Indian Patents. 279096:PRECIPITATED SILICAS HAVING SPECIAL SURFACE PROPERTIES

Title of Invention	PRECIPITATED SILICAS HAVING SPECIAL SURFACE PROPERTIES
Abstract	Precipitated silicas having special surface properties The present invention relates to precipitated silicas having special surface qualities, to a process for preparing them and to their use as reinforcers and thickeners for sealants.

Full Text	Precipitated silicas having special surface properties The present invention relates to precipitated silicas 5 having special surface qualities, to a process for preparing them and to their use for thickening sealants. Sealants are elastic substances that are applied in 10 liquid to highly viscous form for the sealing of buildings or installations against water, atmospheric influence or aggressive media. Silicone rubbers are compositions which are convertible 15 into the elastomeric state and comprise as their base polymers polydiorganosiloxanes containing groups amenable to crosslinking reactions. Suitable such groups include, primarily H atoms, OH groups and vinyl groups, which may be located at the chain ends, or else 20 may be incorporated in the chain. Incorporated into this system are fillers as reinforcers, their nature and amount significantly influencing the mechanical and chemical behaviour of the vulcanizates. Silicone rubbers can be coloured with inorganic pigments. One 25 distinction is between high-temperature vulcanizing and room-temperature vulcanizing (HTV/RTV) silicone rubbers. Among the room-temperature curing or vulcanizing 30 silicone rubber compositions, it is possible to differentiate one-component (IK) and two-component (2K) systems. The first group (RTV-IK) polymerizes slowly at room temperature under the influence of atmospheric moisture, with crosslinking taking place through 35 condensation of SiOH groups to form Si,0 bonds. The SiOH groups are formed by hydrolysis of SiX groups of a species formed as an intermediate from a polymer with terminal OH groups and from what is called a crosslinker R-SiX3 (e.g. = -O-CO-CH3, -NHR). In two-component rubbers (RTV-2K) the crosslinkers used are, for example, mixtures of silicic esters (e.g. ethyl silicate) and organotin compounds, the crosslinking reaction that takes place being the formation of an Si-O-Si bridge from nSi-OR and =Si-OH (- = methyl group; R = organic radical) by elimination of alcohol. The thickeners used for RTV-IK silicone rubber include silicas. In view of the sensitivity to hydrolysis of the silicone sealants, these silicas must introduce as little moisture as possible into the system. For this reason, fumed silicas have, been used almost exclusively to date for this application. Hydrophilic silicas have not been used to date, on account of their high moisture content. WO 2005/061384 shows the preparation and use including use in silicone rubber - of silicas which according to the claim have a water absorption of 300 ml/100 g. The silicas disclosed in the examples of WO 2005/061384, however, all have a water absorption of between 5.7% and 5.9% and are therefore unsuitable for use in RTV-IK formulations. Accordingly, WO 2005/061384 describes only their use in silicone rubber formulations for extrusion processes (HTV). EP 1557446 describes exclusively HTV silicone rubber formulations. The silicas employed therein have a loss on drying of In summary, therefore, it can be stated that the prior art does not disclose any precipitated silicas which meet the exacting requirements for use in RTV-IK silicone rubber. There is therefore a strong need for precipitated silicas of this kind that are suitable for On the basis of the prior art described above, the object of the present invention is to provide precipitated silicas from which the abovementioned disadvantages of the precipitated silicas of the prior art are completely or at least partially eliminated. A further aim is to provide a process for preparing the silicas of the invention. Further objects, not explicitly stated, will emerge from the overall context of the description, examples and claims. Surprisingly it has been found that this object is achieved by the precipitated silicas of the invention that are defined in greater detail in the description below and also in the claims and in the examples. The present invention therefore provides precipitated silicas characterized in that they have an SiOHiSOiated absorbance ratio of greater than or equal to 1. The invention also provides precipitated silicas, preferably hydrophilic precipitated silicas which in addition to the stated parameters, independently of one another, have one or more of the following physicochemical parameters: silanol group density 0.5 - 3.5 SiOH/nm2 modified tapped density BET surface area 50 - 600 m2/g CTAB surface area 50 - 350 m2/g DBP (anhydrous) 150 - 400 g/100 g loss on ignition 0.1% - 3.0% by weight loss on drying 0.1% - 3.0% by weight pH 4-9 fraction of particles 5% to 100% d90 value of the volume-based particle distribution 0.001 to 10 \m The present invention further provides a process for preparing the precipitated silicas of the invention. Additionally provided by the present invention is the use of the silicas of the invention in sealants, especially in silicone rubber and silicone sealants and with particular preference in RTV-1K sealants. Application is possible in different crosslinking systems, e.g. acetoxy-crosslinking, alkoxy-crosslinking and oxime-crosslinking. These systems are employed, for example, in the building industry as joint-sealants, in the automotive industry as adhesives and sealants, and as coating materials for textile fabric, for example. The invention further provides sealants based on silicone rubber which comprise the silicas of the invention, and their use. The precipitated silicas of the invention have the advantage that, on the basis of their special structure and surface qualities, they ensure high storage stability, a firm consistency and an optimum yield point of the silicone rubber when incorporated into silicone rubber compositions, especially those of the A further advantage of the precipitated silicas of the invention is their low modified tapped density. The low modified tapped density comes about as a result of a very loose packing of the silica particles. This means that, although in mutual contact and adhering gently to one another, the silica particles are nevertheless so loosely packed that large cavities are produced. This loose packing comes about in the silicone compound as well and thus contributes to the high level of thixotropy on the part of the silicone compound. In summation, the particular properties of the precipitated silicas of the invention lead to advantages which include the following: • high storage stability of RTV-1K silicone rubber compositions following incorporation of the silicas of the invention; • rapid and effective dispersing and hence high thickening action of the silica in RTV-1K silicone rubber compositions. Moreover, the precipitated silicas of the invention offer a substantial cost advantage over the fumed silicas used to date in RTV1 silicone rubber, being more inexpensive to prepare. The subjects of the invention are described in detail below. In the present invention the terms silica and precipitated silica are used synonymously. By hydrophilic precipitated silicas are meant those whose surface shows hydrophilic behaviour when incorporated by stirring into water, i.e. those whose surface is completely wetted by water and therefore has a contact angle with respect to water of less than 90°. The hydrophilic precipitated silicas of the invention preferably have a carbon content of The silicas of the invention are distinguished by the fact that they have a particularly high proportion of isolated SiOH groups, as expressed by the SiOHisoiated absorbance ratio, on their surface. The SiOHiSOiated absorbance ratio of the silicas of the invention is greater than or equal to 1, preferably between 1.5 and 10, more preferably between 1.5 and 7, very preferably between 1.8 and 5, with especial preference between 2 and 4.5, with very special preference between 2.3 and 4.0 and with particular preference between 2.3 and 3.5. This particular surface quality of the silicas of the invention is a key property, and means that in silicone rubber formulations the silicas lead to a high level of storage stability, and improved firmness " of consistency, and an optimized flow behaviour. Without being tied to any specific theory, the special properties of the silicas of the invention may be explained by the high number of isolated SiOH groups and at the same time their wide spacing. These two properties make it more difficult for hydrogen bonds to form and for water molecules to accumulate on the silica's surface. For the abovementioned reasons it may be advantageous if the silicas of the invention have a low silanol group density, i.e. a broad separation of the silanol groups on the silica surface. For the determination of the silanol group density, the number of silanol groups on the surface of the silica is first determined by means of LiAlH4. This alone, however, is not meaningful, since precipitated silicas with a high surface area generally have a higher absolute number of silanol jroups than do precipitated silicas with a low surface area. Consequently it is necessary to relate the number Df silanol groups to the surface area of the silica. A suitable surface area for this purpose is the BET surface area, since this describes the surface area which is available even to relatively small molecules such as water, for example. The silanol group density of the silicas of the invention is situated preferably in the range from 0.5 to 3.5 SiOH/nm , preferably from 0.5 to 3.0 SiOH/nm2, more preferably from 1.0 to 2.8 SiOH/nm2 and with very particular preference from 1.5 to 2.8 SiOH/nm2. If the number of silanol groups per nm2 is too low, this may result in an excessively low yield point and may consequently have an adverse effect on the consistency of the silicone sealants. The specific BET surface area describes the effect of the silica on the incorporation characteristics into silicone rubber and also on the crude mixing properties (cf. S. Brunauer, P. H. Emmett, E. Teller, "Adsorption of Gases in Multimolecular Layers", J. Am. Chem. Soc. 60, 309 (1938)). Thus the silicas of the invention may have a BET surface area of 50 to 600 m2/g, preferably 50 to 400 m2/g, more preferably 50 to 250 m2/g, very preferably 80 to 230 m2/g, especially of 100 to 180 m2/g, with very especial preference of 125 to 180 m2/g and with particular preference of 140 to 170 m2/g. The specific CTAB surface area is of decisive importance primarily for the reinforcing property of the silica (cf. Janzen, Kraus, Rubber Chem. Technol. 44, 1287 (1971)). The reinforcing potential increases with increasing CTAB surface area. Thus the precipitated silicas of the invention may have a CTAB surface area of 50 to 350 m2/g, more preferably 50 to 250 m2/g, very preferably of 80 to 230 m2/g, especially preferably of 100 to 200m2/g, with very especial preference of 125 to 190 m2/g and with particular preference of 140 to 190 m2/g. It has additionally been found that a high DBP absorption on the part of the silicas of the invention is of benefit in order to obtain effective rheological properties. Excessively high DBP values, however, may lead to an excessive increase in the viscosity of the silicone rubber and ought therefore to be avoided. The silicas of the invention, accordingly, preferably have a DBP absorption of 150 to 400 g/(100 g) , more preferably 200 to 350 g/(100 g), very preferably of 220 to 330 g/(100 g), with especial preference 250 to 330 g/(100 g) and very especially 260 to 320 g/(100 g). In a special embodiment, the precipitated silicas of the invention have a low modified tapped density and hence a very particularly good thixotropy. It should be noted here that the modified tapped density is a reference to the tapped density as measured on the uncompacted material. In order to be able to determine this variable even on materials which have already undergone preliminary compaction as a result of packaging and storage, it is necessary to carry out sample preparation as described in the section "Determining the modified tapped density". The silicas of the invention preferably have a modified tapped density of less than or equal to 70 g/1, preferably of 1 to 60 g/1, more preferably of 5 to 55 g/1, very preferably of 10 to 50 g/1, and in particular of 10 to 30 g/1. The inventors have observed, moreover, that for the consistency of the silicone sealants it may be of particular advantage if the precipitated silicas of the invention contain a sufficient fraction of fine particles, i.e. of particles silicas of the invention in the particle size range It has also been observed that an excessive fraction of coarse particles may adversely affect the performance properties of the precipitated silicas of the invention. For this reason, the silicas of the invention are preferably distinguished by a d90 value, relative to the volume-based particle distribution curve, of between 0.01 and 10 μm, preferably between 1 and 10 μm, more preferably between 2 and 8 μm and with particular preference between 3 and 7 μm. The particle distributions may be monomodal or bimodal, preferably bimodal. It has also been observed that for all of the above-described embodiments of the silicas of the invention it may be of particular advantage if from the outset the silica introduces very little moisture into the silicone sealant. The silicas of the invention may therefore have an initial moisture content, expressed by loss on drying, of 0.1% to 3.0%, preferably of 0.2% to 2.5%, more preferably 0.3% to 2.0%, and with particular preference 0.4% to 1.8% by weight and/or a loss on ignition of 0.1% - 3.0%, preferably 0.2% to 3.0%, more preferably 0.3% to 2.0%, and with particular preference 0.4% to 1.8% by weight. Finally, it has been observed for all of the above-described embodiments of the silicas of the invention that it may be of particular advantage if the silica has a pH value in the range from 4 to 8, preferably 4.5 to 7.5. If the pH is too high then the situation may arise that, following prolonged storage (e.g. after several days), the silicone compound no longer properly crosslinks, but instead remains tacky. The stated ranges of preference may be set independently of one another. The silicas of the invention can be prepared by a process which comprises the steps described below of 1. reacting at least one silicate with at least one acidifier 2. filtering and washing the resulting silica 3. drying the resulting silica or filtercake 4. heat-treating the dried silica. Step 1 here preferably comprises the substeps of la preparing an initial charge of water or of water and at least one silicate and/or a silicate solution, the pH of the resulting initial charge being preferably between 5 and 10 and the temperature of the initial charge being preferably between 80 and 100°C. lb metering at least one silicate and/or a silicate solution and at least one acidifier into the initial charge from substep la) with stirring at 80 to 100°C until the solids content of the precipitation suspension reaches a level which leads to the solids content which is to be reached in substep lc) . With particular preference, silicate and/or silicate solution and acidifier are added simultaneously and/or in such a way that the pH for the period of substep lb) is kept constant at a level between 7 and 10. adding an acidifier at a temperature of the precipitation suspension of 80 to 100°C, so that the pH of the precipitation suspension is lowered to 2 to 6 and the solids content of the precipitation suspension at the end of this substep is between 30 and 70 g/1. Preferably the silicas of the invention are ground. This takes place with particular preference by grinding the silicas of the invention in a step 3a, i.e. between steps 3 and 4, or in a step 5, i.e. after step 4, or both in step 3a, i.e. between steps 3 and 4, and in step 5, i.e. after step 4. All known forms of silicate are suitable for the silicates or silicate solutions used in step 1) of the process of the invention. The silicates used in accordance with the invention are preferably alkaline silicates, e.g. sodium or potassium silicates. With particular preference the silicate in step 1 is sodium silicate (water glass) . Its weight