WO2008013319A1

WO2008013319A1 - Silicone rubber composition, method for producing the same, heat-curable silicone rubber composition, and rubber sealing part for automobiles

Info

Publication number: WO2008013319A1
Application number: PCT/JP2007/065120
Authority: WO
Inventors: Kenji Ota; Takashi Mizushima; Hideyuki Imai; Takahiro Iwata
Original assignee: Dow Corning Toray Co., Ltd.; Toyoda Gosei Co., Ltd.
Priority date: 2006-07-26
Filing date: 2007-07-26
Publication date: 2008-01-31
Also published as: WO2008013319A9; JP2008031227A

Abstract

A silicone rubber composition comprising (A-1) a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule in which at least 48 % by mole of silicon atom-bonded groups are fluorine atom-substituted alkyl groups, (A-2) a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule and having no silicon atom-bonded fluorine atom-substituted alkyl group with the proviso that the weight ratio of component (A-1) to component (A-2) is from 70:30 to 98:2, (B) a dry-process silica of which surface has been hydrophobically treated with a cyclic dimethylsiloxane oligomer, (C) a silanol group-containing organosiloxane oligomer or organosilane. A heat-curable silicone rubber composition comprising the silicone rubber composition and (D) a curing catalyst. A rubber sealing part for automobiles resulting from curing of the heat-curable silicone rubber composition.

Description

DESCRIPTION

SILICONE RUBBER COMPOSITION, METHOD FOR PRODUCING THE

SAME, HEAT-CURABLE SILICONE RUBBER COMPOSITION, AND RUBBER SEALING PART FOR AUTOMOBILES

Technical Field

[0001] The present invention relates to a silicone rubber composition comprising an alkenyl group -containing diorganopolysiloxane and a dryprocess silica, a method for the preparation thereof, a heat-curable silicone rubber composition comprising the silicone rubber composition and a curing agent, and a rubber sealing part for automobiles resulting from curing of said composition.

Background Art [0002] Rubber sealing parts used in contact with fuel oils in engine compartments of automobiles include gaskets, O-rings, and the like which are employed for installing intake manifolds, throttle bodies, insulator valves, PCV valves, and the like, or constitute them. Such rubber sealing parts are generally made of hydrogenated nitrile rubber. The elevation of atmosphere temperatures in engine compartments due to upgrading of engines, countermeasures against exhaustion gas regulations, and the like is increasing quantity of fluorosilicone rubber sealing parts used in engine compartments of automobiles.

[0003] A fluorosilicone rubber composition comprising a fluoropropylmethylpolysiloxane containing a specific amount of alkenyl groups, a silica-type filler and a curing catalyst for intake manifold gaskets is proposed in JP Kokai 2004-269757(JP 2004-269757 A). But such fluorosilicone rubbers ^■ have problems such that they are expensive, have inferior handling properties and fabrication workability due to strong adhesion to metals before curing, tend to tear while being taken out from a metal mold due to inferior release properties when heat-molded, and have higher percentage defectives in molding.

[0004] In view of these, it is said that a mixture of a fiuorosilicone rubber composition comprising a fluoroalkyl group -containing diorganopolysiloxane and a silicone rubber composition comprising a diorganopolysiloxan free of fluoroalkyl group is cheap, exhibits excellent handling properties and mold-release properties, and can be alternative to fiuorosilicone rubbers. For example, it is proposed in JP Kokai H10-89486(JP H10-89486 A) to constitute a gasket for cylinder head cover with a mixture of a fiuorosilicone rubber and a silicone rubber free of fluoroalkyl group.

[0005] But it has been found that since a fluoroalkyl group -containing diorganopolysiloxane, a predominant component of fiuorosilicone rubbers, and a diorganopolysiloxan free of fluoroalkyl group, a predominant component of fiuorosilicone rubbers have no compatibility each other, molded articles resulting from curing of a mixture of these diorganopolysiloxanes tend to delaminate and break depending on heat-molding conditions or product shapes. This tendency is distinctive when molded articles are kept at high temperature or under a condition such that they tend to swell due to fuel oils.

Concretely speaking, molded articles tend to cleave and break when they are taken out from a metal mold for heat-molding, when flashes thereof are removed at high temperature, or when they are kept under a condition such that they tend to swell, for example, in a state immersed in a fuel oil, or in contact with a fuel oil. This means that there are problems such that these molded articles can not satisfy product functions.

Summary of the Invention

[0006] It is an object of the present invention to provide a heat-curable silicone rubber composition comprising a fluoroalkyl group -containing diorganopolysiloxane, a diorganopolysiloxan free of fluoroalkyl group, and a silica-type filler where silicone rubber molded articles resulting from curing in a metal mold scarcely suffer from a delamination fracture i.e. a fracture involving delamination when they are taken out from the metal mold, flashes on the metal mold and flashes extending from the silicone rubber molded articles are easy to be removed, and silicone rubber molded articles utilized in contact with a fuel oil or in a state immersed in a fuel oil scarcely suffer from a delamination fracture i.e. a fracture involving delamination, to provide a silicone rubber composition for producing the heat-curable silicone rubber composition, and to provide a method for producing the silicone rubber composition. It is a further object of the present invention to provide a rubber sealing part that scarcely suffers from a delamination fracture i.e. a fracture involving delamination even if utilized in contact with a fuel oil or in a state immersed in a fuel oil.

[0007] The aforementioned problems are solved by the following means;

[l] A silicone rubber composition comprising a heated mixture of 100 parts by weight of the following component (A-I) and the following component (A-2) combined, 10 to 100 parts by weight of the following component (B), and the following component (C) in an amount corresponding to 1 to 10 % by weight of component (B).

(A-I) a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule in which at least 48 % by mole of silicon atom-bonded groups are fluorine atom -substituted alkyl groups, (A-2) a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule and having no silicon atom-bonded fluorine atom-substituted alkyl group with the proviso that the weight ratio of component (A"l) to component (A-2) is from 70:30 to 98^2,

(B) a dry-process silica of which surface is hydrophobically treated with a cyclic dimethylsiloxane oligomer, (C) a silanol group -containing organosiloxane oligomer or organosilane. [2] The silicone rubber composition according to [l], wherein component (B) has a BET specific surface area of 40 to 400m² /g, a bulk density of from 70 to 200kg/m³ , a carbon atom content of from 1.7 to 4.0 % by weight, a water content measured in accordance with Kahl Fischer method of 0.30 % by weight or less, a hexane extraction rate of 3.0 % by weight or less.

