WO2008071466A1

WO2008071466A1 - Process for preparing fumed silica dispersions

Info

Publication number: WO2008071466A1
Application number: PCT/EP2007/059306
Authority: WO
Inventors: Wolfgang Lortz; Gabriele Perlet; Uwe Diener
Original assignee: Evonik Degussa Gmbh
Priority date: 2006-12-15
Filing date: 2007-09-05
Publication date: 2008-06-19
Also published as: DE102006059315A1; CN101205068A

Abstract

Process for preparing an aqueous dispersion of fumed silica having a pH of 9 to 11.5, the silica having a BET surface area of 30 to 400 m2/g and being present in the dispersion at 20% < 40% by weight, characterized in that - water is circulated and via a feed means, and with a rotor/stator machine running, fumed silica powder is introduced in an amount such as to result in a preliminary dispersion having a silica content of between 25% and 70% by weight, and, after all of the silica powder has been added, - the feed means is closed and, at a pH of 2 to 4 and a temperature of 10 to 50°C, shearing is carried out with a shear rate of 10 000 to 30 000 s-1, - dilution is carried out with water until the desired silica content is exceeded by 0.1% to 10%, and - subsequently, under the same shearing conditions, an aqueous base is added in an amount and concentration such that the amount needed to attain the desired solids content is reached and a pH of 9 to 11.5 results.

Description

Process for preparing fumed silica dispersions

The invention relates to a process for preparing low- viscosity, highly filled dispersions of fumed silica.

Low-viscosity, highly filled dispersions of fumed metal oxides or metalloid oxides find broad use. By way of example, silica dispersions and alumina dispersions are used in polishing processes (chemical mechanical polishing) or in the paper industry for producing a paper coating. In the glass industry, highly filled silica dispersions or mixed silicon titanium oxide dispersions are used for producing glass mouldings.

US 5,116,535, US 5,246,624 and US 6,248,144 all describe processes for preparing low-viscosity dispersions of fumed silica powder.

Fumed silica powders, like other fumed oxide powders, aluminium oxide or titanium dioxide for example, are prepared preferably by flame hydrolysis. In this operation a homogeneous mixture of a vapour-form starting material of the subsequent oxide, silicon tetrachloride or aluminium chloride for example, is burnt with hydrogen, oxygen and an inert gas using a burner in a cooled combustion chamber. In a first step of this operation, reaction of hydrogen and oxygen produces water, which in a second step hydrolyses the starting material to form the fumed oxide.

Primary particles are formed first of all in this operation, and in the further course of reaction join to form aggregates. Aggregates are fused primary particles. The aggregates may congregate further to form agglomerates. When fumed oxide particles are dispersed, first of all, under the action just of low dispersing energy, the agglomerates are separated. At higher dispersing energies, in addition, larger aggregates are converted into smaller aggregates . The principle underlying documents US 5,116,535, US 5,246,624 and US 6,248,144 is the same, namely the maximum destructuring of the fumed silica powder under the action of high shearing energies. However, in order to allow the high shearing energies to be introduced into the system, the system must have a high viscosity. In the preparation processes of the documents identified above, the high viscosity is achieved through a high fill level of silica powder, which is required to be at least 40% by weight, more preferably 50% to 60% by weight. If the amount of silica powder in these processes is reduced to levels below 40% by weight, the efficiency of the dispersing operation is reduced to such an extent that the destructuring of the silica powder is incomplete, and relatively large aggregates remain in dispersion. This can lead to sedimentation or gelling of the dispersion. Subsequently the dispersion is adjusted by dilution to the desired solids content.

A disadvantage with these processes is the time-consuming and energy-intensive incorporation of the fumed silica powder for the purpose of achieving the necessary viscosity.

In addition there is a process for dispersing fumed metal oxides in an aqueous medium that involves releasing two predispersed suspension streams that are under high pressure via two nozzles. The nozzles in this case must be adjusted so that the dispersion jets impinge exactly on one another and so produce self-grinding of the particles.

