KR100772258B1 - Stabilized, aqueous silicon dioxide dispersion - Google Patents

Abstract

An aqueous dispersion containing silicon dioxide powder that is at least partially soluble in the pH range of 2-6 and stable at a pH range of 2-6, with a zeta potential of 0 or less. The dispersion is prepared by contacting silicon dioxide powder and one or more cation-providing compounds while moving in an aqueous medium. The dispersion can be used for chemical-mechanical polishing of metal surfaces.

Silicon dioxide dispersion

Description

Stabilized aqueous silicon dioxide dispersion {STABILIZED, AQUEOUS SILICON DIOXIDE DISPERSION}

The present invention relates to aqueous dispersions containing silicon dioxide which are stable in the acidic pH range, the preparation and use thereof. The invention also relates to powders that can be used to prepare such dispersions.

Silicon dioxide dispersions are generally not stable in the acidic pH range. The possibility of stabilizing the dispersion is shown, for example, by adding aluminum compounds.

WO 00/20221 (filed '221) discloses aqueous silicon dioxide solutions that are stable in the acidic range. The solution is prepared by contacting silicon dioxide particles with an aluminum compound in an aqueous medium. The amount of aluminum compound needed to prepare the dispersion claimed in the '221 application is according to increasing the zeta potential, and is achieved at the point where the increase in the zeta potential curve moves towards zero or reaches a plateau do. The '221 application also claims a dispersion in which zeta potential is achieved only up to 50% of the maximum achievable value. In all cases, the zeta potential of the claimed dispersion has a strong positive value of 30 mV or less. This means that the original negatively charged silicon dioxide particles were fully cationic by the addition of aluminum compounds. Although the claimed dispersions have good stability, they are no longer silicon dioxide dispersions whose surface is coated with positively-charged aluminum species. This is a disadvantage in applications where the dispersion is in contact with negative charge gas or dispersion. This can lead to, for example, unwanted flocculation or sedimentation.

US 2,892,797, on the other hand, discloses aqueous silicon dioxide dispersions stabilized by treatment with alkali metalates. Sodium aluminate is particularly preferred. Stabilization takes place via an anion, for example [Al (OH) ₄ ] ^- . Dispersions are generally stable in the pH range of 5-9. Thus, the zeta potential of the treated powder is negative. Subsequent removal of the cation by, for example, an ion exchange method may be a disadvantage of the method. For special applications such as, for example, chemical-mechanical polishing, alkali cations are generally undesirable. A further disadvantage is the low stability in more acidic media below pH 5.

It is an object of the present invention to provide a silicon dioxide dispersion that is stable in the acidic range without changing the properties of the silicon dioxide powder by reversing the charge on the particle surface.

The present invention

The dispersion is stable at a pH of 2 to 6,

The dispersion contains at least one compound that is soluble in an aqueous solution in the pH range of 2 to 6 in the form of a polyvalent cation, wherein the cation is stable in a silicate-like environment as an anionic component of the particle surface of the silicon dioxide powder,

The amount of cation-providing compound on the surface of silicon dioxide is from 0.001 to 0.1 mg of cation-providing compound per m 2 of silicon dioxide surface, wherein the cation-providing compound is calculated as oxide,

The zeta potential of the dispersion has a value of zero or less,

An aqueous dispersion containing silicon dioxide powder having a silicon dioxide content of 10 to 60% by weight is provided.

Zeta potential is the externally active potential of a particle and represents a measure of electrostatic interaction between individual particles. This acts in part on the stabilization of dispersions, especially dispersions containing dispersed ultrafine particles. Zeta potential can be measured, for example, by measuring the colloidal vibration current (CVI) of the dispersion, or by measuring its electrophoretic mobility.

Cation-providing compounds according to the invention are always understood to be soluble in aqueous solutions in the pH range of 2 to 6 in polyvalent cation form, wherein the cations are stable in a silicate-like environment with anionic centers. These are compounds having Ca, Sr, Ba, Be, Mg, Zn, Mn, Ni, Co, Sn, Pb, Fe, Cr, Al, Sc, Ce, Ti and zr as cations.

The silicate-like environment is understood to mean that the cations of the above-mentioned metals are present in the form of metal-oxygen bonds with silicon atoms on the silicon dioxide surface. They can also replace silicon atoms of silicon dioxide structures.