[3] The silicone rubber composition according to [l], wherein the fluorine atom-substituted alkyl group of component (A-I) is perfluoroalkyl group, alkenyl group is vinyl group, the other organic group is methyl group, alkenyl group of component (A- 2) is vinyl group, and the other organic group is methyl group. [4] The silicone rubber composition comprising the silicone rubber composition according to [l] , and 100 or less parts by weight of (E) an inorganic filler other than component (B) per 100 parts by weight of component (A-l) and component (A-2) combined. [5] A heat-curable silicone rubber composition comprising the silicone rubber composition according to [l] or [4] , and (D) a curing catalyst in a sufficient amount to cure said silicone rubber composition.

[6] The heat-curable silicone rubber composition according to [5], wherein the curing catalyst is an organic peroxide or a combination of an organohydrogenpolysiloxane and a platinum-type catalyst. [7] The heat-curable silicone rubber composition according to [5] or [6], wherein it is for a rubber sealing part for automobiles.

[8] A method for the preparation of the silicone rubber composition according to [l], wherein component(A-l), component(A-2), component(B), and component(C) are blended at less than 100°C, and subsequently are blended to homogeneity at 100 to 17O⁰C.

[9] A rubber sealing part for automobiles comprising a cured product of the heat-curable silicone rubber composition according to [5] or [6]. [lθ] The rubber sealing part for automobiles according to [9], wherein it is used in contact with fuel oils. [0008] The heat-curable silicone rubber composition of the present invention exhibits excellent molding processability and has a large molding yield since a silicone rubber molded article resulting from curing in a metal mold scarcely suffers from a delamination fracture i.e. a fracture involving delamination when it is taken out without cooling from the metal mold.

Flashes on a metal mold and flashes extending from a silicone rubber molded article, which have resulted from curing of the silicone rubber composition flowing out from the cavity of a metal mold onto surrounding area when an upper die and a bottom die have been contacted closely, can be easily removed, since the hot silicone rubber molded article has excellent tensile strength and scarcely suffers from a delamination fracture i.e. a fracture involving delamination.

The heat-curable silicone rubber composition of the present invention exhibits excellent molding proessability and has a large molding yield. Blending of the silicone rubber composition of the present invention with a curing agent provides a heat-curable silicone rubber composition exhibiting excellent characteristics described above. The method for producing the silicone rubber composition of the present invention enables to produce a silicone rubber composition, which is a predominant component of the heat-curable silicone rubber composition exhibiting excellent characteristics described above, efficiently and adequately.

The rubber sealing part of the present invention, which is a silicone rubber molded article prepared by heat-curing the aforementioned heat-curable silicone rubber composition, scarcely suffers from a delamination fracture i.e. a fracture involving delamination even if utilized in contact with a fuel oil or in a state immersed in a fuel oil, has excellent fuel oil resistance, small compression set, and excellent cold resistance.

Brief Description of the Drawings [0009] Fig. 1 is a plain view of the O-ring used in the testing of the fuel oil resistance in the working examples and comparative examples. Fig. 2 is a X-X' cross-section view of the O-ring in Figure 1. 1 — O-ring

Best Mode for Carrying Out the Invention

[OOIO] The silicone rubber composition of the present invention comprises a heated mixture of 100 parts by weight of the following component (A-l) and the following component (A-2) combined, from 10 to 100 parts by weight of the following component (B), and the following component (C) in an amount corresponding to from 1 to 10 % by weight of component (B). (A-I) a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule in which at least 48 % by mole of silicon atom-bonded groups are fluorine atom-substituted alkyl groups, (A-2) a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule and having no silicon atom-bonded fluorine atom-substituted alkyl group with the proviso that the weight ratio of component (A-l) to component (A-2) is from 70:30 to 98^2,

(B) a dry-process silica of which surface has been hydrophobically treated with a cyclic dimethylsiloxane oligomer,

(C) a silanol group -containing organosiloxane oligomer or organosilane.

The silicone rubber composition of the present invention can further comprise (E) an inorganic filler.

[0011] Component (A-l) and component (A-2) are predominant components of the silicone rubber composition and the heat-curable silicone rubber composition of the present invention. Component (A-l) and component (A-2) are cross-linked under the influence of (D) a curing agent, since they have at least two silicon atom-bonded alkenyl groups per molecule. The molecular structure each of component (A-l) and component (A- 2) is linear, or partially branched linear.

[0012] Component (A-l) is a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule in which at least 48 % by mole of silicon atom-bonded groups are fluorine atom-substituted alkyl groups, namely, fluoroalkyl groups. Examples of fluoro alkyl groups include 1-fluoromethyl, 1,1-difluoromethyl, 1,1,1-trifluoromethyl, 2-fluoroethyl, 2,2-difluoroethyl, 2,2,2-trifluoroethyl, fluoropropyl, 3,3,3-trifiuoropropyl, perfluorobutyl, perfluoropentyl, perfluorohexyl, perfluorooctyl, perfluorodecyl, and other fluorine atom -substituted alkyl groups with from 1 to 10 carbon atoms. 3,3,3-trifluoropropyl is preferable among them from the viewpoint of quality and easiness to produce. When Component (A-l) contains fluorine atom-substituted alkyl groups in an amount of less than 48 % by mole of silicon atom-bonded groups, a cured product has insufficient properties such as fuel oil-resistance ant the like. It is not easy to produce a diorganopolysiloxane having fluorine atom-substituted alkyl groups exceeding 55 % by mole of silicon atom-bonded groups, and such diorganopolysiloxane is not practical.

[0013] Examples of alkenyl groups bonded to silicon atoms in the diorganopolysiloxane include vinyl, propenyl, butenyl, and hexenyl, with vinyl being preferred from the viewpoint of cross-linking reactivity and easiness to produce. The alkenyl groups can be located at the molecular terminal positions, pendant positions or the both. Examples of groups bonded to silicon atoms other than fluorine atom-substituted alkyl groups and alkenyl groups include methyl, ethyl, propyl, butyl, hexyl, and other alkyl groups with 1 to 6 carbon atoms; phenyl, naphtyl, and other aryl groups with from 6 to 10 carbon atoms; tolyl, benzyl, and other aryl groups with 7 to 10 carbon atoms with methyl being preferred from the viewpoint of easiness to produce. A hydroxyl group, namely, silanol group can bond to one molecular terminal or two molecular terminals of the diorganopolysiloxane.

