Abstract:
A highly heat conductive silicone rubber composition is provided comprising (A) 100 pbw of a heat curable organopolysiloxane composition and (B) 10-2,000 pbw of a metallic silicon powder having an average particle size of 2-100 μm. It cures into a product having a high thermal conductivity, heat resistance and low compression set and remains fully stable during shelf storage. It is useful as a heat conducting or dissipating material in automobiles and electric and electronic equipment where heat resistance, heat conductivity and rubber elasticity are required.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2005-337072 filed in Japan on Nov. 22, 2005, the entire contents of which are hereby incorporated by reference.  
       TECHNICAL FIELD  
       [0002]     This invention relates to a silicone rubber composition capable of forming a silicone rubber molding having a high thermal conductivity, low compression set, and stability at elevated temperature. More particularly, it relates to a highly heat conductive silicone rubber composition of the heat curable type, especially addition or peroxide cure type, capable of forming silicone rubber which undergoes little changes of physical properties upon exposure to elevated temperature.  
       BACKGROUND ART  
       [0003]     Liquid silicone rubber compositions of the heat curable type are used in a variety of fields since they are easy to mold and after molding, offer cured products having heat resistance, weather resistance and electrical insulation. In particular, silicone rubber compositions can readily cure into highly heat conductive silicone rubbers merely when loaded with highly heat conductive fillers. They often find use in electric and electronic applications, for example, as heat dissipating sheets for CPU and power transistor chips.  
         [0004]     Heat conductive fillers used in prior art heat conductive silicone rubbers include silica, alumina, aluminum, silicon carbide, silicon nitride, magnesium oxide, magnesium carbonate, zinc oxide, aluminum nitride and the like. Silica, due to its low heat conductivity, must be compounded in high loadings, which in turn, makes it difficult to reduce hardness and give too high a viscosity to mold. Alumina and aluminum, which are amphoteric compounds, are susceptible to inherent impurities and when used in silicone rubber compositions, adversely affect heat resistance and compression set. Zinc oxide, due to its high specific gravity of 5.7, tends to settle down and separate out when dispersed in liquid silicone rubber compositions, and inadequately its powder is highly hygroscopic. Silicon carbide has a relatively high specific gravity and is available in such ground form that the powder is likely to agglomerate or settle down when dispersed in silicone rubber compositions. Since agglomerates of silicon carbide become cake-like, they are difficult to disperse again and awkward to handle. Silicon nitride and aluminum nitride are readily reactive with water, suffering from the drawback of extremely poor moisture resistance. Magnesium oxide has also been a candidate for highly heat conductive filler, but like aluminum nitride, it is readily reactive with water and has the drawback of extremely poor moisture resistance. Magnesium carbonate is unstable in that it can be decomposed into magnesium oxide by high-temperature heating.  
         [0005]     Reference should be made to Japanese Patent Nos. 3,354,087 and 3,694,474.  
       SUMMARY OF THE INVENTION  
       [0006]     An object of the invention is to provide a silicone rubber composition which is fully shelf stable and cures into a product (silicone rubber) having a high thermal conductivity, heat resistance and low compression set.  
         [0007]     The inventors have discovered that a silicone rubber composition having a heat curable organopolysiloxane composition loaded with a metallic silicon powder remains fully shelf stable and cures into a product having a high thermal conductivity, heat resistance and low compression set.  
         [0008]     It has been found that a highly heat conductive silicone rubber having a high thermal conductivity, heat resistance and low compression set is obtainable using a metallic silicon powder instead of the conventional heat conductive fillers discussed above. The use of metallic silicon is known in the prior art, for example, the use as a deoxygenating agent for ceramics or refractory; the use of metallic silicon powder as a starting material to silicon nitride powder for use as heat conductivity-imparting filler, as described in Japanese Patent No. 3,354,087; and the preparation of spherical silica by melting metallic silicon and oxidizing in a gas, as described in Japanese Patent No. 3,694,474. None of these patents suggest the use of metallic silicon powder in silicone resins as a heat conductive substance.  
         [0009]     The present invention provides a highly heat conductive silicone rubber composition comprising (A) 100 parts by weight of a heat curable organopolysiloxane composition, and (B) 10 to 2,000 parts by weight of a metallic silicon powder having an average particle size of up to 100 μm.  
         [0010]     In a preferred embodiment, the metallic silicon powder (B) is a ground metallic silicon powder powdered by a grinding method to an average particle size of 2 to 50 μm, or a spherical metallic silicon powder having an average particle size of 2 to 50 μm.  
         [0011]     In a preferred embodiment, the heat curable silicone rubber composition (A) is an addition curing type organopolysiloxane composition or an organic peroxide curing type organopolysiloxane composition.  
         [0012]     The preferred addition curing type organopolysiloxane composition is a composition comprising (1) 100 parts by weight of an organopolysiloxane having at least two alkenyl groups attached to silicon atoms in a molecule, (2) 0.1 to 50 parts by weight of an organohydrogenpolysiloxane having at least two hydrogen atoms attached to silicon atoms in a molecule, and (3) a catalytic amount of an addition reaction catalyst.  
         [0013]     The preferred organic peroxide curing type organopolysiloxane composition is a composition comprising (a) 100 parts by weight of an organopolysiloxane having on average at least two alkenyl groups in a molecule, represented by the average compositional formula (iii): 
 
