Inhomogeneous silicas in dental care compositions
The invention relates to silicas having an inhomogeneous structure or composition, to processes for preparing them, and to their use in dental care compositions.
Readily dispersible silicas are prepared, for example, in accordance with EP 0 901 986 or EP 0 647 591 by precipitating waterglass with sulfuric acid, followed by drying. The dried products are subsequently ground and/or granulated.
In another process, silicas are prepared, likewise by acid precipitation, but are dried by spraying in hot air and at the same time are shaped into beads, which are easily destroyed. Thus EP 018 866 describes the preparation of spray-dried silica having an average particle diameter of more than 80 ^im, the particles being solid and possessing a homogeneous structure.
For use in dental care compositions, important parameters for silicas include not only thickening action and abrasiveness but also the specific surface areas (BET, CTAB) and the oil absorption capacity (DBF] .
Since only specialty silicas, i.e., silicas that are complicated to prepare, combine sufficient thickening action and abrasiveness, dental care compositions normally utilize two different types of silica.
It would be desirable to prepare a silica which at one and the same time covers broad ranges of physicochemical data such as BET or CTAB surface area, and has good abrasiveness in conjunction with a good
thickening action.

O.Z. 5733
It has surprisingly been found that silica that has an inhoinogeneous composition is readily adjustable to meet the requirements that are called for.
Moreover, inventively prepared silicas are low in dust, which facilitates handling and prevents dust contamination of the customer's plants.
The present invention accordingly provides silicas comprising at least two silica fractions which differ by at least 10% in at least one value for BET surface area, CTAB surface area, and DBP absorption.
The silicas of the invention are therefore particularly suitable as fillers in dental care compositions.


The structure of the silicas, comprising at least two silica fractions, results in an inhomogeneity of the silica, which is reflected at the same time in good abrasiveness and thickening action and also in a low fines content.
Silicas of the invention possess a fines content of not more than 10% with a particle diameter of less than or equal to 63 ym (Alpine sieve residue).
A similar concept, i.e. inhomogeneous silicas, is pursued in EP 0 942 029. There, rubber compositions are described which comprise a precipitated silica in two different aggregate sizes. The different aggregate sizes are employed for the ready dispersibility of the silica in the rubber blend.
The different silica fractions of the present invention are not described; moreover, in the present case a different aggregate size of the silica fractions is of secondary importance - what are important are the differences in the physicochemical data.

For the purpose of the present invention, a silica fraction refers to different grades of silicas which, owing to different preparation processes or process variants, have a difference of 10% in at least one of the abovementioned physicochemical data. Such a difference exists preferably for two, with particular preference three, of these parameters.
The differences in the abovementioned parameters may be obtained by means of different processes of preparing the silica fractions. Accordingly, all, one or more of the silica fractions may be precipitated silicas and/or pyrogenic silicas. In the case of precipitated silicas in particular it is possible to obtain different silica fractions by means of different precipitation processes. Silicas of the invention may also be prepared from fractions of precipitated and pyrogenic silicas.
For precipitated silicas, a variety of precipitation methods are known and may be read about, for example, in EP 0 901 986, EP 0 937 755 or EP 0 643 015 or EP 0 647 591. In the examples, illustratively, two precipitated silicas from different preparation processes have been processed to give the inhomogeneous silica of the invention.
The silica fractions may be precipitated silicas or pyrogenic silicas, and the fractions may be mixed at different steps in the process that are normally carried out in the preparation of silicas.
When using fractions of precipitated silicas, mixing may take place following the precipitation of silicate with an acid [generally waterglass, i.e., sodium silicate, with sulfuric acid) by mixing together the precipitation suspensions or the filtercakes obtained