ratio of SiC>2 to Na20 is between 2 and 4, preferably between 3 and 3.6 and more preferably between 3.3 and 3.5. The Si02 content is preferably between 20% and 40% by weight, preferably between 25% and 30% by weight. Acidifiers are acidic compounds of organic or inorganic type which can be used to lower the pH of the precipitation suspension. With preference it is possible to use inorganic acids such as hydrochloric acid, phosphoric acid, sulphuric acid or nitric acid, or organic acids such as acetic acid, formic acid or carbonic acid or carbon dioxide. Both dilute and concentrated acids can be used. With particular preference the process of the invention uses sulphuric acid. In the majority of cases the silicate and/or the silicate solution and acidifier used in substeps la) to lc) are identical. The pH of the initial charge in substep la) is preferably between 7 and 10, more preferably between 8 and 9. The temperature of the initial charge is set at 80 to 100°C, preferably at 85 to 95°C, In substep lb) silicate and acidifier are metered in preferably simultaneously. The addition of the two components preferably takes place continuously constantly over the entire period of substep lb). During this period the temperature remains at 80 to 100 °C, preferably at 85 to 95 °C. The period of the addition lasts until the solids content to be achieved at the end of step lc) has been achieved. It may in this case be necessary to continue the precipitation beyond the viscosity rise point. This viscosity rise point corresponds to the point in time at which a sharp rise is observed in the viscosity of the precipitation suspension in the course of precipitation; cf. EP 0643015. During substep lb), in which the precipitation of the silica begins, the pH is as far as possible kept constant at a level of between 7 and 10, preferably constant at a level of between 7.5 and 9.5 and with very particular preference at a pH of between 8 and 9. Corrections to an off-target pH are generally made by increasing or lowering the addition of the acidifier, so that the set pH preferably fluctuates only by ± 0.2 pH units, more preferably by only i 0.1 pH units. Through addition of an acidifier at a temperature of the precipitation suspension of 80 to 100°C its pH is lowered in substep lc) to 2 to 6, preferably 3 to 6, more preferably to 3 to 4. The solids content of the precipitation suspension at the end of this substep is between 30 and 70 g/1, preferably between 45 and 60 g/1 and with very particular preference between 45 and 55 g/1. Without in any way being tied to one particular theory, the intention is that, by suitable choice of the process parameters, a chain-like structure of the aggregates should be constructed in substep lb) . A reinforcement of this hitherto quite loose aggregate structure is attained by the correspondingly slow further precipitation even after the viscosity rise point. The metering rates in step lb) are to be selected in all embodiments of the process of the invention both before and after the viscosity rise point such that the solids content which is to be achieved after acidification in step lc) , of 30 to 70 g/1, is reached. The filtration, liquefaction (e.g. in accordance with DE 2447613) and both longer and accelerated drying of the silicas of the invention are familiar to the person skilled in the art and can be looked up, for example in the documents specified in the description. The filtration and the washing of the silica take place preferably in such a way that the conductivity of the end product is The silica of the invention is dried preferably in a pneumatic conveying drier, spray drier, rack drier, belt drier, rotary tube drier, flash drier, spin-flash drier or nozzle tower drier. These drying variants include operation with an atomizer, with a single-fluid or two-fluid nozzle or of an integrated fluid bed. Spray drying may be carried out in accordance for example with US 4094771. If the selected mode of drying is spray drying then the filtercake should be redispersed beforehand. Redispersion takes place preferably in water or aqueous acid so that the dispersion has a pH of 4 to 7. It should be ensured here that the silica dispersion when redispersion is at an end has a solids content of 5% to 18%, preferably 8% to 13% by weight, more preferably 9% to 11%, and that in the course of redispersion the shearing forces acting on the silica are not too great. This can be achieved, for example, by stirring with a rotary speed of A substantial step for setting the silanol group density and arrangement of silanol groups on the silica surface, in addition to the precipitation, in which the chain-like structure is constructed, is the heat treatment to be carried out in step 4. This heat treatment may be carried out batchwise or continuously. For the heat treatment it is possible, for example, to use a fluidized-bed, fluid-bed or rotary-tube reactor. It should be ensured that in the course of the heat treatment, the temperature distribution and the process gas atmosphere are homogeneous, so that all of the silica particles are exposed to identical conditions. The process gas must have a sufficient steam concentration. The steam concentration is preferably 10% to 95% by volume, more preferably 40% to 90% by weight, very preferably 50% to 90% by weight. Particularly when using a rotary-tube reactor it must be ensured that the temperatures everywhere are the same, i.e. that no "cold zones" exist in which the steam could condense. The condensed steam may lead to the agglomeration of the silicas. The particular conditions during the heat treatment of the invention also ensure that a silica which has already been ground prior to heat treatment need not be ground once again after heat treatment, in other words that no instances of caking or agglomeration arise, such caking or agglomeration otherwise having to be removed by grinding again after heat treatment. Preference is given to using a fluidized-bed or fluid-bed reactor. By a fluidized bed is meant the following: If a flow of gases from below traverses fine-particled bulk product lying on horizontal, perforated plates, under certain flow conditions a condition comes about which is similar to that of a boiling liquid; the layer bubbles; the particles of the bulk material are located within the layer in a continually fluidizing up-and-down motion and thus remain, so to speak, in suspension. Terms also used are therefore suspension bed, fluidized bed, fluid bed, and fluidizing. The associated large surface area of the fluidized product also facilitates the drying and heat treatment of solids. It is important that during heat treatment all particles of silica are exposed to the same temperature and the same process gas. The temperature differences between the hottest and coldest point ought to be as small as possible. Consequently the temperature of the filter candles as well must not be below the product temperature. With very particular preference, the heat treatment in step 4 of the process of the invention takes place in accordance with substeps 4a) to 4e) below: 4a. introducing the silica into the fluidized-bed reactor. 4b. preheating the reactor to 300 to 800°C/ the reactor being traversed at the same time by a flow of inert gas and/or nitrogen/air mixture in such a way as to produce a fluidization velocity of 0.02 to 0.06 m/s. 4c. feeding in a gas mixture I comprising steam and an inert gas, e.g. nitrogen, or a gas mixture II comprising steam, an inert gas and air, at 300 to 800°C for a period of 0.25 to 6 h, the gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and the gas mixtures I and II having a steam concentration of 10 to 95% by volume and in the case of gas mixture II an oxygen content of 0.01% to 21% by volume. 4d. interrupting the addition of steam and expelling the steam by means of an inert gas, nitrogen for example, and/or of an inert gas/air mixture at 300 to 800°C, the gas or gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume. 4e. cooling the heat-treated silica to room temperature in a dry process atmosphere, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume. After the silica has been introduced into the fluidized-bed reactor (substep 4a)), the reactor is heated in substep 4b) to an operating temperature of 300 to 800°C, preferably of 350 to 690°C and more preferably of 400 to 650°C. During the heating operation the reactor is traversed by a flow of inert gas, preferably nitrogen and/or a mixture of an inert gas and dry air, in such a way that a fluidization velocity of 0,02 to 0.06 m/s is set. After the operating temperature has been reached, in substep 4c) a gas mixture I comprising steam and an inert gas, preferably nitrogen, or a gas mixture II comprising steam, an inert gas and air is passed through the reactor for a period of 0.25 to 6 h, preferably 0.5 to 5 h, more preferably 1 to 4 h, very preferably 2 to 4 h. The fluidization velocity of the gas mixture is 0.02 to 0.06 m/s. The gas mixtures I and II have a steam concentration of 10% to 95% by volume, preferably 40% to 90% by weight, very preferably 50% to 90% by weight and, in the case of gas mixture II an oxygen content of 0.01% to 21% by volume. The techniques for optional grinding of the silicas of the invention are known to the person skilled in the art and can be looked up for example in Ullmann, 5th edition, B2, 5-20. For the grinding of the silicas of the invention in step 3a) and/or in step 5) it is preferred to use grinding systems (grinding apparatus) comprising or consisting of impact mills or jet mills, preferably opposed-jet mills. Particular preference is given to using fluid-bed opposed-jet mills. With very particular preference, grinding takes place by means of a grinding system (grinding apparatus), with particular preference a grinding system comprising a jet mill, characterized in that the mill of the grinding system is operated in the grinding phase with an operational medium selected from the group consisting of gas and/or vapour, preferably steam, and/or a gas comprising steam, and in that the grinding chamber is heated in a heating phase, i.e. before the actual operation with the operational medium, such that the temperature in the grinding chamber and/or at the mill outlet is higher than the dew point of the vapour and/or operational medium. Grinding takes place with particular preference in accordance with the method described in DE 10 2006 048 850.4, using the grinding system (mill) described therein the operational medium used being, with especial preference, steam. In order to avoid pure repetitions of text, the content of the three cited patents is hereby explicitly incorporated as part of the content of the present specification. The grinding parameters are preferably chosen such that the ground product has a fine-particle fraction in the region smaller than 1 \im in the volume-based particle distribution, of 5% to 100%, preferably 10% to 95%, more preferably 15% to 95%, with very particular preference 20% to 90%, and in particular from 40% to 80%, and/or a dgo value in the volume-based particle distribution curve of between 0.01 and 10 ]xm. In one especially preferred embodiment, in preparation for actual grinding with superheated steam, a fluid-bed opposed-jet mill as shown in Figure 7, with an integrated dynamic pneumatic classifier as shown in Figures 8a and 8b, is first heated via the two heating nozzles (5a) (of which only one is depicted in Figure 7) which are charged with hot compressed air, preferably at 10 bar and 160°C, until the mill exit temperature is higher than the dew point of the steam and/or operational medium, preferably about 105°C. Connected downstream of the mill, for the separation of the ground material, is a filter system (not shown in Figure 7) whose filter housing is heated in its lower third indirectly, via attached heating coils, by means of saturated steam (preferably 6 bar saturated steam), likewise for the purpose of preventing condensation. apparatus surfaces in the region of the mill, the separation filter, and the supply lines for steam and hot compressed air have special insulation. After the desired heating temperature has been reached, the supply of hot compressed air to the heating nozzles is shut off and the charging of the three grinding nozzles with superheated steam, preferably at 38 bar (abs) and 325°C, is commenced. In order to protect the filter medium used in the separation filter and also in order to set a defined level of residual water in the ground material, of preferably 2% to 6%, water is introduced in the starting phase, and during grinding, into the grinding chamber of the mill, via a two-fluid nozzle operated with compressed air, as a function of the mill exit temperature. The feed quantity is regulated as a function of the classifier flow which comes about. The classifier flow regulates the feed quantity such that it is not possible to exceed approximately 70% of the nominal flow. The introduction member (4) which functions here is a speed-regulated bucket wheel which meters the feed material from a reservoir container via a cyclical lock, which serves as a barometric endpoint, into the grinding chamber, which is at superatmospheric pressure. The coarse material is comminuted in the expanding steam jets (grinding gas). Together with the depressurized grinding gas, the product particles ascend in the centre of the mill vessel to the classifying wheel. Depending on the classifier speed and grinding steam quantity which have been set, the particles whose fineness is sufficient enter along with the grinding steam into the fines exit, and from there they pass into the downstream separating system, while particles which are too coarse pass back into the grinding zone and are subjected to a repeat comminution. The discharge of the separated fines from the separation filter into the subsequent silo storage and bagging operation takes place by means of a bucket-wheel lock. The grinding pressure of the grinding gas that obtains at the grinding nozzles, and the resulting volume of grinding gas, in conjunction with the speed of the dynamic paddle wheel classifier, determine the fineness of the particle-size distribution function and also the upper particle-size limit. The silicas of the invention can be used in sealants, particularly in silicone rubber and silicone sealants and with particular preference in RTV-1K sealants. Their application is possible in various crosslinking systems, e.g. acetoxy-crosslinking, alkoxy-crosslinking and oxime-crossiinking. These systems find application, for example in the building industry as joint-sealants, in the automotive industry as adhesives and sealants and as coating compositions for textile fabrics, for example. The reaction conditions and the physical/chemical data of the precipitated silicas of the invention are determined by means of the following methods: Determining filtercake solids content In accordance with this method the solids content of filtercakes is determined by removal of the volatile fractions at 105°C. For this purpose 100.00 g of the filtercake are weighed out (initial mass E) into a dry, tared porcelain dish (20 cm diameter) . The filtercake is broken up with a spatula if necessary to give loose lumps of not more than 1 cm3. The sample is dried to constant weight in a drying cabinet at 105 ± 2°C. Subsequently the sample is cooled to room temperature in a desiccator cabinet with silica gel as desiccant. The final mass A is determined gravimetrically. The solids content (SC) in % is determined in accordance with SC = A/E * 100%, where A = final mass in g and E = initial mass in g. Determining precipitation suspension solids content The solids content of the precipitation suspension is determined gravimetrically after the sample has been filtered. 100.0 ml of the homogenized precipitation suspension (VSUspension) are measured off at room temperature with the aid of a measuring cylinder. The sample is filtered through a circular filter (TYP 572, Schleicher & Schuell) in a porcelain suction filter unit, but is not sucked dry, so as to prevent cracking of the filtercake. Subsequently the filtercake is washed with 100.0 ml of distilled water. The washed filtercake is transferred to a tared porcelain dish and dried to constant * weight in a drying oven at 105 ± 2°C. The weight of the dried silica (msampie) is determined after cooling to room temperature. The solids content is determined in accordance with: solids content in g/1 = (msampie in g) / (Vsuspension in 1). Determining silica-feed solids content The silica feed is dried to constant weight in an IR drier. The loss on drying consists predominantly of water moisture. 