This process for preparing dispersions which comprise fumed silica is described for example in EP-A-773270. There an aqueous preliminary dispersion is divided into two sub- streams which are merged again under high pressure. In the course of this merger the particles undergo self-grinding. In another embodiment, the preliminary dispersion is again placed under high pressure, but the collision of the particles takes place against armoured wall regions. Dispersion can take place over the entire pH range, the alkaline range being preferred. Where the desire is for a dispersion with a high solids content in the acidic range, it is advantageous to lower the viscosity by means of suitable additions.

A problem with this process is the exact adjustment of the two predispersed suspension streams. Only in the case of exact adjustment is it possible for there to be uniform grinding of the silica powder. A further complicating factor is that, when the nozzles are stressed in the extreme at pressures up to 3500 kg/cm², they exhibit severe wear, which has an adverse effect on the abovementioned adjustment and can lead to impurities being carried into the dispersion.

In the embodiment where the particles are collided against armoured wall regions, it has been found that the wall regions are subject to severe wear and that this embodiment is not suitable for dispersing fumed silica.

For both high-pressure processes it is the case that the dimensions of the available apparatus do not allow sizeable amounts of dispersion to be prepared inexpensively.

It is an object of the invention to provide a process for preparing fine dispersions, comprising fumed metal oxides as their solid phase, that avoids the disadvantages of the prior art. In particular, the incorporation of fumed metal oxides or metalloid oxides into an aqueous phase as rapidly as possible ought to be possible; the entrainment of impurities ought to be minimal; and the process ought to be amenable to economic implementation.

This object is achieved by means of a process for preparing an aqueous dispersion of fumed silica having a pH of 9 to 11.5, the silica having a BET surface area of 30 to 400 m²/g and being present in the dispersion at 20% < 40% by weight, characterized in that

- water from a reservoir is circulated via a rotor/stator machine and, via a feed means, continuously or discon- tinuously and with the rotor/stator machine running, fumed silica powder is introduced in an amount such as to result in a preliminary dispersion having a silica content of between 25% and 70% by weight, and, after all of the silica powder has been added, - the feed means is closed and, at a pH of 2 to 4 and a temperature of 10 to 50⁰C, shearing is carried out with a shear rate of 10 000 to 30 000 s^"1,

- dilution is carried out with water until the desired silica content is exceeded by 0.1% to 10%, and - subsequently, under the same shearing conditions, an aqueous base is added in an amount and concentration such that the amount needed to attain the desired solids content is reached and a pH of 9 to 11.5, preferably of 9.5 to 11.0, results. In one preferred embodiment the shear rate can be between 20 000 and 30 000 s-¹.

If desired, dilution can be carried out further with water, in order to set a desired content and a desired pH.

The process of the invention can be performed with prefer- ence in such a way that the preliminary dispersion possesses a silica content of 25% to 60%, preferably of 28% to 38% and especially of 32% to 35% by weight.

The process of the invention can also be performed with preference in such a way that, prior to the addition of the aqueous base, the silica content of the dispersion is 0.2% to 5% and more preferably 0.4% to 2.5% above the desired content . The percentages are based on the desired solids content. For example, in the case of the desired solids content of 30% by weight, dilution should be carried out with sufficient water that the solids content is 30% + 0.1% = 30.03% by weight to 30% + 10% = 33% by weight, preferably 30% + 0.2% = 30.06% by weight to 30% + 5% = 31.5% by weight, and more preferably 30% + 0.4% = 30.12% by weight to 30% + 2.5% = 30.75% by weight.

If the solution in which the silica is present does not already have a pH of 2 to 4, such a pH can be set by addition of an acid. In general the silica used already has a pH in the range from 2 to 4. In these cases it is unnecessary to add an acid. The nature of the acid itself is not critical. Typically, hydrochloric, sulphuric or nitric acid is used.

After the end of the dispersing operation in the acidic range, first water and then base are added to give a pH of 9 to 11.5. The required amount of base is advantageously added not in portions in this case; instead, the entire amount of base is added all at once and rapidly.

