Curable fluorinated organosiloxane compositions exhibiting improved adhesion

Abstract: The adhesion developed during curing of organo- siloxane compositions comprising a polyorganosiloxane containing silicon-bonded alkenyl and fluorinated alkyl radicals, at least one organohydrogensiloxane curing agent, a platinum-containing hydrosilation catalyst and a conventional organosiloxane adhesion promoter is substantially improved when the alkenyl-substituted siloxane units of the polyorganosiloxane are separated from the closest fluorine-containing siloxane units by at least two dimethylsiloxane units.

Description: This invention relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction. One of the ingredients of the claimed compositions is a polyorganosiloxane containing silicon-bonded fluorinated hydrocarbon radicals and at least two silicon-bonded alkenyl radicals per molecule. The curable organosiloxane compositions of this invention provide excellent adhesion to a variety of substrates that are in contact with the compositions during curing. A shortcoming of organosiloxane compositions in the art is their relatively poor adhesion to metals and synthetic organic polymers. One method for enhancing adhesion of these compositions is to apply a primer to the substrate surface before coating with the composition. Alkoxysilanes containing an organofunctional group such as alkenyl, epoxy, amino, acryloxy or methacryloxy which are bonded to silicon by a carbon are frequently used as the active ingredient of these primer compositions. The type of organofunctional group present on the silane is determined at least in part by the type of substrate. As an alternate approach, the additional steps of preparing the primer composition and applying it to the substrate can be eliminated by simply the organofunctional silane ingredient directly to the curable organosiloxane composition as an adhesion promoter. A preferred class of adhesion promoters are silanes or organosiloxanes containing one or more silicon-bonded alkoxy groups per molecule and at least one functional group that is reactive with the substrate to which bonding is desired. These functional groups typically include alkenyl radicals, ethylenically unsaturated groups such as 3-methacryoxypropyl and epoxy groups such as glycidoxypropyl. Typical classes of adhesion promoters for use with organosiloxane compositions that cure by a platinum catalyzed hydrosilation reaction include those described in U.S. Patent Nos. 4,087,585; 4,082,726; 4,311,739; 4,677,161; 4,701,503; 4,732,932 and 4,786,701. The curable compositions in these patents contain a polyorganosiloxane with at least two alkenyl radicals per molecule, an organohydrogensiloxane and a hydrosilation catalyst that is typically a metal from the platinum group of the periodic table or a compound of such a metal. The hydrocarbon radicals that are bonded to the silicon atoms of the alkenyl-substituted polyorganosiloxanes and organohydrogensiloxanes described in these patents for adhesion promoters contain from 1 to 10 carbon atoms and can be substituted with halogen atoms, yet their examples are typically limited to polyorganosiloxanes and organohydrogensiloxanes containing methyl radicals as the silicon-bonded hydrocarbon radicals. We have unexpectedly found that little, if any, additional adhesion is obtained using some of the known adhesion promoters when the silicon atoms adjacent to silicon atoms containing a vinyl or other alkenyl radical are bonded to a fluorinated hydrocarbon radical such as 3,3,3-trifluoropropyl. Such curable organosiloxane compositions are described in U.S. Patent No. 4,988,758. The characterizing feature of these compositions is the presence of an average of at least two dimethylsiloxane units between the vinyl-substituted silicon atom and the nearest silicon atom containing a fluorinated hydrocarbon radical. These compositions are converted to cured materials by reaction of the polyorganosiloxane with an organic peroxide or with an organohydrogensiloxane in the presence of a platinum-containing hydrosilation catalyst. The advantage alleged in U. S. Patent No. 4,988,758 involves a faster cure rate relative to compositions containing polyorganosiloxanes wherein the silicon atoms containing alkenyl radicals are adjacent to silicon atoms containing fluorinated