2.0 g of silica feed are charged to a tared aluminium dish and the lid of the IR drying unit (Mettler, type LP 16) is closed. After the start button has been pressed, drying of the suspension at 105°C commences, and is ended automatically when the weight decrease per unit time falls below a value of 2 mg/(120 s). The weight decrease in % is displayed directly by the instrument when the 0-100% mode is selected. The solids content is given by solids content in % = 100% - weight decrease in %. Determining pH The pH of the silica, as a 5% aqueous suspension, is determined at room temperature in a method based on DIN EN ISO 787-9. In contrast to the specifications of the aforementioned standard, the initial masses were changed (5.00 g of silica to 100 ml of deionized water). Determining electrical conductivity The electrical conductivity of silica, as a 4% aqueous suspension, is determined at room temperature in a method based on DIN EN ISO 787-14. In contrast to the specifications of the aforementioned standard, the initial masses were changed (4.00 g of silica to 100 ml of deionized water). Determining the moisture content or loss on drying The moisture content of silica is determined in accordance with ISO 787-2 after 2 hour drying in a forced-air drying cabinet at 105°C. This loss on drying is composed predominantly of moisture water. Determining the loss on ignition By this method the loss in weight of silica is determined in a method based on DIN EN ISO 3262-1 at 1000°C. At this temperature, water bound physically and chemically, and also other volatile constituents escape. The moisture content (LD) of the sample investigated is determined by the above-described method "Determining the moisture content or loss on drying", based on DIN EN ISO 787-2. 0.5 g of the pulverulent, spherical or granular silica is weighed out to an accuracy of 0.1 mg into a tared porcelain crucible which has been purified beforehand by calcining (initial mass E). The sample is heated in a muffle furnace at 1000 ± 50°C for 2 h. The porcelain crucible is subsequently cooled to room temperature in a desiccator with silica gel as desiccant. The final mass A is determined gravimetrically. The loss on ignition (DIN) LOI in % is obtained in accordance with LOI = (1 - A/F) * 100. F denotes the corrected initial mass in g based on dried matter, and is calculated according to F = E * (1 - LD/100). In the calculations A denotes final mass in g, E denotes initial mass in g and LD denotes loss on drying, in %. Determining the BET surface area The specific nitrogen surface (called the BET surface area below) of the pulverulent, spherical or granular silica is determined by a method based on ISO 5794-1/Annexe D using the TRISTAR 3000 instrument (from Micromeritics) in accordance with the multipoint determination of DIN-ISO 9277. Determining the CTAB surface area The method is based on the adsorption of CTAB (N-hexadecyl-N,N,N-trimethylammonium bromide) on the "external" surface of the silica, in a method based on ASTM 3765 or NFT 45-007 (section 5.12.1.3). CTAB is adsorbed in aqueous solution with stirring and ultrasound treatment. Excess, unadsorbed CTAB is determined by back-titration with SDSS (sodium dioctylsulphosuccinate solution, M Aerosol OT" solution) using a titroprocessor, the endpoint being given by the turbidity maximum of the solution and determined using a phototrode. The temperature throughout all of the operations conducted is 23 -25 °C, to prevent crystallization of CTAB. The back-titration is based on the following reaction equation: (C2oH3704)S03Na + BrN(CH3)3 (C16H33) => (C20H37O4) S03N (CH3) 3(^33) + NaBr SDSS CTAB Apparatus Mettler Toledo DL55 titroprocessor and Mettler Toledo DL7 0 titroprocessor, each equipped with pH electrode, Mettler, type DG 111 and phototrode, Mettler, type DP 550 100 ml polypropylene titration beaker Glass titration vessel, 150 ml, with lid Pressure filtration device, 100 ml capacity Cellulose nitrate membrane filter, pore size 0.1 \|im, 47 mm 0, e.g. Whatman (Order No. 7181-004) Reagents The solutions of CTAB (CCTAB = 0.015 mol/1 in deionized water) and SDSS (concentration = 0.00423 mol/1 in deionized water) are purchased in ready-to-use form (Bernd Kraft GmbH, 47167 Duisberg: Order No. 6056.4700 CTAB solution of concentration 0.015 mol/1; Order No. 6057.4700 SDSS solution 0.00423 mol/1), stored at 25°C and used within one month. Procedure 1. Blank titration The consumption of SDSS solution for titrating 5 ml of CTAB solution should be checked 1 x daily before each series of measurements. This is done by setting the phototrode, before beginning the titration, at 1000 ± 20 mV (corresponding to a transparency of 100%). Precisely 5.00 ml of CTAB solution are pipetted into a titration beaker and 50.0 ml of deionized water are added. Titration with SDSS solution is carried out with stirring by the measurement method familiar to the skilled person, using the DL 55 titroprocessor, until the solution reaches maximum turbidity. The consumption VA of SDSS solution in ml is determined. Each titration should be performed in triplicate. 2. Adsorption 10.0 g of the pulverulent, spherical or granular silica with a moisture content of 5 i 21 (if appropriate, the moisture content is adjusted by drying at 105°C in a drying cabinet or by uniform wetting) are comminuted for 30 seconds using a mill (Krups, model KM 75, article no. 2030-70). Precisely 500.0 mg of the comminuted sample (initial mass E) are transferred to a 150 ml titration vessel with magnetic stirrer rod and precisely 100.0 ml of CTAB solution (Ti) are metered in. The titration vessel is closed with a lid and stirred using an Ultra Turrax T 25 stirrer (stirrer shaft KV-18G, 18 mm diameter) at 18 000 rpm for not more than 1 minute until wetting is complete. The titration vessel is screwed on to the DL 70 titroprocessor and the pH of the suspension is adjusted with KOH (0.1 mol/1) to a figure of 9 i 0.05. The suspension is sonicated for 4 minutes in the titration vessel in an ultrasound bath (Bandelin, Sonorex RK 106 S, 35 kHz, 100 W effective and 200 W peak output) at 25°C. This is followed immediately by pressure filtration through a membrane filter under a nitrogen pressure of 1.2 bar. The initial fraction of 5 ml is discarded. 3. Titration 5.00 ml of the remaining filtrate are pipetted into a 100 ml titration beaker and made up to 50.00 ml with deionized water. The titration beaker is screwed on to . the DL 55 titroprocessor and titrated with SDSS solution, with stirring, until maximum turbidity is reached. The consumption VB of SDSS solution, in ml, is determined. Each titration should be performed in triplicate. VA = consumption of SDSS solution in ml in titrating the blank sample VB = consumption of SDSS solution in ml when using the filtrate CCTAB = concentration of CTAB solution in mol/1 MCTAB = molar mass of CTAB = 364.46 g/mol Ti - amount of CTAB solution added in 1 P = surface occupancy of CTAB = 578.435 m2/g E = initial mass of silica The CTAB surface area is based on the anhydrous silica, which is why the following correction is made. The moisture content of the silica is determined in accordance with the above-described method "Determining the Moisture Content". Determining the DBP absorption The DBP absorption (DBP number), which is a measure of the absorbency of the precipitated silica, is determined by a method based on the DIN 53601 standard, as follows: 12.50 g of pulverulent or spherical silica with a moisture content of 0 - 10% (the moisture content is adjusted/ if appropriate, by drying at 105°C in a drying cabinet) are introduced into the kneader chamber (article number 279061) of the Brabender Absorptometer "E" (without damping of the outlet filter of the torque sensor) . In the case of granules, the sieve fraction from 1 to 3.15 mm (stainless steel sieves from Retsch) is used (by gently pressing the granules with a plastic spatula through the sieve with pore size of 3.15 mm). With continual mixing (kneader paddles rotating at a speed of 125 rpm), dibutyl phthalate is added dropwise to the mixture at a rate of 4 ml/min and at room temperature by means of the Brabender T 90/50 Dosimat. The incorporation of the DBP by mixing takes place with only a small amount of force, and is monitored by means of the digital display. Towards the end of the determination the mixture becomes pasty which is indicated by a sharp increase in the required force. At a display reading of 600 digits (torque of 0.6 Nm) an electrical contact shuts off both the kneader and the DBP feed. The synchronous motor for the DBP feed is coupled to a digital counter, so that the consumption of DBP in ml can be read off. The DBP absorption is reported in g/ (100 g) and is calculated using the following formula: where DBP = DBP absorption in g/(100 g) V = consumption of DBP in ml The DBP absorption is defined for the anhydrous, dried silica. If moisture precipitated silicas are used it is necessary to take into account the correction value K for calculating the DBP absorption. This value can be determined using the correction table below: for example, silica having a water content of 5,8% would require an add-on of 33 g/ (100 g) for the DBP absorption. The moisture content of the silica is determined in accordance with the method ^Determining the Moisture Content or Loss on Drying". Moisture correction table for dibutyl phthalate absorption (anhydrous) IR determination By means of IR spectroscopy it is possible to ascertain the different kinds of SiOH groups (isolated, bridged, + H20). To determine the intensities of the different silanol groups, the silicas are subjected to measurement in the form of powder layers. The absorbance values of the different silanol groups are divided (standardized) by the absorbance value of the SiO combination vibration band at 1870 cm"1. The IR-spectroscopic determination takes place by means of a Bruker IFS 85 FR-IR spectrometer. Measurement takes place using a transparent NaCl monocrystal disc (round d=25 mm, h=5 mm) from K.Korth, Kiel, Germany, a 0.5 mm Teflon spacer and a mount for the discs. The spacer is placed on one clean, polished transparent NaCl monocrystal disc. The sample material is dusted on between the spacer and is covered with a further clean, polished transparent NaCl monocrystal disc; there must be no air bubbles included. The two transparent NaCl monocrystal discs with the powder layer are clamped into the sample mount. The sample mount is brought into the IR beam path and the sample chamber is closed. Prior to the measurement, the sample chamber is flushed with air cleaned to remove steam and carbon dioxide. In the alignment mode an "Align" is carried out, and measurement is commenced. Measurement is carried out using the following parameters: Resolution: 2 cm-1 Scanner'speed: 6; 10.51 Hz Measuring range: 4500 cm"1 to 100 cm"1 Apodization function: triangular Number of scans: 128 The spectrum is expressed, in the wavenumber range from 4000 to 1400 cm"1, in continuous wavenumbers. The SiOHiSOiated absorbance ratio is determined as follows (Figure 1): First of all, two baselines are set. This is done by applying two tangents to the absorption plot. The first tangent (1st baseline) touches the absorption plot first in the region from 4000 cm"1 to 3800 cm"1 and secondly in the region from 3000 cm"1 to 2100 cm"1. It should be ensured that the tangent does not intersect the absorption plot either in the region from 4000 cm"1 to 3800 cm"1 or in the region from 3000 cm"1 to 2100 cm-1. The second tangent (2nd baseline) touches the absorption plot first in the region from 2200 cm"1 to 2000 cm"1 and secondly in the region from 1850 cm"1 to 1650 cm"1. It should be ensured that the tangent does not intersect the absorption plot either in the region from 2200 cm"1 to 2000 cm"1 or in the region from 1850 cm"1 to 1650 cm"1. After the baselines have been set, a perpendicular line is taken down from the maximum of the bands in question (3750 and 1870 cm"1) to the respective baseline, and a measurement is made with the respective heights from the maximum to the baseline, in mm. A ratio is formed as follows: height from maximum to base line in mm at 3750 cm '" Absorption ratio (5iOHjM.,t J«-—;—; r ; ; — ; TTT^T. J height from maximum to base line in mm at 1870 cm - For each sample six IR spectra are recorded, measurement taking place in each case with new sample material. Each IR spectrum is evaluated five times in accordance with the procedure described above. The absorbance ratio (SiOiSOiated) is reported, finally, as the average value of all the evaluations. Determining the contact angle The contact angle is determined as described in W. T. Yen, R. S • Chahal, T. Salman, Can. Met. Quart. , Vol. 12, No. 3, 1973. Determining the silanol group density First of all the moisture content of the silica sample is determined in accordance with the section "Determining the Moisture Content or Loss on Drying". Thereafter 2 - 4 g of the sample (to an accuracy of 1 mg) are transferred to a pressure-tight glass apparatus (glass, flask with dropping funnel) with a pressure measurement means attached. In this apparatus it is dried under reduced pressure ( calculated as follows: „., 7 j silanol group concentration Silanol group density = s— BET surface area Determining the particle size distribution by means of laser diffraction The particle distribution is determined in accordance with the principle of laser diffraction on a laser diffractometer (Horiba, LA-920). First of all the silica sample is dispersed in 100 ml of water without additional dispersing additives in a 150 ml glass beaker (diameter: 6 cm) in such a way as to give a dispersion having a weight fraction of 1% by weight Si02. This dispersion is then dispersed using an ultrasonic probe (Dr. Hielscher UP400s, Sonotrode H7) for a duration of 5 minutes intensely (300 W, without pulsing). For this purpose the ultrasound probe is to be mounted such that its lower end is immersed to a distance of approximately 1 cm above the base of the glass beaker. Immediately following the dispersing operation the particle size distribution of a sample of the ultrasonicated dispersion is determined using a laser diffractometer (Horiba LA-920). For the evaluation, using the standard software supplied with the Horiba LA-920, a refractive index of 1.09 should be selected. All measurements take place at room temperature. The particle size distribution and also the relevant variables such as, for example, the particle size d90 are automatically calculated and depicted in graph form by the instrument. Attention should be paid to the notes in the operating instructions. Determining the modified tapped density With the "conventional" tapped density determination of DIN EN ISO 787-11, the result can be falsified by the fact that the silica has already undergone preliminary compaction in the course, for example, of being packed. In order to rule this out, a "modified tapped density" is determined for the silicas of the invention. A porcelain suction filter (nominal size 110, diameter = 12 cm, height = 5.5 cm) fitted with a circular filter (e.g. type 598, Schleicher + Schull) is filled loosely with silica to approximately 1 cm from the top edge, and is covered with elastic film (Parafilm®). The shape and dimensions of the elastic film are to be selected such that it finishes very closely or completely flush with the edge of the porcelain suction filter unit. The unit is mounted on a suction bottle and then a vacuum of -0.7 bar is applied for a period of 5 minutes. In the course of this operation, the silica is compacted uniformly by virtue of the film under suction. Then air is cautiously readmitted and the resulting silica plaque is removed from the filter unit by being tipped forcefully into a porcelain dish. The slightly precomminuted material is redispersed uniformly (in the manner of a silica/air aerosol) via a centrifugal mill (ZM1, Retsch, 0.5 mm screen insert, speed setting 1, without cyclone, without internal funnel insert) with an internal collecting dish (the silica (starting material) is introduced slowly spatula by spatula - into the mill feed; the internal product collection dish should never become completely full) . During this operation the power consumption of the mill should not exceed 3 amperes. This operation is less a conventional grinding than a defined loosening of the silica structure (of air-jet-milled silicas, for example), since the energy input here is substantially weaker than in the case of jet milling. 