JPH0120187B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to curable silicone rubber compositions. In particular, the present invention relates to self-bonding, one-component room temperature vulcanizable (RTV) compositions comprising a diorganosiloxane polymer, a highly functional polysiloxane, a filler, a crosslinking agent, and a crosslinking catalyst. Heat stabilizers may also be included. A family of silicone rubber compositions disclosed in Nitzsche and Wick, U.S. Pat. No. 3,065,194, comprises (1) a linear organosiloxane polymer containing terminal hydroxyl groups; (2)
It consists of an essentially anhydrous mixture of a polyfunctional organosilicone crosslinker and (3) a metal salt, thylate, organometallic compound, acid or base that acts as a crosslinking catalyst. Such compositions are particularly useful because they vulcanize or harden to a rubbery solid when exposed to moisture, can be maintained in a single container for extended periods of time, and
For example, it may be enclosed in a caulking tube and ready for use, after which the user can apply the material and cure it by contacting with water or steam. Such compositions are useful in sealants, electrical insulation, coatings, dental cements, caulking compounds, expansion joints, gaskets, cushioning materials, adhesives, and many other forms. Further technical background regarding single-pack room temperature vulcanizable silicone compositions is provided by Bruner, U.S. Pat. No. 3,035,016;
Ceyzeriat, U.S. Pat. No. 3,133,891, which deals with a one-pack moisture-curable composition comprising the reaction product of an acyloxy-substituted silane and a hydroxylated siloxane, which composition Curing is facilitated by various agents such as organic derivatives of tin. Also of interest is U.S. Pat. No. 3,161,614 to Browh et al., which shows the pre-reaction of a silanol-terminated diorganopolysiloxane with a crosslinking agent in combination with a crosslinking catalyst.
Cooper, U.S. Pat. No. 3,383,355 deals with the preparation of alkoxy-terminated linear siloxane polymers using neutral, finely divided solid catalysts such as Fuller's earth, and Matherly, U.S. Pat. Using an end-capped diorganopolysiloxane and a metal-containing curing catalyst with boron nitride, Cooper et al., U.S. Pat.
A mixture of a linear siloxane with a chemically non-functional, inert end-capping unit at one end and a di- or tri-functional end-capping unit at the other end, including a catalyst and a crosslinking agent. There is. More related is
No. 3,122,522 to Browh et al., in which an organopolysiloxane intermediate containing condensable cellulosorboxyl groups is combined with a catalyst;
U.S. Pat. No. 3,170,894 to Brown et al. combines an organopolysiloxane intermediate having condensable polyhydrocarbonoxy-type groups with a catalyst and further
Weyenberg, US Pat. No. 3,175,993, combines an alkoxylated silicon carbon end-capped organopolysiloxane intermediate with a catalyst. Smith and Hamilton U.S. Patents 3,689,454 and 3,779,986; Weyenberg, U.S. Pat.
Nos. 3,294,739 and 3,334,067, and US Pat. No. 3,719,635 to Clark et al. also disclose single package compositions. Also relevant is U.S. Pat. No. 3,382,205, in which the one-pack system disclosed is moisture curable and further
It contains di- and trialkylpolysiloxanes, with the latter fluid functioning as an adhesion promoter and processing aid. Special classes of compositions are known that provide sealants with significant high temperature resistance, but these have limited use as gasket materials in the automotive industry where odor and corrosion pose problems. . For example, silanol-terminated polydimethylsiloxane,
Compositions containing methyltriacetoxysilane, fumed silica filler, and dibutyltin dilaurate are currently used in most applications, but are not completely satisfactory in some areas because they release acetic acid upon curing. . This curing byproduct causes the unpleasant odor and corrosion mentioned above. The compositions currently in use also tend to have a relatively high degree of adhesion at low temperatures, making them ideal for use in oil pans, for example, if a new car engine needs to be dismantled before being placed and operated. It is difficult to repair without damaging things like the valve cover. Here, we present a low-odor, non-corrosive, fast-curing, one-component RTV that exhibits excellent oil resistance after moisture curing and especially high-temperature resistance properties with heat stabilizers.
