Fluorosilicone rubber compositions

Abstract: Heat curable fluorosilicone rubber compositions curable to products having reduced shrinkage at elevated temperatures, increased solvent resistance and increased resiliency as well as excellent elongation, tear resistance, and hardness properties, are provided herein by reacting a binary filler system composed of a reinforcing silica filler and a potassium aluminosilicate filler with an alkenyl- and silanol-terminated diorganopolysiloxanes base polymer gum and an alkenyl- and silanol-terminated diorganopolysiloxanes crosslinking gum and then combining the resulting composition with an organic peroxide catalyst.

Description: Background of the Invention The present invention relates to fluorosilicone heat cured rubber compositions. More particularly, the present invention relates to fluorosilicone heat cured rubber compositions containing a filler system which provides the rubber composition with reduced shrinkage and enhanced solvent resistance. Fluorosilicone heat cured rubber compositions are known in the art. Reference is made, for example, to U.S. Patent Nos. 4,317,899 (Bluestein et al.), 4,525,528 (Bush et al.), and 4,355,121 (Evans). US-A-4,960,811 discloses a fluorosilicone heat cured rubber composition prepared using a chain modifier mixture comprising vinyl terminated polysiloxanes and silanol terminated siloxanes which allow vinyl-on-chain-ends functionality to easily crosslink unhindered by perfluoroalkyl groups. GB-A-827402 discloses a process for preparing a heat-curable silicone gum composition, which comprises introducing into a silicone gum convertible by heat to the solid, elastic state, a zeolitic molecular sieve containing an organic peroxidic curing agent adsorbed therein. Fluorosilicone heat cured rubbers are useful in the production of such items as electrical connectors, molded silicone rubber inserts, tubing, hoses, sheeting, gaskets, and the like. Such items are frequently used in applications which expose them to high temperatures, such as occurs in the automotive and aerospace industries. A disadvantage with molded articles prepared from heat cured fluorosilicone compositions is that such articles tend to undergo shrinkage at elevated temperatures, which makes the articles unsuitable for continued use. It is desirable to provide a heat cured rubber fluorosilicone composition which undergoes reduced shrinkage at elevated temperatures. It is further desirable to provide a heat cured rubber fluorosilicone composition which undergoes reduced shrinkage at elevated temperatures and which has enhanced resistance to solvent swell and improved resiliency. The present invention provides such a heat cured rubber composition. The present invention is based on the discovery that a binary filler system composed of treated fumed silica filler and micronized potassium aluminosilicate will impart a fluorosilicone rubber composition with a lower degree of shrinkage, improved resiliency, and increased solvent resistance. Summary of the Invention In one aspect of the present invention there is provided a heat curable fluorosilicone rubber composition, comprising comprising by weight: (1) the composition obtained by reacting at a temperature in the range of from 25°C to 180°C : (A) from 93 to 98 parts of an alkenyl-terminated and silanol-terminated diorganopolysiloxane copolymer having a viscosity of from 1,000,000 to 300,000,000 mPas (centipoises) at 25°C., comprising units of the formulae RR 1 SiO and R 2 2 SiO, wherein R is a fluoroalkyl radical, and R 1 and R 2 are each monovalent hydrocarbon radicals, the RR 1 SiO units being present in an amount sufficient to provide a fluorine content of from 36 to 37 weight percent, the ratio of alkenyl endgroups to silanol endgroups being from 65:35 to 75:25, the alkenyl-on-chain content of the copolymer being from 0% to 0.6% by weight; the total silicon-bonded alkenyl content of the copolymer being from 0.007 to 0.02% by weight; (B) from 2.0 to 7.0 parts of an alkenyl-terminated and silanol-terminated diorganopolysiloxane copolymer having a viscosity of from 1,000,000 to 300,000,000 mPa·s (centipoise) at 25°C., comprising units of the formulae RR 1 SiO and R 2 2 SiO, wherein R is a fluoroalkyl radical, and R 1 and R 2 are each monovalent hydrocarbon radicals, the RR 1 SiO units being present in an amount sufficient to provide a fluorine content of from 34 to 35.2 weight percent, the ratio of vinyl endgroups to silanol endgroups being from 65:35 to 75:25, the alkenyl-on-chain content of the copolymer being from 1.2% to 1.6% by weight; the total silicon-bonded alkenyl content of the copolymer being from 1.45% to 1.65% by weight; the sum of (A) and (B) being 100 parts; (C) from about 1 to 40 parts of a potassium