Precipitated silica pigment for silicone rubber

Claim 2

Finely-divided amorphous, precipitated, high structure silica consisting essentially of at least 85 weight percent SiO 2 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.

Claim 3

The silica of claim 1 wherein the molar equivalent ratio of the metal to the alkali metal is from about 1:1 to about 2:1.

Claim 4

The silica of claims 1 or 2 wherein the alkali metal is sodium.

Claim 5

The silica of claim 3 wherein the silica contains less than 2 weight percent of a soluble sulfate salt, less than 0.5 weight percent of a soluble chloride salt, and less than 2 weight percent of alkali metal oxide, the total amount of said alkali metal salts and oxide in said silica being from about 1 to 3 weight percent and the water volume swell of the silicone rubber is less than 2 percent.

Claim 6

The silica of claim 4 wherein the metal is calcium and the relationship of the concentrations of the chloride, sulfate, sodium and calcium ionic species is expressed by the following: ##EQU3## wherein C 1 , C 2 , C 3 and C 4 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 - , SO 4 = , Na + and Ca ++ are the weight percent of said ions in the silica expressed as sodium chloride, sodium sulfate, sodium oxide and calcium oxide.

Claim 7

The silica of claim 5 wherein the silica has a BET surface area of from about 120 to 160 square meters per gram.

Claim 8

The silica of claim 5 wherein the amount of calcium present in the silica, expressed as the oxide, is from about 0.5 to 3 weight percent.

Claim 9

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 SiO 2 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.

Claim 10

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 SiO 2 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.

Claim 11

The silicone elastomer composition of claim 8 wherein the alkali metal is sodium and the ratio of molar equivalents to the alkali metal is from about 1:1 to about 2:1.

Claim 12

The silicone elastomer composition of claim 9 wherein the metal is calcium and the relationship of the concentrations of the chloride, sulfate, sodium and calcium ionic species is expressed by the following: ##EQU4## wherein C 1 , C 2 , C 3 and C 4 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 C - , SO 4 = , Na + and Ca ++ are the weight percent of said ions in the silica expressed as sodium chloride, sodium sulfate, sodium oxide and calcium oxide.

Claim 13

The silicone elastomer composition of claim 10 wherein the amount of calcium present in the silica, expressed as the oxide, is from about 0.5 to 3 weight percent.

Claim 14

The silicone elastomer composition of claims 8 or 10 wherein the amount of silica used ranges from 20 to 70 parts of silica per 100 parts of silicone gum.

Claim 15

A silicone rubber composition comprising the cured silicone elastomer gum of claim 8.

Claim 16

A silicone rubber composition comprising the cured silicone elastomer gum of claim 10 further characterized by having a water volume swell of less than 2 percent.

Claim 17

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 C 1 -C 4 carboxylic acid salt of said metals.

Claim 18

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 C 1 -C 4 carboxylic acid salt of said metals.

Claim 19

The method of claim 15 wherein the alkali metal is sodium and the molar equivalent ratio of the metal to sodium is from about 1:1 to about 2:1.

Claim 20

The method of claim 16 wherein the metal oxide is deposited on the precipitated silica by slurrying the silica in an aqueous solution of an ionizable or hydroxide of the metal.

Claim 21

The method of claim 16 wherein the metal oxide is deposited on the precipitated silica by washing a wet cake of the silica with an aqueous solution of an ionizable oxide or hydroxide of the metal.

Claim 22

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: ##EQU5## wherein C 1 , C 2 , C 3 and C 4 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 - , SO 4 = , Na + and Ca ++ are the weight percent of said ions in the silica expressed as sodium chloride, sodium sulfate, sodium oxide and calcium oxide.

Claim 23

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: ##EQU5## wherein C 1 , C 2 , C 3 and C 4 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 - , SO 4 = , Na + and Ca ++ are the weight percent of said ions in the silica expressed as sodium chloride, sodium sulfate, sodium oxide and calcium oxide.

Claim 24

The method of claim 19 wherein the amount of calcium present on the silica, calculated as calcium oxide, is from about 0.5 to 3 weight percent.