Title of Invention	A PYROGENICALLY PRODUCED SILICON DIOXIDE POWDER
Abstract	The invention relates to a pyrogenically produced silicon dioxide powder in the form of aggregates of primary particles having a BET surface area of 300 ± 25 m2/g, the aggregates display an average surface area of 4800 to 6000 nm2, an average equivalent circle diameter (ECD) of 60 to 80 nm and an average circumference of 580 to 750 nm.

Full Text

The invention concerns a pyrogenically produced silicon dioxide powder, its production and use. Flame hydrolysis for the production of silicon dioxide is a process that has long been known and is performed on an industrial scale. In this process an evaporated or gaseous hydrolysable silicon halide is mixed with a flame formed by burning a water-forming, hydrogen-containing fuel and an oxygen-containing gas. The combustion flame provides water for hydrolysis of the silicon halide and sufficient heat for the hydrolysis reaction. The silicon dioxide powder carried in the residual reaction gases is subjected to conventional cooling and solids separation processes. Silicon tetrachloride is usually used. The use of dichlorosilane, trichlorosilane is also known, however. If carbon-containing feed materials are used, such as e.g. methyl trichlorosilane, dimethyl dichlorosilane, methyl dichlorosilane, dibutyl dichlorosilane, ethyl trichlorosilane, propyl trichlorosilane, an oxidation process to convert the carbon into carbon dioxide also takes place. As a consequence, a silicon dioxide powder which is formed in a process wherein both reaction types, namely flame hydrolysis and oxidation, take place is described as a pyrogenically produced silicon dioxide. In the reaction, highly dispersed, non-porous primary particles are initially formed, which as the reaction continues coalesce to form aggregates which in turn can congregate to form agglomerates. The BET surface area of these primary particles is generally between 5 and 600 m2/g. The powder displays free hydroxyl groups on its surface. The silicon dioxide powder produced in this way is used in many areas of application. In many applications, the time needed for incorporation into liquid media is a substantial cost factor. It has been found that for silicon dioxide powders produced by the prior art, even though they were produced by the same type of reaction and also display parameters that are identical or similar to the parameters that are conventionally cited for characterisation, such as the BET surface area for example, very long times are needed for incorporation into liquid media. The object of the invention is therefore to provide a silicon dioxide powder than can be incorporated quickly into liquid media. A further object of the invention is to provide a process for the production of this powder. The invention provides a pyrogenically produced silicon dioxide powder in the form of aggregates of primary particles having a BET surface area of 300 ± 25 m2/g, wherein the aggregates display - an average surface area of 4800 to 6000 nm2, an average equivalent circle diameter (ECD) of 60 to 80 nm and an average circumference of 580 to 750 nm. The BET surface area is determined here in accordance with DIN 66131. The aggregate sizes are determined by image analysis using an H 75 00 TEM device supplied by Hitachi and a MegaView II CCD camera supplied by SIS. The image magnification for analysis is 30000:1 with a pixel density of 3.2 nm. The number of particles analysed is greater than 1000. Preparation takes place in accordance with ASTM 3849-89. The lower threshold limit in terms of detection is 50 pixels. The BET surface area can preferably be 300 + 15 m2/g and particularly preferably 300 ± 10 m2/g. Furthermore, a pyrogenically produced silicon dioxide powder according to the invention can be preferred in which the aggregates display an average surface area of 5000 to 5700 nm2, an average equivalent circle diameter of 65 to 75 nm and an average circumference of 600 to 720 nm. Furthermore, a pyrogenically produced silicon dioxide powder according to the invention can be preferred in which the maximum aggregate diameter is between 100 and 140 nm and the minimum aggregate diameter is between 60 and 90 nm. Furthermore, a pyrogenically produced silicon dioxide powder according to the invention can be preferred in which the chloride content is less than 250 ppm. A chloride content of less than 15 0 ppm is particularly preferred. Furthermore, a pyrogenically produced silicon dioxide powder according to the invention can be preferred in which the carbon content is less than 5 00 ppm. A carbon content of less than 300 ppm is particularly preferred. The invention also provides a process for the production of the silicon dioxide powder according to the invention wherein a mixture of silicon compounds is evaporated, separately or together, the vapours are transferred by means of a carrier gas to a mixing chamber, with SiCl4 as the first component in a proportion of 60 to 100 wt.% relative to the mixture, and a second component chosen from the group comprising H3SiCl, H2SiCl2, HSiCl3, CH3SiCl3, (CH3)2SiCl2, (CH3)3SiCl, (n-C3H7) SiCl3, in a proportion of 0 to 40 wt.%, preferably 5 to 3 0 wt.