hydrocarbon radicals. That patent does not discuss the subject of adhesion or recommend the use of any ingredients to improve the adhesion to substrates that are in contact with the organosiloxane composition during curing. An objective of this invention is to improve the adhesion to organic and inorganic substrates that is developed during curing by a platinum-catalyzed hydrosilation reaction of organosiloxane compositions wherein at least 20 mole percent of the silicon atoms of the alkenyl-functional polyorganosiloxanes are bonded to a fluorinated hydrocarbon radical. It has now been found that this objective can be achieved by using conventional adhesion promoters for organosiloxane compositions in combination with a curable composition containing a fluorinated polyorganosiloxane wherein the alkenyl-substituted siloxane units are separated by an average of at least two dimethylsiloxane units from the closest siloxane unit containing a fluorinated hydrocarbon radical. This invention provides a curable organosiloxane composition comprising A. a polyorganosiloxane containing at least two silicon-bonded alkenyl radicals per molecule where at least 20 mole percent of the siloxane units contain a silicon-bonded perfluoroalkylethyl radical and the silicon atoms containing said alkenyl radicals are separated from the closest silicon atom containing said perfluoroalkylethyl group by at least two dimethylsiloxane units, B. an organohydrogensiloxane containing an average of at least two silicon-bonded hydrogen atoms per molecule, C. an amount of a hydrosilation catalyst sufficient to promote curing of said composition, where said catalyst is selected from metals of the platinum group of the periodic table and compounds of these metals, and D. (1) an alkoxy-substituted organosilicon compound containing at least one organofunctional substituent bonded to silicon through carbon, where said substituent is selected from epoxy, amino and alkenyl, or (2) copolymers derived from said alkoxy-substituted organosilicon compound and alkyl esters of ethylenically unsaturated carboxylic acids and where the concentration of said organosilicon compound (D) is sufficient to achieve adhesion between said composition and a substrate in contact with said composition during curing. The characterizing feature of the present organosiloxane compositions is the substantial improvement in adhesion to a variety of substrates that is developed during curing as compared to compositions containing the same adhesion promoter and a polyorganosiloxane wherein an organosiloxane unit containing a perfluoroalkylethyl radical is adjacent to a silicon atom bonded to an alkenyl radical. The adhesion promoters of the present invention are organosilicon compounds containing at least one silicon-bonded alkoxy group and at least one adhesion-promoting organofunctional substituent that is bonded to silicon through carbon and contains an epoxy, amino or an ethylenically unsaturated group such as alkenyl, acryloxy or methacryloxy. Ingredient A is the feature considered responsible for improved adhesion developed by the present compositions. More specifically, the presence in the polyorganosiloxane containing alkenyl and fluorinated hydrocarbon radicals of an average of at least two dimethylsiloxane units, -(CH 3 ) 2 SiO-, between the silicon atoms containing the alkenyl radical, which can be represented by the formula R 1 R 2 SiO or R 1 (R 2 ) 2 SiO 1/2 and the closest silicon atom containing a perfluoroalkylethyl radical is the essence of the invention. In these formulae R 1 represents an alkenyl radical containing from 2 to 8 carbon atoms and R 2 represents an alkyl radical containing 1 or 2 carbon atoms. Preferably, R 1 is vinyl or 5-hexenyl and R 2 is methyl. A preferred embodiment of ingredient A is a polydiorganosiloxane wherein from 20 to 95 mole percent of the diorganosiloxane units are methyl-3,3,3-trifluoropropylsiloxane, the remaining diorganosiloxane units are dimethylsiloxane, the terminal units are dimethylvinylsiloxy and the terminal units are separated from the closest fluorine-containing siloxane unit by at least two dimethylsiloxane units. Preferably, at least 40 mole percent of the units are methyl-3,3,3-trifluoropropylsiloxane. Ingredient A can be prepared by polymerizing the corresponding cyclic oligomeric disiloxanes