Title of Invention	FLAME-HYDROLYTICALLY PRODUCED TITANIUM DIOXIDE POWDER
Abstract	Flame-hydrolytically produced titanium dioxide powder that is present in the form of aggregates of primary particles, and has a BET surface of 20 to 200 m2/g, a half width (HW) [nm] of the primary particle distribution of HW = a × BETf where a = 670x10-9 m3/g and -1.3 ≤ f ≤ -1.0 and the proportion of particles with a diameter of more than 45 µm lies in a range from 0.0001 to 0.05 wt.%. The powder is produced by a process in which a titanium halide is vapourised at temperatures of less than 200°C, the vapours are transferred to a mixing chamber by means of a carrier gas of defined moisture content and, separately from this, hydrogen, primary air, which may optionally be enriched with oxygen and/or preheated, and steam are added to the mixing chamber, following which the reaction mixture is combusted in a reaction chamber sealed from the ambient air, secondary air is in addition introduced into the reaction chamber, the solid is then separated from gaseous substances, and following this the solid is treated with steam. The titanium dioxide powder may be used for the heat stabilisation of polymers.

Title of Invention

FLAME-HYDROLYTICALLY PRODUCED TITANIUM DIOXIDE POWDER

Abstract

Flame-hydrolytically produced titanium dioxide powder that is present in the form of aggregates of primary particles, and has a BET surface of 20 to 200 m2/g, a half width (HW) [nm] of the primary particle distribution of HW = a × BETf where a = 670x10-9 m3/g and -1.3 ≤ f ≤ -1.0 and the proportion of particles with a diameter of more than 45 µm lies in a range from 0.0001 to 0.05 wt.%. The powder is produced by a process in which a titanium halide is vapourised at temperatures of less than 200°C, the vapours are transferred to a mixing chamber by means of a carrier gas of defined moisture content and, separately from this, hydrogen, primary air, which may optionally be enriched with oxygen and/or preheated, and steam are added to the mixing chamber, following which the reaction mixture is combusted in a reaction chamber sealed from the ambient air, secondary air is in addition introduced into the reaction chamber, the solid is then separated from gaseous substances, and following this the solid is treated with steam. The titanium dioxide powder may be used for the heat stabilisation of polymers.