[0014] Component (A-I) needs to have at least two alkenyl groups per molecule from the standpoint of cross-linking properties. It is preferable that from 0.1 to 1 % by mole of groups bonded to silicon atoms are alkenyl groups, and it is more preferable that from 0.125 to 0.5 % by mole of groups bonded to silicon atoms are alkenyl groups. The basis for this is as follows; if the molar percentage of alkenyl groups contained in groups bonded to silicon atoms is less than the aforementioned lower limit, the value of compression set of a cured product tends to become too large, while if the molar percentage of alkenyl groups contained in groups bonded to silicon atoms exceeds the aforementioned upper limit, the hardness of a cured product tends to exceed a practical range, the breakage elongation of a cured product tends to become too small, and the tensile strength, tear strength, and other mechanical strength of a cured product tends to decline.

[0015] The weight average molecular weight of component (A-I) is preferably 100,000 or more, more preferably from 200,000 to 9,000,000, and specifically preferably 450,000 to 4,500,000. The weight average molecular weight of component (A-I) can be determined by measuring with gel permeation chromatography (GPC).

Tosoh HLC-8020 Gel Permeation (GPC) system from Tosoh Corporation attached with a refraction detector and two "TSKgel GMHXL-L" columns from Tosoh Corporation can be utilized as a GPC instrument.

Samples are preferably subjected to measurements in a 2-wt.% chloroform solution. Calibration curves are prepared using standard polystyrene of known weight- average molecular weights. The weight-average molecular weight is determined by referencing to the standard polystyrene. [0016] Examples of the diorganopolysiloxane as component (A-I) include methyl(3,3,3-trifluoropropyl)siloxane/methylvinylsiloxane copolymer endblocked with silanol groups at both terminals, methyl(3 ,3,3- trifluoropropyl) siloxane/methylvinylsiloxane/dimethylsiloxane copolymer endblocked with silanol groups at both terminals, methyl(3,3,3-trifluoropropyl)siloxane/vinylphenylsiloxane copolymer endblocked with silanol groups at both terminals, methyl(3,3,3-trifluoropropyl)polysiloxane endblocked with dimethylvinylsiloxy groups at both terminals, methyl(3,3,3-trifluoropropyl)siloxane/dimethylsiloxane copolymer endblocked with dimethylvinylsiloxy groups at both terminals, methyl(3 ,3,3- trifluor op ropyl) siloxane/methylvinylsiloxane cop oly mer endblocked with dimethylvinylsiloxy groups at both terminals, methyl(3,3,3-trifluoropropyl)siloxane/methylvinylsiloxane/dimethylsiloxane copolymer endblocked with dimethylvinylsiloxy groups at both terminals.

[0017] Component (A-2) is a diorganopolysiloxane having at least two silicon-atom bonded alkenyl groups per molecule and being free from halogen atom -substituted organic group. Examples of alkenyl groups include vinyl group, allyl group, butenyl group, hexenyl group, and so on.

Examples of groups bonded to silicon-atoms other than alkenyl groups include methyl, ethyl, propyl, butyl, hexyl, and other alkyl groups having from 1 to 6 carbon atoms! phenyl, naphtyl, and other aryl groups having from 6 to 10 carbon atoms; and tolyl, benzyl, and other aralkyl groups having from 7 to 10 carbon atoms.

Component (A-2) needs to have at least two alkenyl groups per molecule. It is preferable that from 0.1 to 1 % by mole of groups bonded to silicon atoms are alkenyl groups, and more preferable that from 0.125 to 0.8 % by mole of groups bonded to silicon atoms are alkenyl groups. The basis for this is as follows; if the molar percentage of alkenyl groups contained in groups bonded to silicon atoms is less than the aforementioned lower limit, the value of compression set of a cured product tends to become too large, and if the molar percentage of alkenyl groups contained in groups bonded to silicon atoms exceeds the aforementioned upper limit, the hardness of a cured product tends to exceed a practical range, the breakage elongation of a cured product tends to become too small, and the tensile strength, tear strength, and other mechanical strength of a cured product tends to decline.

[0018] The weight average molecular weight of component (A- 2) is preferably 100,000 or more, more preferably the range from 200,000 to

9,000,000, and specifically preferably he range from 450,000 to 4,500,000. The weight average molecular weight of component (A-I) can be determined by measuring with the aforementioned gel permeation chromatography (GPC).

[0019] Examples of the diorganopolysiloxane as component (A-2) include dimethylsiloxane/methylvinylsiloxane copolymer endblocked with silanol groups at both terminals, methylphenylsiloxane/methylvinylsiloxane copolymer endblocked with silanol groups at both terminals, diphenylsiloxane/methylvinylsiloxane copolymer endblocked with silanol groups at both terminals, dimethylsiloxane/diphenylsiloxane/methylvinylsiloxane endblocked with dimethylvinylsiloxy groups at both terminals, dimethylpolysiloxane endblocked with dimethylvinylsiloxy groups at both terminals, dimethylsiloxane/methylvinylsiloxane copolymer endblocked with dimethylvinylsiloxy groups at both terminals, methylphenylpolysiloxane endblocked with dimethylvinylsiloxy groups at both terminals, dimethylsiloxane/methylphenylsiloxane/methylvinylsiloxane copolymer endblocked with dimethylvinylsiloxy groups at both terminals, diphenylsiloxane/methylvinylsiloxane copolymer endblocked with dimethylvinylsiloxy groups at both terminals, The combination such that component (A-I) is a diorganopolysiloxane of which the fluorine atom-substituted alkyl group is a perfluoroalkyl group, the alkenyl group is vinyl group, and other organic group is methyl group, and component (A- 2) is a diorganopolysiloxane of which the alkenyl group is vinyl group, and other organic group is methyl group is preferable from the viewpoint of compatibility and physical properties of a cured product.

[0020] Component (A-I) and component (A-2) are combined with other components in such weight ratio that component (A-I) to component (A-2) is from 70:30 to 98:2. The basis for this is as follows; if the ratio of component (Al) to component (A2) is less than the lower limit of the aforementioned range, a silicone rubber resulting from curing of a heat-curable silicone rubber composition containing this component tends to swell so much when it is immersed in a fuel oil or continues to be in contact with a fuel oil, while if the ratio of component (Al) to component (A2) exceeds the upper limit of the aforementioned range, a cost tends to become high, and a heat-curable silicone rubber composition containing this component tends to have a declined roll-workability.