The nature of the base is not critical. Bases which have proven to be suitable include aqueous potassium hydroxide solution, sodium hydroxide solution, ammonia, aqueous ammonia, amines such as monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, N, N-dimethylisopropanolamine, tetraalkylammonium hydroxides, morpholine, and amino alcohols, such as 3-amino-l-propanol, l-amino-2-propanol and 2-amino-2-methyl-l-propanol . With preference it is possible to use aqueous ammonia, sodium hydroxide solution or potassium hydroxide solution.

The concentration of the base is not critical. However, it has proven to be advantageous to use bases with a concen- tration of 2 to 20 mol/1, more preferably 5 to 18 mol/1 and very preferably 8 to 15 mol/1.

A factor which may be critical, in contrast, is the temperature of the dispersion during the dispersing operation in the acidic range. It has been observed that, at temperatures of more than 55°C, spontaneous gelling may occur. Consequently it may be advantageous to cool the dispersion during the dispersing operation.

For the process of the invention it is possible with preference to use a fumed silica having a BET surface area of 40 to 60 m²/g.

For the process of the invention it is possible with further preference to use a fumed silica having a BET surface area of 80 to 110 m²/g.

With particular preference it is possible to use a fumed silica in the form of aggregates of primary particles, said silica possessing a BET surface area of 90 ± 15 m²/g, wherein the aggregates have an average area of 10 000 - 20 000 nm², an average, equivalent circle diameter (ECD) of 90 - 130 nm and an average circumference of 1000 - 1400 nm. The preparation of this silica is described in DE-A-102005001410.

The BET surface area here is determined in accordance with DIN 66131. The aggregate sizes are determined by image analysis using a TEM instrument from Hitachi, H 7500, and a CCD camera from SIS, MegaView II. The magnification for evaluation is 30 000:1 at a pixel density of 3.2 nm. The number of particles evaluated is greater than 1000. Sample preparation takes place in accordance with ASTM3849-89. The lower threshold limit in respect of detection is 50 pixels.

For the process of the invention it is possible with further preference to use a fumed silica having a BET surface area of 130 to 170 m²/g. With particular preference it is possible to use a fumed silica in the form of aggregates of primary particles, said silica possessing a BET surface area of 150 ± 15 m²/g, wherein the aggregates have an average area of 12 000 - 20 000 nm², an average, equivalent circle diameter (ECD) of 90 - 120 nm and an average circumference of 1150 - 1700 nm. The preparation of this silica is described in DE-A-102005001409.

For the process of the invention it is possible with further preference to use a fumed silica having a BET surface area of 180 to 230 m²/g.

With particular preference it is possible to use a fumed silica in the form of aggregates of primary particles, said silica possessing a BET surface area of 175 to 225 m²/g, wherein the aggregates have an average area of 7000 to

12 000 nm², an average, equivalent circle diameter (ECD) of 80 to 100 nm and an average circumference of 850 to 1050 nm. The preparation of this silica is described in DE-A-102005001408.

For the process of the invention it is possible with further preference to use a fumed silica having a BET surface area of 270 to 330 m²/g.

With particular preference it is possible to use a fumed silica in the form of aggregates of primary particles, said silica possessing a BET surface area of 275 to 325 m²/g, wherein the aggregates have an average area of 4800 to 6000 nm², an average, equivalent circle diameter (ECD) of 60 to 80 nm and an average circumference of 580 to 750 nm. The preparation of this silica is described in DE-A-102005001414.

The pyrogenic silica used in the process of the invention also embraces mixed silicon aluminium oxides and mixed silicon titanium oxides. In particular it is possible to use a mixed silicon aluminium oxide having a BET surface area of 85 to 110 m²/g and an alumina fraction of 67% by weight .

In addition it may be advantageous to add a surface-active substance to the final and/or preliminary dispersion, said substance being nonionic, cationic, anionic or amphoteric in nature.