Full Text	Flame-hydrolytically Produced Titanium Dioxide Powder The invention relates to flame-hydrolytically produced titanium dioxide powder, and its production and use. It is known that titanium dioxide can be produced by pyrogenic processes. Pyrogenic processes are understood to include flame oxidations or flame hydrolyses. In flame oxidation a titanium dioxide precursor, for example titanium tetrachloride, is oxidised with oxygen according to equation la. In flame hydrolysis the formation of titanium dioxide is effected by hydrolysis of the titanium dioxide precursor, the water necessary for the hydrolysis being derived from the combustion of a fuel gas, for example hydrogen, and oxygen (equation lb). EP-A-1231186 claims a titanium dioxide with a BET surface of between 3 and 200 m2/g with a weight-related D90 diameter of the particles of 2.2 urn or less. D90 diameters of between 0.8 and 2.1 urn are mentioned in the examples of implementation. In addition, a titanium dioxide is obtained with a BET surface of between 3 and 200 m2/g and a distribution constant n of 1.7 or more, calculated according to the formula R = 100 exp(-bDn), where D denotes the particle diameter and b is a constant. The value n is obtained from the three values D10, D50 and D90, which are related to one another by an approximate straight line. The titanium dioxide is obtained by a flame oxidation of titanium tetrachloride and an oxidising gas, the starting materials being pre-heated to a temperature of at least 500°C before the reaction. In preferred embodiments the velocity of the reaction mixture is 10 m/sec or more, and the residence time in the reaction space is 3 sec or less. In EP-A-778812 a process for the production of titanium dioxide by a combination of flame oxidation and flame hydrolysis is described. In this connection titanium tetrachloride in the vapour state and oxygen are mixed in a reaction zone and the mixture is heated in a flame that is generated by combustion of a hydrocarbon as fuel gas. The titanium tetrachloride is fed into the central core of the reactor, the oxygen is fed into a tubular sleeve surrounding the central core, and the fuel gas is fed into a tubular sleeve that surrounds those tubes that convey the titanium tetrachloride and oxygen. A laminar diffusion flame reactor is preferably employed. In this method it is possible to produce titanium dioxide powder of large surface area containing a large proportion of the anatase modification. In EP-A-778812 no information is given regarding the structure and size of the primary particles and aggregates. However, it is these quantities in particular that are important for many applications, for example in cosmetics applications or as an abrasive in dispersions for the electronics industry. The mechanism of the formation of the titanium dioxide according to EP-A- 778812 includes both a flame oxidation (equation 1a) as well as a flame hydrolysis (equation lb). Although the different formation mechanisms enable the anatase fraction to be controlled, a specific distribution of the primary particles and aggregates cannot however be achieved. A further disadvantage of this method, as is mentioned in US- A-20002/0004029, is the incomplete conversion of titanium tetrachloride and fuel gas and the resultant grey colouration of the titanium dioxide. These problems are eliminated according to US-A- 20002/0004029, by now using five tubes instead of the three tubes as described in EP-A-778812. For this, titanium tetrachloride vapour, argon, oxygen, hydrogen and air are simultaneously metered into a flame reactor. The disadvantage of this method is the use of the expensive noble gas argon and a low yield of titanium dioxide due to low concentrations of titanium tetrachloride in the reaction gas. A titanium dioxide powder produced by flame hydrolysis has for a long time been marketed by Degussa under the reference P 25. This is a finely particulate titanium dioxide powder with a specific surface of 50+15 m2/g, a mean size of the primary particles of 21 nm, a compacted bulk density (approximate value) of 130 g/1, an HC1 content of less than or equal to 0.300 wt.% and a screening residue according to Mocker (45 urn) of less than or equal to 0.050%. This powder has good properties for many applications. The prior art demonstrates the wide interest in pyrogenically produced titanium dioxide. In this connection it is found that the common generic term "pyrogenic", i.e. flame hydrolysis and flame oxidation, is not an adequate description of titanium dioxide. On account of the complexity of the pyrogenic processes only a few substance parameters can be specifically adjusted. Titanium dioxide is employed in particular in catalysis, for example photocatalysis, in cosmetics, for example sunscreen agents, as an abrasive in the form of dispersions in the electronics industry, or for heat stabilisation of polymers. In these uses increasing demands are placed on the purity and structure of the titanium dioxide. Thus, it is for example important that, when using titanium dioxide as an abrasive in dispersions, the titanium dioxide has a good dispersibility and is as far as possible free from coarse particles that can scratch the surface to be polished. The object of the present invention is to provide a titanium dioxide powder that has a high purity, is easy to disperse, and is as far as possible free of coarse fractions. The object of the present invention is also to provide a process for the production of the titanium dioxide powder. In this connection the process should be able to be implemented on an industrial scale. The present invention provides a flame-hydrolytically produced titanium dioxide powder that is present in aggregates of primary particles, characterised in that it has a BET surface of 20 to 200 m2/g and the half width HW, in nanometers, of the primary particle distribution has values between the proportion of particles with a diameter of more than 45 urn is in a range from 0.0001 to 0.05 wt. %. The term primary particles in the context of the invention is understood to denote particles that are first of all formed in the reaction and that can coalesce to form aggregates during the further course of the reaction. The term aggregate within the context of the invention is understood to denote primary particles of similar structure and size that have coalesced together, and whose surface is smaller than the sum of the individual, isolated primary particles. Several aggregates or also individual primary particles may combine together further to form agglomerates. Aggregates or primary particles accordingly lie adjacent to one another in the form of point objects. Depending on their degree of coalescence, agglomerates may be broken up by application of energy. Aggregates on the other hand can be broken up only by a high input of energy or even cannot be broken up at all. Intermediate forms exist. The mean half width HW of the primary particle distribution (in numerical terms) is obtained by image analysis of TEM photographs. According to the invention the mean half width is a function of the BET surface with a constant f, where -1.3 ≤ f ≤ -1.0. Preferably the half width may lie in the range -1.2 ≤ f ≤ -1.1. It is the high BET surface, the narrow distribution of the primary particle distribution and the small proportion of aggregates with a diameter of more than 45 µm, which lies in a range from 0.0001 to 0.05 wt.%, that are relevant for the positive properties of the powder according to the invention, for example when polishing surfaces. No flame- hydrolytically produced titanium dioxide powders are known in the prior art that exhibit these features simultaneously. It is of course possible for example to remove to a large extent powders according to the prior art mechanically from aggregates with a diameter of more than 45 urn, though the resultant powder would however not be able to achieve the ranges claimed by the present invention as regards BET surface and half width values of the primary particles. The BET surface of the titanium dioxide powder according to the invention lies in a wide range from 20 to 200 m2/g. It has proved advantageous if the BET surface lies in a range from 40 to 60 m2/g. A range of 45 to 55 m2/g may be particularly advantageous. For a titanium dioxide powder according to the invention with a BET surface between 40 and 60 m2/g, the 90% spread of the number distribution of the primary particle diameters may lie between 10 and 100 nm. As a rule the 90% spread of the number distribution of the primary particle diameters is between 10 and 40 nm. Furthermore, the equivalent circular diameter of the aggregates (ECD) of such a titanium dioxide powder may be less than 80 nm. The mean aggregate area of a titanium dioxide powder according to the invention with a BET surface of 40 to 60 m2/g may be less than 6500 nm2 and the mean aggregate circumference may be less than 450 nm. In addition the BET surface of the titanium dioxide powder according to the invention may lie in a range from 80 to 120 m2/g. A range of 85 to 95 m2/g may be particularly preferred. For a titanium dioxide powder according to the invention with a BET surface between 80 and 120 m2/g, a 90% spread of the number distribution of the primary particle diameters may have values between 4 and 25 nm. Furthermore, such a titanium dioxide powder may have an equivalent circular diameter of the aggregates (ECD) of less than 70 nm. The mean aggregate area of a titanium dioxide powder according to the invention with a BET surface of 80 to 120 m2/g may be less than 6000 nm2 and the mean aggregate circumference may be less than 400 nm. The proportion of aggregates and/or agglomerates of the titanium dioxide powder according to the invention with a diameter of more than 45 jam lies in a range from 0.0001 to 0.05 wt.%. A range from 0.001 to 0.01 wt.% may be preferred, and a range from 0.002 to 0.005 wt.