Title of Invention	PYROGENIC SILICON DIOXIDE POWDER AND DISPERSION THEREOF
Abstract	Pyrogenically produced silicon dioxide powder with a specific surface area of between 5 and 600 m2/g and a carbon content of less than 500 ppm, which displays a specific dibutyl phthalate absorption of less than or equal to 1.2 g dibutyl phthalate/100 g SiO2 per m2 of specific surface area and a specific thickening action of less than 15 mPas/m2 of specific surface area. It is produced by supplying vaporous tetramethoxysilane and/or tetraethoxysilane together with air and separately hydrogen to a burner, and allowing the mixture of gases to react in a flame in a reaction chamber connected in series to the burner, and separating the solid reaction product from the gas stream by known means, the lambda value in the burner being between 0.95 and 1.5 and sufficient secondary air also being supplied to the reaction chamber that the lambda value in the reaction chamber is between 0.8 and 1.6. The invention also provides a dispersion containing the silicon dioxide powder and the use of the powder and of the dispersion.

Title of Invention

PYROGENIC SILICON DIOXIDE POWDER AND DISPERSION THEREOF

Abstract

Pyrogenically produced silicon dioxide powder with a specific surface area of between 5 and 600 m2/g and a carbon content of less than 500 ppm, which displays a specific dibutyl phthalate absorption of less than or equal to 1.2 g dibutyl phthalate/100 g SiO2 per m2 of specific surface area and a specific thickening action of less than 15 mPas/m2 of specific surface area. It is produced by supplying vaporous tetramethoxysilane and/or tetraethoxysilane together with air and separately hydrogen to a burner, and allowing the mixture of gases to react in a flame in a reaction chamber connected in series to the burner, and separating the solid reaction product from the gas stream by known means, the lambda value in the burner being between 0.95 and 1.5 and sufficient secondary air also being supplied to the reaction chamber that the lambda value in the reaction chamber is between 0.8 and 1.6. The invention also provides a dispersion containing the silicon dioxide powder and the use of the powder and of the dispersion.