in the presence of an alkali metal compound and a chain terminating agent that is a dimethylvinylsiloxy-terminated polydimethylsiloxane containing an average of from 6 to 20 dimethylsiloxane units per molecule. More specific methods are described in U.S. Patent No. 4,988,758. The viscosity of ingredient A can range from a free flowing liquid to a semi-solid gum. A viscosity within the range of 0.1 to 100,000 Pa·s is preferred to obtain cured organosiloxane materials exhibiting useful physical properties by conventional processing equipment. The organosiloxane compositions of this invention are cured by a hydrosilation reaction catalyzed by a metal from the platinum group of the periodic table or a compound of such a metal. Ingredient B is an organohydrogensiloxane containing an average of more than two silicon bonded hydrogen atoms per molecule. The organohydrogensiloxane contains as few as four silicon atoms per molecule up to an average of 20 and can have a viscosity of up to 10 Pa·s or higher at 25°C. The repeating units of this ingredient can include HSiO 1.5 , R 3 HSiO and/or (R 3 ) 2 HSiO 0.5 in addition to one or more monoorganosiloxy, diorganosiloxane, triorganosiloxy and SiO 4/2 units. In these formulae each R 3 represents an unsubstituted monovalent hydrocarbon radical or a perfluoroalkylethyl radical. A preferred class of organohydrogensiloxanes are copolymers having the repeating units R 3 HSiO and (R 3 ) 2 SiO units, where the R 3 radicals are free of ethylenic unsaturation and are individually selected from monovalent hydrocarbon and perfluoroalkylethyl radicals and the (R 3 ) 2 SiO units constitute from 0.5 to 50 mole percent of the copolymer. Copolymers of this type can be prepared by a controlled hydrolysis of a mixture comprising the corresponding organosilicon halides, such as the chlorides or the corresponding alkoxides or a mixture of the corresponding cyclic organosiloxanes and a suitable chain terminating agent. These and other methods for preparing the preferred organohydrogensiloxanes of this invention are sufficiently well known that a detailed description is not required in this specification. The non-fluorinated hydrocarbon radical is preferably methyl and the fluorinated hydrocarbon radical is 3,3,3-trifluoropropyl. Proper curing of the present compositions also requires that ingredients A and B be miscible with one another. The molar ratio of silicon bonded hydrogen atoms present in the organohydrogensiloxane to vinyl or other alkenyl radicals, referred to as the SiH/alkenyl ratio, is determined by a number of considerations, including the physical properties of the cured material prepared from the present compositions. The SiH/alkenyl ratio is from 0.4 to 3.5. The optimum ratio for particular compositions will be determined at least in part by the molecular weights of ingredients A and B. The optimum range of this ratio can readily be determined by those skilled in the art. In addition to the organohydrogensiloxane containing an average of more than two silicon-bonded hydrogen atoms per molecule, the present compositions may also contain a polyorganosiloxane, preferably a polydiorganosiloxane, having silicon-bonded hydrogen atoms only at the terminal positions. This type of organohydrogensiloxane is referred to as a chain extender and does not provide crosslinking between adjacent molecules of ingredient A. The chain extender is typically present when it is desired to decrease the hardness and/or increase the elongation of the cured material. 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, rhodium and compounds of these metals have been shown to catalyze hydrosilation reactions. Platinum compounds such as hexachloroplatinic acid and particularly complexes of these compounds with relatively low molecular weight vinyl-containing organosiloxane compounds are preferred catalysts because of their high activity and compatibility with the organosiloxane reactants. These complexes are described in U.S. Patent No. 3,419,593. Complexes with low molecular weight organosiloxanes wherein the silicon bonded hydrocarbon radicals are vinyl and either methyl or 3,3,3-trifluoropropyl are particularly preferred because of their ability to catalyze a rapid curing of the elastomer at temperatures of at least 70°C. The catalyst can be present in an amount equivalent to