R 3   d SiO (4-d)/2    (iii) 
 
 wherein R 3  is a substituted or unsubstituted monovalent hydrocarbon group, 0.0001 to 10 mol % of R 3  being alkenyl groups and at least 80 mol % of R 3  being methyl groups, and d is a positive number of 1.9 to 2.4, and (b) an effective amount of an organic peroxide. 
 
       BENEFITS OF THE INVENTION  
       [0014]     The highly heat conductive silicone rubber composition of the invention cures into a product having a high thermal conductivity, heat resistance and low compression set and remains fully stable during shelf storage. Owing to these properties, the composition is useful as a heat conductive material or heat dissipating material in automobiles and electric and electronic equipment where satisfactory heat resistance, heat conductivity and rubber elasticity are required. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     In a preferred embodiment, the heat curable silicone rubber composition as component (A) may be either an addition reaction curing type organopolysiloxane composition or an organic peroxide curing type organopolysiloxane composition. The addition reaction curing type organopolysiloxane composition is preferably defined as comprising (1) 100 parts by weight of an organopolysiloxane having at least two alkenyl groups attached to silicon atoms in a molecule, (2) 0.1 to 50 parts by weight of an organohydrogenpolysiloxane having at least two hydrogen atoms attached to silicon atoms (i.e., SiH groups) in a molecule, and (3) a catalytic amount of an addition reaction catalyst. The organic peroxide curing type organopolysiloxane composition is preferably defined as comprising (a) 100 parts by weight of an organopolysiloxane having on average at least two alkenyl groups attached to silicon atoms in a molecule, and (b) an effective amount of an organic peroxide.  
         [0016]     In the addition reaction curing type organopolysiloxane composition, the organopolysiloxane having at least two alkenyl groups in a molecule is used as component (1) or base polymer. It has the following average compositional formula (i): 
 
R 1   a SiO (4-a)/2    (i) 
 
 wherein R 1 , which may be the same or different, is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, and “a” is a positive number of 1.5 to 2.8, preferably 1.8 to 2.5, and more preferably 1.95 to 2.05. 
 