%, relative to the mixture, and a combustion gas and primary air, which can optionally be enriched with oxygen and/or be preheated, are transferred separately to the mixing chamber, the mixture comprising the vapour of silicon compounds, combustion gas and primary air is ignited in a burner and the flame burns into a reaction chamber, secondary air, which surrounds the flame, is introduced into the reaction chamber, the ratio of secondary air to primary air being in a range from 0.05 to 4, preferably 0.15 to 3, the solid is then separated from gaseous substances and the solid is then steam-treated, wherein the total amount of oxygen is at least sufficient for the complete combustion of the combustion gas and the silicon compounds and the amount of feed materials consisting of silicon compounds, combustion gas, primary air and secondary air is chosen such that an adiabatic flame temperature Tad of 1390 to 1450°C is obtained, where Tad = the temperature of the feed materials + the sum of the reaction enthalpies of the partial reactions/ heat capacity of the substances leaving the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, carbon dioxide, oxygen, nitrogen, and optionally the carrier gas if it is not air or nitrogen, taking the specific heat capacity of these substances at 1000°C as a basis. The reaction of the silicon compounds in the presence of oxygen and a combustion gas yields silicon dioxide, water, hydrochloric acid and, in the case of carbon-containing silicon compounds and/or carbon-containing combustion gases, carbon dioxide. The reaction enthalpies of these reactions can be calculated by means of standard works known to the person skilled in the art. Table 1 shows selected reaction enthalpy values for the reaction of silicon compounds in the presence of hydrogen and oxygen. Methyl trichlorosilane (MTCS, CH3SiCl3) , trichlorosilane (TCS, SiHCl3) and/or dichlorosilane (DCS, SiH2Cl2) can particularly preferably be used. Suitable combustion gases are hydrogen, methane, ethane, propane and/or natural gas, with hydrogen being preferred. There is no restriction on the temperature of the feed materials, provided that it is above the boiling point of the highest boiling silicon compound. A temperature of the feed materials of 90°C + 40°C has proved to be advantageous. It can also be advantageous if the discharge velocity of the reaction mixture from the mixing chamber into the reaction chamber is 10 to 80 m/s. The invention also provides the use of the pyrogenically produced silicon dioxide powder according to the invention as a filler in rubber, silicone rubber and plastics, to adjust the rheology in paints and coatings, as a polyester, as a support for catalysts and to produce dispersions. Examples The BET surface area is determined in accordance with DIN 66131. Determining the incorporation time in water 10 g of silicon dioxide powder are placed in a 500 ml vessel having a diameter of 80 mm. 50 g of aqueous nitric acid (pH 3.90) are added whilst stirring with a high-speed mixer fitted with a 70 mm disc at 500 rpm, and the time needed to wet the powder is measured. The incorporation time for Aerosil® 300, Degussa, is used as a comparison and is standardised to 100. Determining the viscosity in polyvinyl alcohol 60 g of an aqueous five-percent polyvinyl alcohol solution and 80 g of demineralised water are placed in a 500 ml vessel having a diameter of 80 mm. 10 g of silicon dioxide powder are added to this solution and the mixture is then dispersed for five minutes in a high-speed mixer fitted with a 50 mm disc at 2500 rpm. The viscosity of the mixture is then determined using a Haake rheometer at 2.7 s-1 and 23°C. The viscosity of Aerosil® 300, Degussa, is used as a comparison and is standardised to 100. Example 1: Production of a silicon dioxide powder 180 kg/h of silicon tetrachloride are evaporated and transferred to the mixing chamber of a burner by means of nitrogen. At the same time, 58 Nm3/h of hydrogen and 190 Nm3/h of primary air are introduced into the mixing chamber. The mixture displays a temperature of 90°C. It is ignited and burnt into a reaction chamber in a flame. The discharge velocity from the burner is 30.3 m/s. In addition, 60 Nm3/h of secondary air, which surrounds the flame, are introduced into the reaction chamber. The ratio of secondary air to primary air is 0.32. 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 and then steam-treated at a temperature of 560°C. Examples 2 to 9 are performed in the same way. Table 2 reproduces the feed materials and the amounts from Examples 1 to 9. Table 3 reproduces the reaction enthalpies, heat capacity values and adiabatic flame temperatures from Examples 1 to 9. Examples 1 to 5 produce powders according to the invention. Examples 6 to 9 are comparative examples. Two silicon components are used in Examples 2 and 4, three in Example 3. In Examples 1 and 3, high and low proportions of the first silicon component, silicon tetrachloride, are used respectively. In Example 5 a high ratio of secondary air to primary air within the claimed range is established. In Examples 6 and 7 settings are chosen which lead to an adiabatic flame temperature outside the claimed range. In Example 8 no secondary air is introduced. In Example 9 the ratio of