Stable aqueous dispersions of precipitated silica
The invention relates to stabilized dispersions of precipitated silicon dioxide, to a process for production thereof and to the use thereof, especially for production of paper coatings for glossy photo paper for inkjet printing.
1. A process for producing a dispersion comprising precipitated silica particles, comprising the steps of:
a) producing a preliminary dispersion of at least one precipitated silica, wherein the preliminary dispersion has a content of inorganic salts of 0.00015 to 0.6% by weight based on the total weight of the preliminary dispersion, and
b) supplying the preliminary dispersion to a grinding device with subsequent grinding,
and wherein, in the course of the process, at least part of a surface of the precipitated silica particles, a surface of precipitated silica particles newly generated during the grinding, or a combination thereof are modified with at least one aminosilane in such a way that the aminosilane is attached covalently to the precipitated silica particles.
2. The process according to claim 1 , wherein the inorganic salt comprises a polyvalent anion.
3. The process according to claim 1 , wherein the preliminary dispersion is produced using at least one dried precipitated silica, at least one filtercake of a precipitated silica, or a combination thereof.
4. The process according to claim 1 , wherein the precipitated silica particles present in the preliminary dispersion have a mean particle diameter d 50 of greater than 100 nm.
5. The process according to claim 1 , wherein the precipitated silica in the preliminary dispersion has a solids content based on the total weight of the preliminary dispersion of 5 to 40% by weight.
6. The process according to claim 1 , wherein a solution of at least one aminosilane is added at a time selected from any one or a combination of before, during, and after the grinding step b).
7. The process according to claim 6 , wherein one or more of the preliminary dispersion, the dispersion comprising precipitated silica particles, and the solution of at least one aminosilane has a pH that is adjusted to a value less than 7 at a time selected from any one or a combination of before, during and after the solution of at least one aminosilane is added to the preliminary dispersion or the dispersion comprising precipitated silica particles.
8. The process according to claim 1 , wherein the aminosilane has the general formula R a SiX (4-a) in which the X radicals are the same or different and are each hydrolysable groups or hydroxyl groups, the R radicals are the same or different and are each unhydrolysable groups, where at least one R radical comprises at least one amino group, and a has a value of 1, 2 or 3.
9. The process according to claim 1 , wherein the preliminary dispersion has a content of inorganic salts of 0.0024 to 0.03% by weight based on the total weight of the preliminary dispersion.
10. The process according to claim 2 , wherein the polyvalent anion is selected from a divalent anion and a trivalent anion.
11. The process according to claim 4 , wherein the mean particle diameter d 50 of the precipitated silica particles present in the preliminary dispersion is 1 to 500 μm.
12. The process according to claim 5 , wherein the solids content of the precipitated silica in the preliminary dispersion, based on the total weight of the preliminary dispersion, is 25 to 35% by weight.
13. The process according to claim 6 , wherein the solution of at least one aminosilane is added at a time selected from any one or a combination of: during step b), after step b), to the preliminary dispersion before step b), before the production of the preliminary dispersion, and during the production of the preliminary dispersion.
14. The process according to claim 7 , wherein the pH of one or more of the preliminary dispersion, the dispersion comprising precipitated silica particles, and the solution of at least one aminosilane is adjusted to a value between 2 and 6, at a time selected from any one or a combination of before, during and after the aminosilane solution is added to the preliminary dispersion or the dispersion comprising precipitated silica particles.
15. A dispersion of precipitated silica particles, wherein
a) at least one aminosilane is bonded covalently to at least some sites on a surface of the precipitated silica particles,
b) the precipitated silica particles have a mean diameter d 50 of between 50 and 500 nm, a d 90 of a particle size distribution curve of between 150 and 800 nm, or a combination thereof,
c) the dispersion has a solids content of greater than or equal to 20% by weight,
d) the dispersion has a pH of less than or equal to 5, and
e) the dispersion has a content of inorganic salts of 0.00015 to 0.6% by weight based on the total weight of the dispersion.
16. The dispersion according to claim 15 , wherein the mean diameter d 50 of the precipitated silica particles is between 110 to 160 nm, the d 90 of the particle size distribution curve of the precipitated silica particles is between 150 to 200 nm, or a combination thereof.
17. A method of producing a paper coating, comprising incorporating the precipitated silica dispersions according to claim 15 in to the paper coating.
18. The method according to claim 17 , wherein the paper coating is a topcoat for glossy inkjet paper.
19. A method of producing an anticorrosive primer, comprising incorporating the dispersion of precipitated silica particles according to claim 15 in to the anticorrosive primer.