Coated composite high voltage electrical insulator

A method of protecting a composition high voltage insulators from the effects of a corrosive atmospheric environment comprising

• (1) applying to the surface of the 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″O[(R)2SiO]nR′
• in which R is a monovalent alkyl or alkylene radical having 1 to 8 carbon atoms or a phenyl radical, R′ is a monovalent alkyl or alkylene radical having 1 to 8 carbon atoms or a phenyl radical, R″ is OH or a monovalent alkyl or alkylene radical having 1 to 8 carbon atoms or a phenol 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 R″ equal to OH and n has an average value such that the viscosity is in the range from 1,000 to 100,000 centipoise at 25° C., preferably from 3,000 to 40,000 centipoise at 25° C.;
• b) from 0 to about 40 weight percent of a cyclo-organosiloxane of the formula
• [(R)2SiO]n
• in which R is a monovalent alkyl or alkylene 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 SiO2 reinforcing filler having a surface area of between about 100 to 250 m2/g and a particle size range between about 0.01 and 0.03 microns;
• e) from about 1 to about 7 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 alkylene 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
• 6
• in which R2 and R3 are independently selected from monovalent alkyl or alkylene 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 R1 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 percent Al2O3, 34.5 percent combined H2O, 0.3 percent Na2O, 0.02 percent CaO, 0.01 percent SiO2 and having a specific gravity of 2.42.

A method according to claim 18 wherein the one part organopolysiloxane rubber composition comprises:

• a) about 30 weight percent of a hydroxyl terminated dimethyl polysiloxane fluid having a viscosity of 10,000 to 20,000 Centipoise at 25° C.;
• b) about 2 weight percent of a mixture of amorphous and crystalline SiO2 fillers having a specific gravity of 2.2 and surface area of up to about 130 m2/g;
• c) about 2 weight percent of methyl tris-(methyl ethyl ketoxime)silane;
• d) about 1 weight percent of N-(2 aminoethyl-3 aminopropyl) trimethoxysilane;
• e) about 0.1 weight percent of dibutyltindilaurate;
• f) about 29 weight percent of dimethylsiloxane cyclics;
• and
• i) about 0.8 weight percent of a gray pigment;
• j) about 35 weight percent of alumina trihydrate, the alumina trihydrate having a median particle size of 13. μm, containing 65.1 percent Al2O3, 34.5 percent combined H2O, 0.3 percent Na2O, 0.02 percent CaO, 0.01 percent SiO2 and having a specific gravity of 2.42.