Thermally stable fluorinated organosiloxane gel composition

Abstract: Polyorganosiloxane gels exhibiting a resistance to discoloration, hardening and surface deformation at temperatures of 100°C. and above can be prepared by curing compositions comprising a polydiorganosiloxane containing silicon bonded fluorinated hydrocarbon radicals and an organohydrogensiloxane containing at least three silicon-­ bonded hydrogen atoms per molecule in the presence of a platinum containing catalyst and up to 0.1 percent, based on the weight of said composition, of copper acetylacetonate.

Description: The term "polyorganosiloxane gel" typically defines a class of elastomeric, readily deformable materials exhibiting a surface hardness of below about 20 on the Shore 00 durometer scale. The hardness value of many gels is too low to be measured on this scale and is often expressed as the depth to which a penetrometer probe of specified dimensions will penetrate or deflect the surface of the gel under a specified loading. The unique physical and chemical properties of the polyorganosiloxane gels make them desirable as coatings and encapsulants for electrical and electronic devices to protect these devices from moisture, other contaminants and mechanical abuse, all of which can cause the device to malfunction or become inoperative. Electronic components associated with electronic ignition and emission control systems of automobiles and other vehicles powered by internal combustion engines are often located in the engine compartment of the vehicle where they are exposed to temperatures of 100°C. and above in addition to hydrocarbon fuels and lubricating fluids. Many prior art polyorganosiloxane gels harden or soften to the extent that they no longer provide the desired protection. It was known from EP-A 0 231 519 that the heat resistance of silicone rubber obtained from a composition of a vinyl-containing polyorganosiloxane, a hydrogen-polyorganosiloxane, and a platinum-containing catalyst can be improved by the use of a combination of carbon black and an acetylacetonate salt of a metal. One objective of this invention is to provide improved fluorine containing polyorganosiloxane compositions that upon curing yield gels exhibiting a resistance to discoloration, hardening and surface deformation when heated and to do so without requiring reactants other than a vinyl containing polyorganosiloxane, an organohydrogensiloxane and a curing catalyst. It has now been found that polyorganosiloxane gels exhibiting a resistance to discoloration, hardening and surface deformation at temperatures of 100°C. and above are obtained by curing compositions comprising a polydiorganosiloxane containing silicon bonded fluorinated hydrocarbon radicals, an organohydrogensiloxane containing at least three silicon bonded hydrogen atoms per molecule and a platinum-containing catalyst when up to 0.1 percent, based on the weight of said composition, of copper acetylacetonate is present in the curable composition. In preferred embodiments the polydiorganosiloxane is prepared by the ammonia catalyzed polymerization of at least one cyclic fluorine-containing diorganosiloxane followed by reaction of the resultant silanol terminated polymer with a hexaorganodisilazane containing a vinyl radical bonded to each of the two silicon atoms. This invention provides an organosiloxane composition that is curable to a clear thermally stable fluorosilicone gel, said composition being composed of A. a liquid diorganovinylsiloxy terminated polydiorganosiloxane wherein at least 25 percent of the silicon atoms are bonded to a fluorine containing hydrocarbon radical; B. a compatible liquid fluorine-containing organohydrogensiloxane having an average of more than two silicon bonded hydrogen atoms per molecule in an amount sufficient to provide a molar ratio of silicon bonded hydrogen atoms to vinyl radicals in said composition of from 0.3 to 1.0; and C. an amount of a platinum-containing catalyst sufficient to promote curing of said composition and optionally D. platinum catalyst inhibitors excluding acetylenic compounds and/or E. nonreactive liquid triorganosiloxy terminated poly(methyl-3,3,3-trifluoropropyl)siloxanes, wherein said composition contains from 0.01 to 0.1 percent, based on the weight of said composition, of copper acetylacetonate to minimize hardening and surface deformation of the cured gel at temperatures above 100°C. The novelty of the present curable compositions resides in the presence of copper acetylacetonate as the heat stabilizer in an amount sufficient to suppress thermally induced hardening and surface deformation of gels prepared by curing the compositions of this invention, which encompass the compositions disclosed in U.S.