It is understood that the anionic component is a component measured by zeta potential that does not change the negative charge on the surface of the silicon dioxide powder or shifts it to a more negative value.

Stability is understood to mean that the particles of the silicon dioxide powder do not more solidify in the dispersion within at least one week and the viscosity of the dispersion does not change or only slightly changes (a viscosity increase of less than 10%).

Preferred cation-providing compounds are amphoteric compounds having Be, Zn, Al, Pb, Fe or Ti as cations and mixtures of these compounds.

Amphoteric compounds act as bases for stronger acids and acids for stronger bases at a given pH.

Particularly preferred cation-providing compounds are aluminum compounds, for example aluminum chloride, aluminum hydroxychloride of the general formula Al (OH) _x Cl, where x = 2-8, aluminum chlorate, aluminum sulphate, aluminum nitrate Latex, aluminum hydroxynitrate of the general formula Al (OH) _x NO ₃ , where x = 2-8, aluminum acetate, alums, for example aluminum potassium sulfate or aluminum ammonium sulfate, Aluminum formate, aluminum lactate, aluminum oxide, aluminum hydroxide acetate, aluminum isopropylate, aluminum hydroxide, aluminum silicate and mixtures thereof. Aluminum silicates are for example cipernat obtained from Degussa AG, a particulate aluminum silicate containing about 9.5 wt% aluminum with Al ₂ O ₃ and about 8 wt% sodium with Na ₂ O. Sipernat) 820, or sodium aluminum silicate in the form of zeolite A.

There is no restriction on the form of the silicon dioxide powder in the dispersion according to the present invention. Therefore, silicon dioxide powder prepared by the sol-gel method, the precipitation method or the exothermic method can be used. It may be a metal oxide powder completely or partially stacked in silicon dioxide, provided its zeta potential is less than or equal to zero in the pH range of 2-6.

Preference is given to silicon dioxide powders produced exothermicly.

Exothermic according to the invention is understood to mean the formation of silicon dioxide by flame hydrolysis of a compound or compounds containing silicon in a flame produced by the reaction of a combustion gas and an oxygen-containing gas in the gas phase. Suitable silicone containing compounds are, for example, silicon tetrachloride, methyltrichlorosilane, ethyltrichlorosilane, propyltrichlorosilane, dimethyldichlorosilane, alkoxysilanes and mixtures thereof. Silicon tetrachloride is particularly preferred. Suitable combustion gases are hydrogen, methane, ethane, propane, with hydrogen being particularly preferred. During flame hydrolysis, highly-dispersed, nonporous primary particles are first formed, which grow together as a reaction process to form aggregates, which can further coalesce to form agglomerates. The surface of the silicon dioxide particles produced exothermicly has silanol groups (Si-OH) and siloxane groups (Si-O-Si).

The pyrogenically produced silicon dioxide powders also contain doped silicon dioxide powders and pyrogenically produced silicon-metal mixed oxide powders, provided their zeta potentials are zero or less in the pH range of 2-6.

The preparation of the doped powder is disclosed, for example, in DE-A-196 50 500. Typical doping components are, for example, aluminum, potassium, sodium or lithium. The content of the doping component is generally 1% by weight or less.

Exothermically prepared mixed oxide powders are understood to mean that both precursors of the mixed oxides are hydrolyzed together in flames. Typical mixed oxide powders are silicon-aluminum mixed oxides or silicon-titanium mixed oxides.

According to certain embodiments, the ratio of the cation-providing compound to the surface of the silicon dioxide is preferably from 0.0025 to 0.04, in particular 0.005 to 0.02 mg, per m 2 of silicon dioxide surface.

The silicon dioxide surface corresponds to the specific surface area of the silicon dioxide powder determined according to DIN 66131. The BET specific surface area is 5 to 600 m 2 / g, preferably in the range of 30 to 400 m 2 / g, particularly preferably in the range of 50 to 300 m 2 / g.

The pH value of the dispersion according to the invention is 2-6. Preferably it is 3-5.5. In particular, the BET specific surface area of the silicon dioxide powder of 50 m 2 / g or less may be 3 to 4, the BET specific surface area of 50 to 100 m 2 / g may be 3.5 to 4.5, the BET ratio of 100 to 200 m 2 / g It can be 4 to 5 at the surface area and 4.5 to 5.5 at a BET specific surface area of greater than 200 m 2 / g.