5 g of the resulting material are weighed out to an accuracy of 0.1 g into the 250 ml volumetric cylinder of the jolting volumeter (STAV 2003 from Engelsmann). In a method based on DIN ISO 787-11, after jolting 1250 times, the resulting volume of the silica, in ml, is read off on the scale. The examples below are intended to illustrate the invention without restricting its scope. The water glass and the sulphuric acid used at various points in the directions of the examples below are characterized as follows: Water glass: density 1.348 kg/1, 27.0% by weight Si02, 8.05% by weight Na20 Sulphuric acid: density 1.83 kg/1, 94% by weight Example 1: A 2m3 precipitating vessel (diameter 160 cm) with inclined base, MIG inclined-blade stirrer system and Ekato fluid shear turbine is charged with 1679 1 of deionized water and this initial charge is heated to 92°C. After the temperature has been reached, and over a period of 100 minutes, water glass is metered in at a rate of 3.93 kg/min, and sulphuric acid is metered in at a rate of 0.526 kg/min with stirring. The rate of metering of sulphuric acid must be corrected if appropriate so that during the entire precipitation time a pH of 8.5 is maintained. Thereafter the water glass feed is shut off, with retention of the temperature, and with the same rate of metering of sulphuric acid the precipitation suspension is acidified to a pH of 3. The precipitation suspension has a solids content of 54 g/1. The resulting suspension is filtered with a membrane filter press and the filtercake is washed with deionized water until the wash water is found to have a conductivity of Before drying by means of a spray drier, the filtercake is redispersed with deionized water to a solids content of 8% - 13%, during which it must be ensured that it is not exposed to any strong shearing forces. The metering of the liquefied filtercake into the spray drier takes place in such a way that the temperature measured at the drier outlet is approximately 150°C. Subsequently the material is treated in a fluidized-bed reactor (expanded fluidized bed height approximately 1.5 m, fluidized-bed diameter approximately 0,5 m). For this purpose the following conditions should be observed: First of all, 30 kg of the spray-dried powder are introduced into the fluidized-bed reactor with fluidizing base. The fluidizing base is traversed by a gas mixture comprising dry nitrogen and dry air. These two gases are metered prior to their entry into the reactor in such a way that a resulting oxygen content of 6% by volume is not exceeded and in such a way as to give a fluidization velocity in the reactor of 0.05 m/s. The reactor is then heated from room temperature to 450°C. The flow rates of the fluidizing gas should be regulated during the heating phase such that the fluidization velocity in the reactor remains constant at 0.05 m/s. After 450°C has been reached, a preheated gas mixture of steam, nitrogen and air is fed into the reactor for a period of 3 hours. The three components are mixed so as to set a steam concentration of 50% and an oxygen content of 3%. Volumes of nitrogen and of air are adapted so that, again, a fluid gas velocity of 0.05 m/s comes about. Thereafter the addition of steam is interrupted. Nitrogen and air volumes are adapted so as to result, again, in a fluidization velocity of 0.05 m/s and an oxygen content of approximately 6%. After at least five times the volume of dry process gas has been passed through the fluidized-bed reactor, after the interruption of the steam supply, the product is cooled to room temperature in a dry process gas atmosphere. Cooling takes place with fluidization in a nitrogen/air mixture in which the oxygen content is approximately 6%. In the cooling phase, particular care should be taken to ensure that at this point there is no longer any steam present. Following the surface treatment in the fluidized bed, the material is ground on a fluid-bed opposed-jet mill AFG 50 from Alpine. The chemicophysical data of Example 1 are listed in Table 1 and an IR spectrum is depicted in Figure 2. Example 2 A 2m3 precipitating vessel (diameter 160 cm) with inclined base, MIG inclined-blade stirrer system and Ekato fluid shear turbine is charged with 1680 1 of deionized water and this initial charge is heated to 92°C. After the temperature has been reached, and over a period of 100 minutes, waterglass is metered in at a rate of 3-93 kg/min, and sulphuric acid is metered in, at a rate of 0.526 kg/min with stirring. The rate of metering of sulphuric ,acid must be corrected if appropriate so that during the entire precipitation time a pH of 8.5 is maintained. Thereafter the waterglass feed is shut off, and with the same rate of metering of sulphuric acid, and still at 92°C, the precipitation suspension is acidifed to a pH of 3. The precipitation suspension at this point has a solids content of 54 g/1. The resulting suspension is filtered with a membrane filter press and the filtercake is washed with deionized water until the wash water is found to have a conductivity of Before drying by means of a spray drier, the filtercake is redispersed with deionized water to a solids content of 8%-13%, during which it must be ensured that it is not exposed to any shearing forces. The metering of the liquefied filtercake into the spray drier takes place in such a way that the temperature measured at the drier outlet is approximately 150°C. The spray-dried material is subjected to preliminary grinding via a mechanical beater mill to an average particle size of 10-12 μm. After this preliminary grinding, the material is subjected to ultrafine grinding on a steam-operated fluid-bed opposed-jet mill at a superatmospheric pressure of 38 bar. Details of the grinding system (mill) and of the grinding method used can be found in DE 10 2006 048 850.4 and also in the description which follows. The entire content of the three cited patent applications is hereby incorporated explicitly to become part of the present specification. In preparation for actual grinding with superheated steam, a fluid-bed opposed-jet mill as shown in Figure 7, with an integrated dynamic pneumatic classifier as shown in Figures 8a and 8b, is first heated via the two heating nozzles 5a (of which only one is shown in Figure 7) , which are charged with hot compressed air at 10 bar and 160°C, until the mill exit temperature is approximately 105°C. For the separation of the ground material, a filter system is connected downstream of the mill (but not shown in Figure 7), its filter housing being heated in the lower third indirectly via attached heating coils by means of 6 bar saturated steam, likewise for the purpose of preventing condensation. All of the apparatus surfaces in the region of the mill, the separating filter, and the supply lines for steam and hot compressed air have special insulation. After the heating temperature has been reached, the supply of hot compressed air to the heating nozzles is shut off and the charging of the three grinding nozzles with the grinding medium of superheated steam (37.9 bar (abs), 325°C) is commenced. In order to protect the filter means used in the separating filter, and also in order to set a defined level of residual water in the ground material (see Table 1), water is introduced in the starting phase and during grinding, into the grinding chamber of the mill, via a two-fluid nozzle operated with compressed air, as a function of the mill exit temperature. The following mill configurations and operating parameters are used: grinding nozzle diameters 2.5 mm, nozzle type = Laval, number of nozzles 3 units; internal mill pressure = 1.306 bar (abs.), grinding medium entry pressure = 37.9 bar (abs.), grinding medium entry temperature = 325°C, grinding medium mill exit temperature = 149.8°C, classifier speed = 3500 min"1, classifier flow = 54.5 A%, outlet port diameter (immersed pipe diameter) = 100 mm. Product feed is commenced when the abovementioned operational parameters are constant. The feed quantity is regulated as a function of the classifier flow which comes about. The classifier flow regulates the feed quantity such that it is not possible to exceed approximately 70% of the nominal flow. The introduction member (4) which functions here is a speed-regulated bucket wheel which meters the feed material from a reservoir container via a cyclical lock, which serves as a barometric endpoint, into the grinding chamber, which is at superatmospheric pressure. The coarse material is comminuted in the expanding steam jets (grinding gas). Together with the depressurized grinding gas, the product particles ascend in the centre of the mill vessel to the classifying wheel. Depending on the classifier speed and grinding steam quantity which have been set, the particles whose fineness is sufficient enter along with the grinding steam into the fines exit, and from there they pass into the downstream separating system, while particles which are too coarse pass back into the grinding zone and are subjected to a repeat comminution. The discharge of the separated fines from the separation filter into the subsequent silo storage and bagging operation takes place by means of a bucket-wheel lock. The grinding pressure of the grinding gas that obtains at the grinding nozzles, and the resulting volume of grinding gas, in conjunction with the speed of the dynamic paddle wheel classifier, determine the fineness of the particle-size distribution function and also the upper particle-size limit. The material is ground to the particle size defined in Table 1 by the d9o value and by the fraction of particles Subsequently the material is treated in a fluidized-bed reactor. For this purpose the following conditions should be observed: First of all, 5 kg of the powder are introduced into the fluidized-bed reactor with fluidizing base. The fluidizing base is traversed by a gas mixture comprising dry nitrogen and dry air. These two gases are metered prior to their entry into the reactor in such a way that a resulting oxygen content of 6% by volume is not exceeded and in such a way as to give a fluidization velocity in the reactor of 0.05 m/s. The reactor is then heated from room temperature to 600°C. The flows of the fluidizing gas should be regulated in the heating phase such that the fluidization velocity in the reactor remains constant at 0.05 m/s. After 600°C has been reached, a preheated gas mixture of steam and nitrogen is fed into the reactor for a period of 2 hours. The two components are mixed so as to set a steam concentration of 90% and a nitrogen content of 10%. The gas volumes are adapted so that, again, a fluid gas velocity of 0.05 m/s comes about. Thereafter the addition of steam is interrupted, and for 30 minutes pure nitrogen at 600°C is passed through the fluidized-bed reactor. The material is then cooled to room temperature in a stream of dry nitrogen and is discharged from the reactor. In the cooling phase, particular care should be taken to ensure that at this point there is no longer any steam present. The chemicophysical data of Example 2 are listed in Table 1 and an IR spectrum is depicted in Figure 6. Comparative Examples 1-2 The following commercially available silicas were analysed (see Table 1) and incorporated into sealants in Example 4: Comparative Example 1: Siloa™ 72 X (Rhodia AG) Comparative Example 2: Ultrasil® VN 3 (Degussa AG) Example 3: Performance -bests 3.1 Preparation of acetate-crosslinking RTV-1K silicone sealants with precipitated silicas The amounts required for preparing the formulation below are indicated in Table 2. In the course of preparation, cooling with mains water should be carried out so that the formulation does not undergo warming substantially beyond room temperature. Preparation takes place at room temperature and at a relative humidity of 40% to 60%. A planetary dissolver (from H. Linden, type LPMD 2SP) equipped with a 2 1 stirring vessel with jacket, cooling water connection and independently controllable planetary drive and a dissolver drive is charged with silicone polymer, plasticizer (silicone oil) and crosslinker and this initial charge, is homogenized for 1 minute at a speed of 50 min"1 (planetary drive) and 500 min"1 (dissolver drive) . Then the catalyst is added and the batch is homogenized for 15 minutes under an N2 atmosphere with the same planetary and dissolver drive speeds. Thereafter the stabilizer and the silica are incorporated, again at the same speeds. As soon as the silica is fully wetted a vacuum of approximately 200 mbar is applied and dispersion takes place for 10 minutes at 100 min"1 of the planetary stirrer mechanism and 2000 min-1 of the dissolver. Immediately after the end of the dispersing operation the stirring vessel is flushed with nitrogen. Using a drum press, the sealant is dispensed as quickly as possible into aluminium tubes (cartridges). 3.2 Production of vulcanizates from RTV-1K In order to examine the performance properties of RTV-1K silicone sealants in which the silicas of the invention are used it is necessary to produce vulcanizates from the sealants prepared above. These vulcanizates are processed into test specimens. For this purpose, first of all the silicone sealant is applied to a smooth support plate in a sufficient amount and is coated with a coating bar (slot height: 2 mm) to form a strip 2 mm in height, approximately 80 mm in width and with a length of approximately 300 mm. Care should be taken to ensure that no air bubbles are formed. The shapes needed for the respective test are then punched out from this silicone strip. The support plate ought to be made from polypropylene, polyethylene, Teflon or another plastic from which the vulcanized sealant is readily detachable. The silicone strips are stored for 7 days for complete curing and then for at least 2 days under standard conditions (23°C, 50% relative humidity). 3.3 Determining the rheological properties and the storage stability of RTV-1K sealants The sealants prepared in accordance with Example 3, section 1, "Preparation of Acetate-Crosslinking RTV-1K Silicone Sealants with Precipitated Silicas" are stored prior to testing for at least 24 hours in a controlled-climate chamber at 23°C/50% relative humidity. To test the storage stability of the sealants, two tubes are stored for 35 days in a cont rolled-climate chamber at 23°C at 50% relative humidity and are tested after storage periods respectively of 1, 7, 14, 21, 28 and 35 days. Additionally, two further tubes are stored in a forced-air oven at 50°C for 35 days and likewise tested after 1, 7, 14, 21, 28 and 35 days of storage. The rheological properties are determined using a Haake RheoStress 1 rheometer (controlled via PC using the RheoWin Pro program). The operation of the instrument and of the software is described in detail in the Haake operating instructions. For the measurement it is necessary to use a die having a diameter of 35 mm, and the measuring-plate attachment MPC 35. Measurement is conducted under the following conditions: Slot distance between die and measuring-plate attachment: 0.5 mm Measurement temperature: 23°C Measurement range (shear rate) 0-10 1/s Number of measurement points: 400 The measurement points are plotted in a diagram which shows the shear rate y on the x axis and the shear stress x on the y axis. At a shear rate of 10 1/s the shear stress is read off and from this figure the viscosity r\| at 10 1/s is calculated using r\| = x/y. Two tubes are measured, with at least three measurements being carried out per tube. From the six individual results the highest and lowest values are discarded. The remaining four results are used to calculate the average value. For the determination of the yield point the Casson model is used. The data basis for calculating the Casson flow curve is the range from 0.2 to 2 1/s from the shear rate/shear stress diagram. The following relationship is defined: The value on the y axis, at which it intersects the flow curve calculated by the method of Casson is reported as the Casson yield point. The determination both of the viscosity at 10 1/s and of the Casson yield point is made automatically under the conditions indicated above by the RheoWin Pro software. 