         [0017]     Examples of the substituted or unsubstituted monovalent hydrocarbon groups attached to silicon atoms, represented by R 1 , include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl and decyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl; and substituted forms of the foregoing groups in which some or all of the hydrogen atoms are replaced by halogen atoms (e.g., fluoro, bromo and chloro), cyano groups or the like, such as halo-substituted alkyl groups, e.g., chloromethyl, chloropropyl, bromoethyl and trifluoropropyl, and cyanoethyl.  
         [0018]     At least two of the R 1  groups must be alkenyl groups, preferably of 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms. It is noted that the content of alkenyl groups is preferably 0.0001 to 20 mol %, more preferably 0.001 to 10 mol %, especially 0.01 to 5 mol % based on the entire organic groups R 1  (i.e., the substituted or unsubstituted monovalent hydrocarbon groups described above). The alkenyl groups may be attached to silicon atoms at ends of the molecular chain and/or silicon atoms midway the molecular chain. The inclusion of alkenyl groups attached to the silicon atoms at both ends of the molecular chain is preferred.  
         [0019]     No particular limit is imposed on the degree of polymerization. Use may be made of organopolysiloxanes ranging from liquid to gum-like at room temperature. Often, those organopolysiloxanes having an average degree of polymerization of 50 to 30,000, preferably 100 to 20,000, and more preferably 100 to 10,000 are used.  
         [0020]     With respect to the structure, the organopolysiloxane has a generally straight chain structure whose backbone is comprised of recurring diorganosiloxane (R 1   2 SiO 2/2 ) units (e.g., dimethylsiloxane, diphenylsiloxane, methylphenylsiloxane, methyltrifluoropropylsiloxane, vinylmethylsiloxane units) and which is blocked with a triorganosiloxy (R 1   3 SiO 1/2 ) group (e.g., trimethylsiloxy, vinyldimethylsiloxy, divinylmethylsiloxy, trivinylsiloxy, vinyldiphenylsiloxy, vinylmethylphenylsiloxy, phenyldimethylsiloxy, diphenylmethylsiloxy groups) at each end of the molecular chain. However, it may have a partially branched or cyclic structure.  
         [0021]     The organohydrogenpolysiloxane (2) serves as a crosslinker for causing the composition to cure through hydrosilylating addition reaction with the alkenyl-containing organopolysiloxane (1). Typically the organohydrogenpolysiloxane has the following average compositional formula (ii): 
 
R 2   b H c SiO (4-b-c)/2    (ii) 
 
 wherein R 2  is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms, the letter b is a positive number of 0.7 to 2.1, c is a positive number of 0.001 to 1.0, and b+c is 0.8 to 3.0. The organohydrogenpolysiloxane should have at least 2, preferably at least 3 (ordinarily 3 to about 200), more preferably 3 to 100 silicon-bonded hydrogen atoms (i.e., SiH groups) in a molecule. The hydrogen atoms may be attached to silicon atoms at ends of the molecular chain and/or silicon atoms midway the molecular chain. 
 
         [0022]     Herein R 2  is a substituted or unsubstituted monovalent hydrocarbon group, examples of which are as described for R 1  in formula (i), and preferably those free of aliphatic unsaturation (like alkenyl).  
         [0023]     Exemplary organohydrogenpolysiloxanes (2) include tris(dimethylhydrogensiloxy)methylsilane, tris(dimethylhydrogensiloxy)phenylsilane, 1,1,3,3-tetramethyldisiloxane, 1,3,5,7-tetramethylcyclotetrasiloxane, methylhydrogencyclopolysiloxane, methylhydrogensiloxane-dimethylsiloxane cyclic copolymers, both end trimethylsiloxy-blocked methylhydrogenpolysiloxane, both end trimethylsiloxy-blocked dimethylsiloxane-methylhydrogensiloxane copolymers, both end dimethylhydrogensiloxy-blocked dimethylpolysiloxane, both end dimethylhydrogensiloxy-blocked dimethylsiloxane-methylhydrogensiloxane copolymers, both end trimethylsiloxy-blocked methylhydrogensiloxane-diphenylsiloxane copolymers, both end trimethylsiloxy-blocked methylhydrogensiloxane-diphenylsiloxane-dimethylsiloxane copolymers, copolymers of (CH 3 ) 2 HSiO 1/2  units and SiO 4/2  units, and copolymers of (CH 3 ) 2 HSiO 1/2  units, SiO 4/2  units, and (C 6 H 5 )SiO 3/2  units, and substituted forms of the foregoing in which some or all methyl groups are substituted by different alkyl groups (e.g., ethyl, propyl), aryl groups (e.g., phenyl), or halo-substituted alkyl groups (e.g., 3,3,3-trifluoropropyl).  
         [0024]     The molecular structure of the organohydrogenpolysiloxane may be linear, cyclic or branched, or three-dimensional network structure while the number of silicon atoms per molecule, that is, the degree of polymerization is 2 to about 1,000, preferably 3 to about 500, and more preferably 3 to about 300.  
         [0025]     The organohydrogenpolysiloxane (2) is preferably blended in an amount of 0.1 to 50 parts, more preferably 0.3 to 30 parts by weight per 100 parts by weight of the organopolysiloxane (1). Differently stated, the organohydrogenpolysiloxane (2) may be blended in such amounts that the molar ratio of silicon-attached hydrogen atoms (SiH groups) in component (2) to silicon-attached alkenyl groups in component (1) is from 0.5 to 5 mol/mol, preferably from 0.8 to 4 mol/mol, and more preferably from 1 to 3 mol/mol.  
         [0026]     The addition reaction catalyst (3) is to promote hydrosilylating addition reaction between alkenyl groups in component (1) and SiH groups in component (2). Typical addition reaction catalysts are platinum group metal catalysts including platinum catalysts, for example, platinum black, platinic chloride, chloroplatinic acid, reaction products of chloroplatinic acid with monohydric alcohols, complexes of chloroplatinic acid with olefins, and platinum bisacetoacetate; palladium catalysts; and rhodium catalysts. The addition reaction catalyst is used in a catalytic amount, typically about 0.5 to 1,000 parts, preferably about 1 to 500 parts by weight of platinum group metal per million parts by weight of components (1) and (2) combined.  
         [0027]     The organic peroxide curing type organopolysiloxane composition contains (a) an organopolysiloxane having on average at least two alkenyl groups attached to silicon atoms in a molecule as a base polymer and (b) an organic peroxide. The organopolysiloxane (a) has the average compositional formula (iii): 
 