The pH value can be adjusted with acid or base if necessary. Preferred acids are hydrochloric acid, sulfuric acid, nitric acid or carboxylic acid, for example acetic acid, oxalic acid or citric acid. Preferred bases are alkali hydroxides such as KOH or NaOH, ammonia, ammonium salts or amines. If necessary, a buffer system can be formed by adding salts.

According to certain embodiments, at a shear energy of 1.28 s ⁻¹ , the viscosity of the dispersion according to the invention may be at least 10% lower than the viscosity of a dispersion that does not contain the same composition or cation-providing compound. The viscosity is preferably 25%, in particular 50% lower than dispersions which do not contain the same composition or cation-providing compound.

According to certain embodiments, the number of agglomerates whose size exceeds 1 μm in the dispersion according to the invention may be at least 50% less than the number of dispersions which do not contain the same composition or cation-providing compound. The number of agglomerates whose size exceeds 1 μm is preferably 75%, in particular 90% less than in dispersions which do not contain the same composition or cation-providing compound.

The dispersion according to the invention may further comprise a preservative. Suitable preservatives are, for example, benzyl alcohol mono (poly) hemiformal, tetramethylolacetylenediurea, formamide monomethylol, trimethylolurea, N-hydroxymethylformamide, 2-bromo-2-nitro Propane-1,3-diol, 1,6-dihydroxy-2,5-dioxahexane, chloromethylisothiazolinone, orthophenylphenol, chloroacetamide, sodium benzoate, octylisothiazolone, pro Piconazole, iodopropynyl butylcarbamate, methoxycarbonyl aminobenzimidazole, 1,3,5-triazine derivative, methylisothiazolinone, benzoisothiazolinone and mixtures thereof.

The invention also provides a process for the preparation of the dispersion according to the invention, wherein the silicon dioxide powder, and at least one cation providing compound, are calculated as oxides and contacted in an amount of 0.001 to 0.1 mg per m 2 of silicon dioxide surface, while Move it in the middle.

Moving while in contact is understood to mean, for example, stirring or dispersion. Dissolvers, toothed gear disks, rotor-stator machines, ball mills or mechanically disturbed ball mills are suitable, for example, for dispersion. High energy injection is possible with planetary kneaders / mixers. However, the effectiveness of the system depends on the treatment mixture having a high viscosity sufficient to incorporate the high shear energy needed to disperse the particles. High pressure homogenizers can be used to obtain aqueous dispersions having aggregate sizes in dispersions of less than 200 nm.

With this apparatus, at least two pre-dispersed suspension streams are discharged through the nozzle under high pressure. The two dispersion jets collide exactly with each other and the particles mill themselves. In other embodiments, the pre-dispersion is also placed under high pressure, but collision of particles occurs with respect to the protected side of the wall. The operation can be repeated as often as necessary to obtain smaller particles.

The process for the preparation of the dispersion according to the invention can be carried out in such a way that the cation-providing compound as an aqueous solution is added to the aqueous dispersion of silicon dioxide.

It may also be carried out in such a way that the silicon dioxide powder is added to the aqueous solution of the cation-providing compound at one time or in part.

Furthermore, it is possible to add the silicon dioxide powder and the cation-providing compound onto the liquid dispersion simultaneously, in part or in succession.

In this case, "simultaneously" is understood to mean that the silicon-dioxide powder and the cation-providing compound can be pre-mixed in the form of a physical or chemical mixture.

The present invention also provides powders of this type containing at least one cation providing compound and silicon dioxide powder, wherein the content of cation-providing compound is 0.001 to 0.1 mg per m 2 of silicon dioxide surface, calculated as oxide. to provide.

This includes customary impurities of starting materials and impurities introduced during manufacture. The amount of impurities is less than 1% by weight, typically less than 0.1% by weight.

The cation-providing compound is preferably an aluminum compound and silicon dioxide, a silicon dioxide powder made pyrogenic.

Powders according to the invention can be quickly incorporated into aqueous media.

In the simplest case, it can be prepared by physical mixing of silicon dioxide powder and one or more cation-providing compounds. Here, it is useful to make full use of the individual packages. Thus, it is not necessary to exhibit a uniform distribution of silicon-dioxide and cation-providing compounds.

