JP7526115B2 - Heat-resistant millable silicone rubber composition - Google Patents

Description

The present invention relates to a millable silicone rubber composition.

Silicone rubber has excellent weather resistance, electrical properties, low compression set, heat resistance, cold resistance, etc., and is therefore widely used in a variety of fields, including electrical equipment, automobiles, construction, medicine, and food. For example, it is used in rubber contacts used as rubber contacts in remote controllers and musical instruments, construction gaskets, office equipment rolls such as fixing rolls, developing rolls, transfer rolls, charging rolls, and paper feed rolls, vibration-proof rubber for audio equipment, packing for compact discs, and electrical wire coating materials.

In order to further improve the heat resistance of silicone rubber, it is known to compound additives such as zirconium oxide (Japanese Patent No. 5,174,270 (Patent Document 1)), cerium oxide (Japanese Unexamined Patent Publication Nos. 2000-212444 (Patent Document 2), 2002-179917 (Patent Document 3), and 2016-030774 (Patent Document 4)), zirconium oxide-cerium oxide solid solution (Patent Document 4 and Japanese Unexamined Patent Publication No. 2013-035890 (Patent Document 5)), cerium hydroxide (Patent Document 4), iron oxide, and carbon black (Patent Document 4).
However, the demand for heat resistance of silicone rubber is increasing year by year, and these conventional techniques are not able to fully meet this demand.

Patent No. 5174270 JP 2000-212444 A JP 2002-179917 A JP 2016-030774 A JP 2013-035890 A

The present invention has been made in consideration of the above circumstances, and aims to provide a millable type silicone rubber composition that has low hardness and high strength, exhibits little change in physical properties (rubber properties) even when exposed to high temperatures, and gives a silicone rubber cured product with excellent heat resistance.

As a result of intensive research into achieving the above-mentioned object, the present inventors have discovered that a silicone rubber composition containing (A) a branched organopolysiloxane having a degree of polymerization of 1000 or more, containing two or more alkenyl groups bonded to silicon atoms in each molecule and containing a specific amount of R ¹ SiO _3/2 units, (B) a reinforcing silica having a specific surface area measured by the BET method of 50 m ² /g or more, (C) cerium oxide and/or cerium hydroxide, and (D) a curing agent, has excellent heat resistance, and have thus completed the present invention.

That is, the present invention provides:
(A) 100 parts by mass of a branched organopolysiloxane having two or more alkenyl groups bonded to silicon atoms in one molecule, the branched organopolysiloxane being characterized in that it contains R ¹ SiO _3/2 units in an amount of 0.001 to 0.5 mol % based on the total moles of all siloxane units (wherein R ¹ is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 12 carbon atoms) and has an average degree of polymerization of 1,000 to 100,000; (B) 10 to 100 parts by mass of a reinforcing silica having a specific surface area of 50 m ² /g or more as measured by the BET method;
The present invention provides a millable type silicone rubber composition, which contains: (C) cerium oxide and/or cerium hydroxide: 0.01 to 10 parts by mass; and (D) a curing agent: an effective amount for curing the component (A). The present invention also provides a cured product thereof.

The silicone rubber composition of the present invention can provide a millable silicone rubber cured product that has low hardness, high strength, and excellent heat resistance. In particular, the obtained millable silicone rubber cured product shows little change in physical properties such as hardness, tensile strength, and elongation at break even when exposed to high temperatures.

The present invention will be described in detail below, but the present invention is not limited thereto.
In this specification, a millable composition means a highly viscous, non-liquid composition that does not have self-flowing properties at room temperature (25°C) and that can be uniformly kneaded under shear stress using a kneading machine such as a roll mill (e.g., a two-roll or three-roll).

[(A) Alkenyl-containing branched organopolysiloxane]
Component (A) is a branched organopolysiloxane having two or more alkenyl groups bonded to silicon atoms and at least one R ¹ SiO _3/2 unit per molecule, characterized in that the R ¹ SiO _3/2 units account for 0.001 to 0.5 mol % of the total moles of all siloxane units (wherein R ¹ is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 12 carbon atoms) and have an average degree of polymerization of 1,000 to 100,000. The branched organopolysiloxane preferably contains 97 to 99.997 mol % of (R ¹ ) ₂ SiO _2/2 units and 0.002 to 2 mol % of (R ¹ ) ₃ SiO _1/2 units. In the above, R ¹ 's are each independently an unsubstituted or substituted monovalent hydrocarbon group having 1 to 12 carbon atoms, provided that two or more of R ^1's in one molecule are alkenyl groups.

