GB2229432A

GB2229432A - Aqueous colloidal dispersion of fumed silica, acid and stabilizer

Info

Publication number: GB2229432A
Application number: GB9006122A
Authority: GB
Inventors: Dennis G Miller; William F Moll
Original assignee: Cabot Corp
Current assignee: Cabot Corp
Priority date: 1989-03-21
Filing date: 1990-03-19
Publication date: 1990-09-26
Anticipated expiration: 2010-03-19
Also published as: BR9001302A; GB9006122D0; FR2644768A1; IT9019748A1; KR0148692B1; BE1002854A3; AU5201090A; CA2012719A1; GB2229432B; CN1027162C; CN1045754A; IT1239546B; JPH0360420A; AU625980B2; IT9019748A0; JP2949633B2; KR900014248A; DE4006392A1; CA2012719C; FR2644768B1

Description

AQUEOUS COLLOIDAL DISPERSION OF FUMED SILICA, ACID AND STABILIZER The

present invention relates to an aqueous colloidal dispersion of fumed silica, acid and stabilizer. A process for making such an aqueous colloidal dispersion is also disclosed.

There are many applications for fumed silicas of extremely fine particle size in which it is convenient to aplly the fumed silica in the form of an aqueous colloidal dispersion. Such applications include non-slip floor waxes, foamed rubbe,- lattices, paper coatings, the sol-gel process for the manufacture of optical fibres and quartz glassware, and thermal insulation. Aqueous colloidal dispersions of fumed silica are also utilized for frictionizing and polishing. There are also many occasions where it is convenient to densify fumed silica for storage or transport by combining the fumed silica with water to form an aqueous colloidal dispersion.

Fumed silica is generally produced by the vapour phase hydrolysis of chlorosilanes, such as silicon tetrachloride. in a hydrogen oxygen flame. The overall reaction is:

SiCl + 2H + 0 --> SiO + 4 HCl 4 2 2 2 In this process, submicron sized molten spheres of silica are formed. These particles collide and fuse to form three dimensional, branched, chain-like aggregates, of approximately 0.1 to 0.5 microns in length. Cooling takes place very quickly, limiting the particle growth and insuring the fumed, silica is amorphous. These aggregates in turn form agglomerates ranging in size from 0.5 to 44 microns (325 US mesh). Fumed silicas generally have very high purity, with total impurities, in many cases below 100 ppm (parts per million). This high purity makes fumed silica aqueous dispersions particularly advantageous for many applications.

Another consideration for many applications is the removal of grit from the aqueous dispersion of fumed silica since grit is a major source of impurities. Grit can also interfere with many applications of the dispersion. For example, in coagulation of latex grit will lead to the formation of defects in the structure of the rubber, and in the polishing of semiconductor single crystals grit can cause scratching. Thus it is generally desirable that the aqueous dispersion be of high purity. One method for increasing purity is to pass the aqueous colloidal dispersion 2 t through a filter, also referred to as filtering, to remove grit and other impurities. In order for an aqueous colloidal dispersion to be filterable, the viscosity of the colloidal dispersion must be low enough, and the colloidal dispersion must be non-dilitant to enable the colloidal dispersion to pass through the desired filter. For the purposes of the present invention. a non-dilitant dispersion is a dispersion which will pass through a filter having a pore size of 1000 microns or smaller.

As described above, the ability of a dispersion to pass through a filter is also related to the viscosity of a dispersion. The finer the filter, i.e. the smaller the size of the pores of the filter, the lower the viscosity of the aqueous colloidal dispersion must be to pass through the filter. As will be appreciated by those of ordinary skill in the art, to increase purity, the colloidal dispersion should be passed through as fine a filter as possible. Thus it is generally advantageous to produce aqueous colloidal dispersions with low viscosities. For the purposes of the present invention low viscosities are viscosities below about 1000 centipoise.

Additionally, in order to be useful for the -applications listed above and other potential 3 applications the aqueous colloidal dispersion cannot gel into a solid. The ability of the aqueous colloidal dispersion to resist gelling is generally referred to as the stability of the aqueous colloidal dispersion. More stable aqueous colloidal dispersions will not gel as soon as less stable aqueous colloidal dispersions.

The present invention is an aqueous colloidal dispersion of fumed silica, acid and stabilizer comprising at least about 40%, by weight, fumed silica, acid in an amount of between.0025% and.50% of the amount, by weight of the fumed silica, sufficient stabilizer to raise the pH of the dispersion to between 7 and 12, dispersed in water.