In addition, the powders according to the invention can also be obtained by spraying onto the silicon dioxide powder at least one compound which is soluble in the pH range of <6 or which provides cations by chemical reaction in the pH range of <6. The solution of the cation-providing compound can be sprayed continuously or in batches using a spray device in a heated mixer and dryer. Suitable apparatus may be, for example, a plow mixer, a disc- or fluidized bed dryer.

Solutions of cation-providing compounds are sprayed or atomized using ultrasonic nozzles. In addition, the mixer may optionally be heated.

Furthermore, the powder according to the invention can be obtained by separating the cation-providing compound (eg aluminum chloride) from the vapor in a fluidized bed or in a mixer.

The invention further provides chemical-mechanical polishing of metal surfaces, in particular polishing of copper surfaces, production of ink-jet papers, for gel batteries, purification / purification of wine and fruit juices, suspension of pigments and fillers against aqueous-based dispersion paints. Natural latex and synthetics to stabilize emulsions and dispersions in the biocidal field, to improve the action and to increase scratch-tolerance, to improve the stability and "blackness" of carbon black dispersions for ink-jet inks. As a latex reinforcing agent, to make latex / rubber articles such as gloves, condoms, infant rubber nipples or molded rubbers in sol-gel applications, to remove surface-tackiness (anti-blocking), slipping on paper and cardboard In order to produce the optical fiber, in order to improve the slip resistance, in order to achieve the prevention effect, The use of the dispersion according to the invention is provided.

It is surprising that the silicon dioxide dispersion in contact with the cation-providing compound has good stability in the acidic range, while at the same time the surface of the silicon dioxide particles maintains or rather strengthens the negative surface charge.

The stabilization mechanism has not been described so far. However, this should be distinguished from that disclosed in WO 00/20221. Here, the charge of the silicon dioxide particles is completely reversed by the positively-charged aluminum species that impart the positive-charge shell to the particles.

The mechanism should also be distinguished from that disclosed in US 2,892,797. Here, negatively-charged metalate ions are incorporated into the surface of the silicon dioxide particles. Although the particles thus changed have negative surface charges (like the particles in the dispersion according to the invention), they have little stability in the acidic pH range.

Analytical Method

Zeta potential is measured with a DT-1200 type device obtained from Dispersion Technology Inc. by the CVI method.

Viscosity of the dispersion was measured with a Physica MCR 300 rotary flow meter and a CC 27 measurement beaker and the measurements were carried out at a shear rate of 0.01 to 500 s ⁻¹ and 23 ° C. The viscosity is shown at a shear rate of 1.28 s ⁻¹ . The shear rate is in a range in which the structural viscous effect has an apparent influence.

Particle / coagulum size was measured with Horiba LB 500 and LA 300 devices or Malvern Zetasizer 3000 Hsa.

Dispersion

Dispersers used are, for example, Dispermat AE-3M type dissolution systems from VMA-GETZMANN with dissolution system disc diameters of 80 mm, or Ultra-Turax from IKA-WERKE with S50N-G45G dispersing tools. (Ultra-Turrax) T 50 type rotor / stator dispersion unit. When using a rotor-stator device, the charge container was cooled to room temperature.

Dispersion can also be done using high-energy mills. For charges containing 50 kg silicon dioxide powder each, an amount of DI water is placed in a 60 L special steel charge container. Corresponding amounts of Aerosil powder are aspirated and approximately pre-dispersed using a Ystrahl Conti-TDS 3 dispersion and suction mixer. During powder inflow, the pH value of 3.5 ± 0.3 is maintained by adding sodium hydroxide solution and aluminum chloride solution. After powder inflow, the dispersion is transferred to Conti TDS 3 (stator slit width of 4 mm) with a closed suction nozzle at maximum speed. Before the rotor / stator dispersion, the pH of the dispersion was maintained at 3.5 by addition of additional sodium hydroxide solution, which was maintained after dispersion for 15 minutes. Residual amount of water was added to adjust the concentration of SiO _{2 to} 20% by weight. This pre-dispersion is milled in a HJP-25050 Ultimizer System high-energy mill obtained from Sugino Machine Ltd. at a pressure of 250 MPa and a diamond nozzle diameter of 0.3 mm and two passages through the mill. .

chemical substance

Aerosil types 50, 90, 200 and 300 obtained from Degussa AG were used as silicon dioxide powders. AlCl ₃ in hexahydrate form was used as the water soluble aluminum compound. Dosage and homogenization was simplified using 1% by weight of solution relative to Al ₂ O ₃ . PH was calibrated with 1N NaOH solution or 1N HCl solution.