Full Text	Pyrogenic silicon dioxide powder and dispersion thereof The invention provides a pyrogenically produced silicon dioxide powder, an aqueous dispersion containing this silicon dioxide powder, the production and use of the silicon dioxide powder and the dispersion. The term pyrogenic silicon dioxide or pyrogenic silica (fumed silica) is a collective term for all highly disperse silicas obtained in the gas phase at elevated temperatures by coagulation of monomeric silica. There are two processes for the industrial production of pyrogenic silicas, high- temperature hydrolysis and the arc process. In the high-temperature hydrolysis process a homogeneous mixture of a vaporous silicon compound, hydrogen, oxygen and an inert gas is burned with a burner in a cooled combustion space. Two reactions proceed side by side here. Firstly the reaction of hydrogen and oxygen with formation of water and secondly the hydrolysis of the silicon compound with formation of silicon dioxide. The homogeneity of the gas mixture means that the reaction conditions and hence the formation and growth conditions are largely the same for each SiO2 particle, such that very homogeneous and uniform particles can form. Air is used as the oxygen source in the known process. The pyrogenic silicas produced by the known process display specific surface areas of between 10 and 600 m2/g. The starting material for the silicon dioxide is generally silicon tetrachloride (cf. Ullmann's Encyclopedia of Industrial Chemistry, Vol. A23, pages 635 ff. 5th edition). In addition to silicon tetrachloride methyl trichlorosilane, trichlorosilane or mixtures thereof with silicon tetrachloride can also be used. JP 2002114510 claims a process in which silicon dioxide is obtained with an average particle size of 0.05 to 5 mm. In this process silicon compounds are burned in the presence of oxygen and hydrogen. Siloxanes, silanes or silicon chlorides can be used as the silicon compound. However, the silicon dioxide produced by this process displays no properties that could not also be obtained by processes of the prior art. The process described is itself only of limited suitability for the production of larger quantities. A non-uniform product and, where carbon- containing silicon compounds are used as starting materials, dark products too are then to be expected in particular. When used in dispersions such as are used in the production of glass articles or in chemical mechanical polishing in the semiconductor industry, the powder produced according to JP 2002114510 provides no advantages over the prior art. Due to growing requirements an improvement in the properties of silicon dioxide is demanded in these very sectors. In the glass industry in particular, highly filled, readily manageable dispersions, in other words ones with low viscosity, are required because of their low shrinkage on drying and sintering. The object of the invention is to provide a silicon dioxide powder which is suitable for the production of highly filled dispersions with low viscosity. The object of the invention is also to provide a stable dispersion containing this silicon dioxide powder. The invention provides a pyrogenically produced silicon dioxide powder having a specific surface area of between 5 and 600 m2/g and a carbon content of less than 500 ppm, which is characterised in that it displays a specific dibutyl phthalate absorption of less than or equal to 1.2 g dibutyl phthalate/100 g SiO2 per m2 of specific surface area and a specific thickening action of less than 15 mPas per m2 of specific surface area. The specific dibutyl phthalate absorption represents a measure of the structure of the silicon dioxide powder according to the invention as a function of its specific surface area. The term structure in this connection means the degree of intergrowth of the primary particles. These are initially formed in the pyrogenic process and as the reaction continues can coalesce to form chain-like aggregates, which in turn form agglomerates. The specific dibutyl phthalate absorption of less than or equal to 1.2 g dibutyl phthalate/100 g SiO2 per m2 of specific surface area claimed for the silicon dioxide powder according to the invention is generally lower than pyrogenic silicon dioxide powders obtained by the prior art. The silicon dioxide powder according to the invention arises only in combination with a specific thickening action. This is understood to mean the thickening action per m2 of specific surface area. The thickening action is determined in a dispersion of a silicon dioxide powder in a polyester. In a preferred embodiment the powders according to the invention can display a specific compacted bulk density, defined as the product of the compacted bulk density and specific surface area, of between 1000 and 10000 and particularly preferably between 4000 and 7000 g/1 x m2 of specific surface area. Powders according to the invention displaying a specific compacted bulk density in this range can be incorporated especially readily into dispersions. Furthermore, silicon dioxide powders according to the invention can preferably have a chloride content of less than 50 ppm, particularly preferably less than 20 ppm. The low chloride contents can for example demonstrate advantageous effects when the powders according to the invention are used in the area of chemical mechanical polishing. The invention also provides a process for the production of silicon dioxide powder according to the invention which is characterised in that vaporous tetramethoxysilane (TMOS) and/or tetraethoxysilane (TEOS) together with air or with oxygen-enriched air and separately hydrogen are supplied to a burner, and the mixture of gases is allowed to react in a flame in a reaction chamber connected in series to the burner, and the solid reaction product is separated from the gas stream by known means, the lambda value in the burner being between 0.95 and 1.5 and sufficient secondary air also being supplied to the reaction chamber that the lambda value in the reaction chamber is between 0.8 and 1.6. Figure 1 shows a simplified process flow chart upon which the process according to the invention is based. A = burner; B = flame; C = reaction chamber; 1 = supply of mixture comprising vaporous tetramethoxysilane and/or tetraethoxysilane together with air or with oxygen-enriched air; 2 = supply of hydrogen; 3 = supply of secondary air. In the performance of the process it is substantial that a premixing of silane and air occurs, the stoichiometry of air/hydrogen and the oxygen component, expressed as the lambda value, being maintained in the burner and reaction chamber. Lambda denotes the ratio of oxygen supplied to the burner or the reaction chamber to stoichiometrically required oxygen, which is needed to convert the silane compound completely to silicon dioxide. The lambda value range that must be maintained in the reaction chamber likewise refers to the total amount of the silane to be hydrolysed. In the process according to the invention the volume ratio of oxygen/hydrogen in the burner can be varied between 0.2 and 2.8. In a particularly preferred embodiment the volume ratio of oxygen/hydrogen in the burner is between 0.9 and 1.4. Depending on the desired specific surface area, it can be useful to vary the streams supplied to the burner and the burner geometry in such a way that the discharge velocity of the gases leaving the burner is at least 10 ms-1. Discharge velocities of at least 20 ms-1 are particularly preferred. The invention also provides an aqueous dispersion containing the silicon dioxide powder according to the invention. The aqueous dispersion according to the invention can display a content of silicon dioxide powder of between 2 0 and 80 wt.