Fluorosilicone rubber composition and method of adhering it to a substrate

Abstract: A fluorosilicone rubber composition is disclosed which, in combination with a specific primer, has excellent curability and adhesion to various substrates. This composition comprises a specific polyorganosiloxane, a silica filler, a polyorganohydrogensiloxane, and an organic peroxide. Also is a method of adhering our fluorosilicone rubber composition to a primed substrate such as metal.

Description: The present invention is a fluorosilicone rubber composition that possesses excellent curability and adhesiveness on various substrate materials. In a separate embodiment, we provide a method of adhering said composition with the use of a specific primer. Fluorosilicone rubber, which has a main component of an organopolysiloxane that contains perfluoroalkyl groups, possesses properties resistant to heat, frost, chemical, and oil. These compositions find wide application in automotive and aircraft industries. However, a disadvantage of fluorosilicone rubber is that it does not adhere well to substrates made of metal, synthetic rubber, and the like. Therefore, various methods are proposed for improving adhesion of fluorosilicone rubbers to these and other surfaces. For example, Japanese Patent Publication 1-38149 provides a method for adhering fluorosilicone rubber to a metal surface by contacting such a composition with a metal surface that has been treated with a primer composition containing vinyltrimethoxysilane and 2,4,4-trimethyl-pentyl-2-hydroperoxide. This composition is then heated and cured. A disadvantage of this method is the cost of the primer composition because it contains a specific organic peroxide. Another disadvantage is the non-uniform adhesion of the fluorosilicone rubber to the substrate. As a result, we performed studies aimed at the elimination of the aforementioned disadvantages. It has now been found that a fluorosilicone rubber composition in combination with a specific polyorganohydrogensiloxane demonstrates excellent adhesion to substrates. This property is further improved when the composition is additionally combined with a specific primer. Heating and curing of fluorosilicone rubber composition then provides for strong adhesion to various substrates. Therefore, it is an object of this invention to provide a fluorosilicone rubber composition which, in combination with a specific primer, acquires excellent curability and adhesion to various substrate materials, especially metals. It is also an object of the invention to provide an adhering method for use with said composition. The present invention is a fluorosilicone rubber composition comprising: (A) 100 parts by weight of a polyorganosiloxane which has a degree of polymerization greater than 1000 and the average unit formula R a SiO (4-a)/2 , where R is a substituted or non-substituted monovalent hydrocarbon group, 20 to 50 mole% of the R substituents are 3,3,3-trifluoropropyl and a is a number within the range of 1.95 to 2.05; (B) 5 to 200 parts by weight of a silica filler; (C) 0.01 to 10 parts by weight of a polyorganohydrogensiloxane comprising 0.8 weight percent or greater of silicon-bonded hydrogen atoms; and (D) 0.1 to 10 parts by weight an organic peroxide. This invention also provides a method for adhering said fluorosilicone rubber composition to a substrate by contacting the composition with a substrate previously treated with a primer composition comprising an organoalkoxysilane containing alkenyl groups, and by then heating and curing said composition. The polyorganosiloxane, component (A), of our composition is represented by the average unit formula R a SiO (4-a)/2 , where R is a substituted or non-substituted monovalent hydrocarbon group; 20 to 50 mole%, preferably 40 to 50 mole%, of the R substituents are 3,3,3-trifluoropropyl. In addition to trifluoropropyl, R is selected from a C 1 to C 6 alkyl group; a vinyl or allyl group; and a phenyl group. For efficient crosslinking, it is recommended that component (A) preferably comprise 0.01 to 2 mole% of vinyl groups bonded to silicon. Furthermore, to improve heat resistance and to facilitate cure it is recommended that methyl groups preferably be used in addition to 3,3,3-trifluoropropyl groups and vinyl groups. In the above formula, a is a number within the range of 1.95 to 2.05. To impart sufficient mechanical strength to our fluorosilicone rubber after cure, it is recommended that the degree of polymerization of component (A) be greater than 1000, and preferably greater than 3000. The polyorganosiloxane of (A) is represented, for example, by a copolymer of dimethylsiloxy units and (3,3,3-trifluoropropyl) methylsiloxy