The organopolysiloxane (A) is characterized by having at least one branched structure (R ¹ SiO _3/2 unit) in one molecule. The R ¹ SiO _3/2 unit is present in an amount of 0.001 to 0.5 mol %, preferably 0.005 to 0.3 mol %, more preferably 0.008 mol % to 0.1 mol %, more preferably 0.01 to 0.05% relative to the total moles of the number of siloxane units. Preferably, the organopolysiloxane has 1 to 10 R ¹ SiO _3/2 units, more preferably 1 to 3 R 1 SiO 3/2 units in one molecule. By having the R ¹ SiO _3/2 units in the above range, the heat resistance of the obtained silicone rubber cured product can be improved, and a silicone rubber cured product with little change in mechanical properties at high temperatures can be obtained. If the content of the R ¹ SiO _3/2 unit exceeds the above upper limit, the viscosity of the organopolysiloxane increases, making it impossible to uniformly blend the reinforcing silica described below, which is not preferable.

The organopolysiloxane of component (A) is characterized by an average degree of polymerization of 1,000 to 100,000, more preferably 1,500 to 50,000, more preferably 2,000 to 20,000, and even more preferably 4,000 to 10,000. If the average degree of polymerization is less than the lower limit, the silicone rubber composition of the present invention will not satisfy the properties required for millable rubber, and the resulting cured product will not have sufficient rubber strength. If the average degree of polymerization exceeds the upper limit, compatibility with other components will decrease, making it difficult to use as a raw material for the composition, which is undesirable.

In this specification, the term "average degree of polymerization" refers to a value determined from the weight average molecular weight measured by gel permeation chromatography (GPC) under the following conditions using polystyrene as a standard substance (hereinafter the same).
[Measurement condition]
Developing solvent: toluene Flow rate: 1 mL/min
Detector: Refractive index detector (RI)
Column: KF-805L x 2 (Shodex)
Column temperature: 25°C
Sample injection volume: 20 μL (toluene solution with a concentration of 0.1% by mass)
In the present invention, component (A) is preferably a non-liquid organopolysiloxane gum that has a high degree of polymerization (high viscosity) and has no self-flowing properties at room temperature (25° C.).

In the above formula, R ¹ is, independently of each other, an unsubstituted or substituted monovalent hydrocarbon group having 1 to 12 carbon atoms, preferably a monovalent hydrocarbon group having 1 to 8 carbon atoms. In detail, the monovalent hydrocarbon group includes alkyl groups such as methyl, ethyl, propyl, butyl, hexyl, and octyl groups, cycloalkyl groups such as cyclopentyl and cyclohexyl groups, alkenyl groups such as vinyl, allyl, and propenyl groups, aryl groups such as cycloalkenyl, phenyl, and tolyl groups, aralkyl groups such as benzyl and 2-phenylethyl groups, and groups in which some or all of the hydrogen atoms of these groups have been substituted with halogen atoms or cyano groups (e.g., trifluoropropyl group, etc.). Among these, methyl, vinyl, phenyl, and trifluoropropyl groups are preferred, and methyl and vinyl groups are more preferred. In particular, it is preferred that 50 mol % or more of the monovalent hydrocarbon groups represented by R ¹ in the molecule are methyl groups, more preferably 80 mol % or more are methyl groups, and furthermore it is preferred that all R ^1s other than the alkenyl groups are methyl groups.

More preferably, the main chain of the organopolysiloxane consisting of the (R ¹ ) ₂ SiO _2/2 units is composed of dimethylsiloxane units, or a methylvinylsiloxane unit having a vinyl group introduced into part of the main chain of this dimethylpolysiloxane is preferred.

The organopolysiloxane of component (A) has two or more alkenyl groups (preferably vinyl groups) bonded to silicon atoms per molecule, and it is preferred that 0.01 to 10 mol %, and more preferably 0.02 to 5 mol %, of the groups bonded to silicon atoms (i.e., R ¹ in the above composition formula) are alkenyl groups.

The organopolysiloxane of component (A) is a diorganopolysiloxane having a branched structure in which the molecular chain is made up of repeating diorganosiloxane units and the molecular chain end is blocked with a triorganosiloxy group (R ¹ ₃ SiO _1/2 ) [R ¹ is the same as above]. The molecular chain end is preferably blocked with a trimethylsiloxy group, a dimethylvinylsiloxy group, a dimethylhydroxysiloxy group, a methyldivinylsiloxy group, a trivinylsiloxy group, or the like. These organopolysiloxanes may be used alone or in combination of two or more types with different degrees of polymerization or molecular structures.