aluminosilicate filler having the formula KAl 2 (AlSi 3 O 10 )(OH) 2 or KAl 2 (AlSi 3 O 10 )(F) 2 and a surface area of from 5.0 to 6.5 square meters per gram and particles having an average diameter of from 1.0 to 10.0 micrometers and an average thickness of from 0.5 to 1.0 micrometers; (D) from 5 to 60 parts of a pre-treated or untreated reinforcing silica filler having a surface area of from 180 to 225 square meters per gram; and (E) from 0 to 50.0 parts of a condensable liquid diorganopolysiloxane having the general formula HO(R 4 2 SiO) y H wherein R 4 is a monovalent substituted or unsubstituted hydrocarbon radical and "y" has a value of from 1 to 20; with the proviso that if the reinforcing silica filler (D) is untreated, (E) is present in an amount of from 5 to 50 parts by weight to in situ treat the untreated silica filler (D); (2) a catalytic amount of an organic peroxide curing catalyst. The present invention is also directed to the reaction product (1) described above and to a method for reducing the shrinkage of heat cured fluorosilicone compositions. The compositions provided by this invention have reduced shrinkage at elevated temperatures, increased solvent resistance and increased resiliency as well as excellent elongation, tear resistance, and hardness properties. In addition, the surface of the cured rubber is smoother and has a perceptibly lower coefficient of friction. Detailed Description of the Invention The present invention is directed to heat curable fluorosilicone rubber compositions which are curable to form rubber products having reduced shrinkage at elevated temperatures, increased solvent resistance and increased resiliency . The improved properties of the product is a result of the use of the binary filler system composed of the potassium aluminosilicate (1)(C) and the fumed silica filler (1)(D). The heat curable composition of this invention is composed of two parts, (1) the product obtained by reacting components (A)-(E) at 25°C to 180°C; and part (2) a peroxide curing catalyst. In part (1) of the heat curable composition of this invention, the total sum of (A) and (B) is 100 parts by weight. The amount of (A) is in the range of from 93 to 98, preferably from 94 to 97, and most preferably from 95 to 96, parts by weight. The amount of (B) is in the range of from 2 to 7, preferably from 3 to 6, and most preferably from 4 to 5, parts by weight. The potassium aluminosilicate filler (C) is present in part (1) in an amount within the range of from 1 to 40, preferably from 5 to 20, and most preferably from 7 to 15, parts by weight per 100 parts of (A) and (B). The reinforcing silica filler (D) is present in an amount within the range of from 5 to 60, preferably from 23 to 29, and most preferably from 23 to 25, parts by weight per 100 parts of (A) and (B). The amount of the condensable polydiorganosiloxane (E) will depend on whether the reinforcing silica filler (D) is treated or untreated as defined below. If the silica filler is treated, the presence of (E) is not required but is useful as a plasticizer in the heat curable composition. Thus, if the silica filler is treated, the amount of (E) will vary from 0 to 50 parts by weight. If the silica filler is untreated, (E) will then be used as an in situ treating agent for the silica filler as well as a plasticizer and in that situation will be present in part (1) in an amount sufficient to treat the silica filler. Part (1) of the heat curable composition of this invention is prepared by reacting (A)-(E) at a temperature in the range of from 25°C to 180°C., typically for a period of from 1 to 4 hours. The base polymer (Component (A)) used in the method and composition of this invention is an alkenyl-terminated and silanol-terminated diorganopolysiloxane copolymer comprising units of the formulae RR 1 SiO and R 2 2 SiO, wherein R is a fluoroalkyl radical, and R 1 and R 2 are each monovalent hydrocarbon radicals. In (A), R, R 1 , and R 2 are each preferably a fluoroalkyl radical containing 3 or more, preferably 3 to 8, carbon atoms, and 1 or more, preferably 1 to 3, fluorine atoms. Examples of such radicals include 3-fluoropropyl; 3,3-difluoropropyl; and 3,3,3-trifluoropropyl. Most preferably, R is a substituted alkyl group such as -CH 2 CH 2 R 3 , wherein R 3 is perfluoroalkyl containing from 1 to 6 carbon atoms, such as perfluoromethyl, perfluoroethyl, perfluorohexyl, and the like. Most preferably, the R radical is 3,3,3-trifluoropropyl. The R 1 and R 2 radicals are each preferably methyl. In (A), the RR 1 SiO units are present in an amount sufficient to provide a fluorine content of from 35 to 37, preferably from 35.5 to 37, and most preferably from 36 to 37 weight percent. The ratio of alkenyl endgroups to silanol endgroups is from 