A process for producing aqueous colloidal dispersions of fumed silica, acid and stabilizer, and especially including those having fumed silica concentration of at least 40%, by weight is also disclosed. According to t"he present invention a stable, non-dilatant, low viscosity, filterable, aqueous colloidal dispersion of fumed silica is produced by dispersing fumed silica in an acid-water solution in a mixer in an amount such that the fumed silica concentration of the first dispersion, by weight, exceeds the amount of silica desired in the final dispersion, 4 J 1 diluting the first dispersion in the mixer with an additional amount of water such that the resulting final dispersion contains the desired concentration of fumed silica, and then adding a stabilizer to adjust the pH of the final aqueous colloidal dispersion of fumed silica, acid and stabilizer to between 7 and 12 and preferably between 7.5 and 11. This resulting final dispersion may optionally be filtered to remove grit and agglomerates.

The fumed silica wets and mixeR quicker into the water-acid solution than into water alone. The addition of acid also reduces the viscosity of the aqueous colloidal dispersion of fumed silica in the mixer as the dispersion is adjusted from acidic to alkaline pH by the addition of the stabilizer. The reduced viscosity helps prevent the dispersion from gelling as the pH of the dispersion is raised by the stabilizer.

The process of the present invention may be used with fumed silica having any surface area in order to produce aqueous colloidal dispersions of fumed silica.

In order to produce an aqueous colloidal dispersion of fumed silica having a fumed silica concentration of at least 40% by weight according to the present invention. however, preferably a fumed silica with a surface area below 75 square metres per gram (sq.m/g), more preferably between 10 sq.m/g and 75 sq.m/g, most preferably between 35 sq.m/g and 80 sq.m/g, is utilized.

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 nondilitant. For the purposes of the present invention "stable" means that the dispersion will not gel for a period of at least 1 day. Typically, the aqueous colloidal dispersions of fumed silica produced according to the process of the present invention are stable for a period of at least a week, preferably several weeks, and more preferably several months to years. As previously explained, for the purposes of the present invention IlnondilitanC refers to the ability of a dispersion to pass through a 1000 micron or smaller pore size filter without gelling. Typically the aqueous colloidal dispersions of fumed silica, produced according to the process of the present invention, will pass through a 250 micron or smaller pore size filter, preferably through a 25 micron or smaller pore size filter, and more preferably through a 10 micron or smaller pore size filter. Typically the "low viscosity" of the aqueous colloidal dispersions of fumed silica, produced according to the process of the present 6 1 1 invention, will be below 1000 centipoise, preferably below 250 centipoise.

Another advantage of the present invention is that the aqueous colloidal dispersions of fumed silica, acid and stabilizer, having a fumed silica concentration of at least about 40%, by weight, are stable for a period of months to years, have low viscosities and are nondilatant. The low viscosity and non-dilatant qualities permit the aqueous colloidal dispersion to pass through a fine filter.

Additional advantages of the present invention will become apparent from the following more detailed description of the invention.

According to the present invention a mixer is charged, generally up to about 50%, with a quantity of water, preferably water which has been de-ionized, and acid is added to the water. Preferably the mixer utilized is a high shear mixer, capable of forming the dispersions, such as those generally known to the art.

The acid may be a mineral or organic acid such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid. acetic acid or maleic acid. The quantity of acid added to the water is based on the amount of fumed silica which will be added to the water 7 to form the final aqueous colloidal dispersion. Generally the quantity of acid added to the water is an amount, by weight, between 0.0025% and 0. 50%, and preferably between 0.02% and 0.15% of the amount, by weight, of fumed silica which will be added to the water. After the addition of.acid to the water the mixer may be operated to mix the acid and water to form a water-acid solution.

The percentage water initially charged to the mixer can obviously vary. However, as will become apparent from the following description, there must be room left in the mixer to add fumed silica and additional water. The initial quantity of water chosen is usually based on the amount of fumed silica to be added, and the desired final concentration of fumed silica in the aqueous colloidal dispersion. For example, if the desired final concentration of the aqueous colloidal dispersion of fumed silica is about 50% fumed silica, by weight, and 45. 4Kg (100 lbs.) of fumed silica are to be added to the mixer, then the initial quantity of water is that quantity which will result in a greater than 50% by weight concentration 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 8 f at least about 5% greater than the desired final concentration of fumed silica in the aqueous colloidal dispersion of fumed silica. Thereafter the aqueous colloidal dispersion in the mixer will be diluted by the addition of an additional amount of water to achieve the desired final concentration of fumed silica in the dispersion of about 50% by weight.

