Silica and also epoxy resins

1. A process for preparing a fumed hydrophilic silicon dioxide powder comprising aggregated primary particles having a BET surface area of 175±15 m 2 /g, wherein a thickening effect of the powder, based on the BET surface area, is greater than 18 mPas·g/m 2 , the process comprising:

vaporizing a silicon compound;

transferring the vaporized silicon compound, optionally with a carrier gas, into a mixing chamber,

separately, transferring a fuel gas and primary air into the mixing chamber, and mixing with the silicon compound vapour;

igniting the mixture in a burner wherein a flame is burnt into a reaction chamber;

introducing secondary air surrounding the flame into the reaction chamber;

separating solid silicon dioxide from gaseous substances; and

treating the separated solid with a steam at 250° C. to 750° C.,

wherein

the primary air is optionally preheated or enriched with oxygen,

a ratio of secondary air/primary air is from 0.05 to 3,

a total amount of oxygen is at least sufficient for the complete combustion of the fuel gas and the silicon compound, and

an adiabatic flame temperature T ad is from 1840 to 1900° C., wherein T ad =temperature of feedstocks+sum of the reaction enthalpies of the sub-reactions/heat capacity of the substances which depart from the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, carbon dioxide, oxygen, nitrogen and, a carrier gas, if the latter is not air or nitrogen, on the basis of the specific heat capacity of these substances at 1000° C.

2. The process as claimed in claim 1 , wherein the silicon compound is SiCl 4 , and the fuel gas is H 2 .

3. The process as claimed in claim 2 , wherein for 100 kg/h of SiCl 4 transferred to the mixing chamber, 53 Nm 3 /h of H 2 , 155 Nm 3 of primary air and 55 Nm 3 of secondary air are employed.

4. The process as claimed in claim 2 , wherein the secondary air/primary air ratio is 0.35.

5. The process as claimed in claim 1 , wherein the thickening effect relative to the surface area of the powder is 20.08 mPas·g/m 2 .

6. A hydrophobic fumed silica, wherein

a BET surface area is 110±25 m 2 /g,

an average aggregate surface area is 53 257±500 nm 2 ,

an average aggregate circumference is 3605±500 nm,

an average maximum aggregate diameter is 394±5 nm,

an average minimum aggregate diameter is 242±5 nm, and

an average primary particle diameter is 18±5 nm,

wherein

the hydrophobic fumed silica is obtained by a process comprising:

spraying the hydrophilic fumed silica obtained by the process according to claim 1 with polydimethylsiloxane, followed by mixing and then heat treatment for 1 to 6 hours.

7. The hydrophobic fumed silica according to claim 6 , wherein the heat treatment is conducted at 360° C. for 3 hours under a nitrogen atmosphere.

8. An adhesive composition which comprises the hydrophobic fumed silica according to claim 6 and an epoxy resin.

9. An epoxy resin composition comprising the hydrophobic fumed silica according to claim 6 .

10. A method to adhere two surfaces comprising applying the epoxy resin composition according to claim 9 to at least one of the two surfaces.