The above (A) organopolysiloxane having a branched structure can be obtained by a known method, for example, by (co)hydrolytic condensation of one or more organohalogenosilanes, or by ring-opening polymerization of a cyclic polysiloxane using an alkaline or acidic catalyst.

[(B) Reinforcing Silica]
In the silicone rubber composition of the present invention, component (B) is a reinforcing silica having a specific surface area of 50 ^m2 /g or more as measured by the BET method. (B) Reinforcing silica functions to impart excellent mechanical properties to the resulting silicone rubber. Reinforcing silica is a powdered reinforcing filler, and examples of the reinforcing silica include fumed silica (dry silica or fumed silica), calcined silica, and precipitated silica (wet silica). From the viewpoint of heat resistance, fumed silica is preferred.

The reinforcing silica has a specific surface area of at least 50 m ² /g, preferably at least 100 m ² /g, and more preferably from 100 to 400 m ² /g. If the specific surface area is less than 50 m ² /g, the mechanical strength of the resulting silicone rubber cured product will be insufficient.

The reinforcing silica may be used in an untreated state or may be surface-treated. If necessary, it is preferable to use silica whose surface has been hydrophobized, since this improves dispersibility and reinforcing properties in component (A). The surface treatment of silica may be performed according to a conventional method. For example, silica that has already been surface-hydrophobized directly with a silica surface treatment agent in a powder state may be used. In addition, the surface may be hydrophobized by adding a silica surface treatment agent when mixing the silica fine powder with component (A) and heating and mixing.

There is no particular restriction on the surface treatment method. For example, untreated silica fine powder and a surface treatment agent are placed in a mechanical kneading device or fluidized bed sealed at normal pressure, and mixed at room temperature or by heat treatment in the presence of an inert gas as necessary. In some cases, a catalyst may be used to promote the surface treatment. After kneading, the mixture is dried to obtain a reinforcing silica fine powder with a hydrophobic surface.

Examples of silica surface treatment agents include organopolysiloxanes, alkoxysilanes, chlorosilanes, and organopolysilazanes. The amount of silica surface treatment agent used is 5 to 75 parts by mass, preferably 5 to 60 parts by mass, per 100 parts by mass of untreated silica fine powder.

The amount of reinforcing silica (B) added is 10 to 100 parts by mass, preferably 20 to 70 parts by mass, and more preferably 30 to 60 parts by mass, per 100 parts by mass of component (A). If the amount of component (B) added is less than the lower limit, the resulting silicone rubber will not have a sufficient reinforcing effect. If the amount exceeds the upper limit, the processability of the silicone rubber composition will deteriorate and the mechanical strength of the cured silicone rubber will decrease.

[(C) Cerium oxide and/or cerium hydroxide]
Component (C) is at least one selected from cerium oxide and cerium hydroxide. Both cerium oxide and cerium hydroxide can improve the heat resistance of silicone rubber.
Commercially available cerium oxide products include Showlox FL-2 (manufactured by Showa Denko K.K.), SN-2 (manufactured by Nikki Co., Ltd.), and Cerium Oxide S (manufactured by Anan Kasei Co., Ltd.). Commercially available cerium hydroxide products include cerium hydroxide (manufactured by Nikki Co., Ltd.) and Cerhydrate 90 (manufactured by Treibach Industrie AG).

The amount of component (C) is preferably 0.01 to 10 parts by mass, and more preferably 0.1 to 5 parts by mass, per 100 parts by mass of component (A). If the amount is less than the lower limit, the heat resistance of the silicone rubber will not improve, and if the amount is more than the upper limit, the mechanical properties may be significantly reduced.
Cerium oxide and cerium hydroxide may be used alone or in combination. When cerium oxide and cerium hydroxide are used in combination, the total amount of the two may be within the above range.

[(D) Curing Agent]
Component (D) is a curing agent for the organopolysiloxane, and is not particularly limited as long as it can cure the silicone rubber composition of the present invention. Therefore, it may be any of the known organic peroxide curing agents used in silicone rubber compositions, or addition reaction type curing agents consisting of an organohydrogenpolysiloxane and a hydrosilylation catalyst (e.g., a platinum group metal catalyst), etc.

(D-1) Organic Peroxide Curing Agent Examples of organic peroxides include 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, 1,6-hexanediol-bis-t-butylperoxycarbonate, etc. These may be used alone or in combination of two or more.