% may be particularly preferred. The titanium dioxide powder according to the invention contains rutile and anatase as crystal modifications. In this connection the anatase/rutile proportion for a given surface may lie in a range from 2:98 to 98:2. The range from 80:20 to 95:5 may be particularly preferred. The titanium dioxide powder according to the invention may contain residues of chloride. The chloride content is preferably less than 0.1 wt.%. A titanium dioxide powder according to the invention with a chloride content in the range from 0.01 to 0.05 wt.% may be particularly preferred. The compacted bulk density of the titanium dioxide powder according to the invention is not limited. It has however proved advantageous if the compacted bulk density has values from 20 to 200 g/1. A compacted bulk density of 30 to 120 g/1 may be particularly preferred. The present invention also provides a process for the production of the titanium dioxide powder according to the invention, which is characterised in that a titanium halide, preferably titanium tetrachloride, is vapourised at temperatures of less than 200°C, the vapours are transferred to a mixing chamber by means of a carrier gas with a proportion of steam in a range from 1 to 25 g/m3, and separately from this, hydrogen, primary air, which may optionally be enriched with oxygen and/or pre-heated, and steam are transferred to the mixing chamber, wherein the proportion of steam is in a range from 1 to 25 g/m3 primary air, the lambda value lies in the range from 1 to 9 and the gamma value lies in the range from 1 to 9, following which the mixture consisting of the titanium halide vapour, hydrogen, air and steam is ignited in a burner and the flame burns into a reaction chamber sealed from the ambient air, wherein a vacuum of 1 to 200 mbar exists in the reaction chamber, the exit velocity of the reaction mixture from the mixing chamber into the reaction space lies in a range from 10 to 80 m/sec, in addition secondary air is introduced into the reaction chamber, wherein the ratio of primary air to secondary air is between 10 and 0.5, following which the solid is separated from gaseous substances, and the solid is then treated with steam. An essential feature of the process according to the invention is that the titanium halide is vapourised at temperatures below 200°C and the vapours are conveyed to the mixing chamber by means of a carrier gas, for example air or oxygen, which has a defined carrier gas moisture content. It has been found for example that the product quality decreases at higher vapourisation temperatures. Moreover it has also been found that, within the claimed steam content of 1 to 25 g/m3 of gas, or primary air, there is no noticeable hydrolysis of the titanium halide in the form of caking, whereas on the other hand the steam content influences the subsequent primary particle and aggregate structure. Outside the claimed range, no powder according to the invention can be obtained. In a preferred embodiment the steam content is between 5 and 20 g/m3 of As carrier gas, air may also be used. This permits a higher space-time yield in the reaction chamber than when using an inert gas. Furthermore, the exit velocity of the reaction mixture from the mixing chamber into the reaction space lies in a range from 10 to 80 m/sec. In a preferred embodiment the exit velocity is between 15 and 60 m/sec, and in a particularly preferred embodiment is between 20 and 40 m/sec. At values below this no uniform powder is obtained, but instead a powder is obtained that contains particles of diameter 45 urn or more in an amount of more than 0.05 wt.%. In addition the reaction must be carried out so that the lambda value lies in the range from 1 to 9 and the gamma value lies in the range from 1 to 9. Flame-hydrolytically produced oxides are normally obtained by taking the gaseous starting substances in a stoichiometric ratio with respect to one another so that the added hydrogen is at least sufficient to react with the halogen X present from the titanium halide TiX4 to form HX. The amount of hydrogen reguired for this purpose is termed the stoichiometric amount of hydrogen. The ratio of the added hydrogen to the stoichiometrically necessary hydrogen defined above is termed gamma. Gamma is defined as: Gamma = added hydrogen/stoichiometrically required hydrogen or Gamma = H2 fed in (moles)/H2 stoichiometric (moles) . With flame-hydrolytically produced oxides in addition an amount of oxygen (for example from the air) is normally used that is at least sufficient to convert the titanium halide into titanium dioxide and to convert excess hydrogen that may still be present into water. This amount of oxygen is termed the stoichiometric amount of oxygen. Similarly, the ratio of added oxygen to the stoichiometrically required oxygen is termed lambda, and is defined as follows: Lambda = added oxygen/stoichiometrically required oxygen or Lambda = O2 fed in (moles)/O2 stoichiometric (moles). Moreover, in the process according to the invention, in addition to the primary air in the mixing chamber air (secondary air) is directly introduced into the reaction chamber. It has been found that, without the addition of the additional air to the mixing chamber, no titanium dioxide powder according to the invention is obtained. In this connection it should be noted that the ratio of primary air to secondary air is between 10 and 0.5. The ratio is preferably in a range between 5 and 1. In order to be able accurately to meter in the amount of secondary air, it is necessary to cause the flame to burn back into a reaction chamber sealed from the ambient air. This enables the process to be accurately controlled, which is essential in order to obtain the titanium dioxide powder according to the invention. The vacuum in the reaction chamber is preferably between 10 and 80 mbar. An essential feature is also to treat the titanium dioxide powder after separation from gaseous substances with steam. This treatment is primarily intended to remove halide- containing groups from the surface. At the same time this treatment reduces the number of agglomerates. The process may be carried out continuously in such a way that the powder is treated, in counter-current or co-current, with steam, possibly together with air, in which connection the steam is always introduced from below into an upright, heatable column. The feed of the powder may take place from below or above into the column. The reaction conditions may be chosen so that a fluidised bed is formed. The temperature at which the treatment with steam is carried out between 250 and 750°C, values from 450 to 550°C being preferred. In addition it is preferred to carry out the treatment in counter-current in such a way that no fluidised bed is formed. Moreover it may be advantageous to introduce the steam together with the air into the mixing chamber. Fig. 1A shows diagrammatically an arrangement for carrying out the process according to the invention. In the figure: A = mixing chamber, B = flame, C = reaction chamber, D = solid/gaseous separation, E = post-treatment with steam. The substances used are identified as follows: a = mixture of titanium halide and carrier gas with defined moisture content, b = hydrogen, c = air, d = steam, e = secondary air, f = steam or steam/air. Fig. 1B shows a section of the arrangement of Fig. 1A. In this, the steam (d) together with the air (c) are introduced into the mixing chamber. Fig. 1C shows an open reaction chamber in which the secondary air e is aspirated from the surroundings. With the arrangement according to Fig. 1C no titanium dioxide powder according to the invention can be obtained. The invention also provides for the use of the titanium dioxide powder according to the invention for the heat protection stabilisation of silicones. The invention in addition provides for the use of the titanium dioxide powder according to the invention in sunscreen agents. The invention furthermore provides for the use of the titanium dioxide powder according to the invention as a catalyst, as a catalyst carrier, as a photocatalyst, and as an abrasive for the production of dispersions. Examples Analysis The BET surface is determined according to DIN 66131. The compacted bulk density is determined on the basis of DIN ISO 787/XI K 5101/18 (not screened). The bulk density is determined according to DIN-ISO 787/XI. The pH value is determined on the basis of DIN ISO 787/IX, ASTM D 1280, JIS K 5101/24. The proportion of particles larger than 45 µm is determined according to DIN ISO 787/XVIII, JIS K 5101/20. Determination of the chloride content: ca. 0.3 g of the particles according to the invention is accurately weighed out, 20 ml of 20 per cent sodium hydroxide solution (analysis purity) are added thereto, dissolved, and transferred while stirring to 15 ml of cooled HNO3. The chloride content in the solution is titrated with AgNO3 solution (0.1 mole/1 or 0.01 mole/1). The half width of the primary particle distribution and area, circumference and diameter of the aggregates are determined by means of image analysis. The image analyses are carried out using an H 7500 TEM instrument from Hitachi and a MegaView II CCD camera from SIS. The image magnification for the evaluation is 30000 : 1 at a pixel density of 3.2 nm. The number of evaluated particles is greater than 1000. The preparation is carried out according to ASTM3849-89. The lower threshold boundary as regards detection is 50 pixels. Example A1 (according to the invention) 160 kg/hr of TiCl4 are vapourised in an evaporator at 140°C. The vapours are transferred to a mixing chamber by means of nitrogen (15 Nm3/hr) as carrier gas with a carrier gas moisture content of 15 g/m3 of carrier gas. Separately from this, 52 Nm3/hr of hydrogen and 525 Nm3/hr of primary air are introduced into the mixing chamber. In a central tube the reaction mixture is fed to a burner and ignited. The flame burns in a water-cooled flame tube. In addition 200 Nm3/hr of secondary air are added to the reaction space. The powder formed is separated in a downstream filter and then treated in countercurrent with air and steam at 520°C. The Examples A2 to A9 according to the invention are carried out similarly to A1. The parameters