Surface-modified silicas

1. A surface-modified, fumed silica, wherein:

the surface modified fumed silica is obtained by spraying a fumed silica with a solution of a potassium compound prior to or during a surface modification reaction; and wherein for the surface-modified fumed silica:

a BET surface area is 25 to 400 m 2 /g,

an average primary particle size is 5 to 50 nm,

a pH of a 4% aqueous dispersion is from 3 to 10,

a carbon content is 0.1 to 10% by weight, and

a potassium content calculated as potassium oxide, is 0.000001 to 40% by weight.

2. A process for preparing the surface-modified, fumed silica according to claim 1 , wherein the surface modification reaction comprises:

treating the fumed silica with at least one surface modifier selected from the group consisting of:

a) organosilanes of formula (RO) 3 Si(C n H 2n+1 ) and formula (RO) 3 Si (C n H 2n+1 ),

wherein

R is methyl-, ethyl-, n-propyl-, isopropyl- or butyl-, and

n is 1-20;

b) organosilanes of formula R′ x (RO) y Si(C n H 2n+1 ) and formula R′ x (RO) y Si(C n H 2n−1 ), wherein

R is methyl-, ethyl-, n-propyl-, isopropyl- or butyl-,

R′ is methyl-, ethyl-, n-propyl-, isopropyl-, butyl- or cycloalkyl-,

n is 1-20,

x+y is 3,

x is 1 or 2, and

y is 1 or 2;

c) haloorganosilanes of formula X 3 Si(C n H 2n+1 ) and formula X 3 Si(C n H 2n−1 ),

wherein

X is Cl or Br, and

n is 1-20;

d) haloorganosilanes of formula X 2 (R′)Si(C n H 2n+1 ) and formula X 2 (R′)Si(C n H 2n−1 ),

wherein

X is Cl or Br,

R′ is methyl-, ethyl-, n-propyl-, isopropyl-, butyl- or cycloalkyl-, and

n is 1-20;

e) haloorganosilanes of formula X(R′) 2 Si(C n H 2n+1 ) and formula X(R) 2 Si(C n H 2n−1 ),

wherein

X is Cl or Br,

R′ is methyl-, ethyl-, n-propyl-, isopropyl-, butyl- or cycloalkyl-, and

n is 1-20;

f) organosilanes of formula (RO) 3 Si(CH 2 ) m —R′,

wherein

R is methyl-, ethyl- or propyl-,

m is 0.1-20, and

R′ is methyl-, aryl-, —C 4 F 9 , —OCF 2 —CHF—CF 3 , —C 6 F 13 , —O—CF 2 —CHF 2 ,

—NH 2 , —N 3 , —SCN, —CH═CH 2 , —NH—CH 2 —CH 2 —NH 2 ,

—N—(CH 2 —CH 2 —NH 2 ) 2 ,

—OOC(CH 3 )C═CH 2 ,

—OCH 2 —CH(O)CH 2 ,

—NH—CO—N—CO—(CH 2 ) 5 ,

—NH—COO—CH 3 , —NH—COO—CH 2 —CH 3 , —NH—(CH 2 ) 3 Si(OR) 3 ,

—S x —(CH 2 ) 3 Si(OR) 3 ,

—SH or

—NR″R′″R″″ wherein R″ is alkyl or aryl; R′″ is H, alkyl or aryl; R″″ is H, alkyl, aryl, benzyl, C 2 H 4 NR 2 ′″″ wherein R″″″ is, independently, H or alkyl;