-A-4,719,275. Concentrations of up to 0.1 percent, based on the weight of the curable composition, of copper acetylacetonate are effective. The present inventors discovered that more than 0.1 weight percent of this ingredient will cause softening of the cured gel at temperatures above about 100°C. Less than 0.01 weight percent typically provides no significant reduction in hardening and surface deformation of the gel during heating. The concentration of copper acetylacetonate in the present compositions is preferably from 0.01 to 0.07 percent, based on the total weight of the curable composition. A concentration of at least 0.03 percent is particularly preferred. The copper acetylacetonate is preferably used in combination with a non-reactive liquid triorganosiloxy-terminated poly(methyl-3,3,3-trifluoropropyl)siloxane to increase the adhesion, also referred to as "tack" of the cured gel. The presence of from 10 to 50 weight percent, based on the weight of diorganovinylsiloxy-terminated polydiorganosiloxane, of this non-reactive liquid fluorosilicone compound also increases the extent to which the cured gel softens, as evidenced by an increased penetration value, during long term exposure to temperatures above 100°C. The present inventors discovered that this softening can be reduced by excluding from the curable composition the acetylenically unsaturated alcohols typically used as catalyst inhibitors in this type of organosiloxane composition. The vinyl-terminated polydiorganosiloxane, referred to hereinafter as Ingredient A, can be either a homopolymer or a copolymer and is conveniently represented by the general formula where R represents a monovalent hydrocarbon radical, R' represents the radical RfCH₂CH₂- where Rf represents a monovalent perfluorinated hydrocarbon radical containing from 1 to 5 carbon atoms, Vi represents a vinyl radical, the sum of n and p typically represents a molecular weight equivalent to a viscosity of from 0.2 x 10⁻³ to 0.01 m²/s. at 25°C. and the value of n can be from 0 to 3 p . The radical represented by R can contain from 1 to 20 carbon atoms. A range of from 1 to 10 carbon atoms is preferred based on the availability of the corresponding cyclic diorganosiloxanes used to prepare ingredient A. Preferably, R is methyl, phenyl or a combination of methyl and phenyl and R' is 3,3,3-trifluoropropyl. Most preferably, all of the radicals represented by R are methyl and the value of n is 0. Ingredient A is represented in the foregoing formula as a linear molecule. It will be understood, however, that in reality some of the molecules may contain branched units resulting from small amounts of trifunctional reactants present as impurities in the cyclic diorganosiloxanes used to prepare ingredient A. Ingredient A can be a single polydiorganosiloxane species or a mixture containing two or more polydiorganosiloxanes of different molecular weights, so long as the viscosity of the mixture is within the aforementioned limits. To reduce heat induced discoloration and hardening exhibited by prior art fluorosilicone gels cured by a hydrosilation reaction ingredient A is prepared using ammonia as the polymerization catalyst. The reason for this reduction in discoloration and hardening is not fully understood. Methods for polymerising cyclic diorganosiloxanes to liquid silanol terminated polydiorganosiloxanes using a variety of catalysts are sufficiently described in the literature and a detailed description in this specification is not necessary. When ammonia is used as the catalyst, the polymerization reaction of the cyclic diorganosiloxane is typically conducted at temperatures of from 25 to 100°C. until the desired molecular weight is achieved. Ingredient A is cured by a hydrosilation reaction between the vinyl radicals of this curing agent ingredient and the silicon-bonded hydrogen atoms of the organohydrogensiloxane, referred to hereinafter as ingredient B. Ingredient B contains an average of more than two silicon bonded hydrogen atoms per molecule. It can contain an average of from 3 up to 20 or more silicon atoms per molecule and exhibits a viscosity of up to 10 Pa·s or higher at 25°C. Ingredient B contains repeating units of the formulae HSiO 1.5 , R''HSiO and/or R''₂HSiO 0.5 . The molecules of ingredient B can also include one or more monoorganosiloxane, diorganosiloxane, triorganosiloxy and SiO₂ units that do not contain silicon bonded hydrogen atoms. In these formulae, R'' is a monovalent hydrocarbon radical containing from 1 to 20 carbon atoms or a fluorinated hydrocarbon radical selected from the same group as the R' radical of ingredient A. Alternatively, ingredient B can be a cyclic compound containing diorganosiloxane and organohydrogensiloxane units or a compound of the formula Si(OSiR''₂H)₄. To ensure compatibility between ingredients