as little as one part by weight and as much as 50 parts by weight of platinum group metal per one million parts of curable composition. Catalyst concentrations equivalent to from 5 to 50 parts of platinum group metal per million of curable composition are preferred to achieve a practical curing rate. Higher concentrations provide only marginal improvements in curing rate and are therefore economically unattractive, particularly when the preferred catalysts are used. Mixtures of the aforementioned vinyl-containing reactants, curing agents and hydrosilation catalysts may begin to cure at ambient temperature. To obtain a longer working time or "pot life", the activity of the catalyst under ambient conditions can be retarded or suppressed by addition of a suitable inhibitor. Known catalyst inhibitors include the acetylenic compounds disclosed in U.S. Patent No. 3,445,420. Acetylenic alcohols such as 2-methyl-3-butyn-2-ol constitute a preferred class of inhibitors that will suppress the activity of a platinum group metal catalyst at 25°C. Compositions containing these catalysts typically require heating at temperatures of 70°C. or above to cure at a practical rate. If it desired to increase the working time or pot life of a curable composition under ambient conditions, this can be accomplished by the addition of one or more olefinically substituted siloxanes of the type described in U.S. Patent No. 3,989,667. Cyclic methylvinylsiloxanes are preferred. Inhibitor concentrations as low as one mole of inhibitor per mole of platinum group metal will in some instances impart satisfactory storage stability and cure rate. In other instances inhibitor concentrations of up to 500 moles of inhibitor per mole of platinum group metal are required. The optimum concentration for a given inhibitor in a given composition can readily be determined by routine experimentation and does not constitute part of this invention. When inhibitors will not provide the desired long term storage stability, it may be necessary to separate the organohydrogensiloxane (ingredient B) from the hydrosilation catalyst (ingredient C). This can be achieved by packaging the curable composition in two or more containers with ingredients B and C in separate containers. Alternatively, the catalyst can be microencapsulated in a layer of a thermoplastic or thermosetting organic polymer. Curable compositions containing a microencapsulated platinum-containing hydrosilation catalyst are described in U.S. Patent No. 4,766,176. Ingredient D contributes to the adhesion developed during curing of the present compositions and is an organo-silicon compound containing at least one alkoxy group and at least one organofunctional group selected from epoxy, amino, alkenyl, alkyl esters of ethylenically unsaturated carboxylic acids and polymers derived from homo- or co-polymerization of ethylenically unsaturated carboxylic acids. All of these organofunctional groups are bonded to silicon through carbon. Suitable organosilicon compounds include silanes, hydrolysis products of these silanes and polyorganosiloxanes containing at least one of the organofunctional groups and at least two alkoxy groups per molecule. Preferred organo-silicon compounds are silanes and hydrolysis-condensation products or polymers derived from these silanes. The silanes can be represented by the general formula (R 4 ) a XSi(OR 5 ) 3-a where R 4 and R 5 represent alkyl radicals containing from 1 to 4 carbon atoms and X is selected from epoxy and ethylenically unsaturated groups. The X group is bonded to silicon through a carbon atom. The organofunctional organosilicon compound can be used alone or in combination with polyorganosiloxanes containing the same types of organofunctional groups that can be present on the adhesion promoter. A particularly preferred combination is an epoxy-substituted silane such as 3-glycidoxypropyl-trimethoxysilane and a polyorganosiloxane containing at least one silicon-bonded vinyl radical and at least one silanol group per molecule. Preferred compounds of this type are liquid hydroxyl-terminated polymethylvinylsiloxanes. Adhesion promoters of this type are described in U.S. Patent No. 4,087,585. A second class of adhesion promoters that can be present in the compositions of this invention are copolymers derived at least in part from a methacryloxy