[0021] Component (B) is a dry-process silica of which surface has been hydrophobically treated with a dimethylsiloxane oligomer. It acts to change a mixture of component (A-I) and component (A-2) to clay-like or semi-solid state, and imparts excellent mechanical strength to a silicone rubber resulting from curing of a heat-curable silicone rubber composition containing this component, and it acts to improve fuel oil-resistance of a silicone rubber resulting from curing of a heat-curable silicone rubber composition containing this component, though a mechanism thereof is not clarified.

[0022] It is preferable that component (B) has a BET specific surface area of from 40 to 400m² /g, a bulk density of from 70 to 200kg/m³ , a carbon atom content of from 1.7 to 4.0 % by weight, a water content measured in accordance with Kahl Fischer method of 0.30 % by weight or less, a hexane extraction rate of 3.0 % by weight or less, from the standpoint of mechanical properties and fuel oil resistance of a silicone rubber resulting from curing of a heat-curable silicone rubber composition containing this component.

[0023] Component (B) has a BET specific surface area of preferably from 40 to 400m² /g, and more preferably from 100 to 400m² /g, and a bulk density of preferably from 70 to 200kg/m³ , and more preferably from 80 to 150kg/m³ . The bulk density is measured by the following method.

The dry-process silica of which surface has been hydrophobically treated with a dimethylsiloxane oligomer is slowly filled into a 1 L(l000cc)-measuring cylinder to a scale of lOOOcc. Filling the dry-process silica is continued for 3 minutes after having begun to fill so as to keep a volume of lOOOcc since the dry-process silica continues to sink. A weight of the dry process-silica in the measuring cylinder after 3 minutes is measured, and the measured value represented with kg/m³ is a bulk density.

[0024] A water content of component (B) measured in accordance with Kahl Fischer method is preferably 0.30% by weight or less, and more preferably 0.25% by weight or less. A water content of component (B) is determined by drying a dry-process silica, of which surface has been hydrophobically treated with a dimethylsiloxane oligomer, in an oven set at 110 ⁰C for 10 hours and measuring a water content of the dried dry-process silica with Kahl Fischer method. [0025] A carbon atom content of Component (B) is preferably from 1.7 to 4.0 % by weight, and more preferably from 1.8 to 3.5 % by weight. The carbon atom content of Component (B) is determined by thermally decomposing methyl groups bonded to silicon atoms of a dry-process silica, of which surface has been hydrophobically treated with a dimethylsiloxane oligomer, at 1000 ⁰C in oxygen gas to convert them to carbon dioxide, and by quantitatively analyzing a carbon atom content in the carbon dioxide gas by means of a micro carbon analysis instrument. The carbon atom content indicates a treatment degree or a hydrophobicity degree of the surface of a dry-process silica, of which surface has been hydrophobically treated with a hydrophobically treating agent, namely, a cyclic dimethylsiloxane oligomer

[0026] A hexane extraction rate of component (B) is preferably 3.0 % by weight or less, and more preferably 2.5 % by weight or less. The hexane extraction rate is determined by measuring a carbon atom content (B) of a dry-process silica of which surface has been hydrophobically treated before extraction with hexane, measuring a carbon atom content (A) of the dry-process silica after extraction with hexane, and calculating ( B - A) /⁷B x I O O (unit: %) . The hexane extraction is carried out by loading 20 grams of a dry-process silica and 150 grams of n-hexane into a 500 cc flask equipped with a stirrer and condenser, boiling the hexane for 30 minutes, and filtrating the dry-process silica dispersed in hexane. It is considered that the hexane extraction rate demonstrates a degree of containing hydrophobically treating agent, namely, a cyclic dimethylsiloxane oligomer which sticks to the dry-process silica without bonding to it. A large carbon atom content and small hexane extraction rate means a large degree in chemical bonding of a hydrophobically treating agent, namely, a cyclic dimethylsiloxane oligomer to the surface of a dry-process silica. [0027] Examples of cyclic dimethylsiloxane oligomers used for hydrophobically treating the surface of a dry-process silica include octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, tetradecamethylcycloheptasiloxane, hexamethylcyclotrisiloxane, and a mixture thereof. The upper limit of the polymerization degree thereof is preferably 20, and more preferably 10. Methods for hydrophobically treating the surface of a dry-process silica are known in the art [see, for example, JP Kokoku S36- 15938 (JP S36- 15938 Bl), US 2,938,009 A, JP Kokoku H05-25893 (JP H05-25893 Bl] where a dry-process silica is contacted with a liquid cyclic dimethylsiloxane oligomer and heat-treated for prescribed time period.

A treatment for increasing a bulk density is also conducted in JP Kokoku H05-25893(JP H05-25893 Bl). The dry-process silica of which surface has been hydrophobically treated employed in the present invention has the aforementioned controlled bulk density, carbon atom content, water content and hexane extraction rate as a result of intensively elaborating treating conditions. This component is added in the range of from 10 to 100 parts by weight per 100 parts by weight of component (A-I) and component (A-2) combined. The basis for this is as follows! a silicone rubber composition containing less than 10 parts by weight of this component does not have enough mechanical strength after curing, and it is difficult to add this component exceeding 100 parts by weight to 100 parts by weight of component (A-I) and component (A-2) combined. From this point of view, from 20 to 70 parts by weight is preferable.

[0028] Component (C), a silanol group -containing organosiloxane oligomer or organosilane, is a component for enhancing the compatibility between component (A-l) and component (A-2) and component (B).

Examples of organic groups bonded to silicon atoms other than silanol group of component (C) include 1-fluoromethyl , 1,1-difluoromethyl , 1,1,1-trifluoromethyl, 2-fluoroethyl , 2,2-difluoroethyl , 2,2,2-trifluoroethyl , perfluoroethyl, 3-fluoropropyl, 3,3,3-trifluoropropyl, perfluorobutyl, perfluoropentyl, perfluorohexyl, perfluorooctyl, perfluorodecyl, and other fluorine atom-substituted alkyl groups with from 1 to 10 carbon atoms! vinyl, allyl, propenyl, butenyl, hexenyl, and other alkenyl groups; methyl, ethyl, propyl, butyl, hexyl, and other alkyl groups with 1 to 6 carbon atoms; phenyl, tolyl, naphtyl, and other aryl groups with from 6 to 10 carbon atoms; benzyl, and other aralkyl groups with 7 to 10 carbon atoms. Preferable are groups selected from the group consisting of 3,3,3-trifluoropropyl, vinyl, methyl, and phenyl groups. Herein, the organosiloxane oligomer means a low polymerization degree diorganopolysiloxane having from 2 to 20 on average diorganosiloxane units, namely, a diorganopolysiloxane having a number average polymerization degree calculated from the number average molecular weight of 2 to 20 on average.

