Patent ID: 6939582

Claim:
A method of protecting a composite high voltage insulators from the effects of a corrosive atmospheric environment comprising (1) applying to the surface of the composite insulator a thin layer of a one-part organopolysiloxane rubber composition comprising the product which is obtained by mixing the following: a) from about 20 to about 60 weight percent of one or more polydiorganosiloxane fluids of the formula R″[(R) 2 SiO] n (R) 2 SiR′ in which R is a monovalent alkyl or alkenyl radical having 1 to 8 carbon atoms or a phenyl radical, R′ and R″ which may be the same or different are OH or a monovalent alkyl or alkenyl radical having 1 to 8 carbon atoms or a phenyl radical and n has an average value such that the viscosity is from about 10 to about 100,000 centipoise at 25° C., wherein at least one of the polyorganosiloxane fluid has at least one of R′ and R″ equal to OH and n has an average value such that the viscosity is in the range from 100 to 100,000 centipoise at 25° C. b) from 0 to about 40 weight percent of a cyclo-organosiloxane of the formula [(R) 2 SiO] n in which R is a monovalent alkyl or alkenyl radical having 1 to 8 carbon atoms or a phenyl radical which may optionally be substituted with an alkyl radical having 1 to 8 carbon atoms and n has an average value of 3 to 10 c) from 0 to about 40 weight percent of an inorganic extending or non-reinforcing filler d) from about 0.5 to about 15 weight percent of an amorphous SiO 2 reinforcing filler having a surface area of between about 100 to 250 m 2 /g and a particle size range between about 0.01 and 0.03 microns; e) from about 1 to about 10 weight percent of an oximinosilane cross-linking agent of the formula; RSi(ON═CR′ 2 ) 3 in which R and R′ are independently selected from monovalent alkyl or alkenyl radicals having 1 to 8 carbon atoms or a phenyl radical which may optionally be substituted with an alkyl radical having 1 to 8 carbon atoms; f) from about 0.2 to about 3 weight percent of an adhesion promoter of the formula in which R 2 and R 3 are independently selected from monovalent alkyl or alkenyl radicals having 1 to 8 carbon atoms or a phenyl radical which may optionally be substituted with an alkyl radical having 1 to 8 carbon atoms, b is an integer between 0 and 3, and R 1 is a saturated, unsaturated or aromatic hydrocarbon radical having 1 to 10 carbon atoms which may optionally contain a functional group; g) from about 0.02 to about 3 weight percent of an organotin salt as a condensation catalyst; and h) from about 20 to about 50 weight percent of alumina trihydrate, the alumina trihydrate having a median particle size of about 10 μm to about 30 μm, containing 65.1 weight percent Al 2 O 3 , 34.5 weight percent combined H 2 O, 0.3 weight percent Na 2 O, 0.02 weight percent CaO, 0.01 weight percent SiO 2 and having a specific gravity of 2.42.