R 3   d SiO (4-d)/2    (iii) 
 
 wherein R 3  is a substituted or unsubstituted monovalent hydrocarbon group, 0.0001 to 10 mol % of R 3  being alkenyl groups and at least 80 mol % of R 3  being methyl groups, and d is a positive number of 1.9 to 2.4. 
 
         [0028]     At each occurrence, R 3  may be the same or different and is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms. The subscript d is a positive number in the range of 1.9 to 2.4, preferably 1.95 to 2.05, and more preferably 1.98 to 2.02.  
         [0029]     Examples of the substituted or unsubstituted monovalent hydrocarbon groups attached to silicon atoms, represented by R 3 , are as described for R 1  in average compositional formula (i), and include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl and decyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl; and substituted forms of the foregoing groups in which some or all of the hydrogen atoms are replaced by halogen atoms (e.g., fluoro, bromo and chloro), cyano groups or the like, such as halo-substituted alkyl groups, e.g., chloromethyl, chloropropyl, bromoethyl and trifluoropropyl, and cyanoethyl.  
         [0030]     At least two of the R 3  groups must be alkenyl groups, preferably of 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms. It is noted that the content of alkenyl groups is preferably 0.0001 to 10 mol %, more preferably 0.001 to 5 mol % based on the entire organic groups R 3  (i.e., the substituted or unsubstituted monovalent hydrocarbon groups described above). The alkenyl groups may be attached to silicon atoms at ends of the molecular chain and/or silicon atoms midway the molecular chain.  
         [0031]     No particular limit is imposed on the degree of polymerization. Use may be made of organopolysiloxanes ranging from liquid to gum-like at room temperature. Often, those organopolysiloxanes having an average degree of polymerization of about 50 to 30,000, preferably about 100 to 20,000, and more preferably about 100 to 10,000 are used.  
         [0032]     With respect to the structure, the organopolysiloxane has a generally straight chain structure whose backbone is comprised of recurring diorganosiloxane (R 3   2 SiO 2/2 ) units (as exemplified in conjunction with average compositional formula (i)) and which is blocked with a triorganosiloxy (R 3   3 SiO 1/2 ) group (as exemplified in conjunction with average compositional formula (i)) or hydroxydiorganosiloxy ((HO)R 3   2 SiO 1/2 ) group at each end of the molecular chain. However, it may have a partially branched or cyclic structure.  
         [0033]     The organic peroxide (b) used herein may be selected from well-known peroxides which are used in conventional peroxide cure type organopolysiloxane compositions as a catalyst for promoting crosslinking reaction of organopolysiloxane. Examples include, but are not limited to, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-methylbenzoyl peroxide, o-methylbenzoyl peroxide, 2,4-dicumyl peroxide, 2,5-dimethyl-bis(2,5-t-butylperoxy)-hexane, di-t-butyl peroxide, t-butyl perbenzoate, and 1,1-bis(t-butylperoxycarboxy)hexane.  
         [0034]     The organic peroxide is used in a catalytic amount for causing the organopolysiloxane to cure. Although the amount of peroxide may be determined appropriate in accordance with the desired cure rate, it is usually 0.1 to 10 parts, preferably 0.2 to 2 parts by weight per 100 parts by weight of component (a).  
         [0035]     The silicone rubber composition of the invention is comprised of (A) the heat curable organopolysiloxane composition described above, and (B) a metallic silicon powder as a highly heat conductive inorganic powder. Metallic silicon has good heat conductivity, a low Mohs hardness, and high malleability due to its metallic nature, and is characterized in that a metal powder is unlikely to agglomerate even when high shear forces are applied. For this reason, metallic silicon is easy to atomize or powder by grinding, and effectively dispersible in organopolysiloxanes. In addition, metallic silicon particles have on their surface a very thin spontaneous oxide film, which is resistant to heat, acid and stain like glass, and prevents electricity conduction and remains stable to heat.  
         [0036]     The metallic silicon powder used herein may be prepared by any desired method. Included are metallic silicon powders obtained by a grinding method, for example, a powder obtained by chemically reducing silica into metallic silicon and grinding it in an existing crusher or grinding machine such as a ball mill, and a powder obtained by using as raw material metallic silicon (wafer) fragments and chips resulting from the semiconductor manufacturing process and finely dividing the material; and a spherical metallic silicon powder obtained by melting metallic silicon at high temperature, atomizing the molten silicon in a vapor phase, followed by cooling and solidification into spherical particles. Metallic silicon may have either a single crystal or a polycrystalline structure. Although the purity of metallic silicon powder as atomized is not particularly limited, it is desired from the standpoint of imparting heat conductivity to have a purity of at least 50%, more preferably at least 80%, and even more preferably at least 95%. Metallic silicon powder of a higher purity has a surface spontaneous oxide film which is defect-free, offering better thermal stability at high temperature.  