65:35 to 75:25, preferably 55:45, and most preferably 60:40. The alkenyl-on-chain content of the copolymer (A) is from 0% to 0.6%, preferably from 0 to 0.3%, and most preferably is 0%, by weight of (A). The total silicon-bonded alkenyl content of the copolymer is from 0.007% to 0.009%, preferably from 0.007% to 0.0085%, and most preferably from 0.0077% to 0.0085% by weight. (A) has a viscosity of from 1,000,000 to 300,000,000, preferably 180,000,000 to 230,000,000, and most preferably from 190,000,000 to 220,000,000, mPa·s (centipoise) at 25°C. Component (A) can be prepared by reacting a cyclotrisiloxane containing organo and fluoroalkyl substituent groups, a vinyl-terminated sodium fluorosilanolate, and a chainstopper which is a mixture containing a vinyl-terminated fluorosilicone and silanol-terminated fluorosilicone. The cyclotrisiloxane and the chainstopper are mixed and then heated to a temperature of from 135°-140°C., after which the fluorosilanolate is added to the reaction mixture. The mixture is then held, with agitation, at a temperature in the range of 135°-140°C for about 4 to about 4.5 hours. After the reaction is over, the polymer mixture is neutralized with a silyl phosphate and the cyclics are stripped off. The final product is then extruded through a screen pack having a sieve size of about 150 mesh to remove extraneous materials such as dirt and particulates. The cyclotrisiloxane used in preparing (A) is preferably a 1,3,5-tris(trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane. Such compounds can be prepared from a cracking process using a trifluoropropylmethylsiloxane hydrolyzate derived from the aqueous hydrolysis of the corresponding pure dichlorosilane, i.e., methyl-3,3,3-trifluoropropyldichlorosilane. The amount of aliphatic unsaturation in the cyclotrisiloxane varies from about 0.0 to about 1.0, preferably from 0.0 to 0.6, and most preferably is 0, weight percent in the total cyclotrisiloxane reactant. The vinyl-terminated sodium fluorosilanolate used in making (A) is a low molecular weight telomeric fluid containing both sodium silanolate and vinyldimethylsiloxy endgroups. This fluid functions as catalyst for the polymerization of the cyclotrisiloxane to fluorosilicone homopolymers and fluids. It is prepared through the base-catalyzed ring opening of fluorosilicone cyclic trimer in the presence of appropriate chain modifiers, e.g., divinyltetramethylsiloxane, and excess aqueous sodium hydroxide. The base is a 4% solution of previously prepared fluorosilanolate. The base is added to the cyclic trimer and chain modifiers while at 35-45°C over a period of about one hour or the appearance of oiling. The aqueous base is then added over a controlled rate and the reaction is heated to a maximum of120°C. Nitrogen is used initially as a blanket but is then reverted to a sparge to facilitate the removal of water during the stripping operation. The chain modifier used in making (A) can be a mixture containing 100 parts by weight of a vinyl-terminated fluorosilicone polydimethylsiloxane copolymer and from 2.0 to 5.0 parts by weight of a silanol-terminated fluorosilicone. The vinyl-terminated fluorosilicone polydimethylsilicone copolymer has a fluorosilicone content of 30-60 mole percent. This copolymer and a method for preparing it is disclosed in U.S. Patent No. 4,348,531 (Evans). The silanol-terminated fluorosilicone preferably has the formula wherein x is preferably 3. It is typically prepared by the acid-catalyzed ring opening of a cyclic trimer of trifluoropropyl-methylsiloxane. Component (B) used in the method and composition of this invention functions as a crosslinker. It is an alkenyl-terminated and silanol-terminated diorganopolysiloxane copolymer, comprising units of the formulae RR 1 SiO and R 2 2 SiO, wherein R, R 1 and R 2 are as previously defined herein, the RR 1 SiO units being present in an amount sufficient to provide a fluorine content of from 34.5 to 35.2 weight percent, the ratio of vinyl endgroups to silanol endgroups being from 65:35 to 75:25, the alkenyl-on-chain content of the copolymer being from 0% to 0.6% by weight; the total silicon-bonded alkenyl content of the copolymer being from 0.007% to 0.020% by weight; the sum of (A) and (B) being 100 parts. The preferences for the radicals represented by R, R 1 , and R 2 in vinyl polymer (B) are the same as those for the base polymer (A). The vinyl polymer (B) has a viscosity of from 1,000,000 to 300,000,000, preferably from 170,000,000 to 220,000,000, and most preferably from 180,000,000 to 200,000,000 mPa·s (centipoises) at 25°C. In (B), the RR 1 SiO units are present in an amount sufficient to provide a fluorine content of from 34.0 to 36.0, preferably from 34.5 to 35.5, and most preferably from 35 to 35.2 