A and B, at least a portion of the radicals represented by R'' should be identical to the majority of the hydrocarbon radicals present in ingredient A. When ingredient A is the preferred polydiorganosiloxane containing methyl-3,3,3-trifluoropropylsiloxane units, at least a portion of the R'' radicals in ingredient B should represent 3,3,3-trifluoropropyl radicals. Most preferably, ingredient B is a linear dimethylhydrogensiloxy terminated polyorganosiloxane containing from one to three repeating units per molecule, all of which correspond to the general formula where R'' represents 3,3,3-trifluoropropyl and Me represents methyl. The molecular weights of ingredients A and B together with the number and distribution of the silicon-bonded hydrogen atoms in ingredient B will determine the location of crosslinks in the cured gel. The concentration of crosslinks per unit area is often referred to as the "crosslink density" and determines certain physical properties of the cured gel, particularly hardness and resiliency. The particular types and amounts of ingredients A and B yielding a desired combination of physical properties can readily be determined by routine experimentation with a knowledge of this invention. The molar ratio of silicon bonded hydrogen atoms to vinyl or other ethylenically unsaturated hydrocarbon radicals present in the curable compositions of this invention is a major factor in determining the properties of the cured gel. As disclosed hereinbefore, a preferred type of organosiloxane gels is prepared from curable compositions containing a stoichiometric excess of vinyl radicals relative to silicon bonded hydrogen atoms. In the present compositions, this ratio is typically from 0.3 to 1 silicon bonded hydrogen atom per vinyl radical, preferably from 0.5 to 0.9 silicon bonded hydrogen atom per vinyl radical. The preferred ratio for a given composition will be determined at least in part by the average molecular weight of ingredient A and the type of organohydrogensiloxane curing agent. Hydrosilation reactions are typically conducted in the presence of a catalyst that is a metal from the platinum group of the Periodic Table or a compound of such a metal. Platinum compounds such as hexachloroplatinic acid and particularly complexes of these compounds with relatively low molecular weight, liquid vinyl-containing organosiloxane compounds, are preferred catalysts because of their high activity and compatibility with the organosiloxane reactants. These complexes are described in US-A 3,419,593. Complexes wherein the silicon bonded hydrocarbon radicals are vinyl and either methyl or 3,3,3-trifluoropropyl are particularly preferred because of their ability to catalyse a rapid curing of the elastomer at temperatures of at least 70°C. A particularly preferred catalyst of this type is the complex formed by reacting hexachloroplatinic acid with a liquid dimethylvinylsiloxy terminated poly(methyl-3,3,3-trifluoropropyl)siloxane. The platinum containing catalyst can be present in an amount equivalent to as little as one part by weight of platinum per one million parts of curable composition. Catalyst concentrations equivalent to from 3 to 10 parts of platinum per million of curable composition are preferred to achieve a practical curing rate. Higher concentrations of platinum provide only marginal improvements in curing rate and are, therefore, economically unattractive, particularly when the preferred catalysts are used. Mixtures containing all of the foregoing reactants may begin to cure at room temperatures of about 25°C. To obtain a longer working time or "pot life" once all of the ingredients have been blended, the activity of the catalyst at room temperature can be retarded or suppressed by the optional addition of one of the known platinum catalyst inhibitors. The acetylenic compounds disclosed in US-A 3,445,420, should not be used in preferred embodiments of the present compositions because cured gels containing these inhibitors soften considerably in the presence of the non-reactive liquid triorganosiloxy-terminated poly(methyl-3,3,3-trifluoropropyl)siloxanes used to increase the adhesion of the cured gels. An increase in the pot life of a curable composition can be achieved using an olefinically substituted siloxane of the type described in US-A 3,989,667. Cyclic methylvinylsiloxanes are preferred. The curable compositions of this invention can be prepared by blending together all of the ingredients at room temperature. Compositions containing one of the aforementioned platinum catalyst inhibitors can be stored at conventional room temperatures of about 25°C. for a few hours, or in some cases days, without curing to any substantial extent. When it is desired to store the curable composition for longer periods