000; Williams plasticity according to JIS K 6249-1997: 140); 20 parts of dry-process silica having a specific surface area of 200 m²/g (Aerosil 200, the product of Nippon Aerosil Co., Ltd.); 20 parts of an expanded perlite powder with a 5.0 μm average particle size (Topco # 54; the product of Toko Perlite Kogyo Co., Ltd.; oil absorption: 220 ml/100 g; cake bulk density: 0.35 g/cm³); and 5 parts of a 60 rnPa-s viscosity dimethylsiloxane oligomer having both molecular terminals capped with dimethylhydroxysiloxy groups. The components were kneaded for 60 min. at 175°C. As a result, a uniform silicone rabber base was produced. At room temperature, the obtained silicone rubber base was combined with 15 parts of acetylene black, and the mixture was uniformly kneaded until it reached room temperature. The resulting product comprised a conductive silicone rubber base. A conductive silicone rabber composition for forming a sponged product was prepared by uniformly kneading in a two- roll mill a mixture consisting of 100 parts of the aforementioned conductive silicone rabber base, 1.0 part of a 25 mPa-s viscosity copolymer of methylhydrogensiloxane and dimethylsiloxane having both molecular terminals capped with trimethylsiloxy (0.8 wt.% content of silicon-bonded hydrogen atoms, 0.06 parts of a complex of a chloroplatinic acid with 1,3-divinyltetramethyldisiloxane (0.6 wt.% content of metallic platinum), 0.15 parts of

1-ethynyl-l-cyclohexanol as a hydrosilylation inhibitor, and 2.0 parts of azo-bis- isobutylonitrile as a blowing agent.

[0046] The obtained silicone rabber composition was loaded into a 65 mm diameter single- screw extrader and extruded into a continuous 7.2 m long conductive silicone rabber sponge of a round cross section (10 mm outer diameter) by passing through the heating oven at

250°C with the linear speed of 1.0 m/min. The obtained conductive silicone rabber sponge was free of fluctuations in the outer diameter.

Industrial Applicability

[0047] Since the silicone rabber composition of the present invention is prepared from specific quantities of components (A), (B), (C) and (D), preferably components (A), (B), (C), (Dl), or components (A), (B), (C), (D2) and (D3), and especially due to the provision of the expanded perlite powder of component (C), this composition having excellent flowability and extradability is an excellent material for molding silicone rubber articles, especially for commercial production of elongated articles such as wires insulated with silicone rabber coating and long rods, and silicone rabber articles of complicated shapes.

Claims

1. A silicone rabber composition comprising

(D) a curing agent in an amount sufficient to cure the composition.

2. The silicone rubber composition in accordance with claim 1 wherein component (D) is (Dl), an organic peroxide in an amount of from: 0.1 to 10 parts by weight per 100 parts by weight of component A.

3. The silicone rubber composition in accordance with claim 1 wherein component (A) is an alkenyl-containing diorganopolysiloxane gum with 2 or more alkenyl groups per molecule, and component D is (D2) an organohydrogenpolysiloxane in an amount such that the mole ratio of silicon-bonded hydrogen atoms contained in (D2) to silicon-bonded alkenyl groups in component (A) is within the range of 0.6:1 to 10.0:1; and (D3) a catalytic quantity of a platinum catalyst.

4. The silicone rubber composition according to any of Claims 1 to 3, wherein component (C) has an average particle diameter of below 50 μm.

5. The silicone rabber composition in accordance with any preceding claim wherein component (B) is treated with a hydrophobing agent selected from the group of a silanol- capped dimethylsiloxane oligomer, silanol-capped methylvinylsiloxane oligomer, silanol- capped methylphenylsiloxane oligomer, diphenylsilane diol, hexamethyldisilazane, cyclic dimethylsiloxane oligomer (e.g., octamethylcyclotetrasiloxane), trimethylchlorosilane and/or dimethyldichlorosilane.

6. The silicone rabber composition in accordance with any preceding claim wherein component (C) is prepared by heating to cause expansion and formation of hollow perlite structures, and subsequently crushing said structures to form perlite powder.

7. Use of the silicone rubber composition in accordance with any preceding claim for forming articles by extrusion.

8. A cured silicone rubber article made by curing the silicone rubber composition in accordance with any one of claims 1 to 4.

9. Use of expanded perlite powder to improve the flowability of a silicone rabber composition comprising

(A) 100 parts by weight of a diorganopolysiloxane gum;

(B) 10 to 100 parts by weight of reinforcing fine silica powder; and

(D) a curing agent in an amount sufficient to cure the composition; in which expanded perlite powder is provided in an amount of from 5 to 200 weight.% of the weight of component (B).