FR2644768A1 - AQUEOUS COLLOIDAL DISPERSION OF SMOKE SILICA, ACID AND STABILIZER - Google Patents

Abstract

The invention relates to a collodal aqueous dispersion of fumed silica, an acid and a stabilizer. According to the invention, it contains at least approximately 40% by weight of fumed silica, the acid in an amount between approximately 0.0025% and approximately 0.50% of the amount by weight of the fumed silica, and the stabilizer in an amount by weight raising the pH of the dispersion to between about 7.0 and about 12.0, dispersed in water. The invention applies in particular to fumed silicas of extremely small particle size.

Description

The present invention relates to a colloidal aqueous dispersion of

  fumed silica, an acid and a stabilizer. A method of preparing such

  Colloidal aqueous dispersion is also disclosed.

  There are many applications for fumed silicas of an extremely fine particle size where it is convenient to apply the fumed silica in the form of a colloidal aqueous dispersion. Such applications include non-slip floor waxes, foam rubber latices, paper coatings, the sol-gel process for the manufacture of optical fibers and

  of quartz glass material, and thermal insulation.

  The colloidal aqueous dispersions of fumed silica are

  also used for rubbing and polishing.

  There are also many occasions where it is practical to densify the fumed silica for storage or transport by combining it with water to form a

colloidal aqueous dispersion.

  The fumed silica is generally produced by the vapor phase hydrolysis of chlorosilanes, such as silicon tetrachloride, in a flame of hydrogen and oxygen. The whole reaction is: SiCt 4 + 2H 2 + O 2 SiO 2 + 4 HCl In this process, melted silica spheres of less than one micron size are formed. These particles collide and fuse to form three-dimensional, branched, chain-like aggregates having about 0.1 to 0.5 microns in length. Cooling takes place very quickly, limiting the growth of particles and ensuring that the fumed silica will be amorphous. These aggregates in turn form agglomerates with a

  size between 0.5 and 44 microns (US 325 mesh).

  The fumed silicas generally have a very high purity, with lower total impurities, in many cases, at 100 ppm (parts per million). This high purity

  makes the aqueous dispersions of fumed silica particularly

  advantageously for many applications.

  Another consideration for many applications is the removal of abrasive dust from the aqueous dispersion of fumed silica because abrasive dust is a major source of impurities. Abrasive dust can also interfere with many applications of dispersion. For example, in the coagulation of latex rubber, abrasive dust will lead to the formation of defects in the rubber structure and in the polishing of semiconductor monocrystals, abrasive dust can cause scratches. So, it is usually

  desirable that the aqueous dispersion is of high purity.

  One method of increasing purity is to pass the colloidal aqueous dispersion through a filter, also known as filtration, to remove abrasive dust and other impurities. In order for a colloidal aqueous dispersion to be filtered, its viscosity must be sufficiently low, and it must not expand to allow it to pass through the desired filter. For the present invention, a non-expanding dispersion is a dispersion that will pass through a filter having a pore size of 1000 microns or less. As described above, the ability of a dispersion to pass through a filter is also related to its viscosity. The finer the filter, i.e. the smaller the pore size of the filter, the lower the viscosity of the colloidal aqueous dispersion needs to pass through the filter. As those skilled in the art will be able to note, to increase the purity, the colloidal dispersion must pass through a filter as fine as possible. Thus, it is generally advantageous to produce aqueous colloidal dispersions having low viscosities. For the present invention, the low viscosities are

  viscosities below about 1000 centipoise.

  In addition, in order to be useful for the applications listed above and other potential applications, the colloidal aqueous dispersion can not gel into a solid. The ability of the aqueous colloidal dispersion to resist gelation is generally

  called the stability of the colloidal aqueous dispersion.

  More stable colloidal aqueous dispersions will not gel as quickly as aqueous dispersions

colloidal less stable.

  The present invention relates to a colloidal aqueous dispersion of fumed silica, an acid and a stabilizer comprising at least about 40% by weight of fumed silica, the acid in an amount of from about 0.0025% to about 0, 50% of the amount, by weight, of the fumed silica, sufficient stabilizer to raise the pH of the dispersion from about 7 to about 12, in dispersion

in water.

  A process for producing colloidal aqueous dispersions of fumed silica, an acid and a stabilizer and especially comprising those having a fumed silica concentration of at least about 40% by weight is also disclosed. According to the present invention, a stable, non-expanding, low viscosity, filterable, colloidal, aqueous dispersion of fumed silica is produced by dispersing the fumed silica in an acid-water solution in a mixer in an amount such as the concentration of fumed silica of the first dispersion, by weight, exceeds the desired amount of silica in the final dispersion, by diluting the first dispersion in the mixer with an additional amount of water so that the resulting final dispersion contains the desired concentration of fumed silica, then adding a stabilizer to adjust the pH of the colloidal aqueous dispersion of fumed silica, acid and stabilizer to between about 7 and 12 and preferably about 7.5 to about 11. This resulting final dispersion may optionally be to be filtered to eliminate the

  abrasive dust and agglomerates.

