Silicone rubber compositions for high-voltage electrical insulators

A silicone rubber composition for use as a high-voltage electrical insulator comprising

• (A) 100 parts by weight of an organopolysiloxane of the following average compositional formula (1):
• R.sup.1.sub.a R.sup.2.sub.b SiO.sub.(4-a-b)/2 ( 1)
• wherein R.sup.1 is a halo-substituted or unsubstituted aliphatic unsaturated monovalent hydrocarbon group, R.sup.2 is a halo-substituted or unsubstituted monovalent hydrocarbon group free of an aliphatic unsaturated bond, and letters a and b are positive numbers in the range:
• 0<a.ltoreq.1, 1.ltoreq.b <3, and 1<a+b.ltoreq.3, said organopolysiloxane having at least two aliphatic unsaturated hydrocarbon groups each attached to a silicon atom in a molecule,
• (B) 1 to 100 parts by weight of finely divided silica based on 100 parts by weight of (A),
• (C) 50 to 300 parts by weight of surface treated aluminum hydroxide based on 100 parts by weight of (A), wherein the aluminum hydroxide is of the formula Al.sub.2 O.sub.3hu . 3HO.sub.2 O,
• (D) an organohydrogenpolysiloxane of the following average compositional formula (2):
• R.sup.3.sub.c H.sub.d SiO.sub.(4-c-d)/2 ( 2)
• wherein R.sup.3 is a halo-substituted or unsubstituted monovalent hydrocarbon group exclusive of an aliphatic unsaturated group, and letters c and d are positive numbers in the range:
• 1.ltoreq.c<3, 0<d.ltoreq.1, and 1<c+d.ltoreq.3, said organohydrogenpolysiloxane having at least two hydrogen atoms each attached to a silicon atom in a molecule, in a sufficient amount to provide 0.5 to 20 mol of the silicon atom-attached hydrogen atom per mol of the aliphatic unsaturated hydrocarbon group in component (A) , and
• (E) a catalytic amount of a platinum group catalyst.