3.4 Determining the tensile strength and the breaking extension of vulcanized silicone rubber This determination is made by a method based on DIN 53504 and is used to determine the tensile strength and the breaking elongation of specimens of defined shape made from elastomers, when the specimens are extended at a constant speed until they rupture. The tensile strength and the breaking extension in this case are defined as follows: The tensile strength 5max is the ratio of the measured maximum force Fmax to the initial cross-section A 0 of the specimen. The breaking elongation sR is the ratio of the length change L A measured at the moment of rupture to the original measurement length L 0 of the specimen. Measurement is carried out on a tensile testing machine (Zwick/Roell, type Z010), ensuring that the preselected maximum force is variable, that the clamping device holds the test specimen firmly without mechanical damage even at high stretch, and holds the centre piece of the test specimen at the set measurement length L 0, without mechanical damage, even at high stretch, and that the spring pressure on the clamping jaws of the fine extension gauge is adjustable. The standard dumbbells S 1 described in Figure 4 are to be used. The corresponding test specimens are punched from the vulcanized strip 2 mm thick using a punching iron for S 1 standard dumbbells, and these test specimens are stored for at least 24 h under standard conditions (23°C, 50% relative humidity) before testing. 4-6 specimens are to be tested at a temperature of (23 ± 2)°C. Prior to the tensile test, a measurement should be made of the thickness d and the width b of the specimens. On clamping, the standard dumbbells should be clamped in centrally between the two clamping jaws. The distance between the clamping jaws is L = 50 mm. The fine extension gauge should be set to a measurement length L 0 of 25 mm and should be fixed centrally between the clamping jaws. The displays should stand at zero. The rate of advance of the pulling bracket is v = 500 mm/min. The force Fmax and the length change L A at rupture are recorded. From these figures the tensile strength and breaking extension are calculated as shown below and are reported as the average value of the individual This determination is carried out by a method based on ASTM D 624 B. The tear propagation test on elastomers is used to determine the resistance presented by an incised sample to the continuation of a tear. The tear propagation resistance of an elastomer is dependent on its formulating constituents and on its processing, on the vulcanization, and on the testing speed. The effect tested is the influence of the reinforcing filler on the tear propagation resistance of the vulcanizates. The measurement is carried out on a tensile testing machine (from Zwick/Roell, type Z010), ensuring that the preselected maximum force is variable and that the clamping device holds the test specimen firmly, without mechanical damage, even at high extension. Test specimens (Figure 5) are cut from the vulcanized silicone strips 2 mm thick, using a punching iron compliant with ASTM D 624 B and are stored for at least 24 h under standard conditions (23°C, 50% relative humidity) before testing. At the vertex of the inside radius, the punching iron has a blade with which a slot 0.5 mm ±0.05 mm in depth is incised at this point in the course of punching. 4-6 specimens should be tested, at a temperature of (23 ± 2) °C. The specimens must be stored at the test temperature for 3 hours prior to testing. The thickness a of the specimens should be determined to ± 0.01 mm prior to testing, using a thickness gauge. The sample is clamped into the clamping brackets of the tensile testing machine and ruptured at a rate of advance of v = 500 mm/min, ensuring that the slot is on the left-hand side from the tester. The two test specimens with the highest and lowest values are disregarded for the evaluation. The tear propagation resistance 5wl in [N/mm] is calculated from the variables Fmaxl (maximum force in [N] ) and also al (thickness in [mm] ) and is reported as the average value of the remaining individual measurements: Tear propagation resistance 8w} = 3.6 Assessing the results The consistency of silicone rubber formulations into which the silicas of the invention have been incorporated is assessed using the measurement results for the Casson yield point and for the viscosity at a shear rate of 1/10 (Table 3). By consistency is meant the rheological behaviour of an RTV-IK silicone sealant. Consistency is said to be good if silicone rubber applied to a vertical surface adheres to that surface without running over 24 h in the course of curing. Adequate consistency can be recognized from a viscosity of > 100 Pas and a yield point of > 90 Pa. The values for the Casson yield point for the silicone rubber formulations of Examples 3a to 3d clearly show that the yield point of silicone rubber formulation comprising the inventive silica of Example 1, with a figure of 100 Pas, is much higher than in the case of the formulations with the comparison silicas. In the case of the silicone rubber formulation 3b with the inventive silica of Example 2, with a Casson yield point of 260, this effect is very much more pronounced. In other words, silicone rubber formulations into which the inventive silicas have been incorporated have a better consistency on the basis of the particular properties of the inventive silicas. Formulations of this kind remain in the form in which they have been applied without showing any tendency to run. This is also confirmed by the viscosity values. Thus, the inventive silicas display equal or in Example 2 markedly improved, i.e. higher viscosity than the comparative examples. The mechanical stability (tensile strength and tear propagation resistance) and also the flexibility (breaking extension) of the cured silicone rubber formulations 3a to 3d can be assessed through their behaviour under tensile load under different conditions (Table 4) . The results of measurement for the mechanical properties can be interpreted as follows: for the silicone rubber formulation 3d containing the silica of comparative example 2 it is not possible at all to produce silicone vulcanizates (complete curing directly after preparation) , and accordingly it is not possible to measure the mechanical properties. The measurements for the inventive silica show that the minimum requirements concerning the mechanical stability (tensile strength and tear propagation resistance) and also the flexibility (breaking extension) of the cured silicone rubber formulations are met. In comparison to comparative Example 1 (silicone rubber formulation 3c), the inventive silicas of Examples 1 and 2 in the silicone rubber formulations 3a and 3b leads to better mechanical stability (higher values for tensile strength and tear propagation resistance) and also the same or better flexibility (equal or higher values for breaking extension). The storage stability, i.e. the change in rheological properties such as yield point and viscosity and also the negative cure behaviour in the tube over time, is shown in Tables 5 and 6. Consideration was given here both to the storage at room temperature and to the storage at elevated temperature (50°C). It is clearly apparent that the silicone rubber formulations 3a and 3b with the inventive silicas of Examples 1 and 2 undergo no change, or no substantial change, in their rheological properties, viscosity of 1/10 shear rate and Casson yield point over the storage period under investigation (namely 35 days). In other words, the effective thickening and processing properties (such as extrudability, for example) are still present even after storage under the stated conditions, without the sealant having undergone preliminary crossiinking or full vulcanization while still in the tube. In contrast, for Comparative Example 1, a negative change in the rheological properties is observed within the first 21 days. In comparison to the initial values, the viscosity and yield point increase significantly, which points to premature crosslinking of the sealant. Between day 21 and day 28, the sample cures while still in the tube, and hence is no longer processable. The situation is similar after storage at elevated temperature. There, the premature curing of Comparative Example 1 occurs as early as between day 2 and day 7. Even worse is the precipitated silica Ultrasil® VN 3. Here, the silicone rubber has cured immediately after preparation and can no longer even be dispensed into the cartridge (tube) and this silicone rubber compound is not suitable for RTV-1K applications. List of reference numerals for Figures 7, 8a and 8b 1 jet mill 2 cylindrical housing 3 grinding chamber 4 feed material for grinding 5 grinding jet inlet 5a heating nozzles 6 product outlet 7 pneumatic classifier 8 classifying wheel 9 Inlet opening or inlet nozzle 10 grinding jet 11 heat source 12 heat source 13 supply pipe 14 thermal insulation jacket 15 Inlet 16 outlet 17 centre of grinding chamber 18 reservoir or generating means 19 pipe installations 20 exit port (immersed pipe) 21 classifier housing 22 top housing part 23 Bottom housing part 24 circumferential flange 25 circumferential flange 26 articulated joint 27 Arrow 28 inspection chamber housing 28a carrying arms 29 discharge cone 30 Flange 31 Flange 32 coverplate 33 coverplate 34 Paddle 35 classifying wheel shaft 35a pivot bearing 36 top machined plates 37 Bottom machined plate 38 end section of housing 39 product feed port 40 axis of rotation 41 exit chamber 42 top coverplate 4 3 removable lid 4 4 carrying arms 45 conical annular housing 46 intake filter 47 perforated plate 48 fines discharge pipe 4 9 deflection cone 50 classifying air entry coil 51 coarse product charge 52 Flange 53 Flange 54 dispersion zone 55 flange machined (bevelled) at the inside edge, and lining 56 replaceable protection pipe 57 replaceable protection pipe 58 fines exit/outlet 59 ring of paddles 60 Claims: 1. Precipitated silica characterized in that it has an SiOH1soilated absorbance ratio of greater than or equal to 1. 3. Precipitated silica according to Claim 1, characterized in that the silanol group density is 0.5 to 3.5 SiOH/nm2. 3. Precipitated silica according to Claim 1 or 2, characterized in that the modified tapped density is less than or equal to 70 g/1. 4. Precipitated silica according to either of Claims 1 and 3, characterized in that it has the following properties: BET surface area 50 - 600 m2/g CTAB surface area 50 - 350 m2/g DBP (anhydrous) 150 - 400 g/100 g 5. Precipitated silica according to any one of Claims 1 to 4, characterized in that 5% to 100% of the particles in the volume-based particle distribution curve are 6. Precipitated silica according to any one of Claims 1 to 5, characterized in that the d90 value is not greater than 0.001 - 10 jam. 7. Precipitated silica according to any one of Claims 1 to 6, characterized in that the particle distribution curve is bimodal. 8. Precipitated silica according to any one of Claims 1 to 1, characterized in that the loss on ignition is 0.1% - 3.0% by weight. Precipitated silica according to any one of Claims 1 to 8, characterized in that the loss on drying is 0.1% - 3.0% by weight. Precipitated silica according to any one of Claims 1 to 9, characterized in that the pH is 4-8. Precipitated silica according to any one of Claims 1 to 10, characterized in that it is a hydrophilic precipitated silica. Process for preparing a silica according to any one of Claims 1 to 11, characterized in that the process comprises the following steps: 1. reacting at least one silicate with at least one acidifier 2. filtering and washing the resulting silica 3. drying the resulting silica or filtercake 4. heat-treating the dried silica. Process according to Claim 12, characterized in that step 1 comprises the following substeps: la preparing an initial charge of water or of water and at least one silicate and/or a silicate solution, the pH of the resulting initial charge being preferably between 5 and 10 and the temperature of the initial charge being preferably between 80 and 100°C. lb metering at least one silicate and/or a silicate solution and at least one acidifier into the initial charge from substep la) with stirring at 80 to 100°C until the solids content of the precipitation suspension reaches a level which leads to the solids content which is to be reached in substep lc). 1c adding an acidifier at a temperature of the precipitation suspension of 80 to 100°C, so that the pH of the precipitation suspension is lowered to 2 to 6 and the solids content of the precipitation suspension at the end of this substep is between 30 and 70 g/1. Process according to Claim 13, characterized in that for the period of substep lb) the pH is held constantly at a level between 7 and 10. Process according to any one of Claims 12 to 14, characterized in that the silicas are ground in a step 3a, i.e. between steps 3 and 4, or in a step 5, i.e. after step 4, or both in step 3a, i.e. between steps 3 and 4, and in step 5, i.e. after step 4. Process according to Claim 15, characterized in that the grinding parameters are selected such that the ground product in the range Process according to Claim 15 or 16, characterized in that grinding is carried out using a jet mill, preferably a fluid-bed opposed-jet mill. Process according to Claim 17, characterized in that the fluid-bed opposed-jet mill is operated with steam as operational medium. Process according to any one of Claims 15 to 18, characterized in that grinding is carried out by means of a grinding system (grinding apparatus), preferably a grinding system comprising a jet mill, and in that in the grinding phase the mill is operated with an operational medium selected from the group consisting of gas and/or vapour, preferably steam, and/or a gas comprising steam, and in that in a heating phase, i.e. before the actual operation with the operational medium, the grinding chamber is heated such that the temperature in the grinding chamber and/or at the mill outlet is higher than the dew point of the vapour and/or operational medium. Process according to any one of Claims 12 to 19, characterized in that the heat treatment of the silica in step 4 is carried out in a fluidized-bed, fluid-bed or rotary-tube reactor. Process according to Claim 20, characterized in that a fluidized-bed reactor is used and in that 'the following substeps are carried out: 4a introducing the silica into the fluidized-bed reactor, 4b preheating the reactor to 300 to 800°C, the reactor being traversed at the same time by a flow of inert gas and/or nitrogen/air mixture in such a way as to produce a •fluidization velocity of 0.02 to 0.06 m/s, 4c feeding in a gas mixture I comprising steam and an inert gas, or a gas mixture II comprising steam, an inert gas and air, at 300 to 800°C for a period of 0.25 to 6 h, the gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and the gas mixtures I and II having a steam concentration of 10 to 95% by volume and in the case of gas mixture II an oxygen content of 0.01% to 21% by volume, 4d interrupting the addition of steam and expelling the steam by means of an inert gas, nitrogen for example, and/or of an inert gas/air mixture at 300 to 800°C, the gas or gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume, 4e cooling the heat-treated silica to room temperature in a dry process atmosphere, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume. 22. Precipitated silica characterized in that it is obtainable by the process according to any one of Claims 12 to 21. 23. Use of precipitated silicas according to any one of Claims 1 to 11 or 22 for producing sealants. 24. Use according to Claim 23, characterized in that the sealant is RTV-1K silicone rubber or a silicone sealant of the various crosslinking systems (acetoxy-crossiinking, alkoxy-crossiinking and/or oxime-crosslinking). 25. Sealant comprising at least one precipitated silica according to any one of Claims 1 to 11 or 22. 26. Sealant according to Claim 25, characterized in that the sealant is RTV-1K silicone rubber or a silicone sealant of the various crosslinking systems (acetoxy-crosslinking, alkoxy-crosslinking and/or oxime-crosslinking). 27. Use of the sealant according to Claim 25 or 26, in the building industry as a joint-sealant, in the automotive industry as an adhesive and sealant and/or as a coating material for textile fabric.