The amount of organic peroxide curing agent is preferably 0.1 to 10 parts by mass, and more preferably 0.2 to 5 parts by mass, per 100 parts by mass of component (A). If the amount of the curing agent is less than the lower limit above, the silicone rubber composition may not be cured sufficiently. If the amount of the curing agent is more than the upper limit above, the cured silicone rubber may turn yellow due to decomposition residues of the organic peroxide.

(D-2) Addition reaction type curing agent Examples of the addition reaction type curing agent include a combination of an organohydrogenpolysiloxane and a platinum group metal catalyst. Platinum group metal catalysts include platinum group metals (platinum, palladium, rhodium, etc.) and their compounds, and examples thereof include platinum element alone, platinum compounds, platinum complexes, chloroplatinic acid, alcohol compounds of chloroplatinic acid, aldehyde compounds, ether compounds, and complexes with various olefins. The amount of platinum group metal catalyst added may be an effective amount for promoting the addition reaction. For example, the amount is preferably in the range of 1 to 2,000 ppm (by mass) of platinum group metal atoms relative to the organopolysiloxane of component (A). If the amount of catalyst added is less than the lower limit, the addition reaction may not be sufficiently promoted, and curing may be insufficient, while if it exceeds the upper limit, the effect on reactivity may be reduced, which may be uneconomical.

Organohydrogenpolysiloxane contains 2 or more, preferably 3 or more, more preferably 3 to 200, and even more preferably about 4 to 100 hydrogen atoms (hydrosilyl groups) bonded to silicon atoms in one molecule. The organohydrogenpolysiloxane may be linear, branched, or cyclic, but preferably has an average degree of polymerization of 300 or less.

Examples of organohydrogenpolysiloxanes include dimethylpolysiloxanes blocked at both ends with dimethylhydrogensiloxy groups, copolymers of dimethylsiloxane units blocked at both ends with trimethylsiloxy units and methylhydrogensiloxane units, copolymers of dimethylsiloxane units blocked at both ends with dimethylhydrogen groups and methylhydrogen units, copolymers of dimethylhydrogensiloxane units (H(CH ₃ ) ₂ SiO _1/2 units) and SiO _4/2 units, 1,3,5,7-tetrahydrogen-1,3,5,7-tetramethylcyclotetrasiloxane, 1-propyl-3,5,7-trihydrogen-1,3,5,7-tetramethylcyclotetrasiloxane, 1,5-dihydrogen-3,7-dihexyl-1,3,5,7-tetramethylcyclotetrasiloxane, and the like, as well as compounds in which some or all of the methyl groups in the above exemplary compounds have been substituted with other alkyl groups or aryl groups such as phenyl groups.

The amount of organohydrogenpolysiloxane is preferably 0.1 to 40 parts by weight, particularly 0.2 to 10 parts by weight, per 100 parts by weight of component (A). In addition, the ratio of the number of hydrogen atoms bonded to silicon atoms (hydrosilyl groups) is preferably in the range of 0.5 to 5 per alkenyl group in component (A), and more preferably in the range of 0.7 to 5. If the amount is less than the lower limit, crosslinking may be insufficient and sufficient mechanical strength may not be obtained, and if the amount is more than the upper limit, the physical properties after curing may decrease, and in particular the heat resistance of the silicone rubber may deteriorate and the compression set may become large.

[(E) Dispersant for filler]
In addition to the above components, the silicone rubber composition of the present invention may further contain a filler dispersant, particularly a silica dispersant, as an optional component. The filler dispersant is preferably an organosilane or organosiloxane compound represented by the following general formula (2). Component (E) acts as a dispersant (wetter) to improve the dispersibility of the reinforcing silica of component (B) in component (A), and by blending component (E), the workability, extrusion characteristics, etc. of the silicone rubber composition of the present invention are improved.
R ² O((R ³ ) ₂ SiO) _m R ² (2)
In the formula, R ² 's are each independently a hydrogen atom or an unsubstituted or substituted alkyl group, R ^3's are each independently an unsubstituted or substituted monovalent hydrocarbon group having 1 to 12 carbon atoms, and m is a positive number from 1 to 50.