[0029] The following diorganosiloxane oligomers and organosilanes are exemplifiedas component (C) ; methyl(3,3,3-trifluoropropyl)siloxane oligomer endblocked with silanol groups at both terminals, methyl(3,3,3-trifluoropropyl)siloxane oligomer endblocked with a silanol group at one terminal and a methyl group at another terminal, dimethysiloxane oligomer endblocked with silanol groups at both terminals, dimethysiloxane oligomer endblocked with a silanol group at one terminal and a methyl group at another terminal, methylvinylsiloxane oligomer endblocked with silanol groups at both terminals, methyl vinylsiloxane oligomer endblocked with a silanol group at one terminal and a methyl group at another terminal, methylphenylsiloxane oligomer endblocked with silanol groups at both terminals, methylphenylsiloxane oligomer endblocked with a silanol group at one terminal and a methyl group at another terminal, dimethysiloxane/methylphenylsiloxane cooligomer endblocked with silanol groups at both terminals; trimethylsilanol, and diphenylsilanediol. Methyl(3,3,3-trifluoropropyl)siloxane oligomer endblocked with silanol groups at both terminals is preferable among these since it remarkably enhances the compatibility between component (A-l) and component (A- 2) and component (B). Methyl(3,3,3-trifluoropropyl)siloxane oligomer having a number average polymerization degree of 3 and endblocked with silanol groups at both terminals is specifically preferable.

[0030] Component (C) is added in the range from 1 to 10 % by weight, and preferably from 2 to 8 % by weight of component (B). The basis for this is as follows; if the concentration of component (C) is less than the lower limit of the aforementioned range, the silicone rubber composition and heat-curable silicone rubber composition of the present invention tends to have declined working processability and storage stability, impaired flowability, and declined moldability, and if the concentration of component (C) exceed the upper limit of the aforementioned range, a silicone rubber article resulting from curing the heat-curable silicone rubber composition of the present invention tends to have a declined fuel-oil resistance, namely, declined strength when utilized in contact with a fuel oil or in a state immersed in a fuel oil.

[0031] The silicone rubber composition of the present invention can be produced by blending component (A"l), component (A-2), component (B), and component (C) at less than 100°C, and subsequently blending them to homogeneity at from 100 to 170°C.

In detail, it can be manufactured through the following two processes. Process l: component (A-l), component (A-2), component (B), and component (C) are blended nearly to homogeneity at less than 100°C.

Process 2' the mixture obtained in Process 1 is further blended to homogeneity at from 100°C to 17O⁰C.

It is preferable to use a mixing apparatus equipped with a heating/cooling measure known in the prior art such as Banbary mixer and kneader mixer for blending component (A-I), component (A- 2), component (B), and component (C) in Process 1, and for blending under heating in Process 2.

[0032] Blending of component (A-l), component (A-2), component (B), and component (C) in Process 1 might be conducted at room temperature, but is conducted preferably at not less than 5O⁰C and less than 100⁰C, and more preferably at not less than 8O⁰C and less than 100°C. Heating decreases the viscosity of component (A) to facilitate blending with component (B).

[0033] The heating temperature in Process 1 is from 100°C to 17O⁰C, and preferably from 120°C to 150°C.

The basis for this is as follows; if the heating temperature is less than the lower limit of the aforementioned range, the heat-curable silicone rubber composition of the present invention tends to have insufficient molding processability and fuel-oil resistance, and the silicone rubber composition and heat-curable silicone rubber composition of the present invention tends to become hard and have declined molding processability, and if the heating temperature exceeds the upper limit of the aforementioned range, the silicone rubber composition of the present invention tends to have gels during mixing under heating. The time period for mixing under heating in Process 2 is preferably from 30 minutes to 6 hours, but more preferably from 1 hour to 4 hours.

[0034] The silicone rubber composition of the present invention can further comprises component (E), an inorganic filler other than component (B). Examples of component (E) include finely divided quartz, diatomaceous earth powder, spherical silica particulate and other non-reinforcing silica type fillers; acetylene black, furnace black, channel black, and other carbon black; light calcium carbonate, heavy calcium carbonate, and other calcium carbonate powder; magnesium oxide powder, mica powder. Non-reinforcing silica type fillers such as finely divided quartz, diatomaceous earth powder, and the like are preferable since they are cheap, and act to enhance handling workability of the heat-curable silicone rubber composition, and to depress swelling values when immersed in a fuel oil without impairing mechanical properties of a silicone rubber resulting from curing of the heatxurable silicone rubber composition.

Component (E) is blended preferably in an amount of 100 parts by weight or less, and more preferably in an amount of from 5 to 50 parts by weight per 100 parts by weight of component (A-I) and component (A-2) combined. Of course, The preferable amount of component (E) may differ depending on the amount of component (B), applications of silicone rubber articles, kinds of component (E), and the like.

Blending of the silicone rubber composition and component (E) can be conducted by cooling the silicone rubber composition after Process 2, loading component (E) to the cooled silicone rubber composition, and blending them to homogeneity, and also can be conducted by blending on a two-roll mill or three-roll mill.

[0035] A curing catalyst, Component (D) is combined with the aforementioned silicone rubber composition or a mixture of the aforementioned silicone rubber composition and component (E) to cure under heating. It is exemplified by an organic peroxide, which is one of curing catalysts known in the prior art for silicone rubbers, and a combination of an organohydrogenpolysiloxane and a platinum type catalyst, with organic peroxides being preferred.

[0036] Such organic peroxides are exemplified by benzoyl peroxide, t-butyl perbenzoate, o-niethylbenzoyl peroxide, p-methylbenzoyl peroxide, di-t-butyl peroxide, dicumyl peroxide, l,l-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, 2,5"dimethyl-2,5-di(t-butylperoxy)hexane, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexine. These organic peroxides can be used singly or as a mixture of two or more organic peroxides. The amount of the curing catalyst is sufficient amount to cure the silicone rubber composition. The amount of the aforementioned organic peroxide is preferably 0.1 to 5 parts by weight per 100 parts by weight of component (A-l) and component (A-2) combined.

[0037] The combination of an organohydrogenpolysiloxane and a platinum type catalyst cross-links component (A-l) and component (A-2) under hydrosilylation reaction to cure them. The organohydrogenpolysiloxane is preferably methylhydrogenpolysiloxane, and the platinum type catalyst is preferably hydrogencloroplatinic acid and a platinum* divinyltetramethyldisiloxane complex.