Full Text

Precipitated silicas having special surface properties
The present invention relates to precipitated silicas 5 having special surface qualities, to a process for preparing them and to their use for thickening sealants.
Sealants are elastic substances that are applied in
10 liquid to highly viscous form for the sealing of
buildings or installations against water, atmospheric
influence or aggressive media.
Silicone rubbers are compositions which are convertible
15 into the elastomeric state and comprise as their base
polymers polydiorganosiloxanes containing groups
amenable to crosslinking reactions. Suitable such
groups include, primarily H atoms, OH groups and vinyl
groups, which may be located at the chain ends, or else
20 may be incorporated in the chain. Incorporated into
this system are fillers as reinforcers, their nature
and amount significantly influencing the mechanical and
chemical behaviour of the vulcanizates. Silicone
rubbers can be coloured with inorganic pigments. One
25 distinction is between high-temperature vulcanizing and
room-temperature vulcanizing (HTV/RTV) silicone
rubbers.
Among the room-temperature curing or vulcanizing 30 silicone rubber compositions, it is possible to differentiate one-component (IK) and two-component (2K) systems. The first group (RTV-IK) polymerizes slowly at room temperature under the influence of atmospheric moisture, with crosslinking taking place through 35 condensation of SiOH groups to form Si,0 bonds. The SiOH groups are formed by hydrolysis of SiX groups of a species formed as an intermediate from a polymer with terminal OH groups and from what is called a

crosslinker R-SiX3 (e.g. = -O-CO-CH3, -NHR). In two-component rubbers (RTV-2K) the crosslinkers used are, for example, mixtures of silicic esters (e.g. ethyl silicate) and organotin compounds, the crosslinking reaction that takes place being the formation of an Si-O-Si bridge from nSi-OR and =Si-OH (- = methyl group; R = organic radical) by elimination of alcohol.
The thickeners used for RTV-IK silicone rubber include silicas. In view of the sensitivity to hydrolysis of the silicone sealants, these silicas must introduce as little moisture as possible into the system. For this reason, fumed silicas have, been used almost exclusively to date for this application. Hydrophilic silicas have not been used to date, on account of their high moisture content.
WO 2005/061384 shows the preparation and use including use in silicone rubber - of silicas which according to the claim have a water absorption of 300 ml/100 g. The silicas disclosed in the examples of WO 2005/061384, however, all have a water absorption of between 5.7% and 5.9% and are therefore unsuitable for use in RTV-IK formulations. Accordingly, WO 2005/061384 describes only their use in silicone rubber formulations for extrusion processes (HTV).
EP 1557446 describes exclusively HTV silicone rubber formulations. The silicas employed therein have a loss on drying of In summary, therefore, it can be stated that the prior art does not disclose any precipitated silicas which meet the exacting requirements for use in RTV-IK silicone rubber. There is therefore a strong need for precipitated silicas of this kind that are suitable for

On the basis of the prior art described above, the object of the present invention is to provide precipitated silicas from which the abovementioned disadvantages of the precipitated silicas of the prior art are completely or at least partially eliminated. A further aim is to provide a process for preparing the silicas of the invention.
Further objects, not explicitly stated, will emerge from the overall context of the description, examples
and claims.
Surprisingly it has been found that this object is achieved by the precipitated silicas of the invention that are defined in greater detail in the description below and also in the claims and in the examples.
The present invention therefore provides precipitated silicas characterized in that they have an SiOHiSOiated absorbance ratio of greater than or equal to 1.
The invention also provides precipitated silicas, preferably hydrophilic precipitated silicas which in addition to the stated parameters, independently of one another, have one or more of the following physicochemical parameters:
silanol group density 0.5 - 3.5 SiOH/nm2
modified tapped density BET surface area 50 - 600 m2/g
CTAB surface area 50 - 350 m2/g
DBP (anhydrous) 150 - 400 g/100 g

loss on ignition 0.1% - 3.0% by weight
loss on drying 0.1% - 3.0% by weight
pH 4-9
fraction of particles 5% to 100%
d90 value of the volume-based particle distribution
0.001 to 10 \m
The present invention further provides a process for preparing the precipitated silicas of the invention.
Additionally provided by the present invention is the use of the silicas of the invention in sealants, especially in silicone rubber and silicone sealants and with particular preference in RTV-1K sealants. Application is possible in different crosslinking systems, e.g. acetoxy-crosslinking, alkoxy-crosslinking and oxime-crosslinking. These systems are employed, for example, in the building industry as joint-sealants, in the automotive industry as adhesives and sealants, and as coating materials for textile fabric, for example.
The invention further provides sealants based on silicone rubber which comprise the silicas of the invention, and their use.
The precipitated silicas of the invention have the advantage that, on the basis of their special structure and surface qualities, they ensure high storage stability, a firm consistency and an optimum yield point of the silicone rubber when incorporated into silicone rubber compositions, especially those of the

A further advantage of the precipitated silicas of the invention is their low modified tapped density. The low modified tapped density comes about as a result of a very loose packing of the silica particles. This means that, although in mutual contact and adhering gently to one another, the silica particles are nevertheless so loosely packed that large cavities are produced. This loose packing comes about in the silicone compound as well and thus contributes to the high level of thixotropy on the part of the silicone compound.
In summation, the particular properties of the precipitated silicas of the invention lead to advantages which include the following:
• high storage stability of RTV-1K silicone rubber compositions following incorporation of the silicas of the invention;
• rapid and effective dispersing and hence high thickening action of the silica in RTV-1K silicone rubber compositions.
Moreover, the precipitated silicas of the invention offer a substantial cost advantage over the fumed silicas used to date in RTV1 silicone rubber, being more inexpensive to prepare.
The subjects of the invention are described in detail below.
In the present invention the terms silica and precipitated silica are used synonymously. By hydrophilic precipitated silicas are meant those whose surface shows hydrophilic behaviour when incorporated by stirring into water, i.e. those whose surface is

completely wetted by water and therefore has a contact angle with respect to water of less than 90°. The hydrophilic precipitated silicas of the invention preferably have a carbon content of The silicas of the invention are distinguished by the fact that they have a particularly high proportion of isolated SiOH groups, as expressed by the SiOHisoiated absorbance ratio, on their surface. The SiOHiSOiated absorbance ratio of the silicas of the invention is greater than or equal to 1, preferably between 1.5 and 10, more preferably between 1.5 and 7, very preferably between 1.8 and 5, with especial preference between 2 and 4.5, with very special preference between 2.3 and 4.0 and with particular preference between 2.3 and 3.5. This particular surface quality of the silicas of the invention is a key property, and means that in silicone rubber formulations the silicas lead to a high level of storage stability, and improved firmness " of consistency, and an optimized flow behaviour.
Without being tied to any specific theory, the special properties of the silicas of the invention may be explained by the high number of isolated SiOH groups and at the same time their wide spacing. These two properties make it more difficult for hydrogen bonds to form and for water molecules to accumulate on the silica's surface.
For the abovementioned reasons it may be advantageous if the silicas of the invention have a low silanol group density, i.e. a broad separation of the silanol groups on the silica surface. For the determination of the silanol group density, the number of silanol groups on the surface of the silica is first determined by means of LiAlH4. This alone, however, is not meaningful, since precipitated silicas with a high surface area generally have a higher absolute number of silanol

jroups than do precipitated silicas with a low surface area. Consequently it is necessary to relate the number Df silanol groups to the surface area of the silica. A suitable surface area for this purpose is the BET surface area, since this describes the surface area which is available even to relatively small molecules such as water, for example. The silanol group density of the silicas of the invention is situated preferably in the range from 0.5 to 3.5 SiOH/nm , preferably from 0.5 to 3.0 SiOH/nm2, more preferably from 1.0 to 2.8 SiOH/nm2 and with very particular preference from 1.5 to 2.8 SiOH/nm2. If the number of silanol groups per nm2 is too low, this may result in an excessively low yield point and may consequently have an adverse effect on the consistency of the silicone sealants.
The specific BET surface area describes the effect of the silica on the incorporation characteristics into silicone rubber and also on the crude mixing properties (cf. S. Brunauer, P. H. Emmett, E. Teller, "Adsorption of Gases in Multimolecular Layers", J. Am. Chem. Soc. 60, 309 (1938)). Thus the silicas of the invention may have a BET surface area of 50 to 600 m2/g, preferably 50 to 400 m2/g, more preferably 50 to 250 m2/g, very preferably 80 to 230 m2/g, especially of 100 to 180 m2/g, with very especial preference of 125 to 180 m2/g and with particular preference of 140 to 170 m2/g.
The specific CTAB surface area is of decisive importance primarily for the reinforcing property of the silica (cf. Janzen, Kraus, Rubber Chem. Technol. 44, 1287 (1971)). The reinforcing potential increases with increasing CTAB surface area. Thus the precipitated silicas of the invention may have a CTAB surface area of 50 to 350 m2/g, more preferably 50 to 250 m2/g, very preferably of 80 to 230 m2/g, especially preferably of 100 to 200m2/g, with very especial