         [0037]     The metallic silicon powder used herein should have an average particle size of up to 100 μm, preferably 2 to 50 μm, and more preferably 2 to 25 μm. Particles with an average particle size less than 2 μm are difficult to prepare and may be difficult to incorporate in large amounts whereas particles with an average particle size more than 100 μm may result in cured rubber with low mechanical strength and even having a surface which is irregular on visual observation.  
         [0038]     It is noted that the average particle size is determined as a weight average value or median diameter in particle size distribution measurement by the laser light diffraction method.  
         [0039]     The metallic silicon powder used as component (B) may be treated with suitable surface treating agents for the purpose of improving the dispersibility of the powder in a silicone rubber composition and the thermal stability of the silicone rubber composition. Suitable surface treating agents include silane coupling agents and partial hydrolyzates thereof, alkylalkoxysilanes and partial hydrolyzates thereof, organic silazanes, titanate coupling agents, organopolysiloxane oils, and hydrolyzable functional group-bearing organopolysiloxanes. With respect to the time of surface treatment, the inorganic powder may be treated previously or at the time of mixing with the organopolysiloxane.  
         [0040]     An appropriate amount of the metallic silicon powder (B) loaded is 10 to 2,000 parts, preferably 50 to 1,000 parts, and more preferably 70 to 500 parts by weight per 100 parts by weight of component (A). Less than 10 pbw of the metallic silicon powder fails to provide high heat conductivity whereas more than 2,000 pbw is difficult to incorporate and detracts from rubber strength and physical properties.  
         [0041]     An additional heat conductive substance may be used together as long as it does not adversely affect low compression set and heat resistance. In such a case, the metallic silicon powder should account for at least 50% by volume of the entire heat conductive substances.  
         [0042]     The additional heat conductive substance is not particularly limited and may be any of existing substances. Suitable examples include alumina, aluminum, silicon carbide, silicon nitride, magnesium oxide, magnesium carbonate, zinc oxide, aluminum nitride, graphite, and fibrous graphite.  
         [0043]     In the silicone rubber composition of the invention, there may be further admixed reinforcing or semi-reinforcing fillers including finely divided silica such as fumed silica, precipitated silica, fused silica, fired silica, spherical silica by the sol-gel method, crystalline silica (quartz flour), and diatomaceous earth (inter alia, fused silica and crystalline silica also serving as the additional heat conductive substance), and calcium carbonate; reinforcements such as silicone resins; conductive agents such as carbon black, conductive zinc white and powdered metals; hydrosilylation reaction regulators such as nitrogen-containing compounds, acetylene compounds, phosphorus compounds, nitrile compounds, carboxylates, tin compounds, mercury compounds, and sulfur compounds; heat resistance improvers such as iron oxide and cerium oxide; internal parting agents such as dimethylsilicone oil; tackifiers; thixotropic agents; and the like. These additives may be incorporated insofar as they do not compromise the objects of the invention.  
         [0044]     The silicone rubber composition of the invention may be prepared by uniformly mixing the above essential and optional components in a rubber milling machine such as a planetary mixer, twin-roll mill, Banbury mixer, dough mixer or kneader, optionally followed by heat treatment. It is acceptable to premix part or all of the organopolysiloxane (1) or (a) with the metallic silicon powder and optionally another heat conductive filler and a surface treating agent to form a base compound and thereafter, mix the base compound with the remainder of the organopolysiloxane (1) or (a), organohydrogenpolysiloxane (2) for addition crosslinking and addition reaction catalyst (3), or organic peroxide (b), and other optional components.  
         [0045]     The silicone rubber composition thus obtained can be molded into suitable silicone rubber parts by various molding processes such as injection molding, casting, in-mold compression molding and extrusion molding processes.  