weight percent. The ratio of alkenyl endgroups to silanol endgroups is from 65:35 to 75:25, preferably 65:35 and most preferably 70:30. The alkenyl-on-chain content of (B) is from 1.40% to 1.60%, preferably from 1.45 to 1.60 %, and most preferably from 1.50% to 1.60%, by weight. The total silicon-bonded alkenyl content of (B) is from 1.46% to 1.68%, preferably from 1.50% to 1.68%, and most preferably from 1.52 to 1.62% by weight. Component (B) can be prepared by reacting a cyclotrisiloxane containing organo and fluoroalkyl substituent groups, a vinyl-terminated sodium fluorosilanolate, a methylvinylcyclic trimer, and a vinyl-terminated fluorosilicone polydimethylsiloxane copolymer chainstopper. The cyclotrisiloxane is heated to a temperature of from 130°-135°C., after which the chain modifier, fluorosilanolate and methylvinylcyclic trimer are added to the cyclotrisiloxane. The mixture is then held, with agitation, at a temperature in the range of 135°-140°C for 4 to 4.5 hours. After the reaction is over, the polymer mixture is neutralized with a silyl phosphate and the cyclics are stripped off. The final product is then extruded through a screen pack having a sieve size of about 150 mesh to remove extraneous materials such as dirt and particulates. The cyclotrisiloxane and vinyl-terminated sodium fluorosilanolate used for making (B) can be the same as those used for making (A). The vinyl-terminated fluorosilicone polydimethylsiloxane copolymer chain modifier used in the preparation of (B) contains a fluorosilicone content of 47 to 55 mole percent. It is prepared by the polymerization of a methyl-3,3,3-trifluoropropylmethylsiloxane cyclic trimer with octamethylcyclotetrasiloxane in the presence of a vinyl-type chainstopper. Component (C) in the composition of this invention is a potassium aluminosilicate filler having a specific surface area of from 5.0 to 6.50, preferably from 5.05 to 6.2, and most preferably from 5.07 to 6.11, square meters per gram. The potassium aluminosilicate filler has an average particle diameter of from 1.0 to 20.0, preferably from 1.0 to 10, and most preferably from 1.0 to 5.0 micrometers and an average particle thickness of from 0.5 to 1.0, preferably from 0.5 to 0.8, and most preferably from 0.45 to 0.55, micrometers. The surface area of the potassium aluminisilicate filler is critical to this invention because a surface area which is too large will lead to an increase in the final cured product's durometer, an increase in its modulus, a decrease in its elongation and a decrease in its tear properties. On the other hand, a surface area which is too small will lead to a decrease in the final cured product's durometer, a decrease in its modulus, an increase in its elongation and an increase in its tear properties. The average particle size in the potassium aluminosilicate filler is also critical to this invention because if the average particle size is too large, the permeability and swell properties of the final cured product increase and it becomes more difficult to control the shrinkage of the cured product. If the particle size is too small, the modulus of the cured product increases, which makes the curable composition harder to work with and causes it to crumble easily. In addition, the amount of potassium aluminosilicate is critical to this invention because if an insufficient amount is used, there will be no reduction in the shrinkage of the final cured product. The potassium aluminosilicate (C) has the formula KAl 2 (AlSi 3 O 10 )(OH) 2 or KAl 2 (AlSi 3 O 10 )(F) 2 . The most preferred potassium aluminosilicate filler for use herein is potassium mica (also known as muscovite), having the formula KAl 2 (AlSi 3 O 10 )(OH) 2 , which is a natural hydrous potassium aluminum silicate of the mica group. The potassium aluminosilicate fillers used in the compositions of this invention have a highly delaminated flake structure. The micronized muscovite micas are preferred over the wet-ground muscovites due to the former's smaller average particle size. The laminar structure of the mica helps to create a uniform pore structure in the cured matrix which leads to lower compression set, reduced permeation of gases and liquids, and a lower degree of shrinkage of the cured final product. Potassium aluminosilicate compounds are known in the art and can be obtained commercially. Component (D) in the compositions of this invention is a pre-treated or untreated reinforcing silica filler. The reinforcing silica filler (D) comprises fine powdery silica conventionally known, such as, for example, fumed silica, precipitated silica, silica aerogel, and the like. The most preferred reinforcing silica filler for use in the present invention is fumed silica. The term "pre-treated" as used herein refers