units; a copolymer of dimethylsiloxy units, (3,3,3-trifluoropropyl)methylsiloxy units, and vinylmethylsiloxy units; a copolymer of (3,3,3-trifluoropropyl)methylsiloxy units and vinylmethylsiloxy units; and poly(3,3,3-trifluoropropyl)methylsiloxane. The terminal group on the molecular chains of component (A) are selected from a trimethylsiloxy group, vinyldimethylsiloxy group, dimethylhydroxysiloxy group, and (3,3,3-trifluoropropyl)methylhydroxysiloxy group. The silica filler, component (B), is represented by fillers conventionally used for silicone rubbers such as precipitated silica, silica aerogel, and fumed silica. It is preferable that this filler have a pH below 9. Furthermore, for obtaining a sufficient reinforcement effect, the filler must have a specific surface area greater than 50 m 2 /g. The filler amount is from 5 to 200 parts by weight, and preferably 20 to 80 parts by weight, per 100 parts by weight of component (A). If the amount of the filler exceeds 200 parts by weight, the fluorosilicone rubber will lose its mechanical properties after processing and curing. The polyorganohydrogensiloxane, component (C), of the present invention is required for improving curability and adhesion to various substrates. The amount of hydrogen atoms bonded to silicon atoms in component (C) is 0.8 weight percent or greater, and preferably 1.2 weight percent or greater. If the amount of silicon-bonded hydrogen atoms silicon-bonded atoms of component (C) is less than 0.8 weight percent, the adhesion to the substrate will be impaired. In component (C), groups other than silicon-bonded hydrogen atoms are represented by substituted or non-substituted monovalent hydrocarbon groups such as C 1 to C 6 alkyl groups; vinyl or allyl groups; aryl group such as phenyl; and 3,3,3-trifluoropropyl. Component (C) normally has a linear or cyclic molecular structure, but it can also be partially branched or three-dimensional. It is recommended that component (C) have a degree of polymerization greater than 5 and preferably within the range of 10 to 150. The following are Examples of polyorganohydrogensiloxanes useful in the present invention are: polymethylhydrogensiloxane; a copolymer of methylhydrogensiloxy units and dimethylsiloxy units; and a copolymer of methylhydrogensiloxy units, dimethylsiloxy units, and (3,3,3-trifluoropropyl)methylsiloxy units. Terminal groups of the molecular chain of component (C) are represented by a trimethylsiloxy group or a dimethylhydrogensiloxy group. It is recommended that the amount of component (C) be within the range of 0.01 to 10 parts by weight, per 100 parts by weight of component (A). If component (C) is present at less than 0.01 parts by weight, it is not impossible to improve curing and adhesion properties. If, the amount of component (C) exceeds 10 parts by weight, the mold release properties of the cured composition are impaired. The organic peroxide, component (D), of the present invention is any commercial compound such as benzoyl peroxide, di-tertiary-butyl peroxide, tertiary-butyl per-benzoate, dicumyl peroxide, and 2,5-dimethyl-2,5-di-(tertiary butyloxy)hexane. This component may also be formed into a paste by dispersing in a silicone oil. It is recommended that component (D) be used in an amount of 0.1 to 10 parts by weight, preferably 0.5 to 2.0 parts by weight, per 100 parts by weight of component (A). Our claimed composition comprises components (A) through (D). If necessary, however, this composition is combined with the following additives: plasticizers such as diorganosiloxane oligomers normally used with silicone rubber compositions; metal soaps or processing aids; iron oxide, titanium oxide, magnesium oxide, magnesium carbonate or oil resistance improvers; quartz, diatomaceous earth, calcium carbonate, or similar fillers. The diorganosiloxane oligomers which are used as plasticizers are polymers which do not contain the polyorganohydrogensiloxane of component (C) and which have a viscosity below 500 mPa·s at 25°C. Specific examples of diorganosiloxane oligomers are: (3,3,3-trifluoropropyl)methylsiloxane oligomer having both molecular terminals end-blocked with dimethylhydroxysiloxy groups, or a dimethylsiloxane oligomer having both terminals end-blocked with dimethylhydroxysiloxy groups. These oligomers are used in an amount of 0.1 to 40 parts by weight per 100 parts by weight of component (B). When component (A) is poly(3,3,3-trifluoropropyl)methylsiloxane, our claimed composition may also be combined with an organopolysiloxane which has a degree of polymerization greater than 1,000 and which does not contain 3,3,3-trifluoropropyl groups. Organic groups which are bonded to silicon