After the mixer is charged with water, and acid is added to the water, fumed silica is added to the water- acid solution in the mixer. The fumed silica may be added by mixing the fumed silica into 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 incrementally, in a series of steps, with the mixer operating between each step.

As previously discussed the process of the present invention may be utilized with fumed silicas having any surface areas. To produce aqueous colloidal dispersions of fumed silica having fumed silica concentrations at least about 40% by weight, preferably a fumed silica with a surface area less than 75 sq.m/g is utilized. More preferably a fumed silica with a surface area between 10 tq.a/g and 75 sq.a/g is utilized, and most preferably a 9 fumed silica with a surface area between 35 sq.m/g and 60 sq.z/g is utilized to form the aqueous colloidal dispersions of fumed silica having fumed silica concentrations of at least 40% by weight.

The immediate effect of the addition, or each addition, of fumed silica to the mixer will be to thicken the aqueous colloidal dispersion of fumed silica in the mixer. As the mixer continues to operate however, the aqueous colloidal dispersion of fumed silica in the mixer will thin.

After the concentration of fumed silica, by weight, in the aqueous colloidal dispersion of fumed silica in the mixer has been raised to a point above the desired final concentration of fumed silica, by weight, the mixer is allowed to operate until the dispersion in the mixer thins. As previously explained, typically in the process of the present invention, the dispersion in the mixer, before dilution, will have a fumed silica concentration at least about 5% greater than the desired final concentration of fumed silica in the aqueous colloidal 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 into the aqueous colloidal dispersion in the mixer by operating the il mixer. The amount of water added is the amount which will lower the concentration of fumed silica, by weight, of the aqueous colloidal dispersion of fumed silica in the mixer to the desired final concentration, taking into account the stabilizer which will be added to the dispersion.

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

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

11 The process of the present invention allows the production of stable, non- dilitant, low viscosity, high purity, filterable aqueous colloidal dispersion of fumed silica, acid and stabilizer of any fumed silica concentrations. It is particularly noteworthy that aqueous colloidal dispersions having a fumed silica concentration of at least 40% by weight, and more preferably having a fumed silica concentration between 40% and 65% by weight, may be prepared. The resulting aqueous colloidal dispersions of fumed silica, acid and stabilizer are non- dilitant, have low viscosities, and are stable for a period of between several months to longer than a year.

The effectiveness and advantages of the present invention will be further illustrated by the following Examples. The following Examples illustrate the production of aqueous colloidal dispersions of fumed silica, acid and stabilizer, having fumed silica concentrations of 40% and 65%, by weight respectively. Obviously, however. the amounts of fumed silica, acid, stabilizer, and water used in these examples can be varied to produce aqueous colloidal dispersion of fumed silica with different fumed silica concentrations such as 45%, 50%, 55% and 60% by weight.

12 EXAMPLE 1

The following Example illustrates the preparation of an aqueous colloidal dispersion of fumed silica. acid and stabi lizer having a fumed silica concentration of 40% by weight using a 378.5 litre (100 gallon) capacity high shear mixer capable of forming the dispersions.

The high shear mixer is initially charged with 151.4 litres (40 gallons) of water and about 813 gms (1.35 lbs) of a 37% solution of HCl, which is about.27% of the amount, by weight, of the fumed silica which will be added to the mixer, are added to the water. About 227Kg (500 lbs) of fumed silica, having a surface area of about 50 sq.z/g, is slowly added to the mixer, 45.4Kg, (100 lbs) at a time, while the mixer is operating, to form an aqueous colloidal dispersion of fumed silica in the mixer having a fumed silica concentration of about 80% by weight. At this point, 177.9 (47) additional litres (gallons) of water are slowly added to the mixer, while the mixer is operating, to form an aqueous colloidal dispersion of fumed silica having a fumed silica concentration of about 41% by weight. A sufficient amount of ammonium hydroxide, which in this instance is about 10.6 litres (2.8 gallons) of a 30% ammonium hydroxide solution, is added to this aqueous colloidal dispersion of fumed silica to adjust the pH 13 of the dispersion to about 10.4. After the addition of the ammonium hydroxide the resulting aqueous colloidal dispersion of fumed silica has a fumed silica concentration of about 40% by weight and may optionally be filtered to remove any grit or agglomerated particles. Filtering the aqueous colloidal dispersion will change the fumed silica concentration less than about 0.5% by weight. The filtered or unfiltered aqueous colloidal dispersion of fumed silica, acid and stabilizer may be stored, and/or packaged for transport, in any of the manners generally known in the art.

EXAMPLE 2

The following Example illustrates the preparation of an aqueous colloidal dispersion of fumed silica, acid and stabilizer having a fumed silica concentration of about 85% by weight using a 378.5 litre (100 gallon) capacity high shear mixer capable of forming the dispersions.

