Silicon dioxide powder having special surface properties and toner composition containing said powder

1. A silica powder, in a form of aggregated primary particles having a mean diameter of at least 40 nm and a ratio of a BET surface area to a STSA surface area of at least 3.5.

2. The silica powder of claim 1 , wherein

the BET surface area is 100 to 400 m 2 /g.

3. The silica powder of claim 1 , having

a thickening action based on the STSA surface area of more than 8 mPas·g/m 2 .

4. The silica powder of claim 1 , having

a micropore volume of at least 0.07 cm 3 /g.

5. The silica powder of claim 1 , having a

fractal dimension Dm of at least 2.80.

6. The silica powder of claim 1 , wherein a

DBP absorption based on the STSA surface area is 3.5 g/100 g m 2 g −1 or higher.

7. A process for preparing the silica powder of claim 1 , the process comprising

a) supplying a mixture comprising one or more hydrolysable and/or oxidizable silicon compounds, an oxygen-comprising gas and a primary combustion gas which reacts with the oxygen-comprising gas to form water, to a flame burning from a burner in a flame tube,

b) separately introducing one or more secondary combustion gases into the flame tube from a tube surrounding the burner, and

c) at a later time, supplying one or more tertiary combustion gases to a reaction mixture within the flame tube, and

d) then removing at least one solid from the reaction mixture, where

gamma 1 <1, gamma 2 <1 and gamma 3 <1 and

gamma 1/2 =gamma 1 +gamma 2 <1

gamma total =gamma 1 +gamma 2 +gamma 3 >1 and

lambda 1 >1 where

gamma 1 =primary combustion gas/stoichiometrically required combustion gas

gamma 2 =secondary combustion gas/stoichiometrically required combustion gas

gamma 3 =tertiary combustion gas/stoichiometrically required combustion gas and

lambda 1 =oxygen in the oxygen-containing gas/stoichiometrically required oxygen.

8. The process of claim 7 , wherein

gamma 1 >gamma 2 .

9. The process of claim 7 , wherein

(i) gamma 1 <gamma 3 , (ii) gamma 2 <gamma 3 , or both (i) and (ii).

10. The process of claim 7 , wherein

0.2≦gamma 1 ≦0.6 and 0.4≦gamma 3 ≦0.8.

11. The process of claim 7 , wherein the

secondary combustion gas, the tertiary combustion gas, or both, are supplied by a ring nozzle.

12. The process of claim 7 , wherein

1<lambda 1 ≦10.

13. The process of claim 7 , further comprising introducing

a further oxygen-comprising gas into the flame tube.

14. A process for preparing a surface-modified silica powder in a form of aggregated primary particles, a surface of which or parts thereof is covered by chemically bound linear and/or branched alkylsilyl groups having 1 to 20 carbon atoms, the process comprising spraying the silica powder of claim 1 with one or more surface-modifying agents which are optionally dissolved in an organic solvent and have linear and/or branched alkylsilyl groups having 1 to 20 carbon atoms, and subsequently thermally treating the sprayed silica powder at a temperature of 120 to 400° C. over a period of 0.5 to 8 hours, optionally under a protective gas.

15. The process of claim 14 , wherein

the surface-modifying agent is at least one selected from the group consisting of hexamethyldisilazane, dimethyldimethoxysilane, dimethyldiethoxysilane, trimethylmethoxysilane, methyltrimethoxysilane, butyltrimethoxysilane, dimethyldichlorosilane, trimethylchlorosilane and a silicone oil.

16. A surface-modified silica powder in a form of aggregated primary particles, a surface of which or parts thereof is covered by chemically bound linear and/or branched alkylsilyl groups having 1 to 20 carbon atoms, obtained by the process of claim 14 .

17. A toner composition comprising the silica powder of claim 1 .

18. The silica powder of claim 1 , wherein the aggregated primary particles have a mean diameter of 40 to 70 nm.

19. The silica powder of claim 1 , wherein the ratio of the BET surface area to the STSA surface area is in a range of 3.5 to 7.

20. The silica powder of claim 1 , having a micropore volume of 0.07 to 0.09 cm 3 /g.