%. Dispersions having a content of silicon dioxide powder of between 40 and 60 can be particularly preferred. These dispersions display a particularly high stability with a comparatively low structure. The aqueous dispersion according to the invention can preferably display an average particle size in the aggregates of silicon dioxide powder which is less than 200 nm. For certain applications such as e.g. the chemical mechanical polishing of semiconductor substrates, a value of less than l50 nm can be particularly preferred. The dispersion according to the invention can be stabilised by the addition of bases or cationic polymers or aluminium salts or a mixture of cationic polymers and aluminium salts or acids. Bases that can be used are ammonia, ammonium hydroxide, tetramethyl ammonium hydroxide, primary, secondary or tertiary organic amines, sodium hydroxide solution or potassium hydroxide solution. Cationic polymers that can be used are examples having at least one quaternary ammonium group, phosphonium group, an acid adduct of a primary, secondary or tertiary amine group, polyethylene imines, polydiallylamines or polyallylamines, polyvinylamines, dicyandiamide condensates, dicyandiamide-polyamine cocondensates or polyamide-formaldehyde condensates. Aluminium salts that can be used are aluminium chloride, aluminium hydroxychlorides having the general formula Al(OH)xCl where x=2-8, aluminium chlorate, aluminium sulfate, aluminium nitrate, aluminium hydroxynitrates having the general formula Al(OH)xNO3 where x=2-8, aluminium acetate, alums such as aluminium potassium sulfate or aluminium ammonium sulfate, aluminium formate, aluminium lactate, aluminium oxide, aluminium hydroxide acetate, aluminium isopropylate, aluminium hydroxide, aluminium silicates and mixtures of the aforementioned compounds. Inorganic acids, organic acids or mixtures of the aforementioned can be used as acids. In particular, phosphoric acid, phosphorous acid, nitric acid, sulfuric acid, mixtures thereof and their acid- reacting salts can be used as inorganic acids. Organic acids that are preferably used are carboxylic acids having the general formula CnH2n+1CO2H, where n=0-6 or n= 8, 10, 12, 14, 16, or dicarboxylic acids having the general formula HO2C(CH2)nCO2H, where n=0-4, or hydroxycarboxylic acids having the general formula R1R2C (OH) CO2H, where R1=H, R2=CH3, CH2CO2H, CH(OH)CO2H, or phthalic acid or salicylic acid, or acid-reacting salts of the aforementioned acids or mixtures of the aforementioned acids and salts thereof. Stabilisation of the dispersion according to the invention with tetramethyl ammonium hydroxide or aluminium hydroxychloride in an acid medium can be particularly advantageous. The dispersion can optionally also contain other additives. These can for example be oxidising agents such as hydrogen peroxide or per-acids, oxidation activators whose purpose is to increase the rate of oxidation, corrosion inhibitors such as e.g. benzotriazole. Surface-active substances of a non-ionic, cationic, anionic or amphoteric nature can also be added to the dispersion according to the invention. The invention also provides a process for the production of the dispersion according to the invention, which is characterised in that the silicon dioxide powder according to the invention is incorporated with a dispersing device into water, which can be stabilised by the addition of bases or cationic polymers or aluminium salts or a mixture of cationic polymers and aluminium salts or acids, and is then dispersed further for a period of 5 to 3 0 minutes. There is no restriction on the type of dispersing device. It can be advantageous however, especially for the production of highly filled dispersions, to use dispersing devices with a high energy input. These can for example be rotor-stator systems, planetary compounders or high-energy mills. In the latter, two predispersed streams of suspension under high pressure are decompressed through a nozzle. The two jets of dispersion hit each other exactly and the particles grind themselves. In another embodiment the predispersion is likewise placed under high pressure, but the particles collide against armoured sections of wall. A rotor-stator system can preferably be used to produce the dispersion according to the invention. The invention also provides the use of the silicon dioxide powder according to the invention as a filler in rubber, silicone rubber and plastics, to adjust the rheology in paints and coatings and as a support for catalysts. The invention also provides the use of the dispersion according to the invention for the production of glass articles, for chemical mechanical polishing and for the production of inkjet papers. Examples Analytical determinations The specific surface area of the powders is determined in accordance with DIN 66131. The dibutyl phthalate absorption is measured with a RHEOCORD 90 device supplied by Haake, Karlsruhe. To this end 8 g of the silicon dioxide powder is introduced into a mixing chamber with an accuracy of 0.001 g, the chamber is closed with a lid and dibutyl phthalate is metered in through a hole in the lid at a predefined feed rate of 0.0667 ml/s. The compounder is operated at a motor speed of 125 revolutions per minute. On reaching the maximum torque the compounder and DBP metering are automatically switched off. The DBP absorption is calculated from the consumed amount of DBP and the weighed amount of particles according to the formula below: DBP value (g/100 g) = (DBP consumption in g / weighed amount of particles in g) x 100. The thickening action is determined by the following method: 7.5 g silicon dioxide powder are introduced into 142.5 g of a solution of an unsaturated polyester resin in styrene with a viscosity of 13 00 +/- 100 mPas at a temperature of 22°C and dispersed by means of a high-speed mixer at 3000 min-1. A suitable example of an unsaturated polyester resin is Ludopal® P6, BASF. A further 90 g of the unsaturated polyester resin in styrene are added to 60 g of this dispersion and the dispersion process is repeated. The thickening action is taken to be the viscosity value in mPas of the dispersion at 25°C, measured with a rotary viscometer at a shear rate of 2.7 s-1. The chloride content of the silicon dioxide powder is determined by the following procedure: Approximately 0.3 g of the particles according to the invention are weighed in accurately, topped up with 20 ml of 20 percent reagent- grade sodium hydroxide solution, dissolved and transferred into 15 ml cooled HNO3 whilst being stirred. The chloride content in the solution is titrated with AgNO3 solution (0.1 mo1/1 or 0.01 mol/1). The carbon content of the silicon dioxide powder is determined by the following procedure: Approximately 100 to 1000 mg of the particles according to the invention are weighed accurately into a crucible, combined with 1 g each of ultrapure iron and aggregate (LECOCELL II) and burned in a carbon analyser (LECO) at approx. 1800°C with the aid of oxygen. The CO2 that is generated is measured by IR and the content calculated therefrom. The compacted bulk density is determined by reference to DIN ISO 787/XI K 5101/18 (not screened). The pH is determined by reference to DIN ISO 787/IX, ASTM D 1280, JIS K 5101/24. The viscosity of the dispersions is determined with a Physica Model 300 rotary rheometer and a CC 27 measuring beaker at 25°C. The viscosity value is determined at a shear rate of 10 1/sec. This shear rate is in a range in which the viscosity of the dispersions formed is virtually independent of the shear stress. The particle size prevailing in the dispersion is determined by means of dynamic light scattering. A Zetasizer 3000 HSa (Malvern Instruments, UK) is used. The volume-weighted median value of the peak analysis is stated. Example 1; 1.5 kg/h tetramethoxysilane are evaporated at 180°C and introduced into the central pipe of the burner. 12 m3/h of air are additionally introduced into the central pipe. 1.8 m3/h of hydrogen are fed into a pipe surrounding the central pipe. The gas mixture burns in the reaction chamber, into which 17 m3/h of secondary air are additionally introduced. The reaction gases and the silicon dioxide that is formed are drawn through a cooling system by application of a partial vacuum, cooling them to values between 100 and 160°C. The solid is separated from the waste gas stream in a filter or cyclone. The analytical data for the silicon dioxide powder obtained is reproduced in Table 2. Examples 2 to 9 and comparative examples 10 and 11 were performed in the same way. In comparative examples 12 to 14 silicon tetrachloride is used in place of tetramethoxysilane. In these experiments, following separation from the waste gas stream the silicon dioxide powder is treated at elevated temperature with water vapour-containing air to remove adhering hydrochloric acid residues. The physical-chemical data for the silicon dioxide powders obtained is reproduced in Table 2. Examples 1 to 9 lead to the silicon dioxide powders according to the invention having a low structure, expressed as the specific DBP value, a low specific thickening action and a high specific compacted bulk density. Examples 10 and 11 show that only the process according to the invention leads to these powders. Reducing the secondary air or even omitting it altogether or increasing the burner air does not lead to the silicon dioxide powders according to the invention. In the same way, using silicon tetrachloride, examples 12 to 14, whilst maintaining the conditions with regard to the lambda value in the burner and in the reaction chamber, does not lead to the silicon dioxide powders according to the invention. Example 15: Production of a dispersion in the acid pH range 36 kg of demineralised water are placed in a 60 1 stainless steel batch container. 