following filtration of the suspensions, and also liquefied (resuspended) filtercakes. It is also possible to add ready-prepared and dried silica fractions, as solids, to the suspensions or to the filtercakes.
The mixtures obtained in this way may need to be filtered and dried in a customary manner. Examples of drying processes are spray drying, jet tower, rack drier, rotary tube drier, and spin flash drier processes.
Drying may be followed by a final grinding and/or granulation step.
It is also possible to mix the silica fractions in the dry state. This may be followed by resuspension, with the above drying steps, and/or by grinding/granulation.
Silicas of the invention may have the following
physicochemical datar
BET surface area 30 - 300 mVg, especially 30 -
200 mVg CTAB surface area 30 - 300 m^/g, especially 30 -
200 mVg
DBF absorption 80 - 300 g/100 g
These physicochemical data relate to the silica according to the invention per se, and not to the silica fractions.
In the manner described, the physicochemical data of the silica fractions must differ by at least 10%, preferably by at least lb%, with particular preference by at least 20%.
The physicochemical data are determined by the following methods:

BET surface area
CTAB surface area
DBP number

Areameter, from Strbhlein, to
ISO 5794/Annex D at pH 9 by the method of Jay, Janzen and Kraus in Rubber Chemistry and Technology 44
(1971) 1287 ASTM D 2414-88

The invention additionally provides a process for preparing silicas comprising at least two silica fractions, in which at least two silica fractions which differ by at least 10% in at one least one value for BET surface area, CTAB surface area and DBP absorption are mixed with one another.
The proportion of the respective fractions in the suspension or of the silica should in each case be between 5 and 95% by weight, based on the dry silica.
The silica is preferably obtained, by spray drying, for example, in a particle form having an average diameter of more than 80 pm, in particular more than 100 jjm, with particular preference more than 200 jam. The suspension may be spray-dried in accordance, for example, with US 4 097 771.
The silicas of the invention may therefore be used as dental care compositions, toothpastes or abrasives.
Moreover, the silicas of the invention may be used "in all areas of application in which silicas are customarily used, such as in battery separators, antiblocking agents, flatting agents in paints, paper coating slips or defoamers, for example.

The Alpine sieve residue is determined as follows, using in each case a sieve having the stated mesh size (63 im, 250 \m) .
> Procedure for determining the Alpine sieve residue:
To determine the sieve residue, the silica or silicate sample is passed through a 500 ]im sieve in order to destroy any devQlatiLiz,ation agglomerates that may be present. Then 10 g of the sieved sample are placed on the air jet sieve, with a 63 \im sieve mesh, and are sieved at 200 mm water column underpressure. Particles of silica or silicate which settle on the sieve cover of the apparatus are knocked off by careful tapping on the button of the sieve cover. The sieving operation generally lasts 5 minutes. It is at an end when the residue remains constant, generally evident from the free-flowing appearance. Sieving is then continued for one more minute in order to be on the safe side.
If any agglomerates form, the sieving operation is briefly interrupted and the agglomerates are broken down under gentle pressure using a brush. After sieving, the sieve residue is carefully knocked from the air j et sieve and reweighed. The sieve residue is expressed in percent, always in conjunction with the mesh size of the sieve.


Precision balance
The examples which follow are intended to illustrate the invention without restricting its scope.
Two silica fractions were prepared, A in accordance with US 1 043 282 or DE 24 47 013 and B in accordance with DE 44 23 493, and the suspensions obtained from the precipitations were reacted further in the manner described below.
Example 1
The precipitation suspensions of the silica fractions A and B were mixed in a 50:50 ratio. This was done by mixing 80 kg of the precipitated silica A (solids content approximately 170 g/1) with 80 kg of the precipitated silica B (solids content approximately 63 g/1) in a stirred vessel. The resulting mixture was filtered and the filtercake was liquefied with a small amount of acid and sprayed in a jet tower drier. The analytical data are compiled in Table 1.
Example 2
The precipitation suspensions of the precipitated silicas A and B were mixed in a 70:30 ratio. This was done by mixing 196 kg of the precipitated silica A (solids content approximately 174 g/1) with 84 kg of the precipitated silica B (solids content approximately 63 g/1) in a stirred vessel. The resulting mixture was filtered and the filtercake was liquefied with a small amount of acid and sprayed in a jet tower drier. The analytical data are compiled in Table 1.
Example 3
The precipitation suspensions of the precipitated silicas A and B were mixed in a 30:70 ratio. This was done by mixing 71 kg of the precipitated silica A (solids content approximately 174 g/1) with 142 kg of