to silica fillers which have been treated with a treating agent prior to the silica filler's contact with components (A), (B), (C), and, if present, (E). The treating agent reacts with the silanol end groups on the surface of the silica fillers. Alternatively, the silica filler may be untreated (raw) when it is placed in contact with (A), (B), (C), and (E) and then in situ treated by appropriate amounts of (E). In any case, it is critical to the present invention that the silica filler be treated prior to the addition of the cure catayst to the composition containing (A)-(E). Treatment is necessary because without it, the silanol end groups on the surface of the silica fillers have a tendency to cause structuring of an uncured compound on standing. The silica filler may be treated prior to its contact with (A)-(C) and (E) by methods known in the art. Reference is made, for example, to U.S. Patent No. 4,529,522 (Evans et al.). In the method disclosed in the patent to Evans et al., silica fillers are exposed in the vapor phase to the direct hydrolyzate of diorganosilanes at temperatures of from 240°C to 310°C. These direct hydrolyzates are compounds comprising cyclic and linear diorganopolysiloxanes. Preferably, the silica filler is treated in situ with (E) according to the method taught in U.S. Patent No. 4,724,167 to Evans et al. (Evans). According to the Evans method, silica reinforcing filler is contacted at a temperature of at most about 210°C with a liquid condensable diorganopolysiloxane having hydroxy or alkoxy functionality for a time sufficient to achieve the desired treatment. The silica filler (D) has a surface area of from 180 to 225, preferably from 200 to 220, and most preferably from 210 to 215 square meters per gram. The surface area of the filler is critical to this invention because a surface area which is too large will lead to an increase in the final cured product's durometer, an increase in its modulus, a decrease in its elongation and a decrease in its tear properties. On the other hand, a surface area which is too small will lead to a decrease in the final cured product's durometer, a decrease in its modulus, an increase in its elongation and an increase in its tear properties. The amount of silica filler is also critical to the present invention because an insufficient amount will cause a decrease in the the tear, modulus and elongation of the cured product. As mentioned previously herein, component (E) is a condensable liquid diorganopolysiloxane having the general formula (12) HO(R 4 2 SiO) y H wherein R 4 is a monovalent substituted or unsubstituted hydrocarbon radical and "y" has a value of from 1 to 20. Preferably, the condensable diorganopolysiloxanes represented by formula (12) are all hydroxy terminated diorganopolysiloxanes. It is preferred that R 4 is generally at least 50% by number methyl with the balance selected from alkyl, such as methyl, ethyl, propyl, butyl and hexyl, alkenyl, such as vinyl; aryl, such as phenyl; cycloalkyl, such as cyclopentyl, cyclohexyl and cycloheptyl; halogenated alkyl, such as 3-chloropropyl, 4-chlorobutyl, 3,3-difluoroallyl and 3,3,3-trifluoropropyl; halogenated aryl, such as 3-fluorophenyl; halogenated cycloalkyl. Where R 4 is halogenated, the preferred R 4 is -CH 2 CH 2 R 5 where R 5 is perfluoroalkyl, such as perfluoromethyl, perfluoroethyl and perfluorohexyl. In formula (II), R 4 most preferably represents the -CH 2 CH 2 R 5 radical where R 5 is perfluoroalkyl and most preferably perfluoromethyl. In formula (II), "y" is preferably a number in the range of 2 to 10 and most preferably 3. The amount of (E) used in part (1) will vary according to whether it is to be used as an in situ treating agent for the silica filler. If the silica filler is treated previously, (E) is not required to be present. If it is desired to be present, for example, if it is to be used as a plasticizer and an anti-structuring agent, then (E) will be present in an amount up to 4 parts, preferably from 0.5 to 3.0, and most preferably from 0.5 to 2.0 parts by weight per 100 parts of (A) and (B). If the silica filler is untreated, (E) is used in an amount sufficient to treat the silica filler, which is typically in the range of from 5 to 50 parts by weight, preferably from 17 to 30 parts by weight, and most preferably from 19 to 25 parts by weight for each 100 parts by weight of silica. The combination of the reinforcing silica filler (C) and the potassium aluminosilicate filler (D) yields a % linear shrinkage (as measured using an optical comparator on a 18.41cm x 3.81cm x 6.35 mm (7 1/4" x 1 1/2" x 1/4") molded slab after post baking for 4 hours at 200°C) of 2.0 to 3.5% (dependent upon loading of fillers). The degree of solvent resistance is dependent upon the total