         [0046]     Curing conditions may be selected from a wide range. For example, appropriate curing conditions include a temperature of 80 to 200° C., preferably 100 to 180° C. and a time of about 5 seconds to 60 minutes, preferably about 20 seconds to 30 minutes. This may be followed by post-curing for the purposes of stabilizing physical properties, reducing the compression set, removing decomposition residues of peroxide, and removing the residual low molecular weight siloxanes. Oven conditions suitable for the post-curing include a temperature of 80 to 230° C., preferably 120 to 210° C., more preferably 150 to 200° C. and a time of about 10 minutes to 70 hours, preferably about 30 minutes to 8 hours.  
       EXAMPLE  
       [0047]     Examples of the invention are given below by way of illustration and not by way of limitation. All parts are by weight.  
       Examples 1-6 and Comparative Examples 1-6  
       [0048]     To a planetary mixer were added 40 parts of a dimethylpolysiloxane (1) blocked with a dimethylvinylsiloxy group at each end of its molecular chain and having a viscosity of 1,000 Pa·s at 25° C. (degree of polymerization 500), 40 parts of a dimethylpolysiloxane (2) blocked with a dimethylvinylsiloxy group at each end of its molecular chain and having a viscosity of 100 Pa·s at 25° C. (degree of polymerization 210), 20 parts of a dimethylpolysiloxane (3) blocked with a trimethylsiloxy group at each end of its molecular chain and having a viscosity of 100 Pa·s at 25° C. (degree of polymerization 210), and a heat conductive filler (selected from among metallic silicon powders A, B and C, aluminum oxide, aluminum powder, and ground quartz as characterized in Table 3) in a so controlled amount that the resultant cured rubber might have a heat conductivity of 0.6 W/m·k or 1.0 W/m·k. The mixer continued agitation for 30 minutes. Thereafter, 2.3 parts of methylhydrogenpolysiloxane having SiH groups at both ends and side chains (degree of polymerization 17, SiH content 0.0038 mol/g) as a crosslinker and 0.05 part of ethynyl cyclohexanol as a reaction regulator were added to the mixer, which continued agitation for a further 15 minutes, yielding a silicone rubber composition. The viscosity of the silicone rubber composition was measured by a Brookfield rotational viscometer (TOKIMEC Inc.). The results are shown in Tables 1 and 2. The silicone rubber composition was combined with 0.1 part of a platinum catalyst (Pt concentration 1%), cured in a press at 120° C. for 10 minutes, and post-cured in an oven at 200° C. for 2 hours. The cured rubber was measured for rubber hardness and compression set (180° C.×22 hours, 25% compression) according to JIS K-6249. Using a similarly cured sheet of 12 mm thick, a heat conductivity was measured by means of a heat conductivity meter QTM-3 (Kyoto Electron Co., Ltd.). The results are also shown in Tables 1 and 2.  
       Example 7  
       [0049]     In a kneader, 100 parts of a gum-like diorganopolysiloxane consisting of 99.825 mol % dimethylsiloxane units, 0.15 mol % methylvinylsiloxane units, and 0.025 mol % dimethylvinylsiloxane units and having an average degree of polymerization of about 8,000 was mixed with 2.0 parts of a silanol end-capped dimethylpolysiloxane (average degree of polymerization 10) as a dispersant and metallic silicon powder A in an amount of 180 parts so that the resultant cured rubber might have a heat conductivity of 1.0 W/m·k. The kneader continued operation for 30 minutes, forming a silicone rubber compound. To 100 parts of the compound was added 0.5 part of 2,5-dimethyl-bis(2,5-t-butylperoxy)hexane as an organic peroxide crosslinker. Uniform dispersion was achieved in a two-roll mill. The silicone rubber composition was cured in a press at 170° C. for 10 minutes, and post-cured in an oven at 200° C. for 2 hours. The cured rubber was measured for rubber hardness and compression set (180° C.×22 hours, 25% compression) according to JIS K-6249, and for plasticity according to JIS K-6249, three roll method.  
                                                                                           TABLE 1                                       Example            Component (pbw)   1   2   3   4   5   6   7                    Metallic silicon   110   200           150       180       powder A       Metallic silicon           105   180       powder B       Metallic silicon                       210       powder C       Aluminum oxide                   90       Rubber hardness   35   60   32   58   55   47   67       (Type A)       Compression set (%)   4   4   5   5   9   4   4       Viscosity (Pa · s)   30   230   36   260   200   190   —       Plasticity   —   —   —   —   —   —   320       Heat conductivity   0.6   1.0   0.6   1.0   1.0   1.0   1.0       (W/m · k)                  
 
