Organopolysiloxane rubber composition for coating high voltage electrical insulators having improved electrical properties

An organopolysiloxane rubber composition for coating high voltage electrical insulators comprising the product which is obtained by mixing the following:

• a) 30 to 50 weight percent of a mixture of
• i) 40 to 44 weight percent of polydimethyl siloxane fluid of the formula
• HO[( CH.sub.3).sub.2 SiO].sub.n H
• wherein n has an average value such that the viscosity is in the range, from 10 to 15 Pa.S at 25.degree. C;
• ii) 2 to 3 weight percent of a reinforcing filler of the formula
• SiO.sub.2
• with a specific gravity of 2.2 and a range of particle size 0.5 to 10 microns,
• iii) 1 to 3 weight percent of an organosilicon cross-linking agent of the formula
• RSi(ON.dbd.CR'.sub.2).sub.3
• wherein R and R'represent a group of monovalent hydrocarbon radicals,
• iv) 0.5 to 2.0 weight percent of an adhesion promoter of the formula
• NHCH.sub.2 CH.sub.2 NH.sub.2
• CH.sub.2
• CH.sub.2 CH.sub.2 Si(OMe).sub.3
• wherein Me is the methyl radical,
• v) 0.1 to 0.2 percent by weight of a condensation catalyst consisting of an organotin salt of carboxylic acid of the formula
• (C.sub.4 H.sub.9).sub.2 Sn(OCOC.sub.10 H.sub.20 CH.sub.3).sub.2,
• and
• vi) 36 to 48 weight percent of alumina trihydrate, the alumina trihydrate having a median particle size of 13.mu.m, containing 65.1 percent Al.sub.2 O.sub.3, 34.5 percent combined H.sub.2 O, 0.3 percent Na.sub.2 O, 0.02 percent CaO, 0.01 percent combined H.sub.2 O, 0.3 percent Na.sub.2 O, 0.02 percent CaO, 0.01 percent SiO.sub.2 and having a specific gravity of 2.42, the aluminum trihydrate being present in a proportion of 90 to 110 parts by weight per 100 parts by weight of the polyorganosiloxane base;
• with
• 50to 70 weight percent of a hydrocarbon solvent as a dispersion medium for the above composition.