It is now possible to provide the composition. The composition also has low adhesion at room temperature and improved adhesion at elevated temperatures, which provides a good seal, such as when an automobile engine equipped with a gasket made from the composition of the invention is started. can be achieved. The compositions are useful in a variety of conventional applications, particularly as gasket layers between components having metal surfaces. The fluid compositions provided by the present invention are stable under substantially anhydrous conditions and harden in the presence of moisture to provide a self-bonding, thermally stable composition resistant to hot hydrocarbon oils. becomes an elastic solid, and formula (a) (wherein R and R ¹ are each independently an organic group having up to 8 carbon atoms selected from hydrocarbyl, halohydrocarbyl and cyano-lower alkyl, and n is an average number of about 10 to 15,000) 100 parts by weight of a silanol chain-terminated polydiorganosiloxane, (b) containing a high degree of trifunctionality, tetrafunctionality, or mixtures thereof; (i) 25 to 60 mole percent of monoalkylsiloxy units, siloxy units, or mixtures thereof; , (ii) 1 to 6 mol% trialkylsiloxy units
and (iii) 2 to 20 parts by weight of a fluid polysiloxane comprising 34 to 74 mole percent of dialkylsiloxy units and containing about 0.1 to 2 percent by weight of silicon-bonded hydroxyl groups; (c) 10 to 100 parts by weight of a finely divided silica filler. Parts by weight, formula (d) R ² _n Si(OR ³ ) _4-n (wherein R ² has the same meaning as defined from R and R ¹ above, R ³ is hydrocarboyl and halohydrocarbon Number of carbon atoms selected from Boyle: 6~
3 to 10 parts by weight of a crosslinking silane represented by (30 organic groups, m has a value of 0 or 1), and (e) an organic tin salt or tin salt of an organic acid having 2 to 6 carbon atoms. The curing catalyst consists of 0.01 to 10 parts by weight. In a preferred aspect of the invention, the composition also includes
(f) Finely divided iron oxide thermal stabilizer 1~
Contains 10 parts by weight. These embodiments are particularly useful at operating temperatures of 140°C and above. According to another preferred aspect of the invention, there is provided a method for preparing a rubbery material, which method comprises preparing a composition as defined above under substantially anhydrous conditions;
It then consists of exposing the composition to moisture until it hardens into a rubbery material. According to another aspect of the invention, a plurality of pieces having a metal surface are manufactured, in which at least one surface portion of each piece is brought into close proximity to another surface, with a gasket layer of the composition as defined above interposed therebetween. Articles are provided as well as methods of making such gasket insert manufacturing articles. In preferred embodiments of this aspect, the composition also includes an iron oxide thermal stabilizer. To prepare the room temperature vulcanizable (RTV) compositions of the present invention, one or more of the silanol chain-terminated polyorganosiloxanes of the above formula, iron oxide, a silica filler, a crosslinked silane compound, and a silanol-reactive tin salt are used. Simply mix. Base compound (excluding crosslinkers, adhesion promoters and catalysts if present)
are usually formulated at elevated temperatures to remove moisture and facilitate wetting of the filler. Silanes tend to hydrolyze on contact with moisture, so care must be taken to exclude moisture during the addition of silanes to silanol chain-stopped polydiorganopolysiloxanes. Similarly, mixtures of silanes, silanol-reactive tin salt catalysts, silanol chain-terminated polydiorganosiloxanes, and highly functional siloxane fluids may be stored for extended periods of time prior to conversion to a cured, solid, elastomeric silicone rubber state. Care should be taken to maintain substantially anhydrous conditions. On the other hand, when it is desired that the mixture be cured immediately, it is only necessary to mix it into the shape to be cured and leave it to stand, and no particular precautions need to be taken in advance. As long as the ratios of ingredients specified above are used, a wide range of ingredient choices are possible in preparing the compositions of this invention. These can be found in many documents such as U.S. Patent No. 3779986, U.S. Pat.
No. 3334067, No. 3382205, and No. 3708467. The silanol chain-terminated polyorganosiloxane component (a) has the formula (wherein R and R ¹ are organic groups having up to 20 carbon atoms, preferably up to 8 carbon atoms, selected from hydrocarbyl, halohydrocarbyl and cyano-lower alkyl, and n is generally from about 10 to 15,000, preferably from 100 to about
The number is more preferably 300 to 1500. Silanol chain-terminated polydiorganosiloxanes are well known in the art and include compositions containing a variety of R and R ¹ groups. For example, the R group may be methyl and the R ¹ group may be phenyl and/or betacyanoethyl. Additionally, within the scope of the definition of polydiorganosiloxane useful in the present invention are copolymers composed of diorganosiloxane units of various types, such as silanol chain-terminated copolymers composed of dimethylsiloxane units, diphenylsiloxane units, and methylphenylsiloxane units. Included are copolymers or copolymers of, for example, dimethylsiloxane units, methylphenylsiloxane units, and methylvinylsiloxane units. Preferably, R and R ¹ of the silanol chain-terminated polydiorganosiloxane
At least 50% of the groups are alkyl such as methyl groups. In the above formula, R and R ¹ may be, for example, any of the groups exemplified in the above patents. The silanol chain-terminated polydiorganosiloxanes used in the practice of this invention are determined by the value of n and R and
Depending on the nature of the particular organic group represented by R ¹ , the viscosity may vary from a low viscosity fluid to a viscous gum. The viscosity of component (a) is, for example, 30 to 10,000,000 ops at 25°C.
