Coated composite high voltage electrical insulator

Abstract

The present invention relates to a one-part room temperature vulcanizable organopolysiloxane rubber composition which is VOC free and crosslinks in the presence of moisture to form a coating for the protection of composite high voltage electrodes. The one-part organopolysiloxane rubber composition comprises the product which is obtained by mixing the following: a) from about 20 to about 60 weight percent of one or more polydiorganosiloxane fluids having a viscosity from about 10 to about 100,000 centipoise at 25° C.; b) from 0 to about 40 weight percent of a cyclo-organosiloxane; 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; e) from about 1 to about 7 weight percent of an oximinosilane cross-linking agent; f) from about 0.2 to about 3 weight percent of an adhesion promoter; 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 10 μm to 30 μm, containing 65.1 percent Al 2 O 3 , 34.5 percent combined H 2 O, 0.3 percent Na 2 O, 0.02 percent CaO, 0.01 percent SiO 2 and having a specific gravity of 2.42.

The embodiments of the invention in which an exclusive property or privilege is claimed are as follows:  
     
         1 . A one-part organopolysiloxane rubber composition for use as a protection coating on composition high voltage insulators 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) 2 SiO] n R′   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 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 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) 2 SiO] 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 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 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                          in which R 2  and R 3  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 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 percent Al 2 O 3 , 34.5 percent combined H 2 O, 0.3 percent Na 2 O, 0.02 percent CaO, 0.01 percent SiO 2  and having a specific gravity of 2.42.    
     
     
         2 . A composition according to  claim 1  wherein the composition contains only one polyorganosiloxane fluid in which R and R′ are each alkyl and from about 20 to about 30 weight percent of the cycloorganosiloxane.  
     
     
         3 . A composition according to  claim 2  wherein R and R″ are methyl.  
     
     
         4 . A composition according to  claim 3  wherein the adhesion promoter is a compound of the formula  
       
         
           
           
               
               
           
         
       
       wherein Me is the methyl radical.  
     
     
         5 . A composition according to  claim 4  wherein the organotin salt is an organotin salt of a carboxylic acid selected from the group consisting of dibutyltindiacetate, stannous octoate and dibutyltin dioctoate.  
     
     
         6 . A composition according to  claim 5  wherein the organotin salt of a carboxylic acid is a compound of the formula  
       (C 4 H 9 ) 2 Sn(OCOC 10 H 20 CH 3 ) 2 .  
     
     
         7 . A composition according to  claim 1  comprising: 
 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 SiO 2  fillers having a specific gravity of 2.2 and surface area of up to about 130 m 2 /g;  
 c) about 2 weight percent of methyl tris-(methyl ethyl ketoxime)silane;  
 d) about 1 weight percent of N-(2 aminoethyl-3 aminopropyl) trimethoxysi lane;  
 e) about 0.1 weight percent of dibutyltindilaurate;  
 f) about 29 weight percent of dimethylsiloxane cyclics; and  
 g) about 0.8 weight percent of a gray pigment;  
 h) about 35 weight percent of alumina trihydrate, the alumina trihydrate having a median particle size of 13. μm, containing 65.1 percent Al 2 O 3 , 34.5 percent combined H 2 O, 0.3 percent Na 2 O, 0.02 percent CaO, 0.01 percent SiO 2  and having a specific gravity of 2.42.  
 
     
     
         8 . A composition according to  claim 1  wherein the composition contains a first polydiorganosiloxane fluid wherein R and R′ are each alkyl, R″ is OH and n has an average value such that the viscosity is from about 3000 to 30,000 centipoise at 25° C. and a second polydiorganosiloxane fluid wherein R, R′ and R″ are each alkyl and the average volume of n is between 4 and 20.  
     
     
         9 . A composition according to  claim 8  wherein R and R′ of the first polydiorganosiloxane and R, R′ and R″ of the second polydiorganosiloxane are all methyl.  
     
     
         10 . A composition according to  claim 10  wherein the adhesion promoter is a compound of the formula  
       
         
           
           
               
               
           
         
       
       wherein Me is the methyl radical.  
     
     
         11 . A composition according to  claim 10  wherein the organotin salt of a carboxylic acid is a compound of the formula  
       (C 4 H 9 ) 2 Sn(OCOC 10 H 20 CH 3 ) 2 .  
     
     
         12 . 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) 2 SiO] n R′ 
 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) 2 SiO] 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 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 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                          in which R 2  and R 3  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 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 percent Al 2 O 3 , 34.5 percent combined H 2 O, 0.3 percent Na 2 O, 0.02 percent CaO, 0.01 percent SiO 2  and having a specific gravity of 2.42.    
     
     
         13 . 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 SiO 2  fillers having a specific gravity of 2.2 and surface area of up to about 130 m 2 /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 Al 2 O 3 , 34.5 percent combined H 2 O, 0.3 percent Na 2 O, 0.02 percent CaO, 0.01 percent SiO 2  and having a specific gravity of 2.42.