atoms in the polyorganosiloxane are represented by C 1 to C 6 alkyl groups; vinyl or allyl; and phenyl. The following are specific examples of these polyorganosiloxanes: polydimethylsiloxane, polyvinylmethylsiloxane, a copolymer of dimethylsiloxy units and methylphenylsiloxy units, and a copolymer of dimethylsiloxy units and vinylmethylsiloxy units. It is recommended that the polyorganosiloxanes be used in such an amount that, after compounding, the content of 3,3,3-trifluoropropyl groups in relation to all monovalent hydrocarbon groups bonded to silicon atoms be at least 20 mole%. The present composition is prepared by uniformly mixing components (A) through (D), with appropriate additives when necessary, in conventional mixing equipment such as a two-roll kneader-mixer. It is possible to premix components (A) and (B) by mixing them under heating conditions to obtain a silicone rubber base compound which is then combined with components (C) and (D). The following is a description of our method for adhering the claimed composition to various substrates. This method comprises contacting a fluorosilicone rubber composition comprising components (A) through (D) with a substrate treated with a primer composition having a main component in the form of an organoalkoxysilane that contains alkenyl groups, and then heating and curing. Suitable organoalkoxysilanes with alkenyl groups are vinyltrimethoxysilane, vinylmethyldimethoxysilane, vinyltriethoxysilane, vinyl(tris(β-methoxyethoxy))silane, allyl trimethoxysilane, allyltriethoxysilane, allylmethyldiethoxysilane, as well as the condensation products of their partial hydrolysis. Other components of the primer compositions include organic titanium acid esters, organic solvents, and platinum compounds. Useful organic titanium esters are: tetraisopropyl titanate, tetra-(n-propyl) titanate, tetra-(n-butyl) titanate, tetra-(2-ethylhexyl) titanate, dibutyl diisopropyl titanate, triethanolamine titanate, ethylene glycol titanate, bis-(acetylacetone)diisopropoxy titanium, bis (ethylacetyl-acetonate) diisopropoxy titanium, and tetra (trimethylsiloxy) titanate. The organic solvents are represented by toluene, xylene, benzene, n-heptane, n-hexane, and trichloroethylene. The following are examples of the platinum compounds: platinum black, platinum tetrachloride, a complex of chloroplatinic acid and an olefin, and a complex of chloroplatinic acid and a vinylmethylsiloxane. Curing of our claimed composition, with simultaneous attachment thereof to the substrate surface, is performed by spreading the primer composition over the substrate, drying said primer composition in air for at least 30 minutes, contacting the fluorosilicone rubber composition with the surface of a substrate, and then heating and adhering to the substrate under a pressure of 100 to 250 kgf/cm 2 applied for 5 to 20 minutes at a temperature within a range of 170 to 190°C. As a result of our method, the fluorosilicone rubber composition of this invention cures and adheres strongly to the surfaces of various substrates such as iron, steel, stainless steel, aluminum, nickel, zinc, copper and their alloys; acrylic resins; phenolic resins; epoxy resins; alkali-treated Teflon™ films; glass and ceramics. The invention is now further described by reference to specific examples. In the examples, all parts are given as parts by weight and the values for viscosity are as measured at 25°C. Example 1. A kneader-mixer was loaded with 100 parts by weight of polyorganosiloxane having both terminals end-blocked with dimethylhydroxysiloxy groups and having a degree of polymerization (dp) of 6,000. The polyorganosiloxane comprised 95.5 mole% of (3,3,3-trifluoropropyl)methylsiloxy units and 0.5 mole% of vinylmethylsiloxy units. Forty parts by weight of aerosol silica (pH 4.3) having a specific surface area of 200 m 2 /g was added to the mixer. The contents of the mixer was then combined with 8 parts of (3,3,3-trifluoropropyl)methylsiloxane oligomer having a viscosity of 30 mPa·s at 25°C and end-blocked with dimethylhydroxysiloxy groups. The components were mixed for 2 hours at 50°C. The mixture was thereafter heat treated for 2 hours at 120°C, whereby volatile polyorganosiloxanes and water were removed, and a fluorosilicone rubber base compound was obtained. This compound was next combined with 0.5 part of polymethylhydrogensiloxane end-blocked with trimethylsiloxy groups and having a dp of 25 (amount of silicon-bonded hydrogen atoms equalled 1.50 wt%), and 0.3 part of 2,5-dimethyl-2,5-di-(tertiary-butylperoxy) hexane. These components were uniformly mixed on a two roll mill, thereby forming a curable fluorosilicone rubber composition. A primer