mixed filler loading, but in general a decrease of approximately 6-10% in swell is observed when the alkali aluminosilicate filler is present at about 8-10 parts by weight per 100 parts of (A) and (B). The curing of the fluorosilicone rubber composition of the present invention can be effected by chemical vulcanizing agents or by high energy electron radiation. More often, chemical vulcanizing agents are used for the curing operation and any of the conventional curing agents can be used. The preferred curing agents are organic peroxides (C) conventionally used to cure silicone elastomers. Thus, part (2) of the heat curable composition of this invention is an organic peroxide curing catalyst. The most suitable peroxide catalysts are compounds of the formula, AOOH or AOOA, in which A is an organic radical, especially those compounds in which at least one peroxide oxygen is attached to a tertiary carbon atom. Preferred peroxide catalysts include, for example, t-butyl hydroperoxide, cumene hydroperoxide, decaline hydroperoxide, di-t-butyl peroxide, dicumyl peroxide, and 2,5-dimethyl-2,5-di-(t-butyl peroxy)hexane. The peroxides containing tertiary alkoxy radicals are more efficient in abstracting hydrogen or halogen atoms from the pendant organic groups linked to the silicon atoms, and are therefore preferred. The most preferred of these is 2,5-dimethyl-2,5-di-(t-butyl peroxy)hexane, which is available commercially, e.g., at 45% by weight on an inert filler under the tradenames, VAROX® (R. T. Vanderbilt Co., Inc.) and LUPERCO® 101XL (Lucidol Division, Pennwalt Corp.) The curing catalyst (2) is used in a catalytic amount, which is typically in the range of from 1.0 to 2.0, preferably from 1.0 to 1.5, and most preferably from 1.0 to 1.3, parts by weight per 100 parts of (A) and (B) of part (1) of the heat curable composition. The composition of this invention may further contain flame retardants, e.g., platinum (as platinum or in combination with other materials such as triallylisocyanurate); adhesion promoters, such as organic silazanes, which promote bonding between fillers and the gum; and other additives, including anti-oxidants, processing aids (e.g., silanol fluids) supplemental curing agents (e.g., materials that provide additional vinyl curing sites) such as trimethylol propane trimethacrylate (Sartomer® 350; Sartomer Co.), triallyl trimellitate (Sipomer ® TATM; Alcolac, Inc.), 1,3,5-trimethyl-1,3,5-trivinylcyclotetrasiloxane; 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane, and the like. The heat curable composition of this invention containing part (1) and part (2) may be cured at elevated temperatures, for example, from 100°C to 300°C, or by exposure to radiation, to produce fluorosilicone elastomers having the aforementioned physical properties. In order to better able the artisan to practice the present invention, the following examples are provided by way of illustration and not by way of limitation. All parts and percentages are by weight unless otherwise noted. Experimental In the examples below, the following terms are defined as follows: "FS Polymer I" - refers to a blend containing 98 parts by weight of (i) a vinyl-terminated polymethyl-3,3,3-trifluoropropylsiloxane having a viscosity of about 200,000,000 mPa·s (centipoises) at 25°C, no vinyl-on-chain content, a total vinyl content of 0.007-0.020% by weight, and a fluorine content of 36.5-37 % by weight, and 2.0 parts by weight of (ii) a vinyl-terminated polymethyl-3,3,3-trifluoropropylsiloxane having a viscosity of 200,000,000 mPa·s (centipoises) at 25°C, a vinyl-on-chain content of 1.4-1.6% by weight, a vinyl end group content of 0.007-0.020% by weight, and a fluorine content of 34.5-35.2 % weight. "FS Polymer II" - refers to a vinyl-terminated-polymethylfluoropropylsiloxydimethylsiloxy copolymer having a viscosity of about 160,000,000-200,000,000 mPa.s (centipoises) at 25°C, a total vinyl content of about 0.032% by weight, and a fluorosilicone content of 80% by weight. "Fumed Silica MS-7" - refers to fumed silica filler. "M.M.C-3000" - refers to a muscovite having an average particle thickness of about 0.5 micrometers and an average particle diameter of about 5-10 micrometers. "FS Silanol Fluid" - refers to fluorodisiloxanol telomeric fluid having a silanol content of about 6.0-6.7% by weight, a fluorosilicone content of about 37 % by weight, and a viscosity of about 70-110 mPa.s (centipoises) at 25°C. The fluorodisiloxanol telomeric fluid has the formula "PC L" - refers to % change in length after press cure. "PC W" - refers to % change in width after press cure. "PB L" - refers to % change in length after post bake. "PB W" - refers to % change in width after post bake. Examples 1-7 Eight compositions having the formulations shown