The concentration of the combination of an organohydrogenpolysiloxane and a platinum type catalyst in the heat-curable silicone composition of the present invention is sufficient to cure the composition. Typically, the concentration of the organohydrogenpolysiloxane is sufficient to provide from 0.5 to 10 silicon-bonded hydrogen atoms per alkenyl group contained in component (A-l) and component (A-2) combined. Preferably, the concentration of the organohydrogenpolysiloxane is sufficient to provide from 1 to 3 silicon-bonded hydrogen atoms per alkenyl group contained in component (A-l) and component (A- 2) combined.

The concentration of the platinum type catalyst is sufficient to catalyze the addition reaction of component (A-l) and component (A-2) with the organohydrogenpolysiloxane. Typically, the concentration of platinum type catalyst is sufficient to provide from 0.1 to 1000 ppm of a platinum group metal, preferably from 1 to 500 ppm of a platinum group metal, and more preferably from 5 to 150 ppm of a platinum group metal, based on the combined weight of component (A-l) and component (A-2) and the organohydrogenpolysiloxane. Combination with a hydrosilylation reaction retarder is preferable for retarding curing at ambient temperature. [0038] The silicone rubber composition can further comprise optionally a coloring agent, heat-stability improver, flame retardant, oil resistance improver, acid receptor, thermal conductivity improver, release agent from metal molds, and the like besides optional component (E) and indispensable component (D) insofar as the object of the present invention is not impaired.

[0039] The heat-curable silicone rubber composition of the present invention can be produced by conducting Process 3 after Process 1 and Process 2, namely, cooling the silicone rubber composition obtained by Process 2, and adding component (D) to the cooled silicone rubber composition.

It is preferable to use a two-roll mill or three-roll mill which can be cooled since temperature rising during mixing causes curing of the composition in Process 3. It is preferable to add optional components such as a coloring agent, he at- stability improver, flame retardant, oil resistance improver, acid receptor, thermal conductivity improver, release agent from metal molds, and the like before adding component (D) in Process 3.

[0040] The rubber sealing part, specifically rubber sealing part for automobiles of the present invention comprises a silicone rubber resulting from curing of the heat curable silicone rubber composition of the present invention. It is suitable for rubber sealing parts in engine compartments of automobiles requiring heat resistance and cold resistance. It is suitable for rubber sealing parts in engine compartments of automobiles, and rubber sealing parts used in a state immersed in fuel oils for automobiles, ships, air planes, and the like, or used in contact with fuel oils for automobiles, ships, air planes, and the like. Concretely speaking, gaskets and O -rings which are employed for installing intake manifolds, throttle bodies, insulator valves, PCV valves, and the like, or constituting them are exemplified as the rubber sealing parts. [0041] "Used in contact with fuel oils" in the present invention includes being used in contact with vaporized fuel oils as well as used in a state immersed in liquid fuel oils and used in contact with liquid fuel oils. Examples of fuel oils include gasoline, alcohol-containing gasoline, kerosene, light oil, and heavy oil.

Examples

[0042] The present invention will be explained concretely with reference to working examples and comparative examples, however, is not limited by the following examples. Parts indicating amounts of components to be added in the following examples are based on weight unless otherwise specified.

[0043] Materials and abbreviation thereof employed as components in the following working examples and comparative examples are as follows; (l-l) component (A-I) a — 1 : methyl(3, 3, 3-trifluoropropyl)siloxane/methylvinylsiloxane copolymer endblocked with silanol groups at both terminals (weight average molecular weight: 608,000, content of 3,3,3-trifluoropropyl group: 49.7 % by mole, content of methyl groups: 50.0 % by mole, content of vinyl groups: 0.29 % by mole) (1-2) component (A-2) a —2 : methyl(3,3,3-trifluoropropyl)siloxane/methylvinylsiloxane copolymer endblocked with silanol groups at both terminals (weight average molecular weight: 600,000, content of methyl groups: 99.4 % by mole, content of vinyl groups: 0.57 % by mole)

[0044]

(2) dry-process silica b— 1 (component (B)) : dry-process silica of which surface had been hydrophobically treated with octamethylcyclotetrasiloxane (a product of a silica manufacturer, BET specific surface area of 160m² /g, carbon atom content of from 2.0 % by weight, bulk density of 100kg/m³ , water content of 0.10 % by weight, hexane extraction rate of 1.8 % by weight) b~ 2 : dry-process silica of which surface had been hydrophobically treated with dimethyldichlorosilane (a product of a silica manufacturer, BET specific surface area of 180m² /g, carbon atom content of from 1.6 % by weight, bulk density of 50kg/m³ , water content of 0.19 % by weight, hexane extraction rate of 1.3 % by weight) b — 3 : dry-process silica of which surface had not been treated (a product of NIPPON AEROSIL CO., LTD., product name: AEROSIL 200 (AEROSIL is registered trade mark owned by Degussa Corporation), BET specific surface area of 200m² /g, carbon atom content of from 0 % by weight, bulk density of 50kg/m³ , water content of 0.30 % by weight, hexane extraction rate of 0 % by weight)

[0045]

(3) component(C) silanol-group-containing organosiloxane oligomer: methyl(3,3,3-trifluoropropyl)siloxane/methylvinylsiloxane copolymer endblocked with silanol groups at both terminals (number average polymerization degree: 3)

(4) component(E) quartz powder: MIN-U-SIL 10 (a product of U.S.Silica Company, MIN-U-SIL is a trade mark owned by said company, compressed bulk density: 721kg/m³, mrcompressed bulk density: 673kg/m³, medium size: 3.4μm, specific gravity: 2.65) quartz powder: MIN-U-SIL 5 (a product of U.S.Silica Company, MIN-U-SIL is a trade mark owned by said company, compressed bulk density: 659kg/m³, un-compressed bulk density: 577kg/m³, medium size: 1.7μm, specific gravity: 2.65) diatomaceous earth- Radiolite F (a product of Show Chemical Industry Co., Ltd., Radiolite is a registered trade mark owned by Showa Chemical Industry Co., Ltd., average diameter: 13.7μm) (5) component (D) organic peroxide: 2,5-dimethyl-2,5-di(t-butylperoxy) hexane

[0046] Properties of a cured heat-curable silicone rubber composition were measured and evaluated after curing in the following manner. [O -ring Moldability] A heat-curable silicone rubber composition was subjected to compression molding in a metal mold at 170°C for 10 minutes to form a Oτing made of silicone rubber with an outer diameter of 245 mm, an inner diameter of 145 mm, and a cross-sectional diameter of 5 mm. The O-ring was taken out from the metal mold, and flashes were removed, observing how flashes were cut. The O-ring Moldability was rated "excellent" in the case of easy removal of flashes from the O-ring, and was rated "breakage" in the case of fissure generation in the O-ring or scission of the O-ring during removing the flashes.

