Pyrogenic silicon dioxide and a dispersion thereof
View Patent ↗Pyrogenic silicon dioxide powder with a BET surface area of 30 to 90 m 2 /g, a DBP index of 80 or less, a mean aggregate area of less than 25000 nm 2 and a mean aggregate circumference of less than 1000 nm, wherein at least 70% of the aggregates have a circumference of less than 1300 nm. It is prepared by mixing at least one silicon compound in vapor form, a free-oxygen-containing gas and a combustible gas in a burner of known construction, igniting this gas mixture at the mouth of the burner and burning it in the flame tube of the burner, separating the solid obtained from the gas mixture and optionally purifying, wherein the oxygen content of the free-oxygen-containing gas is adjusted so that the lambda value is greater than or equal to 1, the gamma value is between 1.2 and 1.8, the throughput is between 0.1 and 0.3 kg SiO 2 /m 3 of core gas mixture and the mean normalized rate of flow of gas in the flame tube at the level of the mouth of the burner is at least 5 m/s. The powder can be used as a filler. A dispersion containing the powder according to the invention. The powder can be used as a filler in rubber, silicone rubber and plastics. The dispersion can be used to prepare glass items.
1. A pyrogenic silicon dioxide powder with
a BET surface area of 30 to 90 m 2 /g,
a DBP index of 80 or less
a mean aggregate area of less than 25000 nm 2 ,
a mean aggregate circumference of less than 1000 nm,
wherein at least 70% of the aggregates have a circumference of less than 1300 nm.
2. The pyrogenic silicon dioxide powder according to claim 1 , wherein the BET surface area is between 35 and 75 m 2 /g.
3. The pyrogenic silicon dioxide powder according to claim 1 , wherein the DBP index is between 60 and 80.
4. The pyrogenic silicon dioxide powder according to claim 1 , wherein the BET surface area is between 40 and 60 m 2 /g and the DBP index is 60 to 80.
5. The pyrogenic silicon dioxide powder according to claim 1 , wherein the pyrogenic silicon dioxide powder has a mean aggregate area of at most 20000 nm 2 .
6. The pyrogenic silicon dioxide powder according to claim 1 , wherein the BET surface area is 40 to 60 m 2 /g, the DBP index is 60 to 80 and the mean aggregate area is between 15000 and 20000 nm 2 .
7. The pyrogenic silicon dioxide powder according to claim 1 , wherein the pyrogenic silicon dioxide powder has a mean aggregate circumference of less than 1000 nm.
8. The pyrogenic silicon dioxide powder according to claim 1 , wherein the BET surface area is 40 to 60 m 2 /g, the DBP index is 60 to 80, the mean aggregate area is 1500 to 20000 nm 2 and the mean aggregate circumference is 600 to 1000 nm.
9. The pyrogenic silicon dioxide powder according to claim 1 , wherein the degree of filling of the powder in an aqueous dispersion is up to 90 wt. %.
10. The pyrogenic silicon dioxide powder according to claim 1 , wherein the pyrogenic silicon dioxide powder has a viscosity of less than 100 mPas, with respect to a 30 wt. % aqueous dispersion, at a rate of shear of 5 rpm.
11. The pyrogenic silicon dioxide powder according to claim 1 , wherein the pyrogenic silicon dioxide powder has a pH, measured in a 4% strength aqueous dispersion, of between 3.8 and 5.
12. A process for preparing the pyrogenic silicon dioxide powder according to claim 1 , wherein at least one silicon compound in vapour form, a free-oxygen-containing gas and a combustible gas are mixed in a burner of known construction, this gas mixture is ignited at the mouth of the burner and burnt in the flame tube of the burner, the solid obtained is separated from the gas mixture and optionally purified, wherein
the oxygen content of the free-oxygen-containing gas is adjusted so that the lambda value is greater than or equal to 1,
the gamma value is between 1.2 and 1.8,
the throughput is between 0.1 and 0.3 kg SiO 2 /m 3 of core gas mixture,
the mean, normalised rate of flow of gas in the flame tube at the level of the mouth of the burner is at least 5 m/s.
13. The process according to claim 12 , wherein the oxygen content of the free-oxygen-containing gas is not more than 40 vol. %.
