SiO2 containing dispersion with high salt stability

1. A process for preparing an aqueous dispersion comprising particles of a surface-modified, hydrophilic silica, wherein:

a) the particles of the surface-modified, hydrophilic silica comprise an aluminium atom and a hydrocarbon radical, wherein:

i) the aluminium atom is bound to a silicon atom of the particle surface via an oxygen atom;

ii) the hydrocarbon radical comprises a silicon atom which is bound to a carbon atom of the hydrocarbon radical; and

ii) the average particle diameter d 50 in the dispersion is 40-200 nm; and

b) the pH of the aqueous dispersion is 8 or higher,

and wherein the process comprises the steps:

aa) adding silica particles comprising hydroxyl groups on their surface to an aqueous solution of an alkali metal aluminate and allowing the silica particles to react;

bb) subsequently adding an agent for surface modification in which a silicon atom is bound to a hydrocarbon radical by a carbon atom and the silicon atom is also bound to one or more hydroxyl groups, alkoxy groups, halide groups or mixtures thereof; and

cc) allowing the mixture to react, during which pH may optionally be adjusted and the hydrolysis product may optionally be removed.

2. The process of claim 1 , wherein, in step aa), the silica particles are introduced in the form of an aqueous dispersion.

3. The process of claim 1 , wherein a mixture formed in step bb) is reacted at a pH of 11 or higher, and the mixture is treated thermally at a temperature of 50-95° C. over a period of 1-30 minutes.

4. The process of claim 1 , wherein, in step aa), a mixed Si—Al oxide is used in which an Al atom is part of the particle surface and the weight ratio of Al 2 O 3 /SiO 2 is 0.1:99.9-5:95.

5. The process of claim 1 , wherein, in step aa), a potassium-doped silica having a potassium content, calculated as K 2 O, of 0.005-5 wt % and having a BET surface area of 100 to 350 m 2 /g is used.

6. The process of claim 1 , wherein the agent for surface modification in step bb) has the formula: X 4-a Si—[(CH 2 ) n —Y m —R] a , wherein:

a=1, 2 or 3; n=1, 2 or 3; and m=0 or 1;

X═H, OH, OCH 3 , OC 2 H 5 , OCH 2 CH 2 H 3 , OCH(CH 3 ) 2 , Cl;

Y═—(OCR 1 R 2 —CR 3 R 4 ) o — wherein o=1-30 and R 1 , R 2 , R 3 , R 4 =independently of one another H or CH 3 ; —(OCR 1 R 2 —CR 3 R 4 —CR 5 R 6 ) p —, wherein p=1-30, R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 =independently of one another H or CH 3 ; and

R is a radical which does not impart hydrophobic properties, or is a mixture of the aforementioned radicals R and Y.

7. The process of claim 6 , wherein,

if m=1,

R is —H, —CH 3 , —C 2 H 5 , —OH, —OCH 3 , —OC 2 H 5 , —C(═O)OCH 3 , —C(═O)OC 2 H 5 , —O—C(═O)CH 3 , —O—C(═O)CH 3 , —O—C(═O)CH═CH 2 , —O—C(═O)CH═CH(CH 3 ), —C(═O)CH 3 , —C(═O)H, NH 2 , or

and, if m=0, the aforementioned radicals R are without —H, —CH 3 , —C 2 H 5 .

8. The process of claim 1 , wherein the agent for surface modification in step bb) is selected from the group consisting of: (CH 3 O) 3 Si(CH 2 ) 3 —OCH 3 ;

(CH 3 O) 3 Si(CH 2 ) 3 —(OCH 2 CH 2 ) 3 —OCH 3 ; (CH 3 O) 3 Si(CH 2 ) 3 —(OCH 2 CH 2 ) 6-9 —OCH 3 ; (CH 3 O) 3 Si(CH 2 ) 3 —(OCH 2 CH 2 ) 9-12 —OCH 3 ; (CH 3 O) 3 Si(CH 2 ) 3 —(OCH 2 CH 2 ) 21-24 —OCH 3 ; and (CH 3 CH 2 O) 3 Si(CH 2 ) 3 —(OCH 2 CH 2 ) 8-120 H.

9. The process of claim 1 , wherein the agent for surface modification in step bb) is selected from the group consisting of: (RO) 3 Si—(CH 2 ) 3 —NH 2 ; (RO) 3 Si—(CH 2 ) 3 —CH—CH 2 —NH 2 ; (RO) 3 Si—(CH 2 ) 3 —NH—(CH 2 ) 2 —NH 2 ; (RO) 3 Si—(CH 2 ) 3 —NH—(CH 2 ) 2 NH(CH 2 )—NH 2 ; (RO) 3 Si—(CH 2 ) 3 —N—[(CH 2 ) 2 NH(CH 2 )—NH 2 ] 2 ; and R═CH 3 , C 2 H 5 .

10. The process of claim 1 , wherein the agent for surface modification in step bb) is an aqueous composition which carries organopolysiloxanes having glycidyl ether alkyl radicals, acryloyloxyalkyl radicals and/or methacryloyloxyalkyl radicals, with each silicon in the organopolysiloxane carrying a functional group.

11. The process of claim 10 , wherein, in step bb), an organopolysiloxane is used which was obtained by mixing water-soluble organosilanes of the formula I:

H 2 N(CH 2 ) f (NH) g (CH 2 ) i —Si(CH 3 ) h (OR) 3-h   (I),

wherein if 0≤f≤6, g=0; if f=0, g=1; if f>1, 0≤i≤6 and 0≤h≤1; and

R is a methyl, ethyl, propyl or isopropyl group;

with:

i) water-soluble organosilanes of the formula II:

X—CH 2 O(CH 2 ) 3 —Si(CH 3 ) h (OR) 3-h   (II),

where 0≤h≤1 and R is a methyl, ethyl, propyl or isopropyl radical;

and X=

ii) and/or organosilanes of the formula III:

H 2 ═CR′—COO(CH 2 ) 3 —Si(CH 3 ) h (OR) 3-h   (III),

where 0≤h≤1, R is a methyl, ethyl, propyl or isopropyl radical and R′ is a methyl or hydrogen radical;

iii) and non-water-soluble organosilanes of the formula IV:

R″—Si(CH 3 ) h (OR) 3-h   (IV),

where 0≤h≤1, R is a methyl, ethyl, propyl or isopropyl radical and R″ is a linear, branched or cyclic hydrocarbon radical having 1 to 8 C atoms;

in a molar ratio M=a/(b+c+d), where a is the sum of the number of moles of the organosilanes of formula I, b is the sum of the number of moles of the organosilanes of formula II, and c is the sum of the number of moles of the organosilanes of formula III, and d is the sum of the number of moles of the organosilanes of formula IV, where 0≤M≤3 and at least b>0 or c>0.