6.4 kg of the pyrogenically produced silicon dioxide are then drawn in under shear conditions using the suction pipe of the Ystral Conti-TDS 3 and on completion of the drawing-in process shearing is continued for a further 15 min at 3000 rpm. Example 16: Production of a dispersion in the alkaline pH range 35.5 kg of demineralised water and 52 g of a 30% KOH solution are placed in a 60 1 stainless steel batch container. 6.4 kg of the pyrogenically produced silicon dioxide are then drawn in under shear conditions using the suction pipe of the Ystral Conti-TDS 3 and on completion of the drawing-in process shearing is continued for a further 15 min at 3 000 rpm. During this 15-minute dispersion the pH is adjusted to and held at a pH of 10.4 by addition of further KOH solution. A further 43 g of KOH solution were used in this process and a solids concentration of 15 wt.% established by addition of 0.4 kg water. Example 17: Production of a dispersion in the presence of aluminium salts 35 kg of demineralised water are placed in a 60 1 stainless steel batch container. 6.4 kg of the pyrogenically produced silicon dioxide are then drawn in under shear conditions using the suction pipe of the Ystral Conti-TDS 3. 640 g of a 1 wt.% solution (relative to aluminium oxide) of aluminium chloride are then added with dispersion and on completion of the addition shearing is continued for a further 15 min at 3000 rpm. 0.1 kg demineralised water and 3 05 g 1 N NaOH are then added to obtain a 15 wt.% dispersion with a pH of 3.5. Example 18: Production of a dispersion of Aerosil 90 (comparative example) 35.5 kg of demineralised water and 52 g of a 30% KOH solution are placed in a 60 1 stainless steel batch container. 5.2 kg of AEROSIL® 90 are then drawn in under shear conditions using the suction pipe of the Ystral Conti-TDS 3 and on completion of the drawing-in process shearing is continued for a further 15 min at 3000 rpm. During this 15-minute dispersion the pH is adjusted to and held at a pH of 10.4 by addition of further KOH solution. A further 63 g of KOH solution were used in this process and a solids concentration of 15 wt.% established by addition of 0.6 kg water. The physical-chemical parameters for the dispersions are reproduced in Table 3. *Pyrogenically produced silicon dioxide from Degussa AG, BET surface area approx. 90 m2/g. Example 19: Dispersion with high solids content 35.5 kg of demineralised water in a 60 1 stainless steel batch container are adjusted to a pH of 11 with tetramethyl ammonium hydroxide solution (25%). 37 kg of the pyrogenically produced silicon dioxide are then drawn in under shear conditions using the suction pipe of the Ystral Conti-TDS 3 and on completion of the drawing-in process shearing is continued for a further 15 min at 3000 rpm. During this 15-minute dispersion the pH is held at a pH of between 10 and 11 by addition of tetramethyl ammonium hydroxide solution. A solids concentration of 50 wt.% is established by addition of the remaining amount of water that is needed. The resulting dispersion has a silicon dioxide content of 50 wt.% and a pH of 10.3. It displays a viscosity, determined with a Physica viscometer, of 2450 mPas. The average particle size is 116 nm. The dispersion displays no thickening or sedimentation even after a storage period of 6 months. The silicon dioxide powders according to the invention are characterised by an ability to be incorporated rapidly into aqueous media. In comparison to dispersions with the known silicon dioxide powder, the dispersions according to the invention display more favourable values for viscosity and smaller particle sizes. Example 19 shows that dispersions with a high solids content can also be produced. Under similar conditions the use of known silicon dioxide powders with a comparable BET surface area leads to gel-like compositions, or the powder cannot be incorporated fully. We Claim: 1. Pyrogenically produced silicon dioxide powder having a specific surface area of between 5 and 600 m2/g and a carbon content of less than 500 ppm, characterized in that it displays - a specific dibutyl phthalate absorption of less than or equal to 1.2 g dibutyl phthalate/100 g SiO2 per m2 of specific surface area - and a specific thickening action of less than 15 mPas per m2 of specific surface area. 2. Silicon dioxide powder as claimed in claim 1, wherein the specific compacted bulk density is between 1000 and 10000 g/1 x m2 of specific surface area. 3. Silicon dioxide powder as claimed in claims 1 or 2, wherein the chloride content is less than 50 ppm. 4. Process for the production of the silicon dioxide powder as claimed in claims 1 to 3, characterized in that - vaporous tetramethoxysilane and/or tetraethoxysilane together with air or with oxygen-enriched air and - separately hydrogen - are supplied to a burner, and the mixture of gases is allowed to react in a flame in a reaction chamber connected in series to the burner, and the solid reaction product is separated from the gas stream by known means, - the lambda value in the burner being between 0.95 and 1.5 and - sufficient secondary air also being supplied to the reaction chamber that the lambda value in the reaction chamber is between 0.8 and 1.6 and wherein Lambda denotes the ratio of oxygen supplied to the burner or the reaction chamber to stoichiometrically required oxygen, which is needed to convert the silane compound completely to silicon dioxide. 5. Process as claimed in claim 4, wherein the volume ratio of oxygen/hydrogen in the burner is between 0.2 and 2.8. 6. Process as claimed in claims 4 or 5, wherein the discharge velocity of the gases leaving the burner is at least 10 ms-1. 7. Aqueous dispersion containing the silicon dioxide powder as claimed in claims 1 to 3, wherein the content of silicon dioxide in the dispersion is between 20 and 80 wt. %. 8. Aqueous dispersion as claimed in claim 7, wherein the average aggregate diameter in the dispersion is less than 200 nm. 9. Aqueous dispersion as claimed in claims 7 or 8, wherein it contains a base, an acid, an aluminum salt and/or a cationic polymer. 10. Aqueous dispersion as claimed in claim 9, wherein the base is ammonia, ammonium hydroxide, tetramethyl ammonium hydroxide, primary, secondary or tertiary organic amines, sodium hydroxide solution or potassium hydroxide solution. 11. Aqueous dispersion as claimed in claim 9, wherein the aluminum salt is an aluminium hydroxychloride having the general formula Al (OH) XCI where x=2-8. Pyrogenic silicon dioxide powder and dispersion thereof Pyrogenically produced silicon dioxide powder with a specific surface area of between 5 and 600 m2/g and a carbon content of less than 500 ppm, which displays a specific dibutyl phthalate absorption of less than or equal to 1.2 g dibutyl phthalate/100 g SiO2 per m2 of specific surface area and a specific thickening action of less than 15 mPas/m2 of specific surface area. It is produced by supplying vaporous tetramethoxysilane and/or tetraethoxysilane together with air and separately hydrogen to a burner, and allowing the mixture of gases to react in a flame in a reaction chamber connected in series to the burner, and separating the solid reaction product from the gas stream by known means, the lambda value in the burner being between 0.95 and 1.5 and sufficient secondary air also being supplied to the reaction chamber that the lambda value in the reaction chamber is between 0.8 and 1.6. The invention also provides a dispersion containing the silicon dioxide powder and the use of the powder and of the dispersion.