may vary widely over a range of . Preferably it is in the range of 1000-200000 cps at 25°C, most preferably about 2000-30000 cps. The highly functional polysiloxane component (b) can be prepared by means known to those skilled in the art. For example, a mixture of (i) trialkylchlorosilane, (ii) dialkyldichlorosilane, and (iii) alkyltrichlorosilane, silicon tetrachloride, or a mixture thereof in an appropriate molar ratio is added to toluene and water, and these are simultaneously hydrated. Subject to decomposition. The mixture is then heated to, for example, about 60° C. for a sufficient period of time, for example, 3 hours, to ensure completion of the reaction. The oil phase is separated and neutralized, such as by washing with an aqueous solution of sodium carbonate or bicarbonate. to remove insoluble materials, e.g.
Upon devolatilization, such as by heating to about 140° C. under a vacuum of mmHg, component (b) remains as a residue. For economic reasons, the silicon-bonded hydroxyl content is maintained below 0.6% by weight to minimize the viscosity of the final composition and to maintain a minimum concentration of crosslinking agent. This is done by heating the product to 110°C in the presence of about 1% sodium carbonate. The water resulting from the silanol condensation can be conveniently removed, for example, by azeotropic distillation with toluene. After removing the toluene by distillation, the product is filtered prior to use. Beers US Pat. No. 3,382,205 illustrates this.
Component (b) generally comprises from 2 to 20 parts by weight per 100 parts of component (a) and preferably from about 5 to 15 parts per 100 parts by weight of the composition. Preferably component (b) has a viscosity in the range of 50 to 300 cps at 25°C. Also preferably, in component (b), at least 50% of the alkyl substituents are methyl and the fluid contains 0.2-0.6% by weight of silanol. Particularly preferably monoalkylsiloxy units, siloxy units or hybrid units of such units account for about 35-45 mol%, trialkylsiloxy units account for 3-5 mol% and dialkylsiloxy units account for 45-67 mol%. and the silanol content is about 0.2-0.5% by weight. Silica component (c) is known in the art as a filler for silicone compositions. Component (c) is in finely divided form and is preferably of the type known as fumed silica. Preferably,
The surface area should be approximately 200 square meters/g. In a preferred form, Lucas U.S. Pat.
No. 2938009, Lichten and Walter U.S. Patent No.
3,004,859 and Smith, US Pat. No. 3,635,743. Silica filler (c) is component (a) 100
Generally 10 to 100 preferably 15 to about 10 to 100 parts by weight
It is used in an amount of 40 parts by weight, preferably in an amount of 20 to 30 parts by weight per 100 parts by weight of the composition. In the case of the silane crosslinking agent (d) of the formula R ² _n Si(OR ³ ) _4-n , the meaning for R ² is the same as defined for R and R ¹ individually above, and R ³ is It should contain between 6 and 30 carbon atoms to eliminate the formation of odorous and corrosive by-products during the curing reaction. CH ₃ Si (OCO (CH ₂ ) ₄ CH ₃ ) ₃ Si (OCO (CH ₂ ) ₄ CH ₃ ) ₄ CH ₃ (CH ₂ ) ₆ CH ₂ Si (OCO (CH ₂ ) ₄ CH ₃ ) ₃ CF ₃ (CH ₂ ) ₃ Si (OCO (CH ₂ ) ₄ CH ₃ ) ₃ NCCH ₂ CH ₂ Si (OCO (CH ₂ ) ) ₄ CH ₃ ) ₃ CH ₃ Si (OCOCH(C ₂ H ₅ ) (CH ₂ ) CH ₃ ) ₃ These silanes are well known in the art and include, for example:
It can be manufactured by the technology disclosed in Beers, US Pat. No. 3,382,205. For the crosslinking agent (d), m preferably has a value of 1, and preferred silanes are methyltris(2-ethylhexanoxy)silane and methyltris(benzoxy)silane. This silane contains 100% of component (a)
Generally 3 to 10 parts by weight, preferably 5 to 10 parts by weight
It is used in an amount of 7 parts by weight. As for the silanol-reactive tin catalyst component (e), generally both organic tin salts and tin salts of organic acids can be used, but the former is preferred. A carbon content of 2 to 6 in the organic acid provides the best combination of cure speed and final properties. The organic component of the organotin salt may be, for example, one or two alkyl groups of 2 to 6 carbon atoms, such as monobutyl or dibutyl;
Organic acid groups can have a number of carbon atoms of 2 to 6. Examples of these include tin hexanoate, dibutyltin hexanoate, dibutyltin diacetate, dibutyltin adipate, dibutyltin dipropionate,
Dibutyltin dibutyrate, monobutyltin triacetate, etc. These are commercially available or can be manufactured by those skilled in the art. Preferred is dibutyltin diacetate. The catalyst is generally 0.01 to 100 parts by weight of component (a).