In order to enable a comparison between the viscosities of the dispersions, a uniform pH of 3.5 is optionally set by adding additional 1N NaOH.

Dispersion

Various charge sizes and properties of the resulting dispersions are shown in Tables 1 and 2.

Example 1a (Control Example)

100 g Aerosil 50 (BET specific surface area of about 50 m 2 / g) was partially incorporated into 385 g of DI water using a dissolution system at a setting of about 1800 rpm. pH became 3.5. Residual 15 g DI water was then added to obtain a 20% dispersion, which was dispersed for 15 minutes at 2000 rpm and Ultra Turrax at about 5000 rpm for 15 minutes.

Example 1b-g

100 g Aerosil 50 was incorporated into 385 g DI water and 1.25 g of 1 wt% aqueous aluminum chloride solution (relative to aluminum oxide) using a dissolution system at about 1800 rpm setting. The pH was 3.4 and adjusted to pH 3.5 by addition of 0.7 g 1N NaOH. The remaining 13.1 g of water was then added to obtain 20% by weight of the dispersion and dispersed for 15 minutes using Ultra Turax at 15 rpm at 2000 rpm and at about 5000 rpm.

Example 1c-g was performed in the same manner as 1b.

Example 2a (Control Example)

100 g Aerosil 90 was partially incorporated into 370 g of DI water using a dissolution system at a setting of about 1800 rpm and then dispersed at 2000 rpm for 15 minutes. The pH value was then adjusted to 3.5 with 1N HCl and the dispersion was dispersed for 15 minutes with Ultra Turax at about 5000 rpm. Then, the remaining water was added to obtain a 20 wt% dispersion and the pH value was readjusted to 3.5.

Example 2b

100 g Aerosil 90 was alternately incorporated into the 370 g DI water alternately using a dissolution system at a setting of about 1800 rpm and then dispersed at a setting of about 2000 rpm. Then, 2.50 g of a 1% by weight solution of aluminum chloride (relative to aluminum oxide) was added while dispersing with Ultra Turax at about 5000 rpm, which was dispersed for 15 minutes. 26.3 g DI water and 1.24 g 1N NaOH were added to obtain a 20 wt% dispersion with a pH value of 3.5.

Example 3a (Control Example)

250 g DI water and 20 g 1% by weight aqueous aluminum chloride solution (relative to aluminum oxide) were fed. Aerosil 90 was added in portions using a dissolution system. The pH value was maintained at 3.5 during the course. After adding about 40 g Aerosil 90 powder, the dispersion became very strong and no further addition was possible.

Example 3b, 3c

100 g Aerosil 90 was incorporated into 250 g DI water using a dissolution system, and 10 g of 1 wt% aqueous aluminum chloride solution (for aluminum oxide) and 1 N NaOH were alternately added in portions to pH values between 3.3 and 4.2. It was set as. An additional 100 g aerosil 90 and an additional 10 g of 1 wt% aluminum chloride solution (relative to aluminum oxide) and sufficient 1N NaOH were alternately added in portions at 5000 rpm using ultra-turax at a pH of 3.5 upon completion of the addition. The value was obtained.

In Example 3c the pH value was adjusted to 4.0.

Example 4 (Control Example)

50 g Aerosil 90 was incorporated partly into 350 g DI water using a dissolution system at a setting of about 1800 rpm and then dispersed at 2000 rpm for 15 minutes. Then, 100 g of aluminum chloride 1 wt% solution (relative to aluminum oxide) was added while dispersing in Ultra Turax at about 5000 rpm and dispersed for 15 minutes, with a pH value of 2 using 30% sodium hydroxide solution. To pH 3.5.

Example 5

Example 5a was carried out in the same manner as 2a. Example 5b-d was carried out in the same manner as 2b. For Example 5e-g, about 100 ml of the dispersion of Example 5d was added dropwise with 30% sodium hydroxide solution to the pH values shown in Table 2 and homogenized for about 5 minutes using a magnetic stirrer, each zeta The potential was measured.

Example 6

Example 6a was carried out in the same manner as 2a. Example 6b-d was carried out in the same manner as 2b.