Finally, in connection with the process of the invention, it is also possible to add one or more preservatives. These may be compounds, for example, which are available under the brand name Preventol^® from Bayer or Acticide^® from Thor.

Examples

Analytical determinations

Determination of viscosity of the dispersions: The viscosity of the dispersions produced was determined using a rotational rheometer from Physica, model 300, with the CC 27 measurement cup at 25°C. The viscosity value was determined at a shear rate of 10 s^'1 and 100 s^'1.

Determination of particle size present in the dispersion:

The particle size present in the dispersion is determined by means of light scattering. The instrument used is the Horiba LA 910 (Horiba Ltd., Japan) . The result reported is the median value of the volume distribution, dso(v).

Determination of shear rate: The shear rate in accordance with the process of the invention is expressed as the tip speed divided by the distance between the faces.

The tip speeds can be calculated from the rotary speed of the rotor and rotor diameter. The distance between rotor and stator in the dispersing apparatus used is approximately 1 mm. Dispersing apparatus used: Dispersing is carried out using the Conti-TDS 3 rotor/stator machines from Ystral.

Silica powders used: The following powders are used:

Pl Silica powder from Example 1, DE-A-102005001410 P2 Silica powder from Example 1, DE-A-102005001409 P3 Silica powder from Example 1, DE-A-102005001408 P4 Silica powder from Example 1, DE-A-102005001414 P5 Silica powder from Example 8, DE-A-102005001410 P6 Silica powder from Example 10, DE-A-102005001409 P7 Silica powder from Example 10, DE-A-102005001408

The properties of silica powders Pl to P7 are shown in Table 1.

Examples: The pH of the preliminary dispersion may be between 2 and 4.5, depending on the acidic nature of the fumed silica and on the grade of the raw material. If desired, by adding acid, aqueous hydrochloric acid for example, or else base, aqueous ammonia solution for example, a pH which is constant over the different silica batches can be set, in order to achieve a consistent grinding performance.

For the grinding operation, a preliminary-dispersion pH in the vicinity of the isoelectric point is advantageous, since in that case the particles for grinding can be ground more effectively without having to overcome forces of mutual electrostatic repulsion.

In all of the examples, warming of the dispersion as a result of the high energy input is countered by a heat exchanger, which limits the temperature increase to max. 40⁰C.

Example 1 : A 100 1 stainless steel batching vessel is charged with 36.0 kg of DI [fully demineralized] water. Subsequently, using the suction hose of the Ystral Conti- TDS 3 (stator slot: 4 mm ring and 1 mm ring, rotor/stator distance approximately 1 mm) , 24 kg of Pl are inducted under shearing conditions. After the end of the addition, the induction port is closed and subsequent shearing takes place at 3000 rpm for 20 minutes. The pH is 3.4. The heat generated in the course of dispersing is taken off by means of an external heat exchanger. The dispersion is diluted with 34 kg of DI water and the pH is adjusted to 10.0 using 0.8 kg of aqueous sodium hydroxide solution (25% strength), with shearing. Then 1.2 kg of DI water are added, to give an Siθ2 content of 25% by weight, and shearing is carried out once more for approximately 5 minutes for homogenization .

The average aggregate diameter is 143 nm (determined with Horiba LA 910) .

Even after 6 months, the dispersion shows no signs of gelling or sedimentation.

Example 2 : A 100 1 stainless steel batching vessel is charged with 36.0 kg of DI [fully demineralized] water. Subsequently, using the suction hose of the Ystral Conti- TDS 3 (stator slot: 4 mm ring and 1 mm ring, rotor/stator distance approximately 1 mm) , 24 kg of P2 are inducted under shearing conditions. The pH is 3.3. After the end of the addition, the induction port is closed and subsequent shearing takes place at 3000 rpm for 20 minutes. The heat generated in the course of dispersing is taken off by means of an external heat exchanger. The dispersion is diluted with 18 kg of DI water and the pH is adjusted to 10.0 using 1.4 kg of NH₃ solution (25% strength), with shearing. Then 0.6 kg of DI water are added, to give an Siθ2 content of 30% by weight, and shearing is carried out once more for approximately 5 minutes for homogenization.