functional alkoxysilane such as methacryloxypropyltrimethoxysilane and an ester of an ethylenically unsaturated acid such as methyl methacrylate. These copolymers can be prepared using conventional methods for free radical polymerization of ethylenically unsaturated compounds. Ingredient D can be present as one of the ingredients of the present compositions, in which instance it would constitute from 0.1 to 10 weight percent of the composition. Alternatively, this ingredient can be incorporated into a primer composition that is coated on a substrate prior to application of a curable composition of this invention containing ingredients A, B, C and any additional optional ingredients. Primer compositions typically contain the organofunctional silane, a hydrolysis-condensation catalyst for the silane and a suitable volatile solvent. The present compositions may contain a variety of additional ingredients to modify the properties of the composition or the material obtained by curing the composition. To achieve high levels of tensile strengths and other physical properties desirable in elastomers, the curable composition may contain a reinforcing silica filler. This type of filler is typically treated with one or more of the known silica treating agents to prevent a phenomenon referred to as "creping" or "crepe hardening" during processing of the curable composition. Any finely divided form of silica can be used as the reinforcing filler. Colloidal silicas are preferred because of their relatively high surface area, which is typically at least 50 m 2 /g. Fillers having surface areas of at least 200 m 2 /g are preferred for use in the present method. Colloidal silicas can be prepared by precipitation or a fume process. Both of these preferred types of silica are commercially available. The amount of finely divided silica used in the present compositions is at least in part determined by the physical properties desired in the cured elastomer. Liquid or pumpable polyorganosiloxane compositions typically contain from 5 to 60 percent by weight of silica, based on the weight of polydiorganosiloxane. This value is preferably from 20 to 40 percent. Silica treating agents are typically low molecular weight organosilicon compounds containing silicon-bonded hydroxyl groups or groups that can be hydrolyzed to hydroxyl groups in the presence of water. Typical hydrolyzable groups include halogen atoms such as chlorine amino and other groups containing a silicon-bonded nitrogen atom. The present compositions may also contain a resinous organosiloxane copolymer consisting essentially of R 3 SiO 1/2 , R'R 2 SiO 1/2 and SiO 4/2 units, where R represents a lower alkyl radical and R' represents an alkenyl radical. These copolymers are well known in the art and are described in U.S. Patent No. 2,676,182. Additional optional ingredients that can be present include extending fillers such as quartz, calcium carbonate and diatomaceous earth; pigments such as carbon black, titanium oxide and zinc oxide; ultraviolet light absorbing agents; and flame retarding materials such as hydrated aluminum oxide. The following examples describe preferred embodiments of the present curable organosiloxane compositions. Unless otherwise specified all parts and percentages in the examples are by weight and viscosities were measured at 25°C. Example 1 A curable composition of this invention was prepared using the following ingredients: As ingredient A1, 63.6 parts of a dimethylsiloxane/ methyl-3,3,3-trifluoropropylsiloxane containing 58 mole percent of methyl-3,3,3-trifluoropropylsiloxane units, 41 mole percent of dimethylsiloxane units, 1 mole percent of vinyl radicals and exhibiting a viscosity of 12.1 Pa·s. The terminal groups of this polymer exhibit the formula ViMe 2 SiO(Me 2 SiO) n where Vi represents a vinyl radical, Me is methyl radical and the value of n is from 2 to 4. Ingredient Al was prepared by the base catalyzed polymerization of the corresponding cyclic siloxanes in the presence of a dimethylvinylsiloxy-terminated polydimethylsiloxane containing an average of 8.8 dimethylsiloxane units per molecule as the chain terminator. As ingredient A2, 7.0 parts of a dimethylvinylsiloxy-terminated dimethylsiloxane/methyl-3,3,3-trifluoropropylsiloxane copolymer containing the same molar ratio of these siloxane units as ingredient A1 