  The fumed silica gets wet and mixes more

  quickly in the water-acid solution only in water alone.

  The addition of acid also reduces the viscosity of the colloidal aqueous dispersion of fumed silica in the mixer while the dispersion is adjusted from an acidic to alkaline pH by the addition of the stabilizer. Reduced viscosity helps prevent dispersion from freezing while

  that its pH is increased by the stabilizer.

  The process of the present invention can be used with fumed silica having any surface area to produce colloidal aqueous dispersions of fumed silica. In order to produce colloidal aqueous dispersions of fumed silica having a fumed silica concentration of at least 40% by weight according to the present invention, however, a fumed silica having a surface area of less than about -75 square meters is preferably used. per gram (m2 / g) and better between about m2 / g and about 75 m2 / g and even better between about

35 m2 / g and about 60 m2 / g.

  An advantage of the process of the present invention is that the resulting aqueous colloidal dispersions of fumed silica, acid and stabilizer are stable and do not expand. For the present invention, "stable" means that the dispersion will not gel for at least one day. Typically, the colloidal aqueous dispersions of fumed silica produced according to the process of the present invention are stable for at least one week, preferably several weeks and more preferably several months to years. As previously explained, for the present invention, "non-expanding" indicates the ability of a dispersion to pass through a filter having a pore size of 1000 microns or less, without gelation. Typically, the colloidal aqueous dispersions of fumed silica produced according to the process of the present invention will pass through a filter having a pore size of 250 microns or less, preferably a filter having a pore size of 25 microns or less, and better a filter having a pore size of 10 microns or less. Typically, the "low viscosity" of the colloidal aqueous dispersions of fumed silica, produced according to the process of the present invention, will be less than about 1000 centipoise and preferably

  less than about 250 centipoise.

  Another advantage of the present invention resides in the fact that the colloidal aqueous dispersions of fumed silica, acid and stabilizer, having a fumed silica concentration of at least about 40% by weight, are stable for several months at many

  years, have low viscosities and do not expand.

  The low viscosity and non-dilatation qualities allow the colloidal aqueous dispersion to flow through

a fine filter.

  Other advantages of the present invention

  will become apparent upon reading the description more

detailed below.

  According to the present invention, a quantity of water, preferably water which has been de-ionized, is introduced into a mixer, generally up to about 50%, and an acid is added to the water. Preferably, the mixer used is a high shear mixer capable of

  to form the dispersions, such as those generally known.

  The acid may be a mineral or organic acid such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid or malignant acid. The amount of acid added to the water is based on the amount of fumed silica that will be added to the water to form the final aqueous colloidal dispersion. In general, the amount of acid added to the water is an amount, by weight, of between about 0.0025% and about 0.50% and preferably between about 0.02% and 0.15% of the amount, by weight of fumed silica which will be added to the water. After adding the acid to the water, the mixer can work to mix the acid and water to form a

water-acid solution.

  The percentage of water initially introduced into the mixer may of course vary.

  will become apparent from the description that follows, it must

  leave room in the mixer to add the fumed silica and additional water. The initial amount of water selected is usually based on the amount of fumed silica to be added and the desired final concentration of fumed silica in the colloidal aqueous dispersion. For example, if the desired final concentration of the colloidal aqueous dispersion of fumed silica is about 50% fumed silica, by weight, and 45.4 kg of fumed silica is added to the mixer, the initial amount of The water is then the amount which will result in a concentration of more than 50% by weight of fumed silica in the mixer. Typically, in the process of the present invention, the dispersion in the mixer, before dilution, will have a fumed silica concentration of at least about 5% greater than the desired final concentration of fumed silica in the colloidal aqueous dispersion of fumed silica . Then, the aqueous colloidal dispersion in the mixer will be diluted by the addition of an additional amount of water to obtain the desired final concentration of fumed silica in the mixture.

dispersion of about 50% by weight.

  After loading the water mixer, an acid is added to the water, the fumed silica is added to the water-acid solution in the mixer. The fumed silica may be added by mixing the fumed silica in the water-acid mixture while the mixer is operating or by adding the fumed silica to the water-acid mixture and then operating the mixer. The fumed silica may also be added in increments, in a series of steps, with the mixer operating between each step. As previously described, the process of the present invention can be used with fumed silicas having any surface area. To produce colloidal aqueous dispersions of fumed silica having fumed silica concentrations of at least about 40% by weight, fumed silica having a lower surface area of about 75 m 2 / g is preferably used. Preferably, a fumed silica having a surface area of from about 10 m 2 / g to about 2 m 2 / g and more preferably a fumed silica having a surface area of about 35 m 2 / g to about 2 m 2 / g is used. to form colloidal aqueous dispersions of fumed silica having fumed silica concentrations of

at least about 40% by weight.