Full Text

Pyrogenic silicon dioxide powder and dispersion thereof
The invention provides a pyrogenically produced silicon
dioxide powder, an aqueous dispersion containing this
silicon dioxide powder, the production and use of the
silicon dioxide powder and the dispersion.
The term pyrogenic silicon dioxide or pyrogenic silica
(fumed silica) is a collective term for all highly disperse
silicas obtained in the gas phase at elevated temperatures
by coagulation of monomeric silica. There are two processes
for the industrial production of pyrogenic silicas, high-
temperature hydrolysis and the arc process.
In the high-temperature hydrolysis process a homogeneous
mixture of a vaporous silicon compound, hydrogen, oxygen
and an inert gas is burned with a burner in a cooled
combustion space. Two reactions proceed side by side here.
Firstly the reaction of hydrogen and oxygen with formation
of water and secondly the hydrolysis of the silicon
compound with formation of silicon dioxide.
The homogeneity of the gas mixture means that the reaction
conditions and hence the formation and growth conditions
are largely the same for each SiO2 particle, such that very
homogeneous and uniform particles can form. Air is used as
the oxygen source in the known process. The pyrogenic
silicas produced by the known process display specific
surface areas of between 10 and 600 m2/g.
The starting material for the silicon dioxide is generally
silicon tetrachloride (cf. Ullmann's Encyclopedia of
Industrial Chemistry, Vol. A23, pages 635 ff. 5th edition).
In addition to silicon tetrachloride methyl
trichlorosilane, trichlorosilane or mixtures thereof with
silicon tetrachloride can also be used.
JP 2002114510 claims a process in which silicon dioxide is
obtained with an average particle size of 0.05 to 5 mm. In
this process silicon compounds are burned in the presence
of oxygen and hydrogen. Siloxanes, silanes or silicon
chlorides can be used as the silicon compound. However, the
silicon dioxide produced by this process displays no
properties that could not also be obtained by processes of
the prior art. The process described is itself only of
limited suitability for the production of larger
quantities. A non-uniform product and, where carbon-
containing silicon compounds are used as starting
materials, dark products too are then to be expected in
particular.
When used in dispersions such as are used in the production
of glass articles or in chemical mechanical polishing in
the semiconductor industry, the powder produced according
to JP 2002114510 provides no advantages over the prior art.
Due to growing requirements an improvement in the
properties of silicon dioxide is demanded in these very
sectors. In the glass industry in particular, highly
filled, readily manageable dispersions, in other words ones
with low viscosity, are required because of their low
shrinkage on drying and sintering.
The object of the invention is to provide a silicon dioxide
powder which is suitable for the production of highly
filled dispersions with low viscosity. The object of the
invention is also to provide a stable dispersion containing
this silicon dioxide powder.
The invention provides a pyrogenically produced silicon
dioxide powder having a specific surface area of between 5
and 600 m2/g and a carbon content of less than 500 ppm,
which is characterised in that it displays
a specific dibutyl phthalate absorption of less than
or equal to 1.2 g dibutyl phthalate/100 g SiO2 per
m2 of specific surface area
and a specific thickening action of less than
15 mPas per m2 of specific surface area.
The specific dibutyl phthalate absorption represents a
measure of the structure of the silicon dioxide powder
according to the invention as a function of its specific
surface area. The term structure in this connection means
the degree of intergrowth of the primary particles. These
are initially formed in the pyrogenic process and as the
reaction continues can coalesce to form chain-like
aggregates, which in turn form agglomerates. The specific
dibutyl phthalate absorption of less than or equal to 1.2 g
dibutyl phthalate/100 g SiO2 per m2 of specific surface
area claimed for the silicon dioxide powder according to
the invention is generally lower than pyrogenic silicon
dioxide powders obtained by the prior art.
The silicon dioxide powder according to the invention
arises only in combination with a specific thickening
action. This is understood to mean the thickening action
per m2 of specific surface area. The thickening action is
determined in a dispersion of a silicon dioxide powder in a
polyester.
In a preferred embodiment the powders according to the
invention can display a specific compacted bulk density,
defined as the product of the compacted bulk density and
specific surface area, of between 1000 and 10000 and
particularly preferably between 4000 and 7000 g/1 x m2 of
specific surface area. Powders according to the invention
displaying a specific compacted bulk density in this range
can be incorporated especially readily into dispersions.
Furthermore, silicon dioxide powders according to the
invention can preferably have a chloride content of less
than 50 ppm, particularly preferably less than 20 ppm. The
low chloride contents can for example demonstrate
advantageous effects when the powders according to the
invention are used in the area of chemical mechanical
polishing.
The invention also provides a process for the production of
silicon dioxide powder according to the invention which is
characterised in that
vaporous tetramethoxysilane (TMOS) and/or
tetraethoxysilane (TEOS) together with air or with
oxygen-enriched air and separately
hydrogen
are supplied to a burner, and the mixture of gases
is allowed to react in a flame in a reaction chamber
connected in series to the burner, and the solid
reaction product is separated from the gas stream by
known means,
the lambda value in the burner being between 0.95
and 1.5 and
sufficient secondary air also being supplied to the
reaction chamber that the lambda value in the
reaction chamber is between 0.8 and 1.6.
Figure 1 shows a simplified process flow chart upon which
the process according to the invention is based. A =
burner; B = flame; C = reaction chamber;
1 = supply of mixture comprising vaporous
tetramethoxysilane and/or tetraethoxysilane together with
air or with oxygen-enriched air; 2 = supply of hydrogen; 3
= supply of secondary air.
In the performance of the process it is substantial that a