and exhibiting a viscosity of 0.55 Pa·s. Ingredients A1 and A2 were prepared using the same reactants. As ingredient B, 1.4 parts of an trimethylsiloxy-terminated dimethylsiloxane/methylhydrogensiloxane copolymer containing an average of five methylhydrogensiloxane units and three dimethylsiloxane units per molecule with a silicon-bonded hydrogen atom content in the range of 0.7 to 0.8 weight percent. As the platinum-containing hydrosilation catalyst (ingredient D), 0.2 part of a reaction product of hexachloroplatinic acid and sym-tetramethyldivinyldisiloxane that had been diluted with a liquid dimethylvinylsiloxy terminated polydimethylsiloxane in an amount sufficient to achieve a platinum content of 0.62 weight percent. As the reinforcing filler, 22.2 parts of a fume silica treated to contain trimethylsiloxy and dimethylvinylsiloxy groups. As pigments, 0.74 part of zinc oxide, 1.83 parts of titanium oxide and 0.74 part of carbon black. As platinum catalyst inhibitors, 0.24 part of cyclic dimethylsiloxane tetramer and 0.06 part of 1-ethynylcyclohexanol. As the first adhesion promoter, 2 parts of 3-glycidoxypropyltrimethoxysilane. The composition containing this adhesion promoter will be referred to hereinafter as composition A. As the second adhesion promoter, a 15 weight percent solution in ethyl acetate of a copolymer containing an average of 95 methyl methacrylate units, five 3-trimethoxysilylpropylmethacrylate units and one 3-methoxysilylmercapto terminal unit per molecule. The copolymer was present at a concentration of 0.3 weight percent. The composition containing this adhesion promoter will be referred to as composition B. Curable composition A of the present invention was coated as a 0.06 inch (0.15 cm) thick layers on an aluminum panel and composition B was coated in the same manner on a glass panel. The layer of curable composition was covered with a 1/2 inch (1.3 cm) wide strip of 120 mesh (0.125 mm) stainless steel screening that was longer than the panel. The screening was coated with a 0.06 inch (0.15 cm) thick layer of the curable composition. The resultant composite was heated for 16 hours at a temperature of 100°C. to cure the organosiloxane composition and then remained under ambient conditions for at least 24 hours. The peel strength of the cured composition was determined by bending the exposed, uncoated section of the stainless steel screen through an angle of 180 degrees, securing the uncoated end of the screen and the edge of the aluminum or glass panel adjacent to the uncoated portion of the screen in the jaws of a tensile tester and applying a force sufficient to peel the screen from the panel. The force required to peel the screen from the panel and the type of failure (adhesive or cohesive) are recorded in Table 1. For comparative purposes a curable organosiloxane composition, referred to hereinafter as composition C, containing the types and amounts of ingredients as compositions A and B, with the exception of omitting the adhesion promoter (ingredient D) was prepared, coated on an aluminum panel, cured and tested for peel strength as described in the preceding section of this example. The results of the adhesion test are also recorded in Table 1. Composition Peel Strength (ppi//kN/m) Type of Failure A (Aluminum) 21.4 // 3.7 Cohesive B (Glass) 19.4 // 3.4 Cohesive C (Aluminum) 6.1 // 1.1 Adhesive Example 2 This example demonstrates the superior adhesion exhibited by cured compositions of the present invention relative to a composition wherein the vinyl-containing polydiorganosiloxane does not contain at least two dimethylsiloxane unit between the terminal unit and the first fluorine-containing organosiloxane unit. A curable composition of this invention (composition D) was prepared by blending to homogeneity the ingredients of composition A described in the preceding example 1 with the exception that the adhesion promoter consisted essentially of 1 part of glycidoxypropyltrimethoxysilane and 1 part of a hydroxyl terminated polymethylvinylsiloxane containing 4 weight percent of silicon-bonded hydroxyl groups. A curable organosiloxane composition used for comparative purposes (composition E) was prepared by blending the following ingredients to homogeneity: 72.11 parts of a dimethylvinylsiloxy-terminated polymethyl-3,3,3-trifluoropropylsiloxane exhibiting a