altered in each case are listed in Table 1. The physicochemical data of the powders from Examples Al to A9 are shown in Table 2. The comparison examples B1 to B3 and B5 to B8 are also carried out similarly to A1. The parameters altered in each case are listed in Table 1. The comparison example B4 is carried out using an open burner. The amount of secondary air is not determined. The physicochemical data of the powders from Examples Bl to B8 are shown in Table 2. Table 3 shows calculated half-width values of the primary particles depending on the BET surface with f = -1.0, -1.05, -1.15 and -1.3. The factor 10-9 serves for the conversion from metres into nanometres. Since the factor f can assume only negative values, the unit of BETf can be g/m2. Fig. 2 shows the half width of the primary particles of the titanium dioxide powders produced in the examples. In this connection the titanium dioxide powders according to the invention (identified as ■) lie within the claimed half width HW [nm] = a x BETf where a = 670x10-9 m3/g and -1.3 ≤ f ≤ -1.0, while the comparison examples (identified by +) lie outside. Heat stabilisation of polymers Example C1: without titanium dioxide powder (comparison example) A two-component silicone rubber from Bayer, trade name Silopren® LSR 2040, is used as base component (addition crosslinking). After homogeneously mixing the two components with a dissolver the vulcanisation takes place at 180 °C for 10 minutes. Sample plates (ca. 10 x 15 cm) 6 mm thick are produced. The sample plates are heated at 80 °C in a furnace to constant weight (ca. 1 day). To check the thermal stability a hot storage test is carried out. For this, a sample strip of size 5 x 7 cm is kept in a circulating air oven at 275 °C. The weight loss is measured. Example C2: Addition of titanium dioxide powder according to the prior art (comparison example) A two-component silicone rubber from Bayer, trade name Silopren® LSR 2040, is used as base component (addition crosslinking). 1.5 wt.%, referred to the total batch, of titanium dioxide powder P 25 S (Degussa AG) is incorporated for 5 minutes into one of the components, using a dissolver. Following this the vulcanisation and production of the sample plates take place as described in Example 1. Sample strips of size 5 x 7 cm are stored at 275 °C. The weight loss is measured. The Examples C3-5 are carried out similarly to C1, but using the titanium dioxide powders A1 according to the invention in C3, A3 in C4 and A7 in C5, instead of P25 S. Table 4 shows the length changes of the samples stored at 275°C for 1, 3 and 7 days. The results demonstrate the effective heat protection stabilisation of polymers achieved by using the titanium dioxide powder according to the invention. Photocatalytic activity Example D1: Titanium dioxide powder according to the prior art (comparison example) To determine the photocatalytic activity the sample to be measured is suspended in 2-propanol and irradiated for one hour with UV light. The concentration of acetone formed is then measured. Ca. 250 mg (accuracy 0.1 mg) of titanium dioxide powder P 25S (Degussa AG) are suspended using an Ultra-Turrax stirrer in 350 ml (275.1 g) of 2-propanol. This suspension is conveyed by means of a pump through a cooler thermostatically controlled to 24°C to a glass photoreactor equipped with a radiation source and flushed beforehand with oxygen. An Hg medium-pressure immersion lamp of the type TQ718 (Heraeus) with an output of 500 Watts serves for example as radiation source. A protective tube of borosilicate glass restricts the emitted radiation to wavelengths >300nm. The radiation source is surrounded externally by a cooling tube through which water flows. Oxygen is metered into the reactor via a flow meter. The reaction is started when the radiation source is switched on. At the end of the reaction a small amount of the suspension is immediately removed, filtered, and analysed by gas chromatography. A photoactivity k of 0.68 x 10-3 mole kg-1min-1 is measured. This is taken as base value 1. The titanium dioxide powder according to the invention has a somewhat lower photocatalytic activity of 0.8 to 0.9. Patent Claims 1. Flame-hydrolytically produced titanium dioxide powder present in aggregates of primary particles, characterised in that it has a BET surface of 20 to 200 m2/g and the half width HW, in nanometers, of the primary particle distribution has values between HW [nm] = a x BETf where a = 670x109 m3/g and -1.3 ≤ f ≤ -1.0 and the proportion of particles with a diameter of more than 45 µm is in a range from 0.0001 to 0.05 wt.%. 2. Flame-hydrolytically produced titanium dioxide powder according to claim 1, characterised in that the BET surface is in a range from 40 to 60 m2/g. 3. Flame-hydrolytically produced titanium dioxide powder according to claim 2, characterised in that the 90% spread of the number distribution of the primary particle diameters lies in a range from 5 to 100 nm. 4. Flame-hydrolytically produced titanium dioxide powder according to claim 2 or 3, characterised in that the equivalent circular diameter of the aggregates (ECD) is less than 80 nm. 5. Flame-hydrolytically produced titanium dioxide powder according to claims 2 to 4, characterised in that the mean aggregate area is less than 6500 nm2. 6. Flame-hydrolytically produced titanium dioxide powder according to claims 2 to 5, characterised in that the mean aggregate circumference is less than 450 nm. 7. Flame-hydrolytically produced titanium dioxide powder according to claim 1, characterised in that the BET surface lies in a range from 80 to 120 m2/g. 8. Flame-hydrolytically produced titanium dioxide powder according to claim 7, characterised in that the 90% spread of the number distribution of the primary particles diameters has values from 4 to 25 nm. 9. Flame-hydrolytically produced titanium dioxide powder according to claim 7 or 8, characterised in that the equivalent circular diameter of the aggregates (ECD) is less than 70 nm. 10. Flame-hydrolytically produced titanium dioxide powder according to claims 7 to 9, characterised in that the mean aggregate area is less than 6000 nm2. 11. Flame-hydrolytically produced titanium dioxide powder according to claims 7 to 10, characterised in that the mean aggregate circumference is less than 400 nm. 12. Flame-hydrolytically produced titanium dioxide powder according to claims 1 to 11, characterised in that the proportion of aggregates and/or agglomerates with a diameter of more than 45 µm lies in a range from 0.001 to 0.01 wt.%. 13. Flame-hydrolytically produced titanium dioxide powder according to claims 1 to 12, characterised in that for a given BET surface it has an anatase/rutile ratio of 2:98 to 98:2. 14. Flame-hydrolytically produced titanium dioxide powder according to claims 1 to 13, characterised in that it has a chloride content of less than 0.1 wt.%. 15. Flame-hydrolytically produced titanium dioxide powder according to claims 1 to 14, characterised in that the compacted bulk density has values of 20 to 200 g/l. 16. Process for the production of the flame-hydrolytically produced titanium dioxide powder according to claims 1 to 15, characterised in that a titanium halide, preferably titanium tetrachloride, is vapourised at temperatures of less than 200°C, the vapours are transferred to a mixing chamber by means of a carrier gas with a proportion of steam in a range from 1 to 25 g/m3 of gas, and separately from this, hydrogen, primary air, which may optionally be enriched with oxygen and/or pre-heated, and steam are transferred to the mixing chamber, wherein the proportion of steam is in a range from 1 to 25 g/m3 primary air, the lambda value lies in the range from 1 to 9 and the gamma value lies in the range from 1 to 9, following which the mixture consisting of the titanium halide vapour, hydrogen, air and steam is ignited in a burner and the flame burns into a reaction chamber sealed from the ambient air, wherein a vacuum of 1 to 200 mbar exists in the reaction chamber, the exit velocity of the reaction mixture from the mixing chamber into the reaction space lies in a range from 10 to 80 m/sec, in addition secondary air is introduced into the reaction chamber, wherein the ratio of primary air to secondary air is between 10 and 0.5, following which the solid is separated from the gaseous substances, and the solid is then treated with steam. 17. Process according to claim 16, characterised in that the steam is introduced together with the air into the mixing chamber. 18. Use of the flame-hydrolytically produced titanium dioxide powder according to claims 1 to 15 for the heat protection stabilisation of silicones. 19. Use of the flame-hydrolytically produced titanium dioxide powder according to claims 1 to 15 in sunscreen agents. 20. Use of the flame-hydrolytically produced titanium dioxide powder according to claims 1 to 15 as a catalyst, as a catalyst carrier, as a photocatalyst, and as an abrasive for the production of dispersions. Flame-hydrolytically produced titanium dioxide powder that is present in the form of aggregates of primary particles, and has a BET surface of 20 to 200 m2/g, a half width (HW) [nm] of the primary particle distribution of HW = a × BETf where a = 670x10-9 m3/g and -1.3 ≤ f ≤ -1.0 and the proportion of particles with a diameter of more than 45 µm lies in a range from 0.0001 to 0.05 wt.%. The powder is produced by a process in which a titanium halide is vapourised at temperatures of less than 200°C, the vapours are transferred to a mixing chamber by means of a carrier gas of defined moisture content and, separately from this, hydrogen, primary air, which may optionally be enriched with oxygen and/or preheated, and steam are added to the mixing chamber, following which the reaction mixture is combusted in a reaction chamber sealed from the ambient air, secondary air is in addition introduced into the reaction chamber, the solid is then separated from gaseous substances, and following this the solid is treated with steam. The titanium dioxide powder may be used for the heat stabilisation of polymers.