g) haloorganosilanes of formula X 3 Si(CH 2 ) m —R′,

wherein

X is Cl or Br,

m is 0.1-20, and

R′ is methyl-, aryl-, —C 4 F 9 , —OCF 2 —CHF—CF 3 , —C 6 F 13 , —O—CF 2 —CHF 2 ,

—NH 2 , —N 3 , —SCN, —CH═CH 2 , —NH—CH 2 —CH 2 —NH 2 ,

—N—(CH 2 —CH 2 —NH 2 ) 2 ,

—OOC(CH 3 )C═CH 2 ,

—OCH 2 —CH(O)CH 2 ,

—NH—CO—N—CO—(CH 2 ) 5 ,

—NH—COO—CH 3 , —NH—COO—CH 2 —CH 3 , or

—SH;

h) haloorganosilanes of formula (R)X 2 Si(CH 2 ) m —R′,

wherein

X is Cl or Br,

R is methyl-, ethyl- or propyl-,

m is 0.1-20, and

R′ is methyl-, aryl-, —C 4 F 9 , —OCF 2 —CHF—CF 3 , —C 6 F 13 , —O—CF 2 —CHF 2 ,

—NH 2 , —N 3 , —SCN, —CH═CH 2 , —NH—CH 2 —CH 2 —NH 2 ,

—N—(CH 2 —CH 2 —NH 2 ) 2 ,

—OOC(CH 3 )C═CH 2 ,

—OCH 2 —CH(O)CH 2 ,

—NH—CO—N—CO—(CH 2 ) 5 ,

—SH, or

—NH—COO—CH 3 , —NH—COO—CH 2 —CH 3 , NH—(CH 2 ) 3 Si(OR″) 3 , wherein R″ is methyl-, ethyl-, propyl- or butyl-;

i) haloorganosilanes of formula (R) 2 XSi(CH 2 ) m —R′,

wherein

X is Cl or Br,

R is alkyl,

m is 0.1-20, and

R′ is methyl-, aryl-, —C 4 F 9 , —OCF 2 —CHF—CF 3 , —C 6 F 13 , —O—CF 2 —CHF 2 ,

—NH 2 , —N 3 , —SCN, —CH═CH 2 , —NH—CH 2 —CH 2 —NH 2 ,

—N—(CH 2 —CH 2 —NH 2 ) 2 ,

—OOC(CH 3 )C═CH 2 ,

—OCH 2 —CH(O)CH 2 ,

—NH—CO—N—CO—(CH 2 ) 5 ,

—SH, or

—NH—COO—CH 3 , —NH—COO—CH 2 —CH 3 , NH—(CH 2 ) 3 Si(OR″) 3 , wherein R″ is methyl-, ethyl-, propyl- or butyl-;

j) cyclic polysiloxanes of formula

wherein x is 3, 4 or 5;

l) polysiloxanes or silicone oils of formula

wherein

n, m and u are 0 or an integer,

Y is H or a C 1-20 alkyl,

Y′ is Si(CH 3 ) 3 , Si(CH 3 ) 2 H, Si(CH 3 ) 2 OH, Si(CH 3 ) 2 (OCH 3 ), or Si(CH 3 ) 2 (C k H 2k+1 ), wherein k is 1 to 20, and

R, R′, R″ and R′″ independently is C k H 2k+1 , substituted or unsubstituted aryl, (CH 2 ) k —NH 2 , or H.

3. A silicone rubber composition, comprising the surface-modified, fumed silica according to claim 1 .

4. A LSR silicone rubber composition, comprising the surface-modified, fumed silica according to claim 1 .

5. The process according to claim 2 , wherein the at least one surface modifier is selected from the group consisting of octyltrimethoxysilane, octyltriethoxysilane, hexamethyldisilazane, 3-methacryloyloxypropyltrimethoxysilane, 3-methacryloyloxypropyltriethoxysilane, hexadecyltrimethoxysilane, hexadecyltriethoxysilane, dimethylpolysiloxane, glycidyloxypropyltrimethoxysilane, glycidyloxypropyltriethoxysilane, nonafluorohexyltrimethoxysilane, tridecafluorooctyltrimethoxysilane, tridecafluorooctyltriethoxysilane, aminopropyltriethoxysilane and hexamethyldisilazane.