under ambient conditions, it should be either refrigerated or packaged in at least two parts. The organohydrogensiloxane (ingredient B) and the platinum-containing catalyst are located in separate parts of the composition. Part I of a two part composition will typically include a portion of the total amount of ingredient A, the liquid diorganosiloxy terminated polydiorganosiloxane, the platinum-containing catalyst and, optionally, a catalyst inhibitor that is other than an acetylenic alcohol when the optional non-reactive triorganosiloxy terminated polydiorganosiloxane is present to increase the adhesion of the cured gel. Part II contains the remaining portion of ingredient A and the organohydrogensiloxane (ingredient B). The copper acetylacetonate is preferably present in this part of the composition. Parts I and II each typically exhibit viscosities of from 0.5x10⁻³ to 1.5x10⁻³ m²/sec. at 25°C. In accordance with a preferred method for preparing the present curable compositions substantially equal volumes of parts I and II are blended together. Irrespective of the method by which the present compositions are prepared, they are cured by allowing all of the ingredients to react at ambient or elevated temperature. Compositions containing a catalyst inhibitor typically require heating to temperatures of 70°C. or above to achieve a fully cured state within a reasonable time interval, usually from several minutes to several hours. The fluorine containing polysiloxane gels prepared by curing the present compositions are typically relatively soft, clear and colorless and remain so following prolonged exposure to temperatures from 100 to about 150°C. The hardness of the gels can be expressed as the distance to which a probe of specified dimensions will penetrate the surface of the gel under a specified loading. The device typically used to measure this value is referred to as a penetrometer. A preferred class of cured gels prepared using the present compositions exhibit penetration values of from 2 to about 8 mm. As discussed hereinabove the present gels are particularly suitable for coating or encapsulating solid state electronic devices and substrates on which these devices are mounted. Because the gels will not harden or discolor at temperatures of 100°C. and above and are not swollen and/or degraded by hydrocarbon fuels, devices coated with the present gels can be installed in the engine compartments of automobiles and other vehicles. The following examples describe preferred curable compositions of this invention and demonstrate the resistance to hardening and surface deformation exhibited by cured gels prepared using the compositions of this invention containing copper acetylacetonate relative to fluorosilicone gels prepared using the same reactants in the absence of this additive. are by weight unless otherwise indicated and viscosities were measured at 25°C. Example 1 A curable composition was prepared by blending the following ingredients to homogeneity. As ingredient A, 193 parts of a dimethylvinylsiloxy terminated poly(methyl-3,3,3-trifluoropropyl)siloxane containing 1.3% of vinyl radicals and exhibiting a viscosity of 1.4 x 10⁻³ m²/sec. Ingredient A was prepared by the ammonia catalyzed polymerization of 2,4,6-trimethyl-2,4,6-tris(3,3,3-trifluoropropyl)cyclotrisiloxane followed by reaction of the resultant silanol terminated polydiorganosiloxane with sym-tetramethyldivinyldisilazane; as ingredient B, 6.5 parts of an organohydrogensiloxane curing agent represented by the average formula where Me represents a methyl radical, Prf represents a 3,3,3-trifluoropropyl radical, the average value of x is between 1 and 3 inclusive, and the silicon bonded hydrogen content is 0.6%; and as ingredient C, 0.15 part of a complex prepared by the reaction of hexachloroplatinic acid with a liquid dimethylvinylsiloxy terminated poly(methyl-3,3,3-trifluoropropyl)siloxane. Ingredient C contained 0.67% platinum; and 0.1 %, based on the weight of the composition, of copper acetylacetonate. For comparative purposes a curable composition containing the same types and amounts of ingredients but omitting the copper acetylacetonate was prepared. The molar ratio of silicon bonded hydrogen atoms to vinyl radicals in both compositions was 0.5. A 100 gram portion of the two resultant mixtures were deaired under reduced pressure, poured into a wide mouth container and cured by heating it for one hour at 150°C. The cured gel layer was 2 cm. thick. The penetration value of the gels was measured using a penetrometer manufactured by the Precision Scientific Company, Catalog No. 73,510. The standard cones supplied with the instrument were replaced with a brass head measuring 6.350 mm. in diameter, 4.762 mm. in height and having a flat bottom and rounded