The average aggregate diameter is 121 nm (determined with Horiba LA 910) . Even after 6 months, the dispersion shows no signs of gelling or sedimentation.

Example 3: A 100 1 stainless steel batching vessel is charged with 32.5 kg of DI [fully demineralized] water. Subsequently, using the suction hose of the Ystral Conti- TDS 3 (stator slot: 4 mm ring and 1 mm ring, rotor/stator distance approximately 1 mm), 17.5 kg of P3 are inducted under shearing conditions. The pH is 3.7. After the end of the addition, the induction port is closed and subsequent shearing takes place at 3000 rpm for 20 minutes. The heat generated in the course of dispersing is taken off by means of an external heat exchanger. The dispersion is diluted with 36 kg of DI water and the pH is adjusted to 10.0 using 1.0 kg of NH₃ solution (25% strength), with shearing. Then 0.3 kg of DI water are added, to give an Siθ2 content of 20% by weight, and shearing is carried out once more for approximately 5 minutes for homogenization .

The average aggregate diameter is 105 nm (determined with Horiba LA 910) . Even after 6 months, the dispersion shows no signs of gelling or sedimentation.

Example 4 : A 100 1 stainless steel batching vessel is charged with 32.5 kg of DI [fully demineralized] water. Subsequently, using the suction hose of the Ystral Conti- TDS 3 (stator slot: 4 mm ring and 1 mm ring, rotor/stator distance approximately 1 mm), 17.5 kg of P4 are inducted under shearing conditions. The pH is 3.6. After the end of the addition, the induction port is closed and subsequent shearing takes place at 3000 rpm for 20 minutes. The heat generated in the course of dispersing is taken off by means of an external heat exchanger. The dispersion is diluted with 26 kg of DI water and the pH is adjusted to 10.3 using 1.9 kg of NH₃ solution (25% strength), with shearing. Then 1.6 kg of DI water are added, to give an Siθ2 content of 22% by weight, and shearing is carried out once more for approximately 5 minutes for homogenization .

The average aggregate diameter is 85 nm (determined with Horiba LA 910) . Even after 6 months, the dispersion shows no signs of gelling or sedimentation.

Example 5 (comparative) : A 100 1 stainless steel batching vessel is charged with 36.0 kg of DI [fully demineralized] water. Subsequently, using the suction hose of the Ystral Conti-TDS 3 (stator slot: 4 mm ring and 1 mm ring, rotor/stator distance approximately 1 mm) , 24 kg of P5 are inducted under shearing conditions. After the end of the addition, the induction port is closed and subsequent shearing takes place at 3000 rpm for 20 minutes. The pH is 3.5. The heat generated in the course of dispersing is taken off by means of an external heat exchanger. The dispersion is diluted with 34 kg of DI water and the pH is adjusted to 10.0 using 0.7 kg of aqueous sodium hydroxide solution (25% strength), with shearing. Then 1.3 kg of DI water are added, to give an Siθ2 content of 25% by weight, and shearing is carried out once more for approximately 5 minutes for homogenization.

The average aggregate diameter is 164 nm (determined with Horiba LA 910) . Even after 6 months, the dispersion shows no signs of gelling or sedimentation.

Example 6 (comparative) : A 100 1 stainless steel batching vessel is charged with 36.0 kg of DI [fully demineralized] water. Subsequently, using the suction hose of the Ystral Conti-TDS 3 (stator slot: 4 mm ring and 1 mm ring, rotor/stator distance approximately 1 mm) , 24 kg of P6 are inducted under shearing conditions. The pH is 3.4. After the end of the addition, the induction port is closed and subsequent shearing takes place at 3000 rpm for 20 minutes The heat generated in the course of dispersing is taken off by means of an external heat exchanger. The dispersion is diluted with 18 kg of DI water and the pH is adjusted to 10.0 using 1.3 kg of NH₃ solution (25% strength), with shearing. Then 0.7 kg of DI water are added, to give an

Siθ2 content of 30% by weight, and shearing is carried out once more for approximately 5 minutes for homogenization .