preference of 125 to 190 m2/g and with particular preference of 140 to 190 m2/g.
It has additionally been found that a high DBP absorption on the part of the silicas of the invention is of benefit in order to obtain effective rheological properties. Excessively high DBP values, however, may lead to an excessive increase in the viscosity of the silicone rubber and ought therefore to be avoided. The silicas of the invention, accordingly, preferably have a DBP absorption of 150 to 400 g/(100 g) , more preferably 200 to 350 g/(100 g), very preferably of 220 to 330 g/(100 g), with especial preference 250 to 330 g/(100 g) and very especially 260 to 320 g/(100 g).
In a special embodiment, the precipitated silicas of the invention have a low modified tapped density and hence a very particularly good thixotropy. It should be noted here that the modified tapped density is a reference to the tapped density as measured on the uncompacted material. In order to be able to determine this variable even on materials which have already undergone preliminary compaction as a result of packaging and storage, it is necessary to carry out sample preparation as described in the section "Determining the modified tapped density". The silicas of the invention preferably have a modified tapped density of less than or equal to 70 g/1, preferably of 1 to 60 g/1, more preferably of 5 to 55 g/1, very preferably of 10 to 50 g/1, and in particular of 10 to 30 g/1.
The inventors have observed, moreover, that for the consistency of the silicone sealants it may be of particular advantage if the precipitated silicas of the invention contain a sufficient fraction of fine particles, i.e. of particles
silicas of the invention in the particle size range It has also been observed that an excessive fraction of coarse particles may adversely affect the performance properties of the precipitated silicas of the invention. For this reason, the silicas of the invention are preferably distinguished by a d90 value, relative to the volume-based particle distribution curve, of between 0.01 and 10 μm, preferably between 1 and 10 μm, more preferably between 2 and 8 μm and with particular preference between 3 and 7 μm.
The particle distributions may be monomodal or bimodal, preferably bimodal.
It has also been observed that for all of the above-described embodiments of the silicas of the invention it may be of particular advantage if from the outset the silica introduces very little moisture into the silicone sealant. The silicas of the invention may therefore have an initial moisture content, expressed by loss on drying, of 0.1% to 3.0%, preferably of 0.2% to 2.5%, more preferably 0.3% to 2.0%, and with particular preference 0.4% to 1.8% by weight and/or a loss on ignition of 0.1% - 3.0%, preferably 0.2% to 3.0%, more preferably 0.3% to 2.0%, and with particular preference 0.4% to 1.8% by weight.
Finally, it has been observed for all of the above-described embodiments of the silicas of the invention that it may be of particular advantage if the silica has a pH value in the range from 4 to 8, preferably 4.5

to 7.5. If the pH is too high then the situation may arise that, following prolonged storage (e.g. after several days), the silicone compound no longer properly crosslinks, but instead remains tacky.
The stated ranges of preference may be set independently of one another.
The silicas of the invention can be prepared by a process which comprises the steps described below of
1. reacting at least one silicate with at least one acidifier
2. filtering and washing the resulting silica
3. drying the resulting silica or filtercake
4. heat-treating the dried silica.
Step 1 here preferably comprises the substeps of
la preparing an initial charge of water or of water and at least one silicate and/or a silicate solution, the pH of the resulting initial charge being preferably between 5 and 10 and the temperature of the initial charge being preferably between 80 and 100°C.
lb metering at least one silicate and/or a silicate solution and at least one acidifier into the initial charge from substep la) with stirring at 80 to 100°C until the solids content of the precipitation suspension reaches a level which leads to the solids content which is to be reached in substep lc) . With particular preference, silicate and/or silicate solution and acidifier are added simultaneously and/or in such a way that

the pH for the period of substep lb) is kept constant at a level between 7 and 10.
adding an acidifier at a temperature of the precipitation suspension of 80 to 100°C, so that the pH of the precipitation suspension is lowered to 2 to 6 and the solids content of the precipitation suspension at the end of this substep is between 30 and 70 g/1.
Preferably the silicas of the invention are ground. This takes place with particular preference by grinding the silicas of the invention in a step 3a, i.e. between steps 3 and 4, or in a step 5, i.e. after step 4, or both in step 3a, i.e. between steps 3 and 4, and in step 5, i.e. after step 4.
All known forms of silicate are suitable for the silicates or silicate solutions used in step 1) of the process of the invention. The silicates used in accordance with the invention are preferably alkaline silicates, e.g. sodium or potassium silicates. With particular preference the silicate in step 1 is sodium silicate (water glass) . Its weight ratio of SiC>2 to Na20 is between 2 and 4, preferably between 3 and 3.6 and more preferably between 3.3 and 3.5. The Si02 content is preferably between 20% and 40% by weight, preferably between 25% and 30% by weight.
Acidifiers are acidic compounds of organic or inorganic type which can be used to lower the pH of the precipitation suspension. With preference it is possible to use inorganic acids such as hydrochloric acid, phosphoric acid, sulphuric acid or nitric acid, or organic acids such as acetic acid, formic acid or carbonic acid or carbon dioxide. Both dilute and concentrated acids can be used. With particular preference the process of the invention uses sulphuric

acid.
In the majority of cases the silicate and/or the silicate solution and acidifier used in substeps la) to lc) are identical.
The pH of the initial charge in substep la) is preferably between 7 and 10, more preferably between 8 and 9. The temperature of the initial charge is set at 80 to 100°C, preferably at 85 to 95°C,
In substep lb) silicate and acidifier are metered in preferably simultaneously. The addition of the two components preferably takes place continuously constantly over the entire period of substep lb). During this period the temperature remains at 80 to 100 °C, preferably at 85 to 95 °C. The period of the addition lasts until the solids content to be achieved at the end of step lc) has been achieved. It may in this case be necessary to continue the precipitation beyond the viscosity rise point. This viscosity rise point corresponds to the point in time at which a sharp rise is observed in the viscosity of the precipitation suspension in the course of precipitation; cf. EP 0643015. During substep lb), in which the precipitation of the silica begins, the pH is as far as possible kept constant at a level of between 7 and 10, preferably constant at a level of between 7.5 and 9.5 and with very particular preference at a pH of between 8 and 9. Corrections to an off-target pH are generally made by increasing or lowering the addition of the acidifier, so that the set pH preferably fluctuates only by ± 0.2 pH units, more preferably by only i 0.1 pH units.
Through addition of an acidifier at a temperature of the precipitation suspension of 80 to 100°C its pH is lowered in substep lc) to 2 to 6, preferably 3 to 6,

more preferably to 3 to 4. The solids content of the precipitation suspension at the end of this substep is between 30 and 70 g/1, preferably between 45 and 60 g/1 and with very particular preference between 45 and 55 g/1.
Without in any way being tied to one particular theory, the intention is that, by suitable choice of the process parameters, a chain-like structure of the aggregates should be constructed in substep lb) . A reinforcement of this hitherto quite loose aggregate structure is attained by the correspondingly slow further precipitation even after the viscosity rise point.
The metering rates in step lb) are to be selected in all embodiments of the process of the invention both before and after the viscosity rise point such that the solids content which is to be achieved after acidification in step lc) , of 30 to 70 g/1, is reached.
The filtration, liquefaction (e.g. in accordance with DE 2447613) and both longer and accelerated drying of the silicas of the invention are familiar to the person skilled in the art and can be looked up, for example in the documents specified in the description. The filtration and the washing of the silica take place preferably in such a way that the conductivity of the end product is The silica of the invention is dried preferably in a pneumatic conveying drier, spray drier, rack drier, belt drier, rotary tube drier, flash drier, spin-flash drier or nozzle tower drier. These drying variants include operation with an atomizer, with a single-fluid or two-fluid nozzle or of an integrated fluid bed. Spray drying may be carried out in accordance for

example with US 4094771.
If the selected mode of drying is spray drying then the filtercake should be redispersed beforehand. Redispersion takes place preferably in water or aqueous acid so that the dispersion has a pH of 4 to 7. It should be ensured here that the silica dispersion when redispersion is at an end has a solids content of 5% to 18%, preferably 8% to 13% by weight, more preferably 9% to 11%, and that in the course of redispersion the shearing forces acting on the silica are not too great. This can be achieved, for example, by stirring with a rotary speed of A substantial step for setting the silanol group density and arrangement of silanol groups on the silica surface, in addition to the precipitation, in which the chain-like structure is constructed, is the heat treatment to be carried out in step 4. This heat treatment may be carried out batchwise or continuously. For the heat treatment it is possible, for example, to use a fluidized-bed, fluid-bed or rotary-tube reactor. It should be ensured that in the course of the heat treatment, the temperature distribution and the process gas atmosphere are homogeneous, so that all of the silica particles are exposed to identical conditions. The process gas must have a sufficient steam concentration. The steam concentration is preferably 10% to 95% by volume, more preferably 40% to 90% by weight, very preferably 50% to 90% by weight.
Particularly when using a rotary-tube reactor it must be ensured that the temperatures everywhere are the same, i.e. that no "cold zones" exist in which the

steam could condense. The condensed steam may lead to the agglomeration of the silicas. The particular conditions during the heat treatment of the invention also ensure that a silica which has already been ground prior to heat treatment need not be ground once again after heat treatment, in other words that no instances of caking or agglomeration arise, such caking or agglomeration otherwise having to be removed by grinding again after heat treatment.
Preference is given to using a fluidized-bed or fluid-bed reactor. By a fluidized bed is meant the following:
If a flow of gases from below traverses fine-particled bulk product lying on horizontal, perforated plates, under certain flow conditions a condition comes about which is similar to that of a boiling liquid; the layer bubbles; the particles of the bulk material are located within the layer in a continually fluidizing up-and-down motion and thus remain, so to speak, in suspension. Terms also used are therefore suspension bed, fluidized bed, fluid bed, and fluidizing. The associated large surface area of the fluidized product also facilitates the drying and heat treatment of solids.
It is important that during heat treatment all particles of silica are exposed to the same temperature and the same process gas. The temperature differences between the hottest and coldest point ought to be as small as possible. Consequently the temperature of the filter candles as well must not be below the product temperature.
With very particular preference, the heat treatment in step 4 of the process of the invention takes place in accordance with substeps 4a) to 4e) below:

4a. introducing the silica into the fluidized-bed reactor.
4b. preheating the reactor to 300 to 800°C/ the reactor being traversed at the same time by a flow of inert gas and/or nitrogen/air mixture in such a way as to produce a fluidization velocity of 0.02 to 0.06 m/s.
4c. feeding in a gas mixture I comprising steam and an inert gas, e.g. nitrogen, or a gas mixture II comprising steam, an inert gas and air, at 300 to 800°C for a period of 0.25 to 6 h, the gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and the gas mixtures I and II having a steam concentration of 10 to 95% by volume and in the case of gas mixture II an oxygen content of 0.01% to 21% by volume.
4d. interrupting the addition of steam and expelling the steam by means of an inert gas, nitrogen for example, and/or of an inert gas/air mixture at 300 to 800°C, the gas or gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume.
4e. cooling the heat-treated silica to room temperature in a dry process atmosphere, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume.
After the silica has been introduced into the fluidized-bed reactor (substep 4a)), the reactor is heated in substep 4b) to an operating temperature of 300 to 800°C, preferably of 350 to 690°C and more

preferably of 400 to 650°C. During the heating operation the reactor is traversed by a flow of inert gas, preferably nitrogen and/or a mixture of an inert gas and dry air, in such a way that a fluidization velocity of 0,02 to 0.06 m/s is set.
After the operating temperature has been reached, in substep 4c) a gas mixture I comprising steam and an inert gas, preferably nitrogen, or a gas mixture II comprising steam, an inert gas and air is passed through the reactor for a period of 0.25 to 6 h, preferably 0.5 to 5 h, more preferably 1 to 4 h, very preferably 2 to 4 h. The fluidization velocity of the gas mixture is 0.02 to 0.06 m/s. The gas mixtures I and II have a steam concentration of 10% to 95% by volume, preferably 40% to 90% by weight, very preferably 50% to 90% by weight and, in the case of gas mixture II an oxygen content of 0.01% to 21% by volume.
The techniques for optional grinding of the silicas of the invention are known to the person skilled in the art and can be looked up for example in Ullmann, 5th edition, B2, 5-20. For the grinding of the silicas of the invention in step 3a) and/or in step 5) it is preferred to use grinding systems (grinding apparatus) comprising or consisting of impact mills or jet mills, preferably opposed-jet mills. Particular preference is given to using fluid-bed opposed-jet mills. With very particular preference, grinding takes place by means of a grinding system (grinding apparatus), with particular preference a grinding system comprising a jet mill, characterized in that the mill of the grinding system is operated in the grinding phase with an operational medium selected from the group consisting of gas and/or vapour, preferably steam, and/or a gas comprising steam, and in that the grinding chamber is heated in a heating phase, i.e. before the actual operation with the operational medium, such that the temperature in

the grinding chamber and/or at the mill outlet is higher than the dew point of the vapour and/or operational medium.
Grinding takes place with particular preference in accordance with the method described in DE 10 2006 048 850.4, using the grinding system (mill) described therein the operational medium used being, with especial preference, steam. In order to avoid pure repetitions of text, the content of the three cited patents is hereby explicitly incorporated as part of the content of the present specification. The grinding parameters are preferably chosen such that the ground product has a fine-particle fraction in the region smaller than 1 \im in the volume-based particle distribution, of 5% to 100%, preferably 10% to 95%, more preferably 15% to 95%, with very particular preference 20% to 90%, and in particular from 40% to 80%, and/or a dgo value in the volume-based particle distribution curve of between 0.01 and 10 ]xm.
In one especially preferred embodiment, in preparation for actual grinding with superheated steam, a fluid-bed opposed-jet mill as shown in Figure 7, with an integrated dynamic pneumatic classifier as shown in Figures 8a and 8b, is first heated via the two heating nozzles (5a) (of which only one is depicted in Figure 7) which are charged with hot compressed air, preferably at 10 bar and 160°C, until the mill exit temperature is higher than the dew point of the steam and/or operational medium, preferably about 105°C.
Connected downstream of the mill, for the separation of the ground material, is a filter system (not shown in Figure 7) whose filter housing is heated in its lower third indirectly, via attached heating coils, by means of saturated steam (preferably 6 bar saturated steam), likewise for the purpose of preventing condensation.