Present in an amount of 10.0 parts by weight, preferably 0.02 to 5.0 parts by weight. The catalyst is approximately 0.05 parts per 100 parts by weight of the total composition.
It is particularly preferred to use up to about 0.15 parts by weight. The optional iron oxide thermal stabilizer component (f) is a conventionally used and widely available commercial product in finely divided form for use as a filler in plastic compositions. It is preferred that the PH of the iron oxide be in the range of 6.0 to 7.5 to achieve maximum thermal stability and storage aging stability. The amount used ranges from 1 to 10 parts by weight based on 100 parts by weight of component (a), preferably from 3 to 6 parts by weight based on 100 parts by weight of the total composition. In a preferred embodiment, the compositions of the present invention may also optionally contain from 0.2 to 2 parts per 100 parts of component (a) of an adhesion promoter. These are generally nitrogen-containing compounds, e.g. A group of accelerators represented by the formula, where G is (R ¹¹ O) _3-b R ¹⁰ _b -Si-R ⁹ group, styryl, vinyl,
Allyl, chloroallyl, or cyclohexenyl, or the R ¹⁰ group described below, and R ⁹ is alkylenearylene,
a divalent group selected from alkylene, cyclooxyene and halo-substituted such divalent groups, R ¹⁰ is a group of up to 8 carbon atoms selected from hydrocarbyl or halohydrocarbyl, and R ¹¹ is relative to R ¹⁰ Groups of the defined type are also cyano-lower alkyl, and b is 0-3. These adhesion promoters were used in Berger's U.S. patent.
Disclosed in No. 3517001. Preferred such accelerators are 1,3,5-tris-trimethoxysilylpropylisocyanate and bis-1,3-trimethoxysilylpropylisocyanurate, of which the former is most preferred. Although any conventional adhesion promoter can be used, particular mention is made of the adhesion promoter known as glycidoxypropyltrimethoxysilane. Additional conventional ingredients such as flame retardants, stabilizers,
Pigments, etc. may also be included. The compositions of the invention are stable in the absence of moisture.
As a result, the composition can be stored for long periods of time without adverse effects. During this storage time, no noticeable changes occur in the physical properties of the room temperature vulcanizable composition. This is particularly advantageous commercially, as compositions can be prepared to have a given consistency and setting time without significantly altering either the consistency or the setting time during storage. Storage stability is one of the characteristics that makes the compositions of the present invention useful in a packaged system. Compositions prepared by mixing the catalyst and silane with the silanol chain-terminated polydiorganosiloxane and other ingredients under anhydrous conditions can be prepared by simply placing the composition at the desired location and exposing it to the atmosphere without further modification. It can be used in many sealing, caulking, and coating applications by curing upon exposure to existing moisture. Such exposure results in the formation of a skin on the compositions of the invention within a relatively short period of time, e.g. 10 minutes to about 8 hours, even after previous storage for many months, and at room temperature e.g. It will harden to a rubbery state within a few hours to a few days at ℃. When the compositions of the present invention include tri- and/or tetrafunctional siloxane fluids, silane crosslinkers, and tin catalysts as components other than the silanol-terminated polydiorganosiloxane, these additional components can be added in any desired manner. . However, for ease of manufacture, it is often most convenient to form a base formulation of all ingredients other than the silane, tin catalyst, and adhesion promoter, if present, and then extract moisture from this base formulation. For example, 50-100℃ to remove