[0047] [Mechanical Properties] A heat-curable silicone rubber composition was subjected to compression molding in a metal mold at 170°C for 10 minutes to form a 2 mm thick silicone rubber sheet.

This silicone rubber sheet was cut to prepare prescribed test pieces.

Tensile strength and elongation at break of the test pieces were measured in accordance with the tensile testing method defined in JIS K6251, and the hardness of the test pieces were measured using a type A durometer in accordance with the hardness testing method defined in JIS K6253.

A silicone rubber with a hardness of 60 or more, tensile strength of 5.0 MPa or more, and elongation at break of 150 % or more was judged to be preferable one in terms of product functions. [0048] [Compression Set Percentage]

A heat-curable silicone rubber composition was subjected to compression molding in a metal mold at 17O⁰C for 10 minutes to form a cylindrical silicone rubber test piece with a diameter of 29 mm and a thickness of 12.5 mm. The test piece compressed between metal plates while compressing the test piece to 25 % of the initial thickness was left to stand at 175⁰C for 72 hours, the compressed test piece taken out from between the metal plates was left to stand to room temperature, and the thickness of the cooled test piece was measured to calculate a compression set percentage in accordance with JIS K6262.

[0049] [Fuel Oil Resistance]

The same 2 mm thick silicone rubber sheet as that for measuring mechanical properties was cut to form a rectangular silicone rubber test piece with a length of 50 mm and a width of 20 mm. The test piece was immersed in a fuel oil (2,2,4-trimethylpentene/toluene= 40/60) for testing at 40 °C for 72 hours to measure a volume change rate. The smaller the volume change rate, the better the fuel oil resistance. Volume change rates of +60 % or less were judged to be favorable fuel oil resistance. A cut measuring 1 mm in depth was made at the middle of a side with a length of 50 mm of the silicone rubber test piece having undergone the immersion test, and the both ends were pulled to opposite directions by grasping them to tear the test piece and to examine the fracture surface. Almost smooth fracture surface was rated "ordinary fracture", and non-uniform fracture surface such as a fracture involving delamination to two or more layers was rated "delamination fracture".

[0050] [Working Example 1, Working Example 2, Comparative Example 1 through Comparative Example 4] Heat-curable silicone rubber compositions were prepared from components in each amount shown in Table 1. The preparation method was as follows; The aforementioned components (A-I), (A"2), (B), and (C) were blended nearly to homogeneity at 80°C in a mixer. In working examples and comparative examples indicating "blended at 12O⁰C for 2 hours", the blended components were furthermore blended at 120°C for 2 hours to become a uniform semi-solid mixture. The resultant semi-solid mixture was cooled to 50°C or less, and was combined with component (E) (MIN-U-SIL lθ) to homogeneity. The resultant mixture was taken out from the mixer, and combined with the aforementioned component (D) on a two-roll mil.

[0051] The resultant heat-curable silicone rubber composition was subjected to compression molding in a metal mold at 170⁰C for 10 minutes, and was left to stand in an oven at 200°C for 4 hours for post-curing to form a silicone rubber molding to be evaluated. Mechanical properties, compression set, and fuel oil resistance of the silicone rubber molding were measured or evaluated in accordance with the aforementioned methods. O-ring moldability of the heat-curable silicone rubber composition was evaluated in accordance with the aforementioned method. Measurement results and evaluation results were listed in Table 1.

[0052]

Table 1

[0053] [Comparative Example 5]

Silicone rubber composition 5-1 and Silicone rubber composition 5-2 each were prepared by employing components in each amount listed in Table 2 in the same manner as in the aforementioned working examples.

Silicone rubber composition 5-1 and Silicone rubber composition 5-2 were blended in a weight ratio of 9^1 on a two-roll mill. A heat-curable silicone rubber composition was prepared from the resultant mixture in the same manner as in the aforementioned working examples to form a silicone rubber molding to be evaluated. Mechanical properties, compression set, and fuel oil resistance of the silicone rubber molding were measured or evaluated in accordance with the aforementioned methods. O-ring moldability of the heat-curable silicone rubber composition was evaluated in accordance with the aforementioned method. Measurement results and evaluation results were listed in Table 2.

[0054]

Table 2

[0055] [Working Example 3]

A heat-curable silicone rubber composition was prepared by employing components in each amount listed in Table 3. A method for the preparation is as follows j

The aforementioned components (A-I), (A-2), (B), and (C) were blended nearly to homogeneity at 80°C in a mixer, and the blended components were furthermore blended at 120°C for 2 hours to become a uniform semi-solid mixture. The resultant semi-solid mixture was cooled to 50^cC or less, was combined with the aforementioned component (D), and was blended to homogeneity on a two-roll mill.

The resultant heat-curable silicone rubber composition was subjected to compression molding in a metal mold at 170⁰C for 10 minutes, and was left to stand in an oven at 200⁰C for 4 hours for post-curing to form a silicone rubber molding to be evaluated. Mechanical properties, compression set, and fuel oil resistance of the silicone rubber molding were measured or evaluated in accordance with the aforementioned methods. O-ring moldability of the heat-curable silicone rubber composition was evaluated in accordance with the aforementioned method. Measurement results and evaluation results were listed in Table 3.

[0056] [Working Example 4 and Working Example 5]

A heat-curable silicone rubber composition was prepared by employing components in each amount listed in Table 3. A method for the preparation is as follows;

The aforementioned components (A-I), (A-2), (B), and (C) were blended nearly to homogeneity at 8O⁰C in a mixer, and the blended components were furthermore blended at 120°C for 2 hours to become a uniform semi-solid mixture. The resultant semi-solid mixture was cooled to 5O⁰C or less, was combined with the aforementioned component (E) (MIN-U-SIL 5 or Radiolite F) , and was blended to homogeneity. The resultant mixture was taken out from the mixer, and combined with component (D) on a two-roll mill. The resultant heat-curable silicone rubber composition was subjected to compression molding in a metal mold at 17O⁰C for 10 minutes, and was left to stand in an oven at 200⁰C for 4 hours for post-curing to form a silicone rubber molding to be evaluated. Mechanical properties, compression set, and fuel oil resistance of the silicone rubber molding were measured or evaluated in accordance with the aforementioned methods. O-ring moldability of the heat-curable silicone rubber composition was evaluated in accordance with the aforementioned method. Measurement results and evaluation results were listed in Table 3.