Full Text

Flame-hydrolytically Produced Titanium Dioxide Powder
The invention relates to flame-hydrolytically produced
titanium dioxide powder, and its production and use.
It is known that titanium dioxide can be produced by
pyrogenic processes. Pyrogenic processes are understood to
include flame oxidations or flame hydrolyses. In flame
oxidation a titanium dioxide precursor, for example
titanium tetrachloride, is oxidised with oxygen according
to equation la. In flame hydrolysis the formation of
titanium dioxide is effected by hydrolysis of the titanium
dioxide precursor, the water necessary for the hydrolysis
being derived from the combustion of a fuel gas, for
example hydrogen, and oxygen (equation lb).

EP-A-1231186 claims a titanium dioxide with a BET surface
of between 3 and 200 m2/g with a weight-related D90 diameter
of the particles of 2.2 urn or less. D90 diameters of between
0.8 and 2.1 urn are mentioned in the examples of
implementation. In addition, a titanium dioxide is
obtained with a BET surface of between 3 and 200 m2/g and a
distribution constant n of 1.7 or more, calculated
according to the formula R = 100 exp(-bDn), where D denotes
the particle diameter and b is a constant. The value n is
obtained from the three values D10, D50 and D90, which are
related to one another by an approximate straight line.
The titanium dioxide is obtained by a flame oxidation of
titanium tetrachloride and an oxidising gas, the starting

materials being pre-heated to a temperature of at least
500°C before the reaction. In preferred embodiments the
velocity of the reaction mixture is 10 m/sec or more, and
the residence time in the reaction space is 3 sec or less.
In EP-A-778812 a process for the production of titanium
dioxide by a combination of flame oxidation and flame
hydrolysis is described. In this connection titanium
tetrachloride in the vapour state and oxygen are mixed in a
reaction zone and the mixture is heated in a flame that is
generated by combustion of a hydrocarbon as fuel gas. The
titanium tetrachloride is fed into the central core of the
reactor, the oxygen is fed into a tubular sleeve
surrounding the central core, and the fuel gas is fed into
a tubular sleeve that surrounds those tubes that convey the
titanium tetrachloride and oxygen.
A laminar diffusion flame reactor is preferably employed.
In this method it is possible to produce titanium dioxide
powder of large surface area containing a large proportion
of the anatase modification. In EP-A-778812 no information
is given regarding the structure and size of the primary
particles and aggregates. However, it is these quantities
in particular that are important for many applications, for
example in cosmetics applications or as an abrasive in
dispersions for the electronics industry. The mechanism of
the formation of the titanium dioxide according to EP-A-
778812 includes both a flame oxidation (equation 1a) as
well as a flame hydrolysis (equation lb). Although the
different formation mechanisms enable the anatase fraction
to be controlled, a specific distribution of the primary
particles and aggregates cannot however be achieved. A
further disadvantage of this method, as is mentioned in US-

A-20002/0004029, is the incomplete conversion of titanium
tetrachloride and fuel gas and the resultant grey
colouration of the titanium dioxide.
These problems are eliminated according to US-A-
20002/0004029, by now using five tubes instead of the three
tubes as described in EP-A-778812. For this, titanium
tetrachloride vapour, argon, oxygen, hydrogen and air are
simultaneously metered into a flame reactor. The
disadvantage of this method is the use of the expensive
noble gas argon and a low yield of titanium dioxide due to
low concentrations of titanium tetrachloride in the
reaction gas.
A titanium dioxide powder produced by flame hydrolysis has
for a long time been marketed by Degussa under the
reference P 25.
This is a finely particulate titanium dioxide powder with a
specific surface of 50+15 m2/g, a mean size of the primary
particles of 21 nm, a compacted bulk density (approximate
value) of 130 g/1, an HC1 content of less than or equal to
0.300 wt.% and a screening residue according to Mocker
(45 urn) of less than or equal to 0.050%. This powder has
good properties for many applications.
The prior art demonstrates the wide interest in
pyrogenically produced titanium dioxide. In this
connection it is found that the common generic term
"pyrogenic", i.e. flame hydrolysis and flame oxidation, is
not an adequate description of titanium dioxide. On
account of the complexity of the pyrogenic processes only a
few substance parameters can be specifically adjusted.

Titanium dioxide is employed in particular in catalysis,
for example photocatalysis, in cosmetics, for example
sunscreen agents, as an abrasive in the form of dispersions
in the electronics industry, or for heat stabilisation of
polymers. In these uses increasing demands are placed on
the purity and structure of the titanium dioxide. Thus, it
is for example important that, when using titanium dioxide
as an abrasive in dispersions, the titanium dioxide has a
good dispersibility and is as far as possible free from
coarse particles that can scratch the surface to be
polished.
The object of the present invention is to provide a
titanium dioxide powder that has a high purity, is easy to
disperse, and is as far as possible free of coarse
fractions.
The object of the present invention is also to provide a
process for the production of the titanium dioxide powder.
In this connection the process should be able to be
implemented on an industrial scale.
The present invention provides a flame-hydrolytically
produced titanium dioxide powder that is present in
aggregates of primary particles, characterised in that
it has a BET surface of 20 to 200 m2/g and
the half width HW, in nanometers, of the primary
particle distribution has values between

the proportion of particles with a diameter of more
than 45 urn is in a range from 0.0001 to 0.05 wt. %.
The term primary particles in the context of the invention
is understood to denote particles that are first of all
formed in the reaction and that can coalesce to form
aggregates during the further course of the reaction.
The term aggregate within the context of the invention is
understood to denote primary particles of similar structure
and size that have coalesced together, and whose surface is
smaller than the sum of the individual, isolated primary
particles. Several aggregates or also individual primary
particles may combine together further to form
agglomerates. Aggregates or primary particles accordingly
lie adjacent to one another in the form of point objects.
Depending on their degree of coalescence, agglomerates may
be broken up by application of energy.
Aggregates on the other hand can be broken up only by a
high input of energy or even cannot be broken up at all.
Intermediate forms exist.
The mean half width HW of the primary particle distribution
(in numerical terms) is obtained by image analysis of TEM
photographs. According to the invention the mean half
width is a function of the BET surface with a constant f,
where -1.3 ≤ f ≤ -1.0. Preferably the half width may lie
in the range -1.2 ≤ f ≤ -1.1.

