Hydrophobic silica and its use in silicone rubber

1. A low structure surface-modified pyrogenically prepared silicon dioxide doped with potassium which has been prepared pyrogenically by flame hydrolysis and which has been doped with from 0.000001 to 40 wt. % potassium and is further characterized as having a hydrophobic surface and a DBP absorption of the pyrogenic silica has an endpoint which is not detectable or is less than 85% of the value of a pyrogenic silica educt having a comparable surface area and, wherein the potassium content of the potassium doped silicon dioxide before surface modification is from 0.15 to 0.32 wt. % in the form of K 2 O.

2. A low structure surface-modified pyrogenically prepared silicon dioxide doped with potassium according to claim 1 , wherein surface modification includes contact with one or more compounds from the following groups:

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

R=alkyl

n=1-20

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

R=alkyl

R′=alkyl or

cycloalkyl

n=1-20

x+y=3

x=1,2

y=1,2

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

X=Cl or Br

n=1-20

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

X=Cl or Br

R′=alkyl

R′=cycloalkyl

n=1-20

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

X=Cl or Br

R′=alkyl

R′=cycloalkyl

n=1-20

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

R=alkyl

m=0,1-20

R′=methyl, aryl, substituted 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 , and —NH—(CH 2 ) 3 Si(OR) 3 ,

—S x —(CH 2 ) 3 Si(OR) 3 , wherein X=from 1 to 10 and

R=alkyl,

—SH,

—NR′R″R′″ (R′=alkyl, aryl, substituted aryl; R″=H, alkyl, aryl; R′″=H, alkyl, aryl, benzyl, C 2 H 4 NR″″R′″″ where R″″=H, alkyl and R′″″=H, alkyl)

g) organosilanes of the formula (R″) x (RO) y Si(CH 2 ) m —R′,

R

″

=

alkyl

x

+

y

=

2

=

cycloalkyl

x

=

1

,

2

y

=

1

,

2

m

=

from

⁢

⁢

0

,

1

⁢

⁢

to

⁢

⁢

20

R′=methyl, aryl, substituted 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—

—S x —(CH 2 ) 3 Si(OR) 3 , wherein X=from 1 to 10 and

R=alkyl,

—SH,

—NR′R″R′″ (R′=alkyl, aryl; R″=H, alkyl, aryl; R′″=H, alkyl, aryl, benzyl, C 2 H 4 NR″″R′″″ where R″″=H, alkyl and R′″″=H, alkyl)

h) haloorganosilanes of the formula X 3 Si(CH 2 ) m —R′

X=Cl or Br

m=0,1-20

R′=methyl, aryl substituted 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 , wherein X=from 1 to 10 and

R=alkyl,

—SH

i) haloorganosilanes of the formula (R)X 2 Si(CH 2 ) m —R′

X=C or Br

R=alkyl

m=0,1-20

R′=methyl, substituted 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 , wherein X=from 1 to 10 and

R=alkyl,

—SH

j) haloorganosilanes of the formula (R) 2 X Si(CH 2 ) m —R′

X=Cl, Br

R=alkyl

m=0,1-20

R′=methyl, aryl substituted 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 , wherein X=from 1 to 10 and

R=alkyl,

—SH

k) silazanes of the formula

R=alkyl

R′=alkyl, vinyl

l) cyclic polysiloxanes of the formula D 3, D 4, D 5, having 3, 4 or 5 units of the structure (—O)—(-) Si(CH 3 ) 2 —, respectively; and

m) polysiloxanes or silicone oils of the formula

m=0,1,2,3, . . . ∞

n=0,1,2,3, . . . ∞

u=0,1,2,3, . . . ∞

Y═CH 3 , H, C n H 2n+1 n=1-20

Y═Si(CH 3 ) 3 , Si(CH 3 ) 2 H

Si(CH 3 ) 2 OH, Si(CH 3 ) 2 (OCH 3 )

Si(CH 3 ) 2 (C n H 2n+1 ) n=1-20

R=alkyl, aryl, (CH 2 ) n —NH 2 , H

R′=alkyl, aryl, (CH 2 ) n —NH 2 , H

R′=alkyl, aryl, (CH 2 ) n —NH 2 , H

R′=alkyl, aryl, (CH 2 ) n —NH 2 , H.

3. The low structure surface-modified pyrogenically prepared silicon dioxide doped with potassium according to claim 2 wherein R′ in formula (f), (g), (h), (i) and (j) is —C 6 H 5 or substituted phenyl, and

R is methyl, ethyl, propyl or butyl.

4. Process for the preparation of the low structure surface-modified doped pyrogenically prepared silicon dioxide according to claim 1 , comprising doping pyrogenically prepared silicon dioxide with potassium by means of an aerosol by placing the silicon dioxide in a suitable mixing vessel, then spraying the silicon dioxide, with intensive mixing, optionally first with water and/or dilute acid and then with a surface-modifying reagent or with a mixture of a plurality of surface-modifying reagents, and optionally continuing mixing for a further 15 to 30 minutes, followed by tempering at a temperature of from 100 to 400° C. for a period of from 1 to 6 hours.

5. Process for the preparation of the low structure surface-modified doped pyrogenically prepared silicon dioxide doped with potassium by means of aerosol according to claim 1 , comprising, with the exclusion of oxygen, mixing the silicon dioxide as homogeneously as possible with the surface-modifying reagent to produce a mixture, then heating the mixture together with an inert gas, to temperatures of from 200 to 800° C., in a continuous uniflow process in a treatment chamber which is in the form of a vertical tubular furnace, separating solid and gaseous reaction products from one another, and then optionally deacidifying and drying the solid products.

6. The process according to claim 5 wherein the temperature is 400° to 600° C.

7. A method of preparing a silicon rubber comprising incorporating therein the low structure surface-modified doped pyrogenically prepared silicon dioxide according to claim 1 .

8. Silicone rubber containing as filler the low structure surface-modified doped pyrogenically prepared silica according to claim 1 .

9. Silicone rubber according to claim 8 , wherein the low structure surface-modified doped pyrogenically prepared silica has BET surface area from 10 to 1000 m 2 /g.

10. Silicone rubber according to claim 8 , which is a LSR silicone rubber.

11. A low structure surface-modified pyrogenically prepared silicon dioxide doped with potassium according to claim 1 , wherein surface modification compound is hexamethyldisilazane.

12. Process for the preparation of the low structure surface-modified doped pyrogenically prepared silicon dioxide according to claim 11 , comprising doping pyrogenically prepared silicon dioxide with potassium by means of an aerosol by placing the silicon dioxide in a suitable mixing vessel, then spraying the silicon dioxide, with intensive mixing, optionally first with water and/or dilute acid and then with hexamethyldisilazane or a mixture thereof, and optionally continuing mixing for a further 15 to 30 minutes, followed by tempering at a temperature of from 100 to 400° C. for a period of from 1 to 6 hours.