The average aggregate diameter is 140 nm (determined with Horiba LA 910) . Even after 6 months, the dispersion shows no signs of gelling or sedimentation.

Example 7 (comparative) : A 100 1 stainless steel batching vessel is charged with 32.5 kg of DI [fully demineralized] water. Subsequently, using the suction hose of the Ystral Conti-TDS 3 (stator slot: 4 mm ring and 1 mm ring, rotor/stator distance approximately 1 mm), 17.5 kg of P7 are inducted under shearing conditions. The pH is 3.7. After the end of the addition, the induction port is closed and subsequent shearing takes place at 3000 rpm for 20 minutes. The heat generated in the course of dispersing is taken off by means of an external heat exchanger. The dispersion is diluted with 36 kg of DI water and the pH is adjusted to 10.0 using 1.1 kg of NH₃ solution (25% strength), with shearing. Then 0.2 kg of DI water are added, to give an Siθ2 content of 20% by weight, and shearing is carried out once more for approximately 5 minutes for homogenization.

The average aggregate diameter is 119 nm (determined with Horiba LA 910) . Even after 6 months, the dispersion shows no signs of gelling or sedimentation. Table 1 : Properties of silica powders

Table 2 : Dispersing parameters and properties of silica dispersions

*23 C

Claims

Claims :

1. Process for preparing an aqueous dispersion of fumed silica having a pH of 9 to 11.5, the silica having a BET surface area of 30 to 400 m²/g and being present in the dispersion at 20% < 40% by weight, characterized in that

- water from a reservoir is circulated via a rotor/stator machine and, via a feed means, continuously or discontinuously and with the rotor/stator machine running, fumed silica powder is introduced in an amount such as to result in a preliminary dispersion having a silica content of between 25% and 70% by weight, and, after all of the silica powder has been added, - the feed means is closed and, at a pH of 2 to 4 and a temperature of 10 to 50⁰C, shearing is carried out with a shear rate of 10 000 to 30 000 s^"1,

- dilution is carried out with water until the desired silica content is exceeded by 0.1% to 10%, and - subsequently, under the same shearing conditions, an aqueous base is added in an amount and concentration such that the amount needed to attain the desired solids content is reached and a pH of 9 to 11.5 results .

2. Process according to Claim 1, characterized in that the preliminary dispersion possesses a silica content of 25% to 60% by weight.

3. Process according to Claim 1 or 2, characterized in that the acid used when dispersing in the pH range from 2 to 4 is hydrochloric, sulfuric or nitric acid.

4. Process according to Claims 1 to 3, characterized in that prior to the addition of the aqueous base the silica content of the dispersion is 0.2% to 5% above the desired content.

5. Process according to Claims 1 to 4, characterized in that aqueous ammonia, sodium hydroxide solution or potassium hydroxide solution is used as aqueous base.

6. Process according to Claims 1 to 5, characterized in that the concentration of the aqueous base used is 2 to 20 mol/1.

7. Process according to Claims 1 to 6, characterized in that the dispersing operation is carried out in the pH range from 2 to 4 at a temperature of not more than 55°C.

8. Process according to Claims 1 to 7, characterized in that the fumed silica has a BET surface area of 40 to 60

9. Process according to Claims 1 to 7, characterized in that the fumed silica has a BET surface area of 80 to

110 m²/g.

10. Process according to Claims 1 to I₁ characterized in that the fumed silica has a BET surface area of 130 to 170 m²/g.

11. Process according to Claims 1 to 7, characterized in that the fumed silica has a BET surface area of 180 to 230 m²/g.

12. Process according to Claims 1 to 7, characterized in that the fumed silica has a BET surface area of 270 to 330 m²/g.

13. Process according to Claims 1 to 7, characterized in that the fumed silica is a mixed silicon aluminium oxide .