It is the high BET surface, the narrow distribution of the
primary particle distribution and the small proportion of
aggregates with a diameter of more than 45 µm, which lies
in a range from 0.0001 to 0.05 wt.%, that are relevant for
the positive properties of the powder according to the
invention, for example when polishing surfaces. No flame-
hydrolytically produced titanium dioxide powders are known
in the prior art that exhibit these features
simultaneously. It is of course possible for example to
remove to a large extent powders according to the prior art
mechanically from aggregates with a diameter of more than
45 urn, though the resultant powder would however not be
able to achieve the ranges claimed by the present invention
as regards BET surface and half width values of the primary
particles.
The BET surface of the titanium dioxide powder according to
the invention lies in a wide range from 20 to 200 m2/g. It
has proved advantageous if the BET surface lies in a range
from 40 to 60 m2/g. A range of 45 to 55 m2/g may be
particularly advantageous.
For a titanium dioxide powder according to the invention
with a BET surface between 40 and 60 m2/g, the 90% spread of
the number distribution of the primary particle diameters
may lie between 10 and 100 nm. As a rule the 90% spread of
the number distribution of the primary particle diameters
is between 10 and 40 nm.
Furthermore, the equivalent circular diameter of the
aggregates (ECD) of such a titanium dioxide powder may be
less than 80 nm.

The mean aggregate area of a titanium dioxide powder
according to the invention with a BET surface of 40 to
60 m2/g may be less than 6500 nm2 and the mean aggregate
circumference may be less than 450 nm.
In addition the BET surface of the titanium dioxide powder
according to the invention may lie in a range from 80 to
120 m2/g. A range of 85 to 95 m2/g may be particularly
preferred.
For a titanium dioxide powder according to the invention
with a BET surface between 80 and 120 m2/g, a 90% spread of
the number distribution of the primary particle diameters
may have values between 4 and 25 nm. Furthermore, such a
titanium dioxide powder may have an equivalent circular
diameter of the aggregates (ECD) of less than 70 nm.
The mean aggregate area of a titanium dioxide powder
according to the invention with a BET surface of 80 to
120 m2/g may be less than 6000 nm2 and the mean aggregate
circumference may be less than 400 nm.
The proportion of aggregates and/or agglomerates of the
titanium dioxide powder according to the invention with a
diameter of more than 45 jam lies in a range from 0.0001 to
0.05 wt.%. A range from 0.001 to 0.01 wt.% may be
preferred, and a range from 0.002 to 0.005 wt.% may be
particularly preferred.
The titanium dioxide powder according to the invention
contains rutile and anatase as crystal modifications. In
this connection the anatase/rutile proportion for a given
surface may lie in a range from 2:98 to 98:2. The range
from 80:20 to 95:5 may be particularly preferred.
The titanium dioxide powder according to the invention may
contain residues of chloride. The chloride content is

preferably less than 0.1 wt.%. A titanium dioxide powder
according to the invention with a chloride content in the
range from 0.01 to 0.05 wt.% may be particularly preferred.
The compacted bulk density of the titanium dioxide powder
according to the invention is not limited. It has however
proved advantageous if the compacted bulk density has
values from 20 to 200 g/1. A compacted bulk density of 30
to 120 g/1 may be particularly preferred.
The present invention also provides a process for the
production of the titanium dioxide powder according to the
invention, which is characterised in that
a titanium halide, preferably titanium tetrachloride,
is vapourised at temperatures of less than 200°C, the
vapours are transferred to a mixing chamber by means
of a carrier gas with a proportion of steam in a range
from 1 to 25 g/m3, and
separately from this, hydrogen, primary air, which may
optionally be enriched with oxygen and/or pre-heated,
and steam are transferred to the mixing chamber,
wherein the proportion of steam is in a range
from 1 to 25 g/m3 primary air,
the lambda value lies in the range from 1 to 9
and the gamma value lies in the range from 1
to 9,
following which
the mixture consisting of the titanium halide vapour,
hydrogen, air and steam is ignited in a burner and the
flame burns into a reaction chamber sealed from the

ambient air, wherein
a vacuum of 1 to 200 mbar exists in the reaction
chamber,
the exit velocity of the reaction mixture from
the mixing chamber into the reaction space lies
in a range from 10 to 80 m/sec,
in addition secondary air is introduced into the
reaction chamber, wherein
the ratio of primary air to secondary air is
between 10 and 0.5,
following which the solid is separated from gaseous
substances, and
the solid is then treated with steam.
An essential feature of the process according to the
invention is that the titanium halide is vapourised at
temperatures below 200°C and the vapours are conveyed to
the mixing chamber by means of a carrier gas, for example
air or oxygen, which has a defined carrier gas moisture
content. It has been found for example that the product
quality decreases at higher vapourisation temperatures.
Moreover it has also been found that, within the claimed
steam content of 1 to 25 g/m3 of gas, or primary air, there
is no noticeable hydrolysis of the titanium halide in the
form of caking, whereas on the other hand the steam content
influences the subsequent primary particle and aggregate
structure. Outside the claimed range, no powder according
to the invention can be obtained. In a preferred
embodiment the steam content is between 5 and 20 g/m3 of

As carrier gas, air may also be used. This permits a
higher space-time yield in the reaction chamber than when
using an inert gas.
Furthermore, the exit velocity of the reaction mixture from
the mixing chamber into the reaction space lies in a range
from 10 to 80 m/sec. In a preferred embodiment the exit
velocity is between 15 and 60 m/sec, and in a particularly
preferred embodiment is between 20 and 40 m/sec. At values
below this no uniform powder is obtained, but instead a
powder is obtained that contains particles of diameter
45 urn or more in an amount of more than 0.05 wt.%.
In addition the reaction must be carried out so that the
lambda value lies in the range from 1 to 9 and the gamma
value lies in the range from 1 to 9.
Flame-hydrolytically produced oxides are normally obtained
by taking the gaseous starting substances in a
stoichiometric ratio with respect to one another so that
the added hydrogen is at least sufficient to react with the
halogen X present from the titanium halide TiX4 to form HX.
The amount of hydrogen reguired for this purpose is termed
the stoichiometric amount of hydrogen.
The ratio of the added hydrogen to the stoichiometrically
necessary hydrogen defined above is termed gamma. Gamma is
defined as:
Gamma = added hydrogen/stoichiometrically required
hydrogen
or
Gamma = H2 fed in (moles)/H2 stoichiometric (moles) .

With flame-hydrolytically produced oxides in addition an
amount of oxygen (for example from the air) is normally
used that is at least sufficient to convert the titanium
halide into titanium dioxide and to convert excess hydrogen
that may still be present into water. This amount of
oxygen is termed the stoichiometric amount of oxygen.
Similarly, the ratio of added oxygen to the
stoichiometrically required oxygen is termed lambda, and is
defined as follows:
Lambda = added oxygen/stoichiometrically required oxygen
or
Lambda = O2 fed in (moles)/O2 stoichiometric (moles).
Moreover, in the process according to the invention, in
addition to the primary air in the mixing chamber air
(secondary air) is directly introduced into the reaction
chamber. It has been found that, without the addition of
the additional air to the mixing chamber, no titanium
dioxide powder according to the invention is obtained. In
this connection it should be noted that the ratio of
primary air to secondary air is between 10 and 0.5. The
ratio is preferably in a range between 5 and 1.
In order to be able accurately to meter in the amount of
secondary air, it is necessary to cause the flame to burn
back into a reaction chamber sealed from the ambient air.
This enables the process to be accurately controlled, which
is essential in order to obtain the titanium dioxide powder
according to the invention. The vacuum in the reaction
chamber is preferably between 10 and 80 mbar.
An essential feature is also to treat the titanium dioxide
powder after separation from gaseous substances with steam.