The organosilane or siloxane represented by the above general formula (2) has an alkoxy group or a hydroxyl group at the molecular chain terminal. ^R2 is each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, more specifically a hydrogen atom, a methyl group, an ethyl group, a propyl group, or a butyl group, with a hydrogen atom, a methyl group, and an ethyl group being preferred.
^R3 is an unsubstituted or substituted monovalent hydrocarbon group having 1 to 12 carbon atoms, particularly 1 to 8 carbon atoms. More specifically, it is an alkyl group such as a methyl group, an ethyl group, a propyl group, or a butyl group, a cycloalkyl group such as a cyclohexyl group, or a group in which some or all of the hydrogen atoms bonded to the carbon atoms of these groups have been substituted with halogen atoms, a cyano group, or the like, and examples thereof include a chloromethyl group, a trifluoropropyl group, and a cyanoethyl group. Among the above examples, a methyl group and a trifluoropropyl group are preferred, and a methyl group and a trifluoropropyl group are particularly preferred.

m is a positive number from 1 to 50, preferably a positive number from 1 to 30, and more preferably a positive number from 1 to 20. If m is 50 or less, a sufficient effect of adding component (E) can be obtained without compounding a large amount of component (E), and there is no risk of deterioration of the rubber properties due to compounding a large amount.

The amount of component (E) is preferably 0.1 to 50 parts by mass, more preferably 0.5 to 30 parts by mass, per 100 parts by mass of component (A). If it is equal to or greater than the lower limit, the workability, extrusion characteristics, etc. of the silicone rubber composition of the present invention are sufficiently obtained. If it is equal to or less than the upper limit, there is no risk of the resulting silicone rubber composition becoming tacky, and therefore there is no risk of reduced processability or reduced physical properties of the resulting rubber.

[Other ingredients]
In addition to the above-mentioned components, optional components that may be added to the silicone rubber composition of the present invention, if necessary, include platinum compounds other than the above-mentioned platinum group metal catalysts, flame retardant agents such as iron oxides and halogen compounds, heat resistance improvers other than component (C), antioxidants, ultraviolet absorbers, colorants, release agents, and other known additives used in silicone rubber compositions.

The method for producing the silicone rubber composition of the present invention is not particularly limited, but it can be obtained by kneading the above-mentioned components in a known kneading machine such as a two-roll mill, kneader, or Banbury mixer. If necessary, heat treatment (kneading under heat) may be performed. Specifically, it is preferable to knead components (A) and (B) (if component (E) is added, components (A), (B), and (E) are kneaded), and then heat-treated as necessary, and then component (D) is added at room temperature. In this case, component (C) may be added before or after the heat treatment. When heat-treating, the heat treatment temperature and time are not particularly limited, but it is preferable to perform the heat treatment at 100 to 250°C, especially 140 to 180°C, for about 30 minutes to 5 hours.

When molding the silicone rubber composition of the present invention, an appropriate molding method may be selected depending on the required application (molded product). Specific examples include compression molding, injection molding, transfer molding, normal pressure hot air vulcanization, and steam vulcanization. There are no particular limitations on the curing conditions, which may be selected appropriately depending on the curing method and molded product. Generally, primary vulcanization can be performed at 80 to 600°C, particularly 100 to 450°C, for a few seconds to several days, particularly about 5 seconds to 1 hour. Secondary vulcanization may also be performed if necessary. Secondary vulcanization can usually be performed at 180 to 250°C for about 1 to 10 hours.

As described above, the millable silicone rubber composition of the present invention is a millable silicone rubber composition that gives a cured silicone rubber product with excellent heat resistance, with little change in physical properties (rubber properties) such as hardness, tensile strength, and elongation at break, even when exposed to high temperatures.

The cured product obtained from the millable type silicone rubber composition of the present invention has a type A durometer hardness immediately after curing of 30 or more, preferably 40 to 80, a tensile strength of 6 MPa or more, preferably 6.0 to 15.0 MPa, and an elongation at break of 200% or more, preferably 200 to 1,000%. Furthermore, it is preferable that the hardness change of the cured product after heating at 220°C for 30 days is -5 to 10, the rate of change in tensile strength is within ±30%, and the rate of change in elongation at break is within ±20%. In this specification, the type A durometer hardness, tensile strength, and elongation at break of the silicone rubber cured product are measured in accordance with JIS K 6249:2003.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.
In the following, the type A durometer hardness, tensile strength and elongation at break of the cured silicone rubber products were measured according to methods in accordance with JIS K 6249:2003.