The silane, tin catalyst and optional adhesion promoter are then added just prior to packaging in a container protected from moisture. The compositions of the present invention are particularly suitable for caulking and sealing applications where excellent adhesion to a variety of substrates is important. For example, it is useful in domestic and industrial caulking and sealing in buildings, factories, automobiles, etc., where the substrate is, for example, masonry, glass, plastic, metal, wood, etc. The compositions of the present invention have little or no tendency to corrode sensitive substrates such as metals such as copper and brass. The composition has an excellent application speed and is easily adapted to be applied with conventional caulking equipment under standard conditions. The following examples are illustrative only and should not be construed as limiting the invention. The terms MDT and Q are art-recognized designations, respectively: M = monofunctional organosiloxane unit R ₃ SiO _1/2 D = difunctional organosiloxane unit R ₂ SiO _2/2 T = trifunctional Organosiloxane units RSiO _3/2 Q = Tetrafunctional organosiloxane units SiO _4/2 Example 1 A basic compound is prepared comprising the following components (parts by weight): Silanol-terminated polydimethylsiloxane with a viscosity of 6000 cps at 25°C 100 Trimethylsiloxy 4 mol%, dimethylsiloxy 56 mol%, methylsiloxy 40 mol% and
M, D, T, OH silicone oil containing approximately 0.5% by weight of OH 10 Iron oxide with a pH of 6.0 to 7.5 6 Dimethylsiloxane-treated fumed silica having a surface area of approximately 200 square meters/g 25 A catalyst mixture comprising the following components: (parts by weight). Methyltris-2-ethylhexanoxysilane; crosslinking agent 6 Glycidoxypropyltrimethoxysilane; adhesion promoter 0.25 Dibutyltin diacetate 0.06 100 parts of the base compound are condensed with the catalyst mixture in the absence of air and atmospheric moisture. Mix and package in 3 oz. metal containers. Test sheets are then prepared and cured for 3 days at a temperature of 77±2㎓ and a relative humidity of 50±5%. The following test results are obtained. Shore A Hardness 42 Tensile Strength psi 500 Elongation % 380 Comparative Example A A base compound containing the following ingredients (parts by weight) is prepared. Silanol-terminated polydimethylsiloxane with a viscosity of 6000 cps at 25°C 100 Octamethyltetrasiloxane-treated fumed silica filler with a surface area of about 200 m ² /g 25 Iron oxide 6 A catalyst mixture is prepared comprising the following components (parts by weight). Methyltris(2-ethylhexoxy)silane
6 Dibutyltin diacetate (catalyst) 0.06 Glycidoxypropyltrimethoxysilane (adhesion promoter) 0.25 Mix 100 parts of the base compound with the catalyst mixture in the absence of air and atmospheric moisture.
This anhydrous material is cured and tested as in Example 1. Obtain the following test results. Shore A Hardness 43 Tensile Strength psi 630 Elongation % 370 To determine the relative resistance to hot hydrocarbon oil, the hardened specimens of Example 1 and Comparative Example A were exposed to 300ⓓ of 5W-30 motor oil for various times. It was suspended and the hardness, tensile strength, elongation and volume swelling were measured. The results are shown in the table below.

【table】

Table: The composition of Example 1 has greater resistance to reversion than the composition of Comparative Example. This illustrates the advantages of the high functionality (T-containing) component of the compositions of the present invention. Example 2 The procedure of Example 1 is repeated using 1,3,5-tris-trimethoxysilylpropyl isocyanurate as the adhesion promoter. The properties of the cured product were as follows. Shore A Hardness 46 Tensile strength psi 620 Elongation % 400 Other modifications are possible. For example, iron oxide thermal stabilizers can be omitted. Instead of methyltris(2-ethylhexoxy)silane, methyltris(benzoxy)silane can be used as a crosslinking agent. However, if an M, D, T fluid having a substantially lower T concentration is used instead of the M, D, T fluid used in Example 1, the oil resistance becomes worse. Comparative Example B A base compound having the following composition was prepared. 100 parts silanol-terminated polydimethylsiloxane having a viscosity of 6000 cps at 25°C M, D, T, OH containing approximately 5 mol% trimethylsiloxy, 20 mol% monomethylsiloxy, 75 mol% dimethylsiloxy and 0.5% OH by weight
Silicone oil 10 parts Iron oxide, pH 6.0-7.5 6 parts Dimethylsiloxane-treated fumed silica with a surface area of about 200 m ² /g 25 The same catalyst mixture used in Example 1 in the absence of moisture, in exactly the same percentage composition. and added it to the above base compound. The following room temperature test results were obtained. Shore A Hardness 35 Tensile Pei 600 Elongation % 420 To measure the relative resistance to hot hydrocarbon oil, hardened specimens were suspended in 5W-30 oil at 300°. After 72 hours under these environmental conditions, hardness, tensile strength, elongation, and volume swelling were measured.