premixing of silane and air occurs, the stoichiometry of
air/hydrogen and the oxygen component, expressed as the
lambda value, being maintained in the burner and reaction
chamber.
Lambda denotes the ratio of oxygen supplied to the burner
or the reaction chamber to stoichiometrically required
oxygen, which is needed to convert the silane compound
completely to silicon dioxide. The lambda value range that
must be maintained in the reaction chamber likewise refers
to the total amount of the silane to be hydrolysed.
In the process according to the invention the volume ratio
of oxygen/hydrogen in the burner can be varied between 0.2
and 2.8. In a particularly preferred embodiment the volume
ratio of oxygen/hydrogen in the burner is between 0.9 and
1.4.
Depending on the desired specific surface area, it can be
useful to vary the streams supplied to the burner and the
burner geometry in such a way that the discharge velocity
of the gases leaving the burner is at least 10 ms-1.
Discharge velocities of at least 20 ms-1 are particularly
preferred.
The invention also provides an aqueous dispersion
containing the silicon dioxide powder according to the
invention.
The aqueous dispersion according to the invention can
display a content of silicon dioxide powder of between 2 0
and 80 wt.%. Dispersions having a content of silicon
dioxide powder of between 40 and 60 can be particularly
preferred. These dispersions display a particularly high
stability with a comparatively low structure.
The aqueous dispersion according to the invention can
preferably display an average particle size in the
aggregates of silicon dioxide powder which is less than
200 nm. For certain applications such as e.g. the chemical
mechanical polishing of semiconductor substrates, a value
of less than l50 nm can be particularly preferred.
The dispersion according to the invention can be stabilised
by the addition of bases or cationic polymers or aluminium
salts or a mixture of cationic polymers and aluminium salts
or acids.
Bases that can be used are ammonia, ammonium hydroxide,
tetramethyl ammonium hydroxide, primary, secondary or
tertiary organic amines, sodium hydroxide solution or
potassium hydroxide solution.
Cationic polymers that can be used are examples having at
least one quaternary ammonium group, phosphonium group, an
acid adduct of a primary, secondary or tertiary amine
group, polyethylene imines, polydiallylamines or
polyallylamines, polyvinylamines, dicyandiamide
condensates, dicyandiamide-polyamine cocondensates or
polyamide-formaldehyde condensates.
Aluminium salts that can be used are aluminium chloride,
aluminium hydroxychlorides having the general formula
Al(OH)xCl where x=2-8, aluminium chlorate, aluminium
sulfate, aluminium nitrate, aluminium hydroxynitrates
having the general formula Al(OH)xNO3 where x=2-8,
aluminium acetate, alums such as aluminium potassium
sulfate or aluminium ammonium sulfate, aluminium formate,
aluminium lactate, aluminium oxide, aluminium hydroxide
acetate, aluminium isopropylate, aluminium hydroxide,
aluminium silicates and mixtures of the aforementioned
compounds.
Inorganic acids, organic acids or mixtures of the
aforementioned can be used as acids.
In particular, phosphoric acid, phosphorous acid, nitric
acid, sulfuric acid, mixtures thereof and their acid-
reacting salts can be used as inorganic acids.
Organic acids that are preferably used are carboxylic acids
having the general formula CnH2n+1CO2H, where n=0-6 or n= 8,
10, 12, 14, 16, or dicarboxylic acids having the general
formula HO2C(CH2)nCO2H, where n=0-4, or hydroxycarboxylic
acids having the general formula R1R2C (OH) CO2H, where R1=H,
R2=CH3, CH2CO2H, CH(OH)CO2H, or phthalic acid or salicylic
acid, or acid-reacting salts of the aforementioned acids or
mixtures of the aforementioned acids and salts thereof.
Stabilisation of the dispersion according to the invention
with tetramethyl ammonium hydroxide or aluminium
hydroxychloride in an acid medium can be particularly
advantageous.
The dispersion can optionally also contain other additives.
These can for example be oxidising agents such as hydrogen
peroxide or per-acids, oxidation activators whose purpose
is to increase the rate of oxidation, corrosion inhibitors
such as e.g. benzotriazole. Surface-active substances of a
non-ionic, cationic, anionic or amphoteric nature can also
be added to the dispersion according to the invention.
The invention also provides a process for the production of
the dispersion according to the invention, which is
characterised in that the silicon dioxide powder according
to the invention is incorporated with a dispersing device
into water, which can be stabilised by the addition of
bases or cationic polymers or aluminium salts or a mixture
of cationic polymers and aluminium salts or acids, and is
then dispersed further for a period of 5 to 3 0 minutes.
There is no restriction on the type of dispersing device.
It can be advantageous however, especially for the
production of highly filled dispersions, to use dispersing
devices with a high energy input. These can for example be
rotor-stator systems, planetary compounders or high-energy
mills. In the latter, two predispersed streams of
suspension under high pressure are decompressed through a
nozzle. The two jets of dispersion hit each other exactly
and the particles grind themselves. In another embodiment
the predispersion is likewise placed under high pressure,
but the particles collide against armoured sections of
wall. A rotor-stator system can preferably be used to
produce the dispersion according to the invention.
The invention also provides the use of the silicon dioxide
powder according to the invention as a filler in rubber,
silicone rubber and plastics, to adjust the rheology in
paints and coatings and as a support for catalysts.
The invention also provides the use of the dispersion
according to the invention for the production of glass
articles, for chemical mechanical polishing and for the
production of inkjet papers.
Examples
Analytical determinations
The specific surface area of the powders is determined in
accordance with DIN 66131.
The dibutyl phthalate absorption is measured with a
RHEOCORD 90 device supplied by Haake, Karlsruhe. To this
end 8 g of the silicon dioxide powder is introduced into a
mixing chamber with an accuracy of 0.001 g, the chamber is
closed with a lid and dibutyl phthalate is metered in
through a hole in the lid at a predefined feed rate of