(A) Preparation of branched organopolysiloxane [Preparation Example 1]
1,039 g (3.5 mol) of octamethylcyclotetrasiloxane, 1.812 g (0.00525 mol) of 1,3,5,7-tetravinyl-1,3,5,7- tetramethylcyclotetrasiloxane , 0.504 g (0.00131 mol) of dodecamethylpentasiloxane, and 1.141 g (0.000146 mol) of dimethylpolysiloxane consisting of 23 mol % of MeSiO _3/2 units, 71 mol % of Me ₂ SiO _2/2 units, and 6 mol % of Me ₃ SiO _1/2 units were mixed and adjusted to 120° C. with stirring. Thereafter, 0.874 g of 3% K siliconate (potassium hydroxide diluted with 3% by mass of octamethylcyclotetrasiloxane) was added, and polymerization was carried out at 180° C. for 6 hours to obtain organopolysiloxane raw rubber 1 having a branched structure (consisting of 0.0125 mol % MeSiO _3/2 units, 99.8125 mol % Me ₂ SiO _2/2 units, 0.15 mol % ViMeSiO _2/2 units, and 0.025 mol % Me ₃ SiO _1/2 units, average degree of polymerization 8,000).

[Preparation Example 2]
1,039 g (3.5 mol) of octamethylcyclotetrasiloxane, 1.812 g (0.00525 mol) of 1,3,5,7-tetravinyl-1,3,5,7- tetramethylcyclotetrasiloxane , 1.26 g (0.00328 mol) of dodecamethylpentasiloxane, and 0.57 g (0.00073 mol) of dimethylpolysiloxane consisting of 23 mol % MeSiO _3/2 units, 71 mol % Me ₂ SiO _2/2 units, and 6 mol % Me ₃ SiO _1/2 units were mixed and adjusted to 120° C. with stirring. Thereafter, 0.874 g of 3% K siliconate (potassium hydroxide diluted with 3% by mass of octamethylcyclotetrasiloxane) was added, and polymerization was carried out at 180° C. for 6 hours to obtain organopolysiloxane raw rubber 2 having a branched structure (consisting of 0.0125 mol % MeSiO _3/2 units, 99.8125 mol % Me ₂ SiO _2/2 units, 0.15 mol % ViMeSiO _2/2 units, and 0.025 mol % Me ₃ SiO _1/2 units, average degree of polymerization 4,000).

[Preparation Example 3]
1,039 g (3.5 mol) of octamethylcyclotetrasiloxane, 1.812 g (0.00525 mol) of 1,3,5,7-tetravinyl-1,3,5,7- tetramethylcyclotetrasiloxane , 1.176 g (0.00303 mol) of dodecamethylpentasiloxane, and 1.141 g (0.000146 mol) of dimethylpolysiloxane consisting of 23 mol % MeSiO _3/2 units, 71 mol % Me ₂ SiO _2/2 units, and 6 mol % Me ₃ SiO _1/2 units were mixed and adjusted to 120° C. with stirring. Thereafter, 0.874 g of 3% K siliconate (potassium hydroxide diluted with 3% by mass of octamethylcyclotetrasiloxane) was added, and polymerization was carried out at 180° C. for 6 hours to obtain organopolysiloxane raw rubber 3 having a branched structure (consisting of 0.025 mol % MeSiO _3/2 units, 99.8 mol % Me ₂ SiO _2/2 units, 0.15 mol % ViMeSiO _2/2 units, and 0.025 mol % Me ₃ SiO _1/2 units, with an average degree of polymerization of 4,000).

Preparation of Silicone Rubber Composition [Example 1]
As the component (A), 100 parts by mass of the organopolysiloxane raw rubber 1 having a branched structure obtained in Preparation Example 1 above (consisting of 0.0125 mol % MeSiO _3/2 units, 99.8125 mol % Me ₂ SiO _2/2 units, 0.15 mol % ViMeSiO _2/2 units, and 0.025 mol % Me ₃ SiO _1/2 units, average degree of polymerization 8,000),
As the component (B), 36 parts by mass of fumed silica having a BET specific surface area of 200 m ² /g (Aerosil 200, manufactured by Nippon Aerosil Co., Ltd.);
As component (E), 2.4 parts by mass of diphenylsilanediol and 2.5 parts by mass of dimethylpolysiloxane having silanol at both ends, an average degree of polymerization of 4, and a viscosity of 15 mPa·s at 25°C were added, and the mixture was mixed and heated in a kneader at 165°C for 2 hours to prepare base compound (1).
To the above base compound (1), 1.5 parts by mass of cerium oxide (SN-2, manufactured by Nikki Co., Ltd.) was added per 100 parts by mass of organopolysiloxane raw rubber (A) using a twin roll. 1.3 parts by mass of p-methylbenzoyl peroxide was added as a curing agent to the resulting mixture using a twin roll, and the mixture was mixed uniformly to produce a millable type silicone rubber composition.