in Table 1 below were prepared. Example 1 is a comparative example containing no alkali aluminosilicate filler. Examples 1-7: Formulations Example No. Ingredient 1 2 3 4 5 6 7 FS Polymer I 102.5 102.5 102.5 102.5 102.5 102.5 102.5 Fumed Silica MS-7 26 25 23 23 23 23 23 M.M.C-3000 - 5.0 8.0 8.0 8.0 8.0 8.0 FS Silanol Fluid 4.8 5.8 5.9 5.9 5.9 5.9 5.9 Each composition was catalyzed with 1.0 part by weight of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane having an active oxygen content of 4.9%), press cured for 15 minutes at 177°C, and then post-baked for 4 hours at 200°C. The physical properties of the compositions are presented in Table 2 below. Examples 1-7: Properties Example No. Property 1 2 3 4 5 6 7 Shore A 43 49 42 44 44 41 43 100% (240) (335) (180) (240) (208) (185) (215) Modulus,(psi) kPa 1656 2312 1242 1656 1435 1277 1484 Tensile,(psi) (1555) (1525) (1450) (1500) (1405) (1370) (1295) kPa 10730 10523 10005 10350 9695 9453 8936 Elongation, % 397 392 519 440 480 485 422 Die B Tear, (176) (169) (146) (155) (135) (140) (157) (ppi)kg/m 3143 3018 2607 2768 2411 2500 2804 Specific Gravity 1.462 1.464 1.471 1.465 1.467 1.457 1.465 Compression Set 22 hrs/177°C 13.0 14.5 13.1 11.2 9.8 10.4 - Bashore, % 23 - - 27 27 29 31 % Linear Shrinkage PC L 3.30 3.24 - 2.6 2.8 2.4 2.7 PC W 1.90 1.91 - 1.6 1.8 1.5 1.5 PB L 3.77 3.76 3.25 3.2 3.4 2.9 3.3 PB W 2.34 2.12 2.30 2.6 2.1 2.0 2.1 Volume Swell Volume % 34.0 27.3 The composition of the comparative example (Example 1) displayed the standard degree of shrinkage of about 3.77% after post-baking. Examples 8 and 9 (Comparative) Two compositions having the formulations shown in Table 3 were prepared. Example 8 contains no alkali aluminosilicate filler. Examples 8 and 9: Formulations Example No. Ingredient 8 9 FS Polymer II 101.92 101.92 Fumed Silica MS-7 26.0 23.0 M.M.C-3000 - 8.0 FS Silanol Fluid 5.66 6.0 Each composition was catalyzed with 1.0 part by weight of 2,5-dimethyl-2,5-di(t-butylperoxy)hexane having an active oxygen content of 4.9%), press cured for 15 minutes at 177°C, and then post-baked for 4 hours at 200°C. The physical properties of the compositions prepared in Examples 8 and 9 are presented in Table 4 below. Examples 8 and 9: Properties Example No. Property 8 9 Shore A 42 42 100% Modulus, psi 195 210 Tensile,(psi) kPa (1090) 7521 (820) 5658 Elongation, % 360 349 Die B Tear, (116) (85) (ppi) kg/m 2072 1518 Specific Gravity 1.385 1.386 Compression Set 22 hrs/177°C 15.3 19.2 Bashore, % 37 43 % Linear Shrinkage PC L 3.56 2.4 PC W 2.18 1.54 PB L 4.5 3.1 PB W 2.86 2.0 Volume Swell Volume % 70.0 64.0 Heat Age Shore A, points change +2 +1 Tensile, % change -9.1 +10.6 Elongation, % change -4.0 +3.7 The examples above show that the compositions containing alkali aluminosilicate filler have a reduced degree of shrinkage, increased resilience and increased solvent resistance.

Claims: A heat curable fluorosilicone rubber composition, comprising by weight: (1) the composition obtained by reacting at a temperature in the range of from 25°C to 180°C: (A) from 93 to 98 parts of an alkenyl-terminated and silanol-terminated diorganopolysiloxane copolymer having a viscosity of from 1,000,000 to 300,000,000 mPa.s (centipoises) at 25°C., comprising units of the formulae RR 1 SiO and R 2 2 SiO, wherein R is a fluoroalkyl radical, and R 1 and R 2 are each monovalent hydrocarbon radicals, the RR 1 SiO units being present in an amount sufficient to provide a fluorine content of from 36 to 37 weight percent, the ratio of alkenyl endgroups to silanol endgroups being from 65:35 to 75:25, the alkenyl-on-chain content of the copolymer being from 0% to 0.6% by weight; the total silicon-bonded alkenyl content of the copolymer being from 0.007 to 0.02% by weight; (B) from 2.0 to 7.0 parts of an alkenyl-terminated and silanol-terminated diorganopolysiloxane copolymer having a viscosity of from 1,000,000 to 300,000,000 mPa.s (centipoise) at 25°C., comprising units of the formulae RR 1 SiO and R 2 2 SiO, wherein R is a fluoroalkyl radical, and R 1 and R 2 are each monovalent hydrocarbon radicals, the RR 1 SiO units being present in an amount sufficient to provide a fluorine content of from 34 to 35.2 weight percent, the ratio of vinyl endgroups to silanol endgroups being from 65:35 to 75:25, the alkenyl-on-chain content of the copolymer being from 1.2% to 1.6% by weight; the total silicon-bonded alkenyl content of the copolymer being from 1.45% to 1.65% by weight; the sum of (A) and (B) being 100 parts; (C) from 1 to 40 parts of a potassium aluminosilicate filler having the formula KAl 2 (AlSi 3 O 10 )(OH) 2 or KAl 2 (AlSi 3 O 10 )(F) 2 and having a surface area of from 5.0 to 6.50 square meters per gram and particles having an average diameter of from 1.0 to 20.0 micrometers