secondary air to primary air is outside the claimed range. Table 4 shows analytical data for the silicon dioxide powders produced, and for a commercially available pyrogenically produced silicon dioxide powder (Example 10). Examples 1 to 5 show how a specific narrow adiabatic flame temperature range can be obtained by varying the amounts of feed materials. Comparative examples 6 and 7 also show that with the same composition of silicon compounds as in Example 1, silicon dioxide powders according to the invention are not obtained. The powders obtained display BET surface areas outside the claimed range, Example 6: 266 m2/g and Example 7: 394 m2/g. In comparative examples 6 and 7, the adiabatic flame temperatures are outside the claimed range. In comparative examples 8 and 9, the adiabatic flame temperatures are within the claimed range, but silicon dioxide powders according to the invention are not obtained: In Example 8 no secondary air is added. The powder obtained has a long incorporation time (151%). In Example 9 the ratio of secondary air to primary air of 4.2 is outside the claimed range. The silicon dioxide powder obtained displays a very high relative viscosity of 165%. The powder from Example 1 is a commercially available powder, Aerosil® 300 from Degussa. Although this powder displays a BET surface area that is within the claimed range, it has a markedly higher viscosity and a markedly longer incorporation time than the powders according to the invention. In order to compare the values for the relative viscosity and the incorporation time, powder 10 is standardised to 100%. The average aggregate surface area, average equivalent circle diameter and average aggregate circumference are outside the claimed ranges. WE CLAIM 1. Pyrogenically produced silicon dioxide powder in the form of aggregates of primary particles having a BET surface area of 300 ± 25 m2/g, characterized in that the aggregates display: - an average surface area of 4800 to 6000 nm2, - an average equivalent circle diameter (ECD) of 60 to 80 nm and - an average circumference of 580 to 750 nm. 2. Pyrogenically produced silicon dioxide powder as claimed in claim 1, wherein the aggregates display: - an average surface area of 5000 to 5700 nm2, - an average equivalent circle diameter of 65 to 75 nm and an average circumference of 600 to 720 nm. 3. Pyrogenically produced silicon dioxide powder as claimed in claims 1 or 2, wherein the maximum aggregate diameter is between 100 and 140 nm and the minimum aggregate diameter is between 60 and 90 nm. 4. Pyrogenically produced silicon dioxide powder as claimed in claims 1 to 3, wherein the chloride content is less than 250 ppm. 5. Pyrogencially produced silicon dioxide powder as claimed in claim 1 to 4, wherein the carbon content is less than 500 ppm. 6. Process for the production of the silicon dioxide powder as claimed in claims 1 to 5, wherein: - a mixture of silicon compounds is evaporated, separately or together, the vapours are transferred by means of a carrier gas to a mixing chamber, with - SiCl4 as the first component in a proportion of 60 to 100 wt.% relative to the mixture, and - a second component chosen from the group comprising H3SiC1, H2SiC12, HSiC13, CH3SiC13, (CH3)2SiC12, (CH3)3SiC1, (n-C3H7) SiC13, in a proportion of 0 to 40 wt.%, relative to the mixture, - and a combustion gas and primary air, which can optionally be enriched with oxygen and/or be preheated, are transferred separately to the mixing chamber, - the mixture comprising the vapour of silicon compounds, combustion gas and primary air is ignited in a burner and the flame burns into a reaction chamber, - secondary air, which surrounds the flame, is introduced into the reaction chamber, the ratio of secondary air to primary air being in a range from 0.05 to 4, - the solid is then separated from gaseous substances and the solid is then steam-treated, and wherein, - the total amount of oxygen is at least sufficient for the complete combustion of the combustion gas and the silicon compounds and - the amount of feed materials consisting of silicon compounds, combustion gas, primary air and secondary air is chosen such that an adiabatic flame temperature Tad of 1392 to 1450°C is obtained, where Tad = the temperature of the feed materials + the sum of the reaction enthalpies of the partial reactions / heat capacity of the substances leaving the reaction chamber, comprising silicon dioxide, water, hydrogen chloride, carbon dioxide, oxygen, nitrogen, and optionally the carrier gas if it is not air or nitrogen., taking the specific heat capacity of these substances at 1000°C as a basis. 7. Process as claimed in claim 6, wherein the temperature of the feed materials is 90°C ± 40°C. 8. Process as claimed in claims 6 or 7, wherein the discharge velocity of the reaction mixture from the mixing chamber to the reaction space is 10 to 80 m/s. Dated this 22nd Day of MARCH 2005 The invention relates to a pyrogenically produced silicon dioxide powder in the form of aggregates of primary particles having a BET surface area of 300 ± 25 m2/g, the aggregates display an average surface area of 4800 to 6000 nm2, an average equivalent circle diameter (ECD) of 60 to 80 nm and an average circumference of 580 to 750 nm.

