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Amorphous precipitated high structure silica pigment useful in the reinforcement of silicone rubber is described. The silica pigment is treated with an aqueous solution of a metal, e.g., calcium and barium, which forms a relatively water insoluble, white or colorless sulfate salt in amounts sufficie

Amorphous precipitated high structure silica pigment useful in the reinforcement of silicone rubber is described. The silica pigment is treated with an aqueous solution of a metal, e.g., calcium and barium, which forms a relatively water insoluble, white or colorless sulfate salt in amounts sufficient to provide at least an equal molar amount of the metal compared to the amount of alkali metal in the silica. Silicone rubber reinforced with such silica pigment exhibits a water swell of less than 4 volume percent.

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Finely-divided amorphous, precipitated, high structure silica consisting essentially of at least 85 weight percent SiO2 on an anhydrous basis, less than 2 weight percent of a water soluble alkali metal sulfate salt, less than 1 weight percent of a water soluble alkali metal chloride salt, and from a

Finely-divided amorphous, precipitated, high structure silica consisting essentially of at least 85 weight percent SiO2 on an anhydrous basis, less than 2 weight percent of a water soluble alkali metal sulfate salt, less than 1 weight percent of a water soluble alkali metal chloride salt, and from about 0.05 to 5 weight percent of alkali metal oxide, the total amount of said alkali metal salts and oxide in said silica being from about 1 to about 5 weight percent, and having deposited on its surface the oxide of a metal selected from the group consisting of calcium, barium and mixtures of such metals in amounts sufficient to provide on said silica at least an equal molar amount of said metal based on the total amount of alkali metal present in the silica, said amount of metal being such that a silicone rubber reinforced with said silica as the principal silica filler exhibits a water volume swell of not more than 4 percent and a wet dielectric strength of at least 400 volts/mil. A composition comprising silicone elastomer gum curable with a free radical generator and from 10 to 100 parts per 100 parts of silicone gum, of reinforcing, amorphous precipitated, high structure silica consisting essentially of at least 85 weight percent SiO2 on an anhydrous basis, less than 2 weight percent of a water soluble alkali metal sulfate salt, less than 1 weight percent of a water soluble alkali metal chloride salt, and from about 0.05 to 5 weight percent of alkali metal oxide, the total amount of said alkali metal salts and oxide in said silica being from about 1 to about 5 weight percent, and having deposited on its surface the oxide of a metal selected from the group consisting of calcium, barium and mixtures of such metals in amounts sufficient to provide on said silica at least an equal molar amount of said metal based on the total amount of alkali metal present in the silica, said amount of metal being such that, when said silicone elastomer gum composition is cured, the resulting silicone rubber exhibits a water volume swell of not more than 4 percent and a wet dielectric strength of at least 400 volts/mil. In the method of preparing amorphous, precipitated, high structure silica by acidulating an aqueous alkali metal silicate solution with inorganic acid, the improvement of preparing a precipitated, high structure silica useful for reinforcing silicone rubber, which comprises determining the total amount of alkali metal of the precipitated silica and in response to such determination, depositing on the surface of said precipitated silica metal oxide of a metal selected from the group consisting of calcium, barium and mixtures of such metals in amounts sufficient to provide on said silica at least an equal molar amount of said metal based on the total amount of alkali metal present in the silica, the amount of metal being such that a silicone rubber reinforced with said precipitated silica as the principal silica filler exhibits a water volume swell of not more than 4 percent and a dielectric strength of at least 400 volts/mil, and wherein the metal oxide is deposited from an aqueous solution of an ionizable oxide, hydroxide or C1-C4 carboxylic acid salt of said metals. In the method of preparing amorphous, precipitated, high structure silica by acidulating an aqueous sodium silicate solution with inorganic acid, the improvement of preparing a precipitated, high structure silica useful for reinforcing silicone rubber, which comprises determining the concentration of the chloride, sulfate and sodium ionic species in the silica and, in response to said determination, depositing an oxide of calcium on the surface of said precipitated silica from an aqueous solution of an ionizable oxide or hydroxide of calcium in amounts sufficient to satisfy the following relationship: [Figure] wherein C1, C2, C3 and C4 are numerical coefficients which respectively vary from 8 to 12, 1 to 3, 9.5 to 14.5 and -5.5 to -8.5, and Cl-, SO4=, Na+and Ca++are the weight percent of said ions in the silica expressed as sodium chloride, sodium sulfate, sodium oxide and calcium oxide.