The obtained millable type silicone rubber composition was press-cured for 10 minutes at 165°C and 6.86 MPa to prepare a 2 mm thick test sheet. The test sheet was then post-cured in an oven at 200°C for 4 hours to obtain a silicone rubber cured product. The initial values of hardness, tensile strength, and elongation at break of the cured product were measured according to the methods described above. Furthermore, the cured product was placed in a dryer at 220°C for 30 days, after which the hardness, tensile strength, and elongation at break were measured according to the methods described above. The results are shown in Table 1.

[Example 2]
To the base compound (1) prepared in Example 1, 1.5 parts by mass of cerium oxide (SN-2, manufactured by Nikki Co., Ltd.) was added using a twin roll for 100 parts by mass of (A) organopolysiloxane raw rubber 1. To the resulting mixture, 2.0 parts by mass of organohydrogenpolysiloxane as a curing agent and 0.5 parts by mass of a platinum group catalyst were added using a twin roll and mixed uniformly to produce a millable type silicone rubber composition.

The obtained millable type silicone rubber composition was press-cured for 10 minutes at 120°C and 6.86 MPa to prepare a test sheet with a thickness of 2 mm. The test sheet was then post-cured in an oven at 200°C for 4 hours to obtain a silicone rubber cured product. The initial values of hardness, tensile strength, and elongation at break of the cured product were measured according to the methods described above. Furthermore, the cured product was placed in a dryer at 220°C for 30 days, after which the hardness, tensile strength, and elongation at break were measured according to the methods described above. The results are shown in Table 1.

[Example 3]
Example 1 was repeated to obtain a millable type silicone rubber composition, except that 100 parts by mass of organopolysiloxane raw rubber 2 having a branched structure obtained in Preparation Example 2 above (consisting of 0.0125 mol% MeSiO3 _/ 2 units, 99.8125 mol% _{Me2SiO2 /2} units, 0.15 mol% ViMeSiO2 _/2 units, and 0.025 mol% _{Me3SiO1 /2} units, average degree of polymerization 4,000) was used in place of the component (A) used in Example 1.
The obtained millable type silicone rubber composition was cured in the same manner as in Example 1. According to the above-mentioned methods, the rubber properties were measured initially and after the heat resistance test, and the amount and rate of change from the initial values were calculated. The results are shown in Table 1.

[Example 4]
Instead of the (A) component used in Example 1, 100 parts by mass of the organopolysiloxane raw rubber 3 having a branched structure obtained in Preparation Example 3 (MeSiO3 _/2 unit 0.025 mol%, _{Me2SiO2 /2} unit 99.8 mol%, ViMeSiO2 _/2 unit 0.15 mol%, _{Me3SiO1 /2} unit 0.025 mol%, average polymerization degree 4,000) was used, and the method of Example 1 was repeated to obtain a millable type silicone rubber composition. The obtained millable type silicone rubber composition was cured by the same method as in Example 1. According to the above method, the rubber properties at the initial stage and after the heat resistance test were measured, and the change amount and change rate from the initial value were calculated. The results are shown in Table 1.

[Comparative Example 1]
Example 1 was repeated to obtain a millable silicone rubber composition, except that 100 parts by mass of organopolysiloxane raw rubber (average polymerization degree 8,000) consisting of 99.825 mol% _{Me2SiO2 /2} units, 0.15 mol% ViMeSiO2 _{/ 2} units, and 0.025 mol% ViMe2SiO1/ ₂ units was used instead of the (A) component used in Example 1. The millable silicone rubber composition obtained was cured by the same method as Example 1. According to the above method, the rubber properties at the initial stage and after the heat resistance test were measured, and the change amount and change rate from the initial value were calculated. The results are shown in Table 1.

[Comparative Example 2]
An attempt was made to prepare a base compound by using 100 parts by mass of organopolysiloxane raw rubber (average degree of polymerization 1,000) consisting of 1.1 mol % MeSiO3 _/2 units, 98.55 mol % _{Me2SiO2 / 2} units, 0.15 mol % ViMeSiO2/2 units, and 0.2 mol % ViMe2SiO1 _{/ 2} units, instead of component (A) used in Example 1. However, the viscosity of the organopolysiloxane raw rubber was too high and it was not possible to uniformly mix components (B) and (E).