14. The process according to claim 12 , wherein, 1<lambda≦1.2.
15. The process according to claim 12 , wherein 1.6<gamma≦1.8.
16. The process according to claim 12 , wherein the mean normalised rate of flow of gas in the flame tube at the level of the mouth of the burner is more than 8 m/s.
17. The process according to claim 12 , wherein the mean rate of discharge of the gas mixture at the mouth of the burner is at least 30 m/s.
18. The process according to claim 12 , wherein additional air (secondary air) is introduced into the flame tube.
19. The process according to claim 12 , wherein silicon tetrachloride and/or at least one organosilicon compound is used as a silicon compound.
20. The process according to claim 12 , wherein
silicon tetrachloride is used,
1<lambda≦1.2,
1.6<gamma<1.8,
the throughput is between 0.1 and 0.3 kg SiO 2 /m 3 of core gas mixture,
in addition at least double the amount of air, with respect to the amount of free-oxygen-containing gas introduced into the burner, is introduced into the flame tube and
the rate of flow of feedstocks at the mouth of the burner is 40 to 65 m/s
and the mean normalised rate of flow of gas in the flame tube at the level of the mouth of the burner is between 8 and 12 m/s.
21. An aqueous dispersion comprising the pyrogenic silicon dioxide powder as claimed in claim 1 .
22. The aqueous dispersion according to claim 21 , wherein the concentration of the pyrogenic silicon dioxide powder is between 20 and 80 wt. %.
23. The aqueous dispersion according to claim 21 , wherein the viscosity of a 50 wt. % dispersion is less than 2500 mPas at a rate of shear of 50 rpm.
24. The aqueous dispersion according to claim 21 , wherein the mean particle size of the silicon dioxide powder is less than 200 nm.
25. The aqueous dispersion according to claim 21 , wherein the dispersion is stabilised by adding bases or cationic polymers or aluminum salts or a mixture of cationic polymers and aluminum salts or acids.
26. The aqueous dispersion according to claim 21 , wherein the aqueous dispersion contains additives.
27. A process for preparing the aqueous dispersion in accordance with claim 21 , wherein the pyrogenic silicon dioxide powder is incorporated, using a dispersion device, into water which can be stabilised by adding bases or cationic polymers or aluminum salts or a mixture of cationic polymers and aluminum salts or acids and is then further dispersed for a period of 5 to 30 minutes.
28. The process according to claim 27 , wherein a rotor-stator system is used as a dispersing system.
29. The aqueous dispersion according to claim 21 wherein the concentration of the pyrogenic silicon dioxide powder is between 40 and 60 wt %.
30. A rubber article, a silicon rubber article or a plastic article comprising the pyrogenic silicon dioxide powder as claimed in claim 1 .
31. A method for producing a rubber article, a silicon rubber article or a plastic article comprising adding the pyrogenic silicon dioxide powder as claimed in claim 1 to a rubber formulation, a silicon rubber formulation or a plastic formulation.
32. A method for adjusting the rheology of a dye or a lacquer comprising adding the pyrogenic silicon dioxide powder as claimed in claim 1 to a dye formulation or a lacquer formulation.
33. A dye or a lacquer comprising the pyrogenic silicon dioxide powder as claimed in claim 1 .
34. A catalyst support comprising the pyrogenic silicon dioxide powder as claimed in claim 1 .
35. A method for preparing a catalyst comprising supporting a catalytically active material onto the pyrogenic silicon dioxide powder as claimed in claim 1 .
36. A glass article comprising the pyrogenic silicon dioxide powder as claimed in claim 1 .
37. A method for preparing a glass article comprising adding the pyrogenic silicon dioxide powder as claimed in claim 1 to a glass formulation.
38. A chemical-mechanical polish comprising the pyrogenic silicon dioxide powder as claimed in claim 1 .
39. A method for polishing comprising utilizing the chemical-mechanical polish as claimed in claim 38 as a polish.
40. A coating for ink-jet paper comprising the pyrogenic silicon dioxide powder as claimed in claim 1 .
41. A method for preparing ink-jet paper comprising coating paper with the coating for ink-jet paper as claimed in claim 40 .