Documents:

00207-kol-2005 abstract.pdf

00207-kol-2005 claims.pdf

00207-kol-2005 correspondence-1.1.pdf

00207-kol-2005 correspondence-1.2.pdf

00207-kol-2005 correspondence-1.3.pdf

00207-kol-2005 correspondence.pdf

00207-kol-2005 descriptiopn(complete).pdf

00207-kol-2005 form-1-1.1.pdf

00207-kol-2005 form-1.pdf

00207-kol-2005 form-18.pdf

00207-kol-2005 form-2.pdf

00207-kol-2005 form-3.pdf

00207-kol-2005 form-5-1.1.pdf

00207-kol-2005 form-5.pdf

00207-kol-2005 gpa.pdf

00207-kol-2005 priority document.pdf

207-KOL-2005-CORRESPONDENCE 1.4.pdf

207-KOL-2005-CORRESPONDENCE 1.5.pdf

207-KOL-2005-FORM 27-1.1.pdf

207-KOL-2005-FORM 27.pdf

207-KOL-2005-FORM-27.pdf

207-kol-2005-granted-abstract.pdf

207-kol-2005-granted-claims.pdf

207-kol-2005-granted-correspondence.pdf

207-kol-2005-granted-description (complete).pdf

207-kol-2005-granted-examination report.pdf

207-kol-2005-granted-form 1.pdf

207-kol-2005-granted-form 18.pdf

207-kol-2005-granted-form 2.pdf

207-kol-2005-granted-form 3.pdf

207-kol-2005-granted-form 5.pdf

207-kol-2005-granted-gpa.pdf

207-kol-2005-granted-priority document.pdf

207-kol-2005-granted-reply to examination report.pdf

207-kol-2005-granted-specification.pdf

207-kol-2005-granted-translated copy of priority document.pdf

207-KOL-2005-PA.pdf