[Comparative Example 3]
Example 1 was repeated to obtain a millable type silicone rubber composition, except that 100 parts by mass of organopolysiloxane raw rubber (average polymerization degree 4,000) consisting of 99.825 mol% _{Me2SiO2 /2} units, 0.15 mol% ViMeSiO2 _{/2 units} , and 0.025 mol% ViMe2SiO1/ ₂ units was used instead of the (A) component used in Example 1. The millable type silicone rubber composition obtained was cured by the same method as Example 1. According to the above method, the rubber properties at the initial stage and after the heat resistance test were measured, and the amount and rate of change from the initial value were calculated. The results are shown in Table 1.

As shown in Table 1, in Examples 1 and 2, in which an organopolysiloxane having a branched structure and an average degree of polymerization equivalent to that of conventional raw rubber was used as component (A), the rate of change in rubber properties after the heat resistance test was small, and heat resistance was good, compared to Comparative Example 1, which does not have a branched structure. In particular, the small rate of change in tensile strength is a characteristic. In addition, in Examples 3 and 4, in which an organopolysiloxane having a branched structure and an average degree of polymerization approximately half that of conventional raw rubber was used as component (A), the rate of change in rubber properties after the heat resistance test was small, and heat resistance was good, despite the low molecular weight. In particular, the rate of change in elongation at break was small, compared to Comparative Example 3, which has the same average degree of polymerization and does not have a branched structure. In Comparative Example 2, in which the T unit content exceeded 1 mol % , the viscosity of the raw rubber was too high, and the reinforcing silica of component (B) could not be blended uniformly.
For these reasons, the silicone rubber composition of the present invention can give a cured silicone rubber product that has excellent heat resistance and exhibits little change in physical properties such as hardness, elongation at break, and particularly tensile strength, even when exposed to high temperatures.

The silicone rubber composition of the present invention provides a silicone rubber cured product that has low hardness and high strength, and exhibits little change in rubber properties even when exposed to high temperatures, and has excellent heat resistance. The silicone rubber cured product of the present invention is useful in various fields such as electrical equipment, automobiles, construction, medicine, and food, and is used, for example, in contact rubber switches (contact rubber) for remote controllers and musical instruments, construction gaskets, office rolls such as fixing rolls, developing rolls, transfer rolls, charging rolls, and paper feed rolls, vibration-proof rubber for audio equipment, packing for compact discs, and electric wire coating materials.

Although the present invention has been described above using the above embodiment, the present invention is not limited to this embodiment, and can be modified within the scope of what a person skilled in the art can imagine, including other embodiments, additions, modifications, deletions, etc., and any aspect is within the scope of the present invention as long as it achieves the effects of the present invention.

Claims (7)

(A) 100 parts by mass of a branched organopolysiloxane having two or more alkenyl groups bonded to silicon atoms in one molecule, the branched organopolysiloxane being characterized in that it contains R ¹ SiO _3/2 units in an amount of 0.001 to 0.5 mol % based on the total moles of all siloxane units (wherein R ¹ is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 12 carbon atoms) and has an average degree of polymerization of 1,000 to 100,000; (B) 10 to 100 parts by mass of a reinforcing silica having a specific surface area of 50 m ² /g or more as measured by the BET method;
A millable type silicone rubber composition comprising: (C) cerium oxide and/or cerium hydroxide: 0.01 to 10 parts by mass; and (D) a curing agent: an effective amount for curing the component (A). 2. The millable type silicone rubber composition according to claim 1, wherein component (A) contains 97 to 99.997 mol % of (R ¹ ) ₂ SiO _2/2 units and 0.002 to 2 mol % of (R ¹ ) ₃ SiO _1/2 units based on the total moles of all siloxane units. The millable silicone rubber composition according to claim 1 or 2, wherein the component (D) is an organic peroxide curing agent, and the amount of the component (D) is 0.1 to 10 parts by mass. The millable silicone rubber composition according to claim 1 or 2, in which the component (D) is an addition reaction type curing agent, which is a combination of 0.1 to 40 parts by mass of an organohydrogenpolysiloxane and a catalytic amount of a platinum group metal catalyst. The millable silicone rubber composition according to any one of claims 1 to 4, further comprising 0.1 to 50 parts by mass of (E) a dispersant for fillers. 6. The millable silicone rubber composition according to claim 5, wherein said component (E) is a dispersant for silica.
A cured product of the millable type silicone rubber composition according to any one of claims 1 to 6.