composition was then prepared by uniformly mixing 94 parts of n-heptane, 4 parts of allyltrimethoxysilane, 1.5 parts of tetra-(n-butyl) titanate, and 0.5 part of chloroplatinic acid. An adhesive Teflon™ tape was adhered to both ends on one side of a 6 cm long by 2.5 cm wide stainless steel plate. The primer composition was next applied onto the central portion of the plate, not coated with the Teflon™ tape; and the coating was subsequently dried for 60 minutes at room temperature. After drying of the primer composition, the fluorosilicone rubber composition was applied onto the entire surface of the stainless steel plate in an amount that formed a 2 mm thick coating. The resulting product was then heated at 170°C under a pressure of 200 kgf/cm 2 for 10 minutes. As a result, the coating layer was cured, and an adhesion test specimen was prepared. After the Teflon™ tape was peeled off, the fluorosilicone rubber remained adhered only to the central part of the specimen which was coated with primer composition. While the non-adhered portion of the fluorosilicone rubber was held by the hand, it was pulled up with a force perpendicular to the stainless steel plate surface; and the conditions of peeling the adhered portion of fluorosilicone rubber were observed. The degree of breaking was measured as a ratio of the broken area to the entire surface area of adhesion. From these measurements (Table 1) the curing adhesion capacity was evaluated in accordance with the following four categories: I - 100% breaking through the adhesive layer (100% of the rupture occurred in the rubber layer). II - 50 to 99% of the rupture occurred through the adhesive layer III - 0 to 49% of the rupture occurred through the adhesive layer IV - 100% peeling occurred at the interface surface. Example 2. A fluorosilicone rubber composition was prepared as in Example 1, with the exception that a polymethylhydrogensiloxane, having both molecular terminals end-blocked with trimethylsiloxy groups and a dp of 150 (the silicon-bonded hydrogen atoms were in an amount of 1.65 wt%) was used instead of the polymethylhydrogensiloxane of Example 1. The resultant composition was tested with regard to both curing and adhesive properties. The results are given in Table 1. Example 3. A fluorosilicone rubber composition was prepared as in Example 1, with the exception that a polymethylhydrogensiloxane, having both molecular terminals end-blocked with trimethylsiloxy groups and a dp of 10 (the silicon-bonded hydrogen atoms were in an amount of 1.25 wt%) was used instead of the polymethylhydrogensiloxane of Example 1. The product composition was tested with regard to curing and adhesive properties. The results are in Table 1. Example 4. Adhesion test specimens were prepared by the same method as Example 1, with the exception that an aluminum plate was used instead of stainless steel. The specimens were tested with regard to curing and adhesion as in Example 1. The results showed that 100% of rupture occurred through the adhesive layer (i.e., 100% rupture in the rubber layer). Example 5. A fluorosilicone composition was prepared as in Example 1, with the exception that the polymethylhydrogensiloxane was not used. The composition was tested with regard to curing and adhesion with results shown in Table 1. Example 6. A fluorosilicone rubber composition was prepared as in Example 1, with the exception that a copolymer comprising dimethylsiloxy units and methyl hydrogensiloxy units, end-blocked with trimethylsiloxy groups and having a dp of 10 (the silicon-bonded hydrogen atoms were in an amount of 0.75 wt%), was used instead of the polymethylhydrogensiloxane. The composition was tested with regard to curing and adhesive properties, and results are listed in Table 1. Comparative Example 7. A fluorosilicone rubber composition was prepared as in Example 1, with the exception that a copolymer comprising dimethylsiloxy units and methylhydrogensiloxy units, end-blocked with trimethylsiloxy groups, and having a dp of 10 (the silicon-bonded hydrogen atoms were in an amount of 0.32 wt%) was used instead of the polymethylhydrogensiloxane. The obtained composition was tested with regard to curing and adhesive properties. The results are in Table 1. Example No. Degree of Polymerization of Polyorganohydrogensiloxane (Si-H wt%) Adhesion 1 25(1.50%) I 2 150 (1.65%) I 3 10 (1.25%) II 5 - IV 6 10 (0.75%) III 7 10 (0.32%) IV I - 100% breaking through the adhesive layer (100% of the rupture occurred in the rubber layer). II - 50 to 99% of the rupture occurred through the adhesive layer. III - 0 to 49% of the rupture occurred through the adhesive layer. IV - 100% peeling occurred through the interface surface.