  The immediate effect of the addition or addition of fumed silica to the mixer will be to thicken the dispersion

  colloidal aqueous silica fumed in the mixer.

  However, while the mixer continues to operate, the colloidal aqueous dispersion of fumed silica in the

mixer is diluted.

  After the fumed silica concentration, by weight, in the colloidal aqueous dispersion of fumed silica in the mixer has reached a point above the desired final concentration of the fumed silica, by weight, the mixer is allowed to operate until the dispersion in the mixer is diluted. As previously explained, typically, in the process of the present invention, the dispersion in the mixer, before dilution, will have a fumed silica concentration of at least

2'644768

  About 5% greater than the desired final concentration of fumed silica in the colloidal aqueous dispersion of fumed silica. Then, an additional amount of water is added to the mixer to dilute the dispersion in the mixer. Preferably, this additional water has been deionized. The additional water is then mixed in the aqueous colloidal dispersion in the mixer by operating the latter. The amount of water added is the amount which will reduce the concentration of the fumed silica, by weight, in the colloidal aqueous dispersion of fumed silica in the mixer to the desired final concentration, taking into account

  account of the stabilizer that will be added to the dispersion.

  After adding the additional water, a stabilizer, such as an alkali or an amine, is added to the colloidal aqueous dispersion of fumed silica in an amount that will adjust the pH of the final dispersion to between about 7 and about 12 and preferably about 7.5 and about 11. The particular pH chosen for the final dispersion will depend on the application for which the colloidal aqueous dispersion of fumed silica is intended. Suitable stabilizers include, but are not limited to, alkalis or amines such as sodium hydroxide, potassium hydroxide, lithium hydroxide, hydroxide

  ammonium, triethylamine and dimethylethenol amine.

  After addition of the stabilizer, the colloidal aqueous dispersion of fumed silica may be removed from the mixer and stored or packaged for transport, in any case generally known. If desired, the colloidal aqueous dispersion of fumed silica may also be passed through a filter to remove abrasive dust and all agglomerated particles.

of fumed silica.

  The process of the present invention allows the production of a stable, non-expanding, low-viscosity, high-purity aqueous dispersion of

2644? 68

  Filtering of fumed silica, an acid and a stabilizer to

  any concentration of fumed silica. It is particularly necessary

  It should be noted that aqueous colloidal dispersions having a fumed silica concentration of at least about 40% by weight, and more preferably, having a fumed silica concentration of from about 40% to about 65% by weight,

  can be prepared. The aqueous colloidal dispersions

  the resultants of fumed silica, acid and stabilizer do not expand, have low viscosities

  and are stable for several months to more than a year.

  The effectiveness and benefits of this

  invention will be better illustrated by the following examples.

  The following examples illustrate the production of colloidal aqueous dispersions of fumed silica, an acid and a stabilizer, having fumed silica concentrations of about 40% and about 65%, by weight, respectively. However, it is evidenz that the amounts of fumed silica, acid, stabilizer and water used in these examples can be changed to produce a colloidal aqueous dispersion of fumed silica having different concentrations of fumed silica such as 45%,

%, 55% and 60% by weight.

EXAMPLE 1

  The following example illustrates the preparation of a colloidal aqueous dispersion of fumed silica, an acid and a stabilizer, having a fumed silica concentration of about 40% by weight, using a high shear mixer, with a capacity of 378 liters, capable of

to form the dispersions.

  The high shear mixer is initially charged with 151 liters of water and about 0.61 kg of a 37% solution of HCl is added to the water, which is about 0.27% of the amount, by weight, fumed silica which will be added to the mixer. About 227 kg of fumed silica, having a surface area of about 50 m 2 / g, are slowly added to the mixer at a rate of 45.4 kg.

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  While the mixer is operating, it forms an aqueous colloidal dispersion of fumed silica in the mixer having a fumed silica concentration of about 60% by weight. At this point, an additional 178 liters of water are slowly added to the mixer as it operates to form a colloidal aqueous dispersion of fumed silica having a fumed silica concentration of about 41% by weight. A sufficient amount of ammonium hydroxide, which in this case is about 10.6 liters of a 30% solution of ammonium hydroxide, is added to this fumed silica aqueous colloidal dispersion to adjust the pH. of the dispersion at about 10.4. After addition of the ammonium hydroxide, the resulting colloidal aqueous dispersion of fumed silica has a fumed silica concentration of about 1% by weight and may optionally be filtered to remove any abrasive dust or agglomerated particles. Filtration of the colloidal aqueous dispersion will change the fumed silica concentration by less than about 0.5% by weight. The filtered or unfiltered colloidal aqueous dispersion of fumed silica, acid and stabilizer can be stored and / or packaged for its

  transport in each of the generally known ways.

