Process for preparing redispersible surface-modified silicon dioxide particles
Provided is a process for preparing surface-modified silicon dioxide particles with a mean particle diameter of at most 100 nm, involving (A) high-pressure grinding of a predispersion having (i) surface-modified silicon dioxide particles which are at least partly aggregated, are bonded to the surface-modifying component via Si—O—Si bonds, and still have reactive groups on their surface; (ii) an organosilicon compound which has a silicon-carbon bond and a functional group which can react with the reactive groups to form a covalent Si—O—Si bond; and (iii) a solvent, to form a dispersion; and (B) removing the liquid phase of the dispersion. Also provided are redispersible, surface-modified silicon dioxide particles obtained by this process, and their use in toner powders, silicone rubber, adhesives, and scratch-resistance surface coatings.
1. A process for preparing surface-modified silicon dioxide particles with a mean particle diameter of at most 100 nm, the process comprising:
(I) high-pressure grinding of a predispersion comprising
(A1) initial surface-modified silicon dioxide particles which (A1a) are at least partly aggregated, (A1b) are bonded to a surface-modifying component via Si—O—Si bonds, and (A1c) comprise reactive groups on their surface after the grinding,
(A2) an organosilicon compound which has (A2a) a silicon-carbon bond and (A2b) a functional group which react with the reactive groups to form a covalent Si—O—Si bond, and
(A3) an organic, volatile solvent,
to form a dispersion; and
(II) removing a liquid phase of the dispersion by spray drying, to obtain the surface-modified silicon dioxide particles with the mean particle diameter of at most 100 nm,
wherein a proportion of the organic, volatile solvent (A3) in the predispersion is from 60 to 98% by weight.
2. The process of claim 1 , wherein the initial surface-modified silicon dioxide particles (A1) are of pyrogenic origin.
3. The process of claim 1 , wherein a proportion of the initial surface-modified silicon dioxide particles (A1), based on the predispersion, is 1 to 50% by weight.
4. The process of claim 1 , wherein the organosilicon compound (A2) is a silylamine.
5. The process of claim 1 , wherein a proportion of organosilicon compound (A2), based on the surface-modified silicon dioxide particles (A1), is from 0.01 to 10% by weight.
6. The process of claim 1 , wherein the high-pressure grinding (I) is carried out in such a way that the predispersion is divided into at least two substreams, and the substreams are placed under pressure and decompressed through one nozzle each to a common point of collision.
7. The process of claim 6 , wherein jets of the substreams have a speed of at least 300 m·s −1 .
8. The process of claim 1 , wherein the solvent (A3) consists essentially of methanol, ethanol, acetone, methyl ethyl ketone, 1-propanol, i-propanol, n-butanol, hexane, heptane, isooctane, petroleum ether, dipropyl ether, diisopropyl ether, tetrahydrofuran, tetrahydropyran, methyl acetate, ethyl acetate, or a mixture thereof.
9. The process of claim 1 , wherein the surface-modified silicon dioxide particles have a BET surface area of 80 to 400 m 2 /g.
10. The process of claim 1 , wherein the surface-modified silicon dioxide particles have a BET surface area of 150 to 350 m 2 /g.
11. The process of claim 7 , wherein the jets of the substreams have a speed of at least 400 to 1000 m·s −1 .
12. The process of claim 6 , wherein the jets of the substreams have a speed of at least 600 to 900 m·s −1 .
13. The process of claim 1 , wherein a tamped density of the surface-modified silicon dioxide particles is at most 50% greater than a tamped density silica of the initial surface-modified silicon dioxide particles.
14. The process of claim 1 , wherein a proportion of the organic, volatile solvent (A3) in the predispersion is from 60 to 95% by weight.
15. The process of claim 1 , wherein the surface-modified silicon dioxide particles have a methanol wettability of 20 to 90.
16. The process of claim 1 , wherein the surface-modified silicon dioxide particles have a methanol wettability of 40 to 80.
17. The process of claim 7 , wherein the predispersion is placed under a pressure of 50 to 500 MPa and decompressed through a nozzle,
wherein the nozzle has a bore diameter or a slot width of 0.05 to 1 mm, and
wherein the nozzle has a bore with a length/diameter ratio, or a slot with a depth/slot width ratio, of 1 to 10.