[0057]

[0058] It is understood from the comparison between each working example and Comparative Example 1 that the inventive heat-curable silicone rubber composition comprising the silicone rubber composition resulting from blending components (A-I), (A- 2), (B), and (C) and furthermore blending them at 120⁰C for 2 hours is superior in the O-ring moldability and the uniform fracture cross-section after immersion in the fuel oil for testing over the heat-curable silicone rubber composition comprising the silicone rubber composition omitting furthermore blending them at 12O⁰C for 2 hours.

[0059] It is understood from the comparison between each working example and Comparative Example 2 that the inventive heat-curable silicone rubber composition comprising the silicone rubber composition containing the specified dry-process silica of which surface had been hydrophobically treated as component (B) is superior in the small compression set and the uniform fracture cross-section after immersion in the fuel oil for testing over the heat-curable silicone rubber composition containing the dry-process silica of which surface had been hydrophobically treated and having different hexane extraction rate etc. in place of component (B).

[0060] It is understood from the comparison between each working example and Comparative Example 3 that the inventive heat-curable silicone rubber composition comprising the silicone rubber composition containing the specified amount of component (C) is superior in the uniform fracture cross-section after immersion in the fuel oil for testing over the heat-curable silicone rubber composition comprising the silicone rubber composition containing an excessive amount of component (C).

[0061] It is understood from the comparison between each working example and Comparative Example 4 that the inventive heat-curable silicone rubber composition comprising the silicone rubber composition containing the specified dry-process silica of which surface had been hydrophobically treated as component (B) is superior in the Oring moldability and the uniform fracture cross-section after immersion in the fuel oil for testing over the heat-curable silicone rubber composition containing the dry-process silica of which surface had not been hydrophobically treated.

[0062] It is understood from the comparison between each working example and Comparative Example 5 that the inventive heat-curable silicone rubber composition comprising the silicone rubber composition resulting from blending components (A"l), (A- 2), (B) and (C) at the same time and furthermore blending them under heating is superior in the O-ring moldability and the uniform fracture cross-section after immersion in the fuel oil for testing over the heat-curable silicone rubber composition comprising a mixture of the silicone rubber composition resulting from blending components (A-I), (B) and (C) at the same time and furthermore blending them under heating and the silicone rubber composition resulting from blending components (A-2), (B) and (C) at the same time and furthermore blending them under heating.

[0063] It is understood from the comparison between Working Example 1 and Working Example 2 that there is a tendency such that the larger the amount of component (A-2), the larger the volume change rate after immersion in the fuel-oil for testing.

Industrial Applicability

[0064] The heat-curable silicone rubber composition of the present invention is useful for molding rubber sealing parts used in a state immersed in fuel oils or in contact with fuel oils, and is useful for manufacturing rubber sealing parts needing heat stability, cold resistance, small compression set and the like used in contact with fuel oils in engine compartments of automobiles.

Examples of such rubber sealing parts include gaskets, O-rings, and the like which are employed for installing intake manifolds, throttle bodies, insulator valves, PCV valves, and the like, or constitute them. The silicone rubber composition of the present invention is useful for producing the aforementioned heat-curable silicone rubber composition.

A method for producing the silicone rubber composition of the present invention is useful for producing the silicone rubber composition which is the predominant component of the heat-curable silicone rubber composition of the present invention. Rubber sealing parts of the present invention are useful as rubber sealing parts used in a state immersed in fuel oils or in contact with fuel oils in engine compartments of automobiles.

Claims

1. A silicone rubber composition comprising a heated mixture of 100 parts by weight of the following component (A-I) and the following component (A-2) combined, 10 to 100 parts by weight of the following component (B), and the following component (C) in an amount corresponding to 1 to 10 % by weight of component (B).

(A-I) a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule in which at least 48 % by mole of silicon atom-bonded groups are fluorine atom-substituted alkyl groups,

(A-2) a diorganopolysiloxane having at least two silicon atom-bonded alkenyl groups per molecule and having no silicon atom-bonded fluorine atom -substituted alkyl group with the proviso that the weight ratio of component (A"l) to component (A-2) is from 70^30 to 98^2, (B) a dry-process silica of which surface is hydrophobically treated with a cyclic dimethylsiloxane oligomer, (C) a silanol group -containing organosiloxane oligomer or organosilane.

2. The silicone rubber composition according to claim 1, wherein component (B) has a BET specific surface area of 40 to 400m² /g, a bulk density of from 70 to

200kg/m³ , a carbon atom content of from 1.7 to 4.0 % by weight, a water content measured in accordance with Kahl Fischer method of 0.30 % by weight or less, a hexane extraction rate of 3.0 % by weight or less.

3. The silicone rubber composition according to claim 1, wherein the fluorine atom-substituted alkyl group of component (A-I) is perfluoroalkyl group, alkenyl group is vinyl group, the other organic group is methyl group, alkenyl group of component (A-2) is vinyl group, and the other organic group is methyl group.

4. The silicone rubber composition comprising the silicone rubber composition according to claim 1, and 100 or less parts by weight of (E) an inorganic filler other than component (B) per 100 parts by weight of component (A" l) and component (A-2) combined.

5. A heat-curable silicone rubber composition comprising the silicone rubber composition according to claim 1 or claim 4, and (D) a curing catalyst in a sufficient amount to cure said silicone rubber composition.

6. The heat-curable silicone rubber composition according to claim 5, wherein the curing catalyst is an organic peroxide or a combination of an organohydrogenpolysiloxane and a platinum-type catalyst.

7. The heat-curable silicone rubber composition according to claim 5 or claim 6, wherein it is for a rubber sealing part for automobiles.

8. A method for the preparation of the silicone rubber composition according to claim 1, wherein component(A-l), component(A-2), component(B), and component(C) are blended at less than 100⁰C, and subsequently are blended to homogeneity at 100 to 170°C.

9. A rubber sealing part for automobiles comprising a cured product of the heat-curable silicone rubber composition according to claim 5 or claim 6.

10. The rubber sealing part for automobiles according to claim 9, wherein it is used in contact with fuel oils.