edges. The total weight of the shaft and head was 19.5 grams. The penetration value of the cured gel containing the copper acetylacetonate was 2.7 mm. and the penetration value for the control sample was 3.6 mm. The gel samples were then heated for 28 days in an oven maintained at a temperature of 175°C. At the end of this period the penetration value of the gel containing the copper acetyl-acetonate was 2.3 mm., a decrease of 15%. The control sample had hardened to the extent that the weighted probe of the penetrometer was not able to depress the surface of the cured gel, resulting in a reading of 0 mm. A visual examination of the surface of the two gels revealed a smooth surface on the gel containing copper acetylacetonate and a wrinkled surface on the control gel. Example 2 This example demonstrates the increase in adhesion or "tack" of gels prepared from compositions containing a non-reactive liquid trimethylsiloxy-terminated poly(methyl-3,3,3-trifluoropropyl)siloxane in addition to copper acetylacetonate and the need to exclude acetylenic alcohol type platinum catalyst inhibitors from this type of composition. The penetration values for the cured gels were determined as described herein in the preceding Example 1. Ingredients A, B and C are described in Example 1 and ingredient C was present at a level of 0.02 parts in all samples. The liquid trimethylsiloxy-terminated dimethyl-siloxane/3,3,3-trifluoropropylsiloxane copolymer (ingredient E) exhibited a viscosity of 0.3 Pa·s and was present at a concentration of 90 parts in all samples. All of the gels were "tacky" to the touch after being stored for 1 month at a temperature of 175°C. The amounts of ingredients A, B and methylbutynol (F), a known platinum catalyst inhibitor, are listed in Table 1. The penetration values of the initial cured gel sample and the values obtained following heating at a temperature of 125°C. for 1 and 42 days are recorded in Table 2. Sample No. Ingredient (parts) A B F 1 (Comparison) 93.3 6.3 0.036 2 93.7 6.3 0 3 (Comparison) 93.4 6.6 0.036 4 93.4 6.6 0 Sample No. Penetration Values(@ 125°C.) Initial 1 day 42 days 1 (comparison) 119 Bottom* Bottom* 2 70 70 74 3 (comparison) 66 ND 81 4 42 ND 46 Bottom = The penetrometer probe penetrated the gel layer and contacted the bottom of the container. ND = Penetration value was not determined

Claims: An organosiloxane composition curable to a clear, thermally stable fluorosilicone gel, said composition being composed of A. a liquid diorganovinylsiloxy terminated polydiorganosiloxane wherein at least 25 percent of the silicon atoms are bonded to a fluorine containing hydrocarbon radical; B. a compatible liquid fluorine-containing organohydrogensiloxane having an average of more than two silicon bonded hydrogen atoms per molecule in an amount sufficient to provide a molar ratio of silicon bonded hydrogen atoms to vinyl radicals in said composition of from 0.3 to 1.0; C. an amount sufficient to promote curing of said composition of a platinum-containing catalyst; and optionally D. platinum catalyst inhibitors excluding acetylenic compounds and/or E. non-reactive liquid triorganosiloxy-terminated poly(methyl-3,3,3-trifluoropropyl)siloxanes, wherein said composition contains from 0.01 to 0.1 percent, based on the weight of said composition, of copper acetylacetonate to minimize hardening and surface deformation of the cured gel at temperatures above 100°C. A composition according to claim 1 where said polydiorganosiloxane A. exhibits the general formula where R represents a monovalent hydrocarbon radical, R' represents a monovalent radical of the formula RfCH2CH2-where Rf represents a perfluorinated hydrocarbon radical containing from 1 to 5 carbon atoms, Vi represents a vinyl radical, the sum of n and p is equivalent to a molecular weight that is equivalent to a viscosity of from 0.2 x 10⁻³ to 0.01 m.²/sec. at 25°C. and the value of n is from 0 to 3p, the repeating units of said organohydrogensiloxane B. are of the formula where R'' represents 3,3,3-trifluoropropyl, Me represents methyl and the molar ratio of silicon bonded hydrogen atoms to vinyl radicals present in said composition is from 0.5 to 0.9. A composition according to claim 2 where said composition is packaged in two parts, with said platinum-containing catalyst and said organohydrogensiloxane being located in different parts of said composition. A composition according to claim 3 where each of said parts contains a portion of said polydiorganosiloxane and the copper acetylacetonate is packaged together with said organohydrogensiloxane.

Inventors: FIEDLER LAWRENCE DALE, RASCH DIANE MARIE, FIEDLER, LAWRENCE DALE, RASCH, DIANE MARIE
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1990
Main classification: C08L83
IPC codes: C08K5, C08L83