Claims: A fluorosilicone rubber composition comprising: (A) 100 parts by weight of a polyorganosiloxane having a degree of polymerization greater than 1000 and an average unit formula of R a SiO (4-a)/2 , where R is a substituted or non-substituted monovalent hydrocarbon group and 20 to 50 mole% of the R substituents are 3,3,3-trifluoropropyl group, and a is a number within the range of 1.95 to 2.05; (B) 5 to 200 parts by weight of a silica filler; (C) 0.01 to 10 parts by weight of a polyorganohydrogensiloxane comprising 0.8 weight percent or greater of silicon-bonded hydrogen atoms; and (D) 0.1 to 10 parts by weight of an organic peroxide. The fluorosilicone rubber composition of Claim 1, wherein component (A) has 40 to 50 mole% of the R substituents as 3,3,3-trifluoropropyl and the remaining R substituents are selected from vinyl or methyl. The fluorosilicone rubber composition of Claim 1, wherein component (A) has 0.01 to 2 mole% of the R substituents as vinyl. The fluorosilicone rubber composition of Claim 1, where the silica filler has a surface area greater than 50 m 2 /g and is selected from the group consisting of precipitated silica, silica aerogel, and fumed silica. The fluorosilicone rubber composition of Claim 1, where component (C) is a polyorganohydrogensiloxane having a degree of polymerization within a range of 10 to 150. The fluorosilicone rubber composition of Claim 1, where the organic peroxide is selected from the group consisting of benzoyl peroxide, di-tertiary-butyl peroxide, tertiary-butyl perbenzoate, dicumyl peroxide, and 2,5-dimethyl-2,5-di-(tertiary-butyloxy)hexane. The fluorosilicone rubber composition of Claim 1 further comprising a plasticizer consisting of 0.1 to 40 parts by weight, per 100 parts by weight of component (B), of a diorganosiloxane oligomer having a viscosity less than 500 mPa·s at 25°C and having no hydrogen atoms bonded to silicon atoms. The fluorosilicone rubber composition of claim 1, where component (A) is poly(3,3,3-trifluoropropyl)methylsiloxane and said composition further comprises in admixture with component (A) an organosiloxane having a degree of polymerization greater than 1,000 and containing no 3,3,3-trifluoropropyl groups and the content of 3,3,3-trifluoropropyl groups in relation to all monovalent hydrocarbon groups bonded to silicon in the admixture is at least 20 mole percent. A method of adhering a fluorosilicone rubber composition to a substrate comprising: (A) drying on the surface of a substrate a primer composition comprising an organoalkyoxysilane comprising alkenyl groups, (B) contacting the primed surface of the substrate with the fluorosilicone rubber composition of claim 1; and (C) curing the fluorosilicone rubber composition. The method according to Claim 9, where the organoalkoxysilane comprising alkenyl groups is selected from the group consisting of vinyltrimethoxysilane, vinyl(tris(β-methoxyethoxy))silane, allyltrimethoxysilane, allyltriethoxysilane, and allylmethyldiethoxysilane. The method according to Claim 9 where the primer composition further comprises components selected from the group consisting of organic titanium acid esters or compounds and complexes of platinum.

Inventors: IRIE MASAKAZU, IRIE, MASAKAZU
Applicants: DOW CORNING TORAY SILICONE, DOW CORNING TORAY SILICONE COMPANY, LIMITED
Application year: 1997
Main classification: C08L83
IPC codes: C09J5, C08K3, C09D183, C08L83