         [0050]    
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                   
               
               
                   
                 Comparative Example 
               
             
          
           
               
                 Component (pbw) 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
               
             
          
           
               
                 Aluminum powder 
                 120 
                 250 
                   
                   
                   
                   
               
               
                 Aluminum oxide 
                   
                   
                 230 
                 380 
               
               
                 Ground quartz 
                   
                   
                   
                   
                 160 
                 200&lt;*1 
               
               
                 Rubber hardness 
                 31 
                 49 
                 22 
                 35 
                 42 
                 — 
               
               
                 (Type A) 
               
               
                 Compression set 
                 91 
                 95 
                 15 
                 26 
                 4 
                 — 
               
               
                 (%) 
               
               
                 Viscosity (Pa · s) 
                 25 
                 200 
                 27 
                 215 
                 300 
                 — 
               
               
                 Heat conductivity 
                 0.6 
                 1.0 
                 0.6 
                 1.0 
                 0.6 
                 the target 1.0 
               
               
                 (W/m · k) 
                   
                   
                   
                   
                   
                 (compounding 
               
               
                   
                   
                   
                   
                   
                   
                 impossible) 
               
               
                   
               
               
                   *1The rubber composition became clumpy when 200 parts of ground quartz was compounded, and it could not be compounded beyond the level.    
               
             
          
         
       
     
         [0051]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
               
               
                   
                   
                 Average particle size* 
               
               
                 Heat conductive filler 
                 Type 
                 (μm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Metallic silicon powder A 
                 Ground 
                 12 
               
               
                 Metallic silicon powder B 
                 Ground 
                 5 
               
               
                 Metallic silicon powder C 
                 Ground 
                 10 
               
               
                 Aluminum powder 
                 Spherical particles 
                 5 
               
               
                 Aluminum oxide 
                 Spherical particles 
                 12 
               
               
                 Ground quartz 
                 Ground 
                 5 
               
               
                   
               
               
                   *laser diffraction method    
               
             
          
         
       
     
         [0052]     Japanese Patent Application No. 2005-337072 is incorporated herein by reference.  
         [0053]     Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.