EXAMPLE 2

  The following example illustrates the preparation of a colloidal aqueous dispersion of fumed silica, an acid and a stabilizer having a fumed silica concentration of about 65% by weight using a high shear mixer. a capacity of 378 liters, capable of

to form the dispersions.

  The high shear mixer is initially charged with 98 liters of water and about 0.6 kg of a 37% HCl solution, which is about 0.27% of the amount, by weight, of fumed silica. which will be added in the mixer. About 227 kg of fumed silica having a surface area are slowly added to the mixer.

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  of about 50 m 2 / g, 45.4 kg at a time, while the mixer operates to form a colloidal aqueous dispersion of fumed silica in the mixer, having

  a fumed silica concentration of about 70% by weight.

  At this point, an additional 16 liters of water are slowly added to the mixer while the mixer operates to form an aqueous colloidal dispersion of fumed silica having a fumed silica concentration of about 66.5% by weight. A sufficient amount of ammonium hydroxide is added, which in this case represents about 10.6 liters of a 30% solution of ammonium hydroxide, in this colloidal aqueous dispersion of fumed silica, to adjust the pH of the dispersion at about 10.4. After addition of ammonium hydroxide, the resultant colloidal aqueous diption of fumed silica has a fumed silica concentration of about 1% by weight and can optionally be filtered to remove any abrasive dust or agglomerated particles. Filtration of the colloidal aqueous dispersion will change the fumed silica concentration by less than about 0.5% by weight. The filtered or unfiltered colloidal aqueous dispersion of fumed silica, an acid and the stabilizer can be stored and / or packaged

  for its transport, in any case generally known.

  Similar results were obtained by following the procedures of Examples 1 and 2 and changing the initial amount of water introduced into the mixer, the amount of acid and the amount of additional water added to the mixer to produce aqueous dispersions. colloidal fumed silica, acid and stabilizer with fumed silica concentrations of 45%, 50%,

% and 60% by weight.

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Claims (19)

R E V E N D I C A T I 0 N S   1. Colloidal dispersion, characterized in that it contains at least about 40% by weight of fumed silica, acid in an amount of from about 0.0025% to about 0.50% of the amount, by weight fumed silica, an amount, by weight, of the stabilizer which will raise the pH of the dispersion from about 7.0 to about 12.0, in dispersion in water.   2. Colloidal dispersion according to claim 1, characterized in that the fumed silica is present in one   amount between about 40% and 65% by weight.   3. Colloidal dispersion according to claim 1, characterized er. that the fumed silica is present in one amount of about 40% by weight.   4. Colloidal dispersion according to claim 1, characterized in that the fumed silica is present in one amount of about 45% by weight.   5. Colloidal dispersion according to claim 1, characterized in that the fumed silica is present in one amount of about 50% by weight.   6. Colloidal dispersion according to claim 1, characterized in that the fumed silica is present in one amount of about 55% by weight.   7. Colloidal dispersion according to claim 1, characterized in that the fumed silica is present in one amount of about 60% by weight.   8. Colloidal dispersion according to claim 1, characterized in that the fumed silica is present in one amount of about 65% by weight.   9. Colloidal dispersion according to claim 1, characterized in that the acid is selected from the group   consisting of mineral acids and organic acids.   10. Colloidal dispersion according to claim 1, characterized in that the acid is selected from the group consisting of hydrochloric acid, sulfuric acid, acid - 13 -   nitric acid, phosphoric acid, acetic acid and maleic acid. 11. Colloidal dispersion according to claim 1,   characterized in that the acid is hydrochloric acid.   Colloidal dispersion according to claim 1, characterized in that the acid is present in an amount of between about 0.02% and about 0.15% of the   amount by weight of the fumed silica.   13. Colloidal dispersion according to claim 1, characterized in that the stabilizer is chosen in the   group consisting of alkalis and amines.   The colloidal dispersion according to claim 1, characterized in that the stabilizer is selected from the group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide,   triethylamine, and dimethylethanolamine.   15. Colloidal dispersion according to claim 1, characterized in that the stabilizer is ammonium hydroxide. The colloidal dispersion according to claim 1, characterized in that the stabilizer is added in an amount which will raise the pH of the dispersion between about 7.5 and about 11.   Colloidal dispersion according to Claim 1, characterized in that the fumed silica has an area   superficial smaller than about 75 m2 / g.   18. Colloidal dispersion according to claim 1, characterized in that the fumed silica has a surface area of between approximately 10 m 2 / g and approximately 75 m2 / g.   19. Colloidal dispersion according to claim 1, characterized in that the fumed silica has an area superficial of about 50 m2 / g.