Precipitated silicas having special surface properties

1. A precipitated silica having an SiOH isolated absorbance ratio of greater than or equal to 1.

2. The precipitated silica according to claim 1 , wherein a silanol group density is 0.5 to 3.5 SiOH/nm 2 .

3. The precipitated silica according to claim 1 , wherein a modified tapped density is less than or equal to 70 g/l.

4. The precipitated silica according to claim 1 , having the following properties:

a BET surface area of 50-600 m 2 /g,

a CTAB surface area of 50-350 m 2 /g, and

a DBP (anhydrous) of 150-400 g/100 g.

5. The precipitated silica according to claim 1 , wherein 5% to 100% of particles in a volume-based particle distribution curve are <1 μm.

6. The precipitated silica according to claim 1 , having a d90 value of not greater than 0.001-10 μm.

7. The precipitated silica according to claim 1 , having a bimodal volume-based particle distribution curve.

8. The precipitated silica according to claim 1 , having a loss on ignition of 0.1%-3.0% by weight.

9. The precipitated silica according to claim 1 , having a loss on drying of 0.1%-3.0% by weight.

10. The precipitated silica according to claim 1 , having a pH of 4-8.

11. The precipitated silica according to claim 1 , which is a hydrophilic precipitated silica.

12. A process for preparing a silica according to claim 1 , comprising:

A) reacting at least one silicate with at least one acidifier, to obtain a silica,

B) filtering and washing the silica, to obtain a washed silica or filter cake,

C) drying the washed silica or filter cake, to obtain a dried silica, and

D) heat-treating the dried silica.

13. The process according to claim 12 , wherein step A comprises the following substeps:

Aa preparing an initial charge of i) water or of ii) water and at least one silicate and/or a silicate solution,

Ab metering i) at least one silicate and/or a silicate solution and ii) at least one acidifier into the initial charge from substep Aa) with stirring at 80 to 100° C. until the solids content of an obtained precipitation suspension reaches a level which leads to the solids content which is to be reached in substep Ac).

Ac adding an acidifier at a temperature of the precipitation suspension of 80 to 100° C., so that the pH of the precipitation suspension is lowered to 2 to 6 and the solids content of the precipitation suspension at the end of this substep is between 30 and 70 g/l.

14. The process according to claim 13 , wherein in substep Aa) the pH of the initial charge is between 5 and 10 and the temperature of the initial charge is between 80 and 100° C.

15. The process according to claim 13 , wherein for the period of substep Ab) the pH is held constantly at a level between 7 and 10.

16. The process according to claim 12 , wherein the silica is ground in a step Ca, i.e. between steps C and D, or in a step E, i.e. after step D, or both in step Ca, i.e. between steps C and D, and in step E, i.e. after step D.

17. The process according to claim 16 , wherein the grinding parameters are selected such that the ground product in the range <1 μm of the volume-based particle distribution has a fine-particle fraction of 5% to 100% and/or a d90 value in the volume-based particle distribution curve of between 0.001 and 10 μm.

18. The process according to claim 16 , wherein grinding is carried out using a jet mill, preferably a fluid-bed opposed-jet mill.

19. The process according to claim 18 , wherein the fluid-bed opposed-jet mill is operated with steam as operational medium.

20. The process according to claim 16 , wherein grinding is carried out by using a grinding apparatus;

wherein in the grinding phase the grinding apparatus is operated with an operational medium selected from the group consisting of gas, vapor, and mixtures thereof; and

wherein in a heating phase, i.e. before the actual operation with the operational medium, a grinding chamber of said grinding apparatus is heated such that the temperature in the grinding chamber and/or at a grinding apparatus outlet is higher than the dew point of the operational medium.

21. The process according to claim 16 , wherein grinding is carried out by using a grinding system comprising a jet mill;

wherein in the grinding phase the mill is operated with an operational medium selected from the group consisting of gas, vapor, and mixtures thereof; and

wherein in a heating phase, i.e. before the actual operation with the operational medium, a grinding chamber is heated such that the temperature in the grinding chamber and/or at a mill outlet is higher than the dew point of the operational medium.

22. The process according to claim 20 , wherein said operational medium is steam, a gas comprising steam, or a mixture thereof.

23. The process according to claim 12 , wherein the heat treatment of the silica in step D is carried out in a fluidized-bed, fluid-bed or rotary-tube reactor.

24. The process according to claim 23 , wherein a fluidized-bed reactor is used and the following substeps are carried out:

Da introducing the silica into the fluidized-bed reactor,

Db preheating the reactor to 300 to 800° C., the reactor being traversed at the same time by a flow of inert gas and/or nitrogen/air mixture in such a way as to produce a fluidization velocity of 0.02 to 0.06 m/s,

Dc feeding in a gas mixture I comprising steam and an inert gas, or a gas mixture II comprising steam, an inert gas and air, at 300 to 800° C. for a period of 0.25 to 6 h, the gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and the gas mixtures I and II having a steam concentration of 10 to 95% by volume and in the case of gas mixture II an oxygen content of 0.01% to 21% by volume,

Dd interrupting the addition of steam and expelling the steam using an inert gas, nitrogen for example, and/or of an inert gas/air mixture at 300 to 800° C., the gas or gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume, and

De cooling the heat-treated silica to room temperature in a dry process atmosphere, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume.

25. The process silica wherein it is obtainable by the process according to claim 12 .

26. A method of producing a sealant, comprising:

adding the precipitated silica according to claim 1 to a sealant.

27. The method according to claim 26 , wherein the sealant is RTV-1K silicone rubber or a silicone sealant of a crosslinking system selected from the group consisting of an acetoxy-crosslinking system, an alkoxy-crosslinking system, an oxime-crosslinking system and mixtures thereof.

28. A sealant, comprising:

at least one precipitated silica according to claim 1 .

29. The sealant according to claim 28 , which is RTV-1K silicone rubber or a silicone sealant of a crosslinking system selected from the group consisting of an acetoxy-crosslinking system, an alkoxy-crosslinking system, an oxime-crosslinking system and mixtures thereof.

30. The sealant according to claim 28 , which is a joint-sealant in the building industry, an adhesive or sealant in the automotive industry, or as a coating material for textile fabric.