and an average thickness of from 0.5 to 1.0 micrometers; (D) from 5 to 60 parts of a pretreated or untreated reinforcing silica filler having a surface area of from 180 to 225 square meters per gram; and (E) from 0 to 50 parts of a condensable liquid diorganopolysiloxane having the general formula HO(R 4 2 SiO) y H wherein R 4 is a monovalent substituted or unsubstituted hydrocarbon radical and "y" has a value of from about 1 to about 20; with the proviso that if the reinforcing silica filler (D) is untreated, (E) is present in an amount of from 5 to 50 parts by weight to in situ treat the untreated silica filler (D); and (2) a catalytic amount of an organic peroxide curing catalyst. A composition according to Claim 1 wherein the potassium aluminosilicate filler (1)(C) has a surface area of from 5.05 to 6.2 square meters per gram. A composition according to Claim 1 or Claim 2 wherein the potassium aluminosilicate filler (1)(C) has an average particle diameter of from 1.0 to 10.0 micrometers and an average particle thickness of from 0.5 to 0.8 micrometer. A composition according to any preceding claim wherein the reinforcing silica filler (1)(D) is fumed silica. A composition according to any preceding claim wherein the silica filler (1)(D) is pretreated. A composition according to any one of Claims 1 to 4 wherein the silica filler (1)(D) is in situ treated by the condensable liquid diorganopolysiloxane (1)(E). A composition according to Claim 6 wherein (1) (E) is present in an amount ranging from 5 to 50 parts by weight per 100 parts of silica filler (C). A composition according to any preceding claim wherein the organic peroxide curing catalyst (2) is present in an amount within the range of from 1.0 to 2.0 parts by weight per 100 parts of (1)(A) and (1)(B). A method for providing a curable fluorosilicone composition which will cure to form a product having reduced shrinkage at elevated temperatures, comprising the step of mixing: (1) a composition obtained by reacting at a temperature in the range of from 25°C to 180°C: (A) from 93 to 98 parts of an alkenyl-terminated and silanol-terminated diorganopolysiloxane copolymer having a viscosity of from 1,000,000 to 300,000,000 mPa.s (centipoises) at 25°C., comprising units of the formulae RR 1 SiO and R 2 2 SiO, wherein R is a fluoroalkyl radical, and R 1 and R 2 are each monovalent hydrocarbon radicals, the RR 1 SiO units being present in an amount sufficient to provide a fluorine content of from 36 to 37 weight percent, the ratio of alkenyl endgroups to silanol endgroups being from 65:35 to 75:25, the alkenyl-on-chain content of the copolymer being from 0% to 0.6% by weight; the total silicon-bonded alkenyl content of the copolymer being from 0.007 to 0.02% by weight; (B) from 2.0 to 7.0 parts of an alkenyl-terminated and silanol-terminated diorganopolysiloxane copolymer having a viscosity of from 1 ,000,000 to 300,000,000 mPa.s (centipoise) at 25°C., comprising units of the formulae RR 1 SiO and R 2 2 SiO, wherein R is a fluoroalkyl radical, and R 1 and R 2 are each monovalent hydrocarbon radicals, the RR 1 SiO units being present in an amount sufficient to provide a fluorine content of from 34 to 35.2 weight percent, the ratio of vinyl endgroups to silanol endgroups being from 65:35 to 75:25, the alkenyl-on-chain content of the copolymer being from 1.2% to 1.6% by weight; the total silicon-bonded alkenyl content of the copolymer being from 1.45% to 1.65% by weight; the sum of (A) and (B) being 100 parts; (C) from 1 to 40 parts of a potassium aluminosilicate filler having the formula KAl 2 (AlSi 3 O 10 )(OH) 2 or KAl 2 (AlSi 3 O 10 )(F) 2 and having a surface area of from 5.0 to 6.50 square meters per gram and particles having an average diameter of from 1.0 to 20.0 micrometers and an average thickness of from 0.5 to 1.0 micrometers; (D) from 5 to 60 parts of a pre-treated or untreated reinforcing silica filler having a surface area of from 180 to 225 square meters per gram; and (E) from 0 to 50 parts of a condensable liquid diorganopolysiloxane having the general formula HO(R 4 2 SiO) y H wherein R 4 is a monovalent substituted or unsubstituted hydrocarbon radical and "y" has a value of from about 1 to about 20; with the proviso that if the reinforcing silica filler (D) is untreated, (E) is present in an amount of from 5 to 50 parts by weight to in situ treat the untreated silica filler (D); and (2) a catalytic amount of an organic peroxide curing catalyst. A fluorosilicone with reduced compression set obtained by curing the composition according to the method of claim 9.

Inventors: EVANS EDWIN ROBERT, EVANS, EDWIN ROBERT
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1992
Main classification: C08L83
IPC codes: C08L83, C08F2, C08K3, C08F290, C08F299