The high shear mixer is initially charged with 98.4 litres (28 gallons) of water and about 613 gms (1.35 lbs) of a 37% solution of HCl, which is about.27% of the amount, by weight. of fumed silica which will be added to the mixer, are added to. the water. About 227Kg (500 lbs) of fumed silica, having a surface area of about 50 sq.a/g, is slowly added to the mixer 45.4Kg (100 lbs) 14 at a time, while the mixer is operating, to form an aqueous colloidal dispersion of fumed silica in the mixer having a fumed silica concentration of about 70% by weight. At this point 15.9 (4.2) additional litres (gallons) of water are slowly added to the mixer, while the mixer is operating, 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, which in this instance is about 10.6 litres (2.8 gallons) of a 30% ammonium hydroxide solution, is added to this aqueous colloidal dispersion of fumed silica to adjust the pH of the dispersion to about 10.4. After the addition of ammonium hydroxide the resulting aqueous colloidal dispersion of fumed silica has a fumed silica concentration of about 65% by weight and may optionally be filtered to remove any grit or agglomerated particles. Filtering the aqueous colloidal dispersion will change the fumed silica concentration less than about 0.5% by weight. The filtered or unfiltered aqueous colloidal dispersion of fumed silica, acid and stabilizer may be stored, and/or packaged for transport, in any of the manners generally known in the art.

Similar results have been obtained by following the procedures of Examples 1 and 2 and by varying the initial amount the mixer is charged with water, the amount of acid, and the amount of additional water added to the mixer to produce aqueous colloidal dispersions of fumed silica, acid, and stabilizer with fumed silica concentrations of 45%, 50%, 55% and 60% by weight.

Numerous modifications and variations may obviously be made in the above described Examples without departing from the present invention.

16

Claims

1. A colloidal dispersion comprising: at least 40% by weight fumed silica, acid in an amount between 0.0025% and 0.05% of the amount, by weight, of the fumed silica, an amount, by weight, of stabilizer which will raise the pH of the dispersion to between 7.0 and 12.0, dispersed in water. 2. The colloidal dispersion of claim 1 wherein the fumed silica is present in an amount between 40% and 65% by weight.

3. The colloida silica is present 4. The colloidal silica is present 5. The colloidal silica is present 8. The colloidal silica is present 7. The colloidal silica is present 8. The colloidal silica is present 1 dispersion of claim 1 wherein the fumed in an amount of 40% by weight. dispersion of claim 1 wherein the fumed in an amount of 45% by weight. dispersion of claim 1 wherein the fumed in an amount of 50% by weight. dispersion of claim 1 wherein the fumed in an amount of 55% by weight. dispersion of claim 1 wherein the fumed in an amount of 60% by weight. dispersion of claim 1 wherein the fumed in an amount of 65% by weight.

17 9. The colloidal dispersion of claim 1 wherein the acid is a mineral acid or an organic acid.

10. The colloidal dispersion of claim 1 wherein the acid is hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid or maleic acid.

11. The colloidal dispersion of claim 1 wherein the acid is hydrochloric acid.

12. The colloidal dispersion of claim 1 wherein the acid is present in an amount between 0.02% and 0.15% of the amount by weight of the fumed silica.

13. The colloidal dispersion of claim 1 wherein the stabilizer is an alkali or an amine.

14. The colloidal dispersion of claim 1 wherein the stabilizer is sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, triethylazine or dimethylethanol amine.

15. The colloidal dispersion of claim 1 wherein the stabilizer is ammonium hydroxide.

16. The colloidal dispersion of claim 11 wherein the stabilizer is ammonium hydroxide.

17. The colloidal dispersion of claim 1 wherein the stabilizer is added in an amount which will-raise the pH of the dispersion to between 7.5 and 11.

1 k 18. The colloidal dispersion of claim 1 wherein the fumed silica has a surface area less than 75 sq.m/g. 19. The colloidal dispersion of claim 1 wherein the fumed silica has a surface area between 10 sq.a/g and 75 sq. /g. 20. The colloidal dispersion of claim 1 wherein the fumed silica has a surface area of 50 sq.z/g.

/7 Published 1990 at The Patent =ice. Statelicuse 6671 High Holborn.Londor WC1 R 4TP-Fu.-the.- copies maybe obtained from The Patent OZice Sales Braneb. St Mary Cray- OrpIngtoi.. Kent BR5 3RD Printed by Multiplex techniques ltd. St MEL-y Cray. Kent. Con 1'87