the precipitated silica B (solids content approximately 63 g/1) in a stirred vessel. The resulting mixture was filtered and the filtercake was liquefied with a small amount of acid and sprayed in a jet tower drier. The analytical data are compiled in Table 1.

CLAIMS
1. A silica comprising at least two silica fractions,
wherein said at least two silica fractions differ
by at least 10§ in at least one value for BET
surface area, CTAB surface area and DBF
absorption.
2. The silica as claimed in claim 1, which is in the form of particles having an average diameter of more than 8 0 pm.
3. The silica as claimed in either of claims 1 and 2, which has the following physicochemical data:
BET surface area 30 - 300 m2/g CTAB surface area 30 - 300 mVg DBF absorption 80 - 300 g/100 g.
4. The silica as claimed in any of claims 1 to 3, wherein the respective proportion of one silica fraction in the silica is between 5 and 95% by weight.
5. The silica as claimed in any of claims 1 to 4, wherein one or more silica fractions comprise a precipitated silica.
6. The silica as claimed in any of claims 1 to 5, wherein the silica fractions are prepared by precipitating silicate with an acid and the resulting precipitation suspensions are mixed.
7. The silica as claimed in any of claims 1 to 5, wherein the silica fractions are prepared by precipitating silicate with an acid, the precipitation suspension is filtered, and the resulting filtercakes are mixed.

8. The silica as claimed in any of claims 1 to 5, wherein the silica fractions are prepared by precipitating silicate with an acid, the filtercakes or ready-dried silica are liquefied, and the resulting suspensions are mixed.
9. The silica as claimed in any of claims 1 to 4, wherein one or more silica fractions comprise a pyrogenic silica.
10. The silica as claimed in any of claims 1 to 5 and 9, wherein the silica fractions are mixed in the dry state.
11. A process for preparing silicas comprising at least two silica fractions, which comprises mixing with one another at least two silica fractions which differ by at least 10% in at least one value for BET surface area, CTAB surface area and DBF absorption.
12. The process as claimed in claim 11, wherein the silica is in the form of particles having an average diameter of more than 80 pm.
13. The process as claimed in either of claims 11 and 12, wherein the silica has the following physicochemical data:
BET surface area 30 - 300 mVg CTAB surface area 30 - 300 mVg DBP absorption 80 - 300 g/100 g.
14 . The process as claimed in any of claims 11 to 13, wherein the respective proportion of one silica fraction in the silica is between 5 and 9b% by weight.

15. The process as claimed in any of claims 11 to 14, wherein one or more silica fractions comprise a precipitated silica.
16. The process as claimed in any of claims 11 to 15, wherein the silica fractions sre prepared by precipitating silicate with an acid and the resulting precipitation suspensions are mixed.
17. The process as claimed in any of claims 11 to 15, wherein the silica fractions are prepared by precipitating silicate with an acid, the precipitation suspension is filtered, and the resulting filtercakes are mixed.
IS. The process as claimed in any of claims 11 to 15, wherein the silica fractions are prepared by precipitating silicate with an acid, the filtercakes or ready-dried silica are liquefied, and the resulting suspensions are mixed.
19. The process as claimed in any of claims 11 to 14, wherein one or more silica fractions comprise a pyrogenic silica.
2 0, The process as claimed in any of claims 11 to 15 and 19, wherein the silica fractions are mixed in the dry state.
21. The use of the silica as claimed in any of claims 1 to 10 in toothpastes, dental care compositions or abrasive pastes.

22. A silica, substantially as herein described and exemplified.