0.0667 ml/s. The compounder is operated at a motor speed of
125 revolutions per minute. On reaching the maximum torque
the compounder and DBP metering are automatically switched
off. The DBP absorption is calculated from the consumed
amount of DBP and the weighed amount of particles according
to the formula below:
DBP value (g/100 g) = (DBP consumption in g / weighed
amount of particles in g) x 100.
The thickening action is determined by the following
method: 7.5 g silicon dioxide powder are introduced into
142.5 g of a solution of an unsaturated polyester resin in
styrene with a viscosity of 13 00 +/- 100 mPas at a
temperature of 22°C and dispersed by means of a high-speed
mixer at 3000 min-1. A suitable example of an unsaturated
polyester resin is Ludopal® P6, BASF. A further 90 g of the
unsaturated polyester resin in styrene are added to 60 g of
this dispersion and the dispersion process is repeated. The
thickening action is taken to be the viscosity value in
mPas of the dispersion at 25°C, measured with a rotary
viscometer at a shear rate of 2.7 s-1.
The chloride content of the silicon dioxide powder is
determined by the following procedure: Approximately 0.3 g
of the particles according to the invention are weighed in
accurately, topped up with 20 ml of 20 percent reagent-
grade sodium hydroxide solution, dissolved and transferred
into 15 ml cooled HNO3 whilst being stirred. The chloride
content in the solution is titrated with AgNO3 solution
(0.1 mo1/1 or 0.01 mol/1).
The carbon content of the silicon dioxide powder is
determined by the following procedure: Approximately 100 to
1000 mg of the particles according to the invention are
weighed accurately into a crucible, combined with 1 g each
of ultrapure iron and aggregate (LECOCELL II) and burned in
a carbon analyser (LECO) at approx. 1800°C with the aid of
oxygen. The CO2 that is generated is measured by IR and the
content calculated therefrom.
The compacted bulk density is determined by reference to
DIN ISO 787/XI K 5101/18 (not screened).
The pH is determined by reference to DIN ISO 787/IX, ASTM D
1280, JIS K 5101/24.
The viscosity of the dispersions is determined with a
Physica Model 300 rotary rheometer and a CC 27 measuring
beaker at 25°C. The viscosity value is determined at a
shear rate of 10 1/sec. This shear rate is in a range in
which the viscosity of the dispersions formed is virtually
independent of the shear stress.
The particle size prevailing in the dispersion is
determined by means of dynamic light scattering. A
Zetasizer 3000 HSa (Malvern Instruments, UK) is used. The
volume-weighted median value of the peak analysis is
stated.
Example 1;
1.5 kg/h tetramethoxysilane are evaporated at 180°C and
introduced into the central pipe of the burner. 12 m3/h of
air are additionally introduced into the central pipe.
1.8 m3/h of hydrogen are fed into a pipe surrounding the
central pipe. The gas mixture burns in the reaction
chamber, into which 17 m3/h of secondary air are
additionally introduced.
The reaction gases and the silicon dioxide that is formed
are drawn through a cooling system by application of a
partial vacuum, cooling them to values between 100 and
160°C. The solid is separated from the waste gas stream in
a filter or cyclone.
The analytical data for the silicon dioxide powder obtained
is reproduced in Table 2.
Examples 2 to 9 and comparative examples 10 and 11 were
performed in the same way.
In comparative examples 12 to 14 silicon tetrachloride is
used in place of tetramethoxysilane. In these experiments,
following separation from the waste gas stream the silicon
dioxide powder is treated at elevated temperature with
water vapour-containing air to remove adhering hydrochloric
acid residues.
The physical-chemical data for the silicon dioxide powders
obtained is reproduced in Table 2.
Examples 1 to 9 lead to the silicon dioxide powders
according to the invention having a low structure,
expressed as the specific DBP value, a low specific
thickening action and a high specific compacted bulk
density.
Examples 10 and 11 show that only the process according to
the invention leads to these powders. Reducing the
secondary air or even omitting it altogether or increasing
the burner air does not lead to the silicon dioxide powders
according to the invention.
In the same way, using silicon tetrachloride, examples 12
to 14, whilst maintaining the conditions with regard to the
lambda value in the burner and in the reaction chamber,
does not lead to the silicon dioxide powders according to
the invention.
Example 15: Production of a dispersion in the acid pH range
36 kg of demineralised water are placed in a 60 1 stainless
steel batch container. 6.4 kg of the pyrogenically
produced silicon dioxide are then drawn in under shear
conditions using the suction pipe of the Ystral Conti-TDS 3
and on completion of the drawing-in process shearing is
continued for a further 15 min at 3000 rpm.
Example 16: Production of a dispersion in the alkaline pH
range
35.5 kg of demineralised water and 52 g of a 30% KOH
solution are placed in a 60 1 stainless steel batch
container. 6.4 kg of the pyrogenically produced silicon
dioxide are then drawn in under shear conditions using the
suction pipe of the Ystral Conti-TDS 3 and on completion of
the drawing-in process shearing is continued for a further
15 min at 3 000 rpm. During this 15-minute dispersion the pH
is adjusted to and held at a pH of 10.4 by addition of
further KOH solution. A further 43 g of KOH solution were
used in this process and a solids concentration of 15 wt.%
established by addition of 0.4 kg water.
Example 17: Production of a dispersion in the presence of
aluminium salts
35 kg of demineralised water are placed in a 60 1 stainless
steel batch container. 6.4 kg of the pyrogenically
produced silicon dioxide are then drawn in under shear
conditions using the suction pipe of the Ystral Conti-TDS
3. 640 g of a 1 wt.% solution (relative to aluminium oxide)
of aluminium chloride are then added with dispersion and on
completion of the addition shearing is continued for a
further 15 min at 3000 rpm. 0.1 kg demineralised water and
3 05 g 1 N NaOH are then added to obtain a 15 wt.%
dispersion with a pH of 3.5.
Example 18: Production of a dispersion of Aerosil 90
(comparative example)
35.5 kg of demineralised water and 52 g of a 30% KOH
solution are placed in a 60 1 stainless steel batch
container. 5.2 kg of AEROSIL® 90 are then drawn in under
shear conditions using the suction pipe of the Ystral