viscosity of about 67,800 cps ( 67.8 Pa·s); 5.86 parts of a dimethylvinylsiloxy-terminated polymethyl-3,3,3-trifluoropropylsiloxane exhibiting a viscosity of about 1145 cps (1.145 Pa·s); As the hydrosilation catalyst, 0.12 part of a complex containing 0.67% platinum and prepared by the reaction of hexachloroplatinic acid with a liquid dimethylvinylsiloxy terminated poly(methyl-3,3,3-trifluoropropyl)-siloxane; 0.62 part of zinc oxide; 1.95 parts of titanium dioxide; 14.84 parts of a fumed silica containing dimethyl-3,3,3-trifluoropropylsiloxy and dimethylvinylsiloxy groups; 1.86 parts of an organohydrogensiloxane crosslinking reactant represented by the average formula As a platinum catalyst inhibitor, 0.02 part of methylbutynol; 0.63 part of carbon black; and as the adhesion promoter, 1 part of glycidoxypropyl-trimethoxysilane and 1 part of a hydroxyl-terminated polymethylvinylsiloxane containing 4 weight percent of silicon-bonded hydroxyl groups. The resultant curable compositions D and E were coated, cured and tested for adhesion as described in Example 1 and the results of this evaluation were as follows: Composition Peel Strength (ppi//kN/m) Type of failure D 17.4 // 3.045 35% cohesive, 65 adhesive from the screen, not the panel E 8.3 // 1.45 100% adhesive failure from panel As used herein, adhesive failure refers to a separation of the cured composition from the substrate at the substrate-composition interface. Cohesive failure refers to a separation within the cured composition and not at the substrate.

Claims: A curable organosiloxane composition comprising A. a polyorganosiloxane containing at least two silicon-bonded alkenyl radicals per molecule where at least 20 mole percent of the siloxane units contain a silicon-bonded perfluoroalkylethyl radical and the silicon atoms containing said alkenyl radicals are separated by at least two dimethylsiloxane units from the closest silicon atom containing said perfluoroalkylethyl radical; B. an organohydrogensiloxane containing an average of at least two silicon-bonded hydrogen atoms per molecule and the molar ratio of hydrogen atoms in component B to alkenyl radicals in component A being in the range of 0.4 to 3.5; C. a platinum-containing hydrosilation catalyst, where said catalyst is selected from metals of the platinum group of the periodic table and compounds of these metals in an amount within the range equivalent to 1 to 50 parts of platinum group metal per 1 million parts of said curable organosiloxane composition; and optionally D. an adhesion promoter in an amount of 0.1 to 10 weight percent based on the total weight of the curable composition of an (1) alkoxy-substituted organosilicon compound containing at least one organofunctional substituent bonded to silicon through carbon, where said substituent is selected from epoxy, amino and alkenyl, or (2) copolymers derived from said alkoxy-substituted organosilicon compound and alkyl esters of ethylenically unsaturated carboxylic acids. A composition according to claim 1 where the adhesion promoter contains at least three alkoxy groups per molecule, the polyorganosiloxane is a polydiorganosiloxane wherein from 40 to 95 mole percent of the diorganosiloxane units are methyl-3,3,3-trifluoropropylsiloxane, the remaining diorganosiloxane units are dimethylsiloxane, the terminal units are dimethylvinylsiloxy and the silicon-bonded organic radicals present in said organohydrogensiloxane are selected from methyl and 3,3,3-trifluoropropyl radicals. A composition according to claim 2 where the organofunctional substituent present in said adhesion promoter is selected from 3-glycidoxypropyl, epoxycyclohexyl, vinyl, 3-acryloxypropyl and 3-methacryloxypropyl. A composition according to claim 3 where an adhesion promoter containing said 3-glycidoxypropyl or epoxycyclohexyl substituent is present in combination with a liquid silanol-terminated polydimethylsiloxane containing at least one silicon-bonded vinyl radical per molecule. A composition according to claim 2 where said adhesion promoter is a copolymer derived at least in part from a methacryloxy functional alkoxysilane and an ester of an ethylenically unsaturated acid.

Inventors: WATSON MICHAEL JOHN, WATSON, MICHAEL JOHN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1993
Main classification: C08L83
IPC codes: C08K5, C08L83, C08F8