This treatment is primarily intended to remove halide-
containing groups from the surface. At the same time this
treatment reduces the number of agglomerates. The process
may be carried out continuously in such a way that the
powder is treated, in counter-current or co-current, with
steam, possibly together with air, in which connection the
steam is always introduced from below into an upright,
heatable column. The feed of the powder may take place
from below or above into the column. The reaction
conditions may be chosen so that a fluidised bed is formed.
The temperature at which the treatment with steam is
carried out between 250 and 750°C, values from 450 to 550°C
being preferred. In addition it is preferred to carry out
the treatment in counter-current in such a way that no
fluidised bed is formed.
Moreover it may be advantageous to introduce the steam
together with the air into the mixing chamber.
Fig. 1A shows diagrammatically an arrangement for carrying
out the process according to the invention. In the figure:
A = mixing chamber, B = flame, C = reaction chamber, D =
solid/gaseous separation, E = post-treatment with steam.
The substances used are identified as follows: a = mixture
of titanium halide and carrier gas with defined moisture
content, b = hydrogen, c = air, d = steam, e = secondary
air, f = steam or steam/air. Fig. 1B shows a section of
the arrangement of Fig. 1A. In this, the steam (d)
together with the air (c) are introduced into the mixing
chamber. Fig. 1C shows an open reaction chamber in which
the secondary air e is aspirated from the surroundings.
With the arrangement according to Fig. 1C no titanium
dioxide powder according to the invention can be obtained.
The invention also provides for the use of the titanium
dioxide powder according to the invention for the heat
protection stabilisation of silicones.

The invention in addition provides for the use of the
titanium dioxide powder according to the invention in
sunscreen agents.
The invention furthermore provides for the use of the
titanium dioxide powder according to the invention as a
catalyst, as a catalyst carrier, as a photocatalyst, and as
an abrasive for the production of dispersions.

Examples
Analysis
The BET surface is determined according to DIN 66131.
The compacted bulk density is determined on the basis of
DIN ISO 787/XI K 5101/18 (not screened).
The bulk density is determined according to DIN-ISO 787/XI.
The pH value is determined on the basis of DIN ISO 787/IX,
ASTM D 1280, JIS K 5101/24.
The proportion of particles larger than 45 µm is determined
according to DIN ISO 787/XVIII, JIS K 5101/20.
Determination of the chloride content: ca. 0.3 g of the
particles according to the invention is accurately weighed
out, 20 ml of 20 per cent sodium hydroxide solution
(analysis purity) are added thereto, dissolved, and
transferred while stirring to 15 ml of cooled HNO3. The
chloride content in the solution is titrated with AgNO3
solution (0.1 mole/1 or 0.01 mole/1).
The half width of the primary particle distribution and
area, circumference and diameter of the aggregates are
determined by means of image analysis. The image analyses
are carried out using an H 7500 TEM instrument from Hitachi
and a MegaView II CCD camera from SIS. The image
magnification for the evaluation is 30000 : 1 at a pixel
density of 3.2 nm. The number of evaluated particles is
greater than 1000. The preparation is carried out
according to ASTM3849-89. The lower threshold boundary as
regards detection is 50 pixels.

Example A1 (according to the invention)
160 kg/hr of TiCl4 are vapourised in an evaporator at 140°C.
The vapours are transferred to a mixing chamber by means of
nitrogen (15 Nm3/hr) as carrier gas with a carrier gas
moisture content of 15 g/m3 of carrier gas. Separately from
this, 52 Nm3/hr of hydrogen and 525 Nm3/hr of primary air
are introduced into the mixing chamber. In a central tube
the reaction mixture is fed to a burner and ignited. The
flame burns in a water-cooled flame tube. In addition
200 Nm3/hr of secondary air are added to the reaction space.
The powder formed is separated in a downstream filter and
then treated in countercurrent with air and steam at 520°C.
The Examples A2 to A9 according to the invention are
carried out similarly to A1. The parameters altered in
each case are listed in Table 1.
The physicochemical data of the powders from Examples Al to
A9 are shown in Table 2.
The comparison examples B1 to B3 and B5 to B8 are also
carried out similarly to A1. The parameters altered in
each case are listed in Table 1.
The comparison example B4 is carried out using an open
burner. The amount of secondary air is not determined.
The physicochemical data of the powders from Examples Bl to
B8 are shown in Table 2.
Table 3 shows calculated half-width values of the primary
particles depending on the BET surface with f = -1.0,
-1.05, -1.15 and -1.3. The factor 10-9 serves for the
conversion from metres into nanometres. Since the factor f

can assume only negative values, the unit of BETf can
be g/m2.
Fig. 2 shows the half width of the primary particles of the
titanium dioxide powders produced in the examples. In this
connection the titanium dioxide powders according to the
invention (identified as ■) lie within the claimed half
width HW [nm] = a x BETf where a = 670x10-9 m3/g and
-1.3 ≤ f ≤ -1.0, while the comparison examples (identified
by +) lie outside.

Heat stabilisation of polymers
Example C1: without titanium dioxide powder (comparison
example)
A two-component silicone rubber from Bayer, trade name
Silopren® LSR 2040, is used as base component (addition
crosslinking). After homogeneously mixing the two
components with a dissolver the vulcanisation takes place
at 180 °C for 10 minutes. Sample plates (ca. 10 x 15 cm)
6 mm thick are produced. The sample plates are heated at
80 °C in a furnace to constant weight (ca. 1 day). To
check the thermal stability a hot storage test is carried
out. For this, a sample strip of size 5 x 7 cm is kept in
a circulating air oven at 275 °C. The weight loss is
measured.

Example C2: Addition of titanium dioxide powder according
to the prior art (comparison example)
A two-component silicone rubber from Bayer, trade name
Silopren® LSR 2040, is used as base component (addition
crosslinking). 1.5 wt.%, referred to the total batch, of
titanium dioxide powder P 25 S (Degussa AG) is incorporated
for 5 minutes into one of the components, using a
dissolver. Following this the vulcanisation and production
of the sample plates take place as described in Example 1.
Sample strips of size 5 x 7 cm are stored at 275 °C. The
weight loss is measured.
The Examples C3-5 are carried out similarly to C1, but
using the titanium dioxide powders A1 according to the
invention in C3, A3 in C4 and A7 in C5, instead of P25 S.
Table 4 shows the length changes of the samples stored at
275°C for 1, 3 and 7 days. The results demonstrate the
effective heat protection stabilisation of polymers
achieved by using the titanium dioxide powder according to
the invention.