Conti-TDS 3 and on completion of the drawing-in process
shearing is continued for a further 15 min at 3000 rpm.
During this 15-minute dispersion the pH is adjusted to and
held at a pH of 10.4 by addition of further KOH solution. A
further 63 g of KOH solution were used in this process and
a solids concentration of 15 wt.% established by addition
of 0.6 kg water.
The physical-chemical parameters for the dispersions are
reproduced in Table 3.
*Pyrogenically produced silicon dioxide from Degussa AG,
BET surface area approx. 90 m2/g.
Example 19: Dispersion with high solids content
35.5 kg of demineralised water in a 60 1 stainless steel
batch container are adjusted to a pH of 11 with tetramethyl
ammonium hydroxide solution (25%). 37 kg of the
pyrogenically produced silicon dioxide are then drawn in
under shear conditions using the suction pipe of the Ystral
Conti-TDS 3 and on completion of the drawing-in process
shearing is continued for a further 15 min at 3000 rpm.
During this 15-minute dispersion the pH is held at a pH of
between 10 and 11 by addition of tetramethyl ammonium
hydroxide solution. A solids concentration of 50 wt.% is
established by addition of the remaining amount of water
that is needed.
The resulting dispersion has a silicon dioxide content of
50 wt.% and a pH of 10.3. It displays a viscosity,
determined with a Physica viscometer, of 2450 mPas. The
average particle size is 116 nm. The dispersion displays no
thickening or sedimentation even after a storage period of
6 months.
The silicon dioxide powders according to the invention are
characterised by an ability to be incorporated rapidly into
aqueous media.
In comparison to dispersions with the known silicon dioxide
powder, the dispersions according to the invention display
more favourable values for viscosity and smaller particle
sizes.
Example 19 shows that dispersions with a high solids
content can also be produced. Under similar conditions the
use of known silicon dioxide powders with a comparable BET
surface area leads to gel-like compositions, or the powder
cannot be incorporated fully.
We Claim:
1. Pyrogenically produced silicon dioxide powder having a specific surface area of
between 5 and 600 m2/g and a carbon content of less than 500 ppm,
characterized in that it displays
- a specific dibutyl phthalate absorption of less than or equal to 1.2 g dibutyl
phthalate/100 g SiO2 per m2 of specific surface area
- and a specific thickening action of less than 15 mPas per m2 of specific
surface area.
2. Silicon dioxide powder as claimed in claim 1, wherein the specific compacted
bulk density is between 1000 and 10000 g/1 x m2 of specific surface area.
3. Silicon dioxide powder as claimed in claims 1 or 2, wherein the chloride
content is less than 50 ppm.
4. Process for the production of the silicon dioxide powder as claimed in claims 1
to 3, characterized in that
- vaporous tetramethoxysilane and/or
tetraethoxysilane together with air or with oxygen-enriched air and
- separately hydrogen
- are supplied to a burner, and the mixture of gases is allowed to react in a
flame in a reaction chamber connected in series to the burner, and the
solid reaction product is separated from the gas stream by known means,
- the lambda value in the burner being between 0.95 and 1.5 and
- sufficient secondary air also being supplied to the reaction chamber that
the lambda value in the reaction chamber is between 0.8 and 1.6 and
wherein Lambda denotes the ratio of oxygen supplied to the burner or the
reaction chamber to stoichiometrically required oxygen, which is needed to
convert the silane compound completely to silicon dioxide.
5. Process as claimed in claim 4, wherein the volume ratio of oxygen/hydrogen in
the burner is between 0.2 and 2.8.
6. Process as claimed in claims 4 or 5, wherein the discharge velocity of the
gases leaving the burner is at least 10 ms-1.
7. Aqueous dispersion containing the silicon dioxide powder as claimed in claims
1 to 3, wherein the content of silicon dioxide in the dispersion is between 20 and
80 wt. %.
8. Aqueous dispersion as claimed in claim 7, wherein the average aggregate
diameter in the dispersion is less than 200 nm.
9. Aqueous dispersion as claimed in claims 7 or 8, wherein it contains a base, an
acid, an aluminum salt and/or a cationic polymer.
10. Aqueous dispersion as claimed in claim 9, wherein the base is ammonia,
ammonium hydroxide, tetramethyl ammonium hydroxide, primary, secondary or
tertiary organic amines, sodium hydroxide solution or potassium hydroxide
solution.
11. Aqueous dispersion as claimed in claim 9, wherein the aluminum salt is an
aluminium hydroxychloride having the general formula Al (OH) XCI where x=2-8.
Pyrogenic silicon dioxide powder and dispersion thereof
Pyrogenically produced silicon dioxide powder with a
specific surface area of between 5 and 600 m2/g and a
carbon content of less than 500 ppm, which displays a
specific dibutyl phthalate absorption of less than or equal
to 1.2 g dibutyl phthalate/100 g SiO2 per m2 of specific
surface area and a specific thickening action of less than
15 mPas/m2 of specific surface area. It is produced by
supplying vaporous tetramethoxysilane and/or
tetraethoxysilane together with air and separately hydrogen
to a burner, and allowing the mixture of gases to react in
a flame in a reaction chamber connected in series to the
burner, and separating the solid reaction product from the
gas stream by known means, the lambda value in the burner
being between 0.95 and 1.5 and sufficient secondary air
also being supplied to the reaction chamber that the lambda
value in the reaction chamber is between 0.8 and 1.6. The
invention also provides a dispersion containing the silicon
dioxide powder and the use of the powder and of the
dispersion.

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Patent Number

223370

Indian Patent Application Number

01575/KOLNP/2005

PG Journal Number

37/2008

Publication Date

12-Sep-2008

Grant Date

10-Sep-2008

Date of Filing

08-Aug-2005

Name of Patentee

DEGUSSA AG

Applicant Address

BENNIGSENPLATZ 1, 40474 DÜSSELDORF

Inventors:

#	Inventor's Name	Inventor's Address
1	SCHUMACHER, DR. KAI	BERLINER STRASSE 16, 65719 HOFHEIM
2	KERNER, DR. DIETER	AM HEXENPFAD 21, 63450 HANAU

PCT International Classification Number

C01B 33/00

PCT International Application Number

PCT/EP2004/002664

PCT International Filing date

2004-03-15

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	103 12 970.7	2003-03-24	Germany