1 shows the zeta potential (mV; ◆) and viscosity (mPas; ○) of Examples 1a-g as a function of mg Al ₂ O ₃ / m 2 SiO ₂ -surface.

Example 7-Powder Preparation

500 g silicon dioxide powder (Aerosil 200, Degussa) was added to a 20 L Lodige mixer. 20 g of 5% by weight aluminum chloride solution (relative to Al ₂ O ₃ ) at a spray displacement of about 100 ml / h was applied at a speed of 250 rpm in 10-15 minutes. The powder had a 0.01 mg Al ₂ O ₃ / m 2 silicon dioxide surface, a BET specific surface area of 202 m 2 / g and a firming density of about 60 g / l. The water content is about 4% and can be reduced by heating the wiper if necessary or subsequent drying in a drying cabinet, rotating tube or fluidized bed. The aqueous dispersion (20 wt%) had a pH of 2.6.

TABLE 1

Aqueous Aerosil Dispersion ⁽¹⁾

1) 100 g SiO ₂ in each experiment, except 3: 200 g and 4: 50 g; Example 1 Aerosil 50, Examples 2, 3, 4: Aerosil 90, Example 5: Aerosil 200, Example 6: Aerosil 300, all obtained from Degussa AG;

₂₎ Al 2 O ₃ was used as AlCl _3;

3) 1N HCl instead of 1N NaOH;

4) 30% sodium hydroxide solution;

5) Add a few drops of 30% sodium hydroxide solution to obtain the corresponding pH values in Table 2.

TABLE 2

Analytical Data of Dispersion ⁽¹⁾

1) measured after 1 week;

2) shear rate of 1.28 s ⁻¹ ;

3) 40% dispersion could not be prepared;

4) n.d. = Not determined;

5) gelled dispersion.

Claims (18)

The dispersion is stable in the pH range of 3 to 5, The dispersion further contains at least one cation-providing compound selected from the group consisting of amphoteric compounds having Be, Zn, Al, Pb, Fe or Ti as cations and mixtures of these compounds, The amount of cation-providing compound on the surface of silicon dioxide is from 0.001 to 0.1 mg of cation-providing compound per m 2 of silicon dioxide surface, wherein the cation-providing compound is calculated as oxide, The zeta potential of the dispersion has a value of zero or less, Silicon dioxide powder is a pyrogenically produced silicon dioxide powder, An aqueous dispersion containing silicon dioxide powder having a silicon dioxide content of 10 to 60% by weight. delete The aqueous dispersion of claim 1 wherein the amphoteric compound is an aluminum compound. delete The aqueous dispersion of claim 1 wherein the BET specific surface area is between 5 and 600 m 2 / g. delete 6. The acid according to claim 1, wherein the acid is selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid and C 1 -C 4 carboxylic acid, or an alkali hydroxide, ammonia, ammonium salt and amine. An aqueous dispersion in which the pH value is adjusted using a base selected from the group. 6. The aqueous dispersion according to claim 1, wherein the viscosity at a shear energy of 1.28 s ⁻¹ is at least 10% lower than the viscosity of a dispersion of the same composition that does not contain a cation-providing compound. . 6. The method of claim 1, wherein the number of agglomerates having a size greater than 1 μm is at least 50% greater than that of a dispersion of the same composition that does not contain a cation-providing compound. Less aqueous dispersion. 6. The aqueous dispersion according to claim 1, wherein the average secondary particle size of the silicon dioxide powder is less than 200 nm. 7. The aqueous dispersion according to any one of claims 1, 3 and 5, which contains a preservative. Claims 1, 3 and 3, wherein the silicon dioxide powder, and at least one cation-providing compound, are calculated as oxides and contacted in an aqueous medium while contacting in an amount of 0.001 to 0.1 mg of the cation-providing compound per m 2 of silicon dioxide surface. A process for the preparation of an aqueous dispersion according to claim 5. 13. The process of claim 12, wherein the cation-providing compound is added to the aqueous dispersion of silicon dioxide as an aqueous solution. The process of claim 12, wherein the silicon dioxide powder is added to the aqueous solution of the cation-providing compound one at a time or in portions. 13. The process of claim 12, wherein the silicon dioxide powder and the cation-providing compound are added simultaneously, in part or sequentially on the liquid dispersion. delete delete delete

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