Photocatalytic activity
Example D1: Titanium dioxide powder according to the prior
art (comparison example)
To determine the photocatalytic activity the sample to be
measured is suspended in 2-propanol and irradiated for one
hour with UV light. The concentration of acetone formed is
then measured.
Ca. 250 mg (accuracy 0.1 mg) of titanium dioxide powder
P 25S (Degussa AG) are suspended using an Ultra-Turrax
stirrer in 350 ml (275.1 g) of 2-propanol. This suspension
is conveyed by means of a pump through a cooler
thermostatically controlled to 24°C to a glass photoreactor
equipped with a radiation source and flushed beforehand
with oxygen. An Hg medium-pressure immersion lamp of the
type TQ718 (Heraeus) with an output of 500 Watts serves for
example as radiation source. A protective tube of
borosilicate glass restricts the emitted radiation to
wavelengths >300nm. The radiation source is surrounded
externally by a cooling tube through which water flows.
Oxygen is metered into the reactor via a flow meter. The
reaction is started when the radiation source is switched
on. At the end of the reaction a small amount of the
suspension is immediately removed, filtered, and analysed
by gas chromatography.
A photoactivity k of 0.68 x 10-3 mole kg-1min-1 is measured.
This is taken as base value 1. The titanium dioxide powder
according to the invention has a somewhat lower
photocatalytic activity of 0.8 to 0.9.

Patent Claims
1. Flame-hydrolytically produced titanium dioxide powder
present in aggregates of primary particles,
characterised in that
it has a BET surface of 20 to 200 m2/g and
the half width HW, in nanometers, of the primary
particle distribution has values between
HW [nm] = a x BETf where a = 670x109 m3/g and
-1.3 ≤ f ≤ -1.0 and
the proportion of particles with a diameter of
more than 45 µm is in a range from 0.0001 to
0.05 wt.%.
2. Flame-hydrolytically produced titanium dioxide powder
according to claim 1, characterised in that the BET
surface is in a range from 40 to 60 m2/g.
3. Flame-hydrolytically produced titanium dioxide powder
according to claim 2, characterised in that the 90%
spread of the number distribution of the primary
particle diameters lies in a range from 5 to 100 nm.
4. Flame-hydrolytically produced titanium dioxide powder
according to claim 2 or 3, characterised in that the
equivalent circular diameter of the aggregates (ECD)
is less than 80 nm.
5. Flame-hydrolytically produced titanium dioxide powder
according to claims 2 to 4, characterised in that the
mean aggregate area is less than 6500 nm2.

6. Flame-hydrolytically produced titanium dioxide powder
according to claims 2 to 5, characterised in that the
mean aggregate circumference is less than 450 nm.
7. Flame-hydrolytically produced titanium dioxide powder
according to claim 1, characterised in that the BET
surface lies in a range from 80 to 120 m2/g.
8. Flame-hydrolytically produced titanium dioxide powder
according to claim 7, characterised in that the 90%
spread of the number distribution of the primary
particles diameters has values from 4 to 25 nm.
9. Flame-hydrolytically produced titanium dioxide powder
according to claim 7 or 8, characterised in that the
equivalent circular diameter of the aggregates (ECD)
is less than 70 nm.
10. Flame-hydrolytically produced titanium dioxide powder
according to claims 7 to 9, characterised in that the
mean aggregate area is less than 6000 nm2.
11. Flame-hydrolytically produced titanium dioxide powder
according to claims 7 to 10, characterised in that the
mean aggregate circumference is less than 400 nm.
12. Flame-hydrolytically produced titanium dioxide powder
according to claims 1 to 11, characterised in that the
proportion of aggregates and/or agglomerates with a
diameter of more than 45 µm lies in a range from 0.001
to 0.01 wt.%.
13. Flame-hydrolytically produced titanium dioxide powder
according to claims 1 to 12, characterised in that for
a given BET surface it has an anatase/rutile ratio of
2:98 to 98:2.

14. Flame-hydrolytically produced titanium dioxide powder
according to claims 1 to 13, characterised in that it
has a chloride content of less than 0.1 wt.%.
15. Flame-hydrolytically produced titanium dioxide powder
according to claims 1 to 14, characterised in that the
compacted bulk density has values of 20 to 200 g/l.
16. Process for the production of the flame-hydrolytically
produced titanium dioxide powder according to claims 1
to 15, characterised in that
a titanium halide, preferably titanium
tetrachloride, is vapourised at temperatures of
less than 200°C, the vapours are transferred to a
mixing chamber by means of a carrier gas with a
proportion of steam in a range from 1 to 25 g/m3
of gas, and
separately from this, hydrogen, primary air,
which may optionally be enriched with oxygen
and/or pre-heated, and steam are transferred to
the mixing chamber,
wherein the proportion of steam is in a
range from 1 to 25 g/m3 primary air,
the lambda value lies in the range from 1
to 9 and the gamma value lies in the range
from 1 to 9,
following which
the mixture consisting of the titanium halide
vapour, hydrogen, air and steam is ignited in a
burner and the flame burns into a reaction
chamber sealed from the ambient air, wherein

a vacuum of 1 to 200 mbar exists in the
reaction chamber,
the exit velocity of the reaction mixture
from the mixing chamber into the reaction
space lies in a range from 10 to 80 m/sec,
in addition secondary air is introduced into the
reaction chamber, wherein
the ratio of primary air to secondary air is
between 10 and 0.5,
following which the solid is separated from the
gaseous substances, and
the solid is then treated with steam.
17. Process according to claim 16, characterised in that
the steam is introduced together with the air into the
mixing chamber.
18. Use of the flame-hydrolytically produced titanium
dioxide powder according to claims 1 to 15 for the
heat protection stabilisation of silicones.
19. Use of the flame-hydrolytically produced titanium
dioxide powder according to claims 1 to 15 in
sunscreen agents.
20. Use of the flame-hydrolytically produced titanium
dioxide powder according to claims 1 to 15 as a
catalyst, as a catalyst carrier, as a photocatalyst,
and as an abrasive for the production of dispersions.

Flame-hydrolytically produced titanium dioxide powder that
is present in the form of aggregates of primary particles,
and has a BET surface of 20 to 200 m2/g, a half width (HW)
[nm] of the primary particle distribution of HW = a × BETf
where a = 670x10-9 m3/g and -1.3 ≤ f ≤ -1.0 and the
proportion of particles with a diameter of more than 45 µm
lies in a range from 0.0001 to 0.05 wt.%.
The powder is produced by a process in which a titanium
halide is vapourised at temperatures of less than 200°C,
the vapours are transferred to a mixing chamber by means of
a carrier gas of defined moisture content and, separately
from this, hydrogen, primary air, which may optionally be
enriched with oxygen and/or preheated, and steam are added
to the mixing chamber, following which the reaction mixture
is combusted in a reaction chamber sealed from the ambient
air, secondary air is in addition introduced into the
reaction chamber, the solid is then separated from gaseous
substances, and following this the solid is treated with
steam.
The titanium dioxide powder may be used for the heat
stabilisation of polymers.

FLAME-HYDROLYTICALLY PRODUCED TITANIUM DIOXIDE POWDER

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