The tensile strength was low and the test was discontinued. The data presented below clearly demonstrate the benefits of the high T content oil used in Example 1. The results are shown below. Shore A Hardness 9 Tensile Strength psi 40 Elongation % 240 Swelling % 36 A coating of the composition of Example 1 was distributed on the surface of the first piece. The surfaces of the second piece of surface are brought into close proximity to form a sandwich sandwich with a composition between the pieces to form a gasket between the two corrosion-susceptible metal workpieces. The structure is exposed to a humid atmosphere of about 75°C until a strong bond is formed and the composition cures to a rubbery state without releasing any substantial amounts of corrosive or odorous substances. There are other variations to the present invention in light of the above teachings.
Changes are of course possible. It is therefore understood that changes may be made to the particular embodiments of the invention without departing from the scope of the invention.

Claims (1)

[Claims] 1 Formula (a) (wherein R and R ¹ are individually hydrocarbyl,
(b) 100 parts by weight of a silanol chain-terminated polydiorganosiloxane of up to 8 carbon atoms selected from halohydrocarbyl and cyano-lower alkyl, where n is an average number of about 10 to 15,000; (i) 25 to 60 mol% of monoalkylsiloxy units, siloxy units or mixtures thereof, (ii) 1 to 6 mol% of trialkylsiloxy units, and (iii) ) 2 to 20 parts by weight of a fluid polysiloxane containing from 34 to 74 mole percent of dialkylsiloxy units and from about 0.1 to about 2 percent by weight of silicon-bonded hydroxyl groups; (c) from 10 to 100 parts by weight of a finely divided silica filler; (d) Formula R ² _n Si(OR ³ ) _4-n (wherein R ² has the same meaning as defined from R and R ¹ above, R ³ is hydrocarboyl and halohydrocarboyl Number of carbon atoms selected from 6~
3 to 10 parts by weight of a crosslinking silane represented by 30 organic groups, m having a value of 0 or 1;
and (e) from a thermally stable rubbery moisture-cured composition comprising 0.01 to 10 parts by weight of a curing catalyst consisting of an organic tin or tin salt of an organic acid having 2 to 6 carbon atoms. An article of manufacture comprising a plurality of metal-faced strips, wherein a gasket layer is inserted between at least a portion of the surface portion of each strip in close proximity to another surface portion. 2. Claim 1, wherein the composition also contains 1 to 10 parts by weight of a finely divided iron oxide thermal stabilizer.
Articles of manufacture described in Section 1. 3. (i) providing a piece with a plurality of metal surfaces; (ii) applying a layer of a moisture-curable gasket-forming composition to at least a portion of at least one of the metal surfaces; and (iii) providing a piece with a plurality of metal surfaces; (iv) exposing the article of step (iii) to moisture until the gasket layer has hardened into a rubbery material; In this case, the moisture-curable gasket composition has the formula (a). (wherein R and R ¹ are individually hydrocarbyl,
(b) 100 parts by weight of a silanol chain-terminated polydiorganosiloxane of up to 8 carbon atoms selected from halohydrocarbyl and cyano-lower alkyl, where n is an average number of about 10 to 15,000; (i) 25 to 60 mol% of monoalkylsiloxy units, siloxy units or mixtures thereof, (ii) 1 to 6 mol% of trialkylsiloxy units, and (iii) ) 2 to 20 parts by weight of a fluid polysiloxane containing from 34 to 74 mole percent of dialkylsiloxy units and from about 0.1 to about 2 percent by weight of silicon-bonded hydroxyl groups; (c) from 10 to 100 parts by weight of a finely divided silica filler; (d) Formula R ² _n Si(OR ³ ) _4-n (wherein R ² has the same meaning as defined from R and R ¹ above, R ³ is hydrocarboyl and halohydrocarboyl Number of carbon atoms selected from 6~
3 to 10 parts by weight of a crosslinking silane represented by 30 organic groups, m having a value of 0 or 1;
and (e) 0.01 to 10 parts by weight of a curing catalyst consisting of an organic tin salt or a tin salt of a fatty acid having 2 to 6 carbon atoms. 4. The method of claim 3, wherein the composition also includes 1 to 10 parts by weight of a finely divided iron oxide thermal stabilizer.