apparatus surfaces in the region of the mill, the separation filter, and the supply lines for steam and hot compressed air have special insulation.
After the desired heating temperature has been reached, the supply of hot compressed air to the heating nozzles is shut off and the charging of the three grinding nozzles with superheated steam, preferably at 38 bar (abs) and 325°C, is commenced.
In order to protect the filter medium used in the separation filter and also in order to set a defined level of residual water in the ground material, of preferably 2% to 6%, water is introduced in the starting phase, and during grinding, into the grinding chamber of the mill, via a two-fluid nozzle operated with compressed air, as a function of the mill exit temperature.
The feed quantity is regulated as a function of the classifier flow which comes about. The classifier flow regulates the feed quantity such that it is not possible to exceed approximately 70% of the nominal flow.
The introduction member (4) which functions here is a speed-regulated bucket wheel which meters the feed material from a reservoir container via a cyclical lock, which serves as a barometric endpoint, into the grinding chamber, which is at superatmospheric pressure.
The coarse material is comminuted in the expanding steam jets (grinding gas). Together with the depressurized grinding gas, the product particles ascend in the centre of the mill vessel to the classifying wheel. Depending on the classifier speed and grinding steam quantity which have been set, the

particles whose fineness is sufficient enter along with the grinding steam into the fines exit, and from there they pass into the downstream separating system, while particles which are too coarse pass back into the grinding zone and are subjected to a repeat comminution. The discharge of the separated fines from the separation filter into the subsequent silo storage and bagging operation takes place by means of a bucket-wheel lock.
The grinding pressure of the grinding gas that obtains at the grinding nozzles, and the resulting volume of grinding gas, in conjunction with the speed of the dynamic paddle wheel classifier, determine the fineness of the particle-size distribution function and also the upper particle-size limit.
The silicas of the invention can be used in sealants, particularly in silicone rubber and silicone sealants and with particular preference in RTV-1K sealants. Their application is possible in various crosslinking systems, e.g. acetoxy-crosslinking, alkoxy-crosslinking and oxime-crossiinking. These systems find application, for example in the building industry as joint-sealants, in the automotive industry as adhesives and sealants and as coating compositions for textile fabrics, for example.
The reaction conditions and the physical/chemical data of the precipitated silicas of the invention are determined by means of the following methods:
Determining filtercake solids content
In accordance with this method the solids content of filtercakes is determined by removal of the volatile fractions at 105°C.

For this purpose 100.00 g of the filtercake are weighed out (initial mass E) into a dry, tared porcelain dish (20 cm diameter) . The filtercake is broken up with a spatula if necessary to give loose lumps of not more than 1 cm3. The sample is dried to constant weight in a drying cabinet at 105 ± 2°C. Subsequently the sample is cooled to room temperature in a desiccator cabinet with silica gel as desiccant. The final mass A is determined gravimetrically.
The solids content (SC) in % is determined in accordance with
SC = A/E * 100%,
where A = final mass in g and E = initial mass in g.
Determining precipitation suspension solids content
The solids content of the precipitation suspension is determined gravimetrically after the sample has been filtered.
100.0 ml of the homogenized precipitation suspension (VSUspension) are measured off at room temperature with the aid of a measuring cylinder. The sample is filtered through a circular filter (TYP 572, Schleicher & Schuell) in a porcelain suction filter unit, but is not sucked dry, so as to prevent cracking of the filtercake. Subsequently the filtercake is washed with 100.0 ml of distilled water. The washed filtercake is transferred to a tared porcelain dish and dried to constant * weight in a drying oven at 105 ± 2°C. The weight of the dried silica (msampie) is determined after cooling to room temperature.
The solids content is determined in accordance with:

solids content in g/1 = (msampie in g) / (Vsuspension in 1). Determining silica-feed solids content
The silica feed is dried to constant weight in an IR drier. The loss on drying consists predominantly of water moisture.
2.0 g of silica feed are charged to a tared aluminium dish and the lid of the IR drying unit (Mettler, type LP 16) is closed. After the start button has been pressed, drying of the suspension at 105°C commences, and is ended automatically when the weight decrease per unit time falls below a value of 2 mg/(120 s).
The weight decrease in % is displayed directly by the instrument when the 0-100% mode is selected. The solids content is given by
solids content in % = 100% - weight decrease in %.
Determining pH
The pH of the silica, as a 5% aqueous suspension, is determined at room temperature in a method based on DIN EN ISO 787-9. In contrast to the specifications of the aforementioned standard, the initial masses were changed (5.00 g of silica to 100 ml of deionized water).
Determining electrical conductivity
The electrical conductivity of silica, as a 4% aqueous suspension, is determined at room temperature in a method based on DIN EN ISO 787-14. In contrast to the specifications of the aforementioned standard, the initial masses were changed (4.00 g of silica to 100 ml of deionized water).

Determining the moisture content or loss on drying
The moisture content of silica is determined in accordance with ISO 787-2 after 2 hour drying in a forced-air drying cabinet at 105°C. This loss on drying is composed predominantly of moisture water.
Determining the loss on ignition
By this method the loss in weight of silica is determined in a method based on DIN EN ISO 3262-1 at 1000°C. At this temperature, water bound physically and chemically, and also other volatile constituents escape. The moisture content (LD) of the sample investigated is determined by the above-described method "Determining the moisture content or loss on drying", based on DIN EN ISO 787-2.
0.5 g of the pulverulent, spherical or granular silica is weighed out to an accuracy of 0.1 mg into a tared porcelain crucible which has been purified beforehand by calcining (initial mass E). The sample is heated in a muffle furnace at 1000 ± 50°C for 2 h. The porcelain crucible is subsequently cooled to room temperature in a desiccator with silica gel as desiccant. The final mass A is determined gravimetrically.
The loss on ignition (DIN) LOI in % is obtained in accordance with
LOI = (1 - A/F) * 100.
F denotes the corrected initial mass in g based on dried matter, and is calculated according to
F = E * (1 - LD/100).
In the calculations A denotes final mass in g, E

denotes initial mass in g and LD denotes loss on drying, in %.
Determining the BET surface area
The specific nitrogen surface (called the BET surface area below) of the pulverulent, spherical or granular silica is determined by a method based on ISO 5794-1/Annexe D using the TRISTAR 3000 instrument (from Micromeritics) in accordance with the multipoint determination of DIN-ISO 9277.
Determining the CTAB surface area
The method is based on the adsorption of CTAB (N-hexadecyl-N,N,N-trimethylammonium bromide) on the "external" surface of the silica, in a method based on ASTM 3765 or NFT 45-007 (section 5.12.1.3).
CTAB is adsorbed in aqueous solution with stirring and
ultrasound treatment. Excess, unadsorbed CTAB is
determined by back-titration with SDSS (sodium dioctylsulphosuccinate solution, M Aerosol OT"
solution) using a titroprocessor, the endpoint being given by the turbidity maximum of the solution and determined using a phototrode. The temperature throughout all of the operations conducted is 23 -25 °C, to prevent crystallization of CTAB. The back-titration is based on the following reaction equation:
(C2oH3704)S03Na + BrN(CH3)3 (C16H33) => (C20H37O4) S03N (CH3) 3(^33) + NaBr
SDSS CTAB
Apparatus
Mettler Toledo DL55 titroprocessor and Mettler Toledo DL7 0 titroprocessor, each equipped with pH electrode, Mettler, type DG 111 and phototrode, Mettler, type

DP 550
100 ml polypropylene titration beaker
Glass titration vessel, 150 ml, with lid
Pressure filtration device, 100 ml capacity
Cellulose nitrate membrane filter, pore size 0.1 |im,
47 mm 0, e.g. Whatman (Order No. 7181-004)
Reagents
The solutions of CTAB (CCTAB = 0.015 mol/1 in deionized water) and SDSS (concentration = 0.00423 mol/1 in deionized water) are purchased in ready-to-use form (Bernd Kraft GmbH, 47167 Duisberg: Order No. 6056.4700 CTAB solution of concentration 0.015 mol/1; Order No. 6057.4700 SDSS solution 0.00423 mol/1), stored at 25°C and used within one month.
Procedure
1. Blank titration
The consumption of SDSS solution for titrating 5 ml of CTAB solution should be checked 1 x daily before each series of measurements. This is done by setting the phototrode, before beginning the titration, at 1000 ± 20 mV (corresponding to a transparency of 100%).
Precisely 5.00 ml of CTAB solution are pipetted into a titration beaker and 50.0 ml of deionized water are added. Titration with SDSS solution is carried out with stirring by the measurement method familiar to the skilled person, using the DL 55 titroprocessor, until the solution reaches maximum turbidity. The consumption VA of SDSS solution in ml is determined. Each titration should be performed in triplicate.

2. Adsorption
10.0 g of the pulverulent, spherical or granular silica with a moisture content of 5 i 21 (if appropriate, the moisture content is adjusted by drying at 105°C in a drying cabinet or by uniform wetting) are comminuted for 30 seconds using a mill (Krups, model KM 75, article no. 2030-70). Precisely 500.0 mg of the comminuted sample (initial mass E) are transferred to a 150 ml titration vessel with magnetic stirrer rod and precisely 100.0 ml of CTAB solution (Ti) are metered in. The titration vessel is closed with a lid and stirred using an Ultra Turrax T 25 stirrer (stirrer shaft KV-18G, 18 mm diameter) at 18 000 rpm for not more than 1 minute until wetting is complete. The titration vessel is screwed on to the DL 70 titroprocessor and the pH of the suspension is adjusted with KOH (0.1 mol/1) to a figure of 9 i 0.05.
The suspension is sonicated for 4 minutes in the titration vessel in an ultrasound bath (Bandelin, Sonorex RK 106 S, 35 kHz, 100 W effective and 200 W peak output) at 25°C. This is followed immediately by pressure filtration through a membrane filter under a nitrogen pressure of 1.2 bar. The initial fraction of 5 ml is discarded.
3. Titration
5.00 ml of the remaining filtrate are pipetted into a 100 ml titration beaker and made up to 50.00 ml with deionized water. The titration beaker is screwed on to . the DL 55 titroprocessor and titrated with SDSS solution, with stirring, until maximum turbidity is reached. The consumption VB of SDSS solution, in ml, is determined. Each titration should be performed in triplicate.

VA = consumption of SDSS solution in ml in titrating the blank sample
VB = consumption of SDSS solution in ml when using the filtrate
CCTAB = concentration of CTAB solution in mol/1
MCTAB = molar mass of CTAB = 364.46 g/mol
Ti - amount of CTAB solution added in 1
P = surface occupancy of CTAB = 578.435 m2/g
E = initial mass of silica
The CTAB surface area is based on the anhydrous silica, which is why the following correction is made.

The moisture content of the silica is determined in accordance with the above-described method "Determining the Moisture Content".
Determining the DBP absorption
The DBP absorption (DBP number), which is a measure of the absorbency of the precipitated silica, is determined by a method based on the DIN 53601 standard, as follows:

12.50 g of pulverulent or spherical silica with a moisture content of 0 - 10% (the moisture content is adjusted/ if appropriate, by drying at 105°C in a drying cabinet) are introduced into the kneader chamber (article number 279061) of the Brabender Absorptometer "E" (without damping of the outlet filter of the torque sensor) . In the case of granules, the sieve fraction from 1 to 3.15 mm (stainless steel sieves from Retsch) is used (by gently pressing the granules with a plastic spatula through the sieve with pore size of 3.15 mm). With continual mixing (kneader paddles rotating at a speed of 125 rpm), dibutyl phthalate is added dropwise to the mixture at a rate of 4 ml/min and at room temperature by means of the Brabender T 90/50 Dosimat. The incorporation of the DBP by mixing takes place with only a small amount of force, and is monitored by means of the digital display. Towards the end of the determination the mixture becomes pasty which is indicated by a sharp increase in the required force. At a display reading of 600 digits (torque of 0.6 Nm) an electrical contact shuts off both the kneader and the DBP feed. The synchronous motor for the DBP feed is coupled to a digital counter, so that the consumption of DBP in ml can be read off.
The DBP absorption is reported in g/ (100 g) and is calculated using the following formula:

where
DBP = DBP absorption in g/(100 g)
V = consumption of DBP in ml

The DBP absorption is defined for the anhydrous, dried silica. If moisture precipitated silicas are used it is necessary to take into account the correction value K for calculating the DBP absorption. This value can be determined using the correction table below: for example, silica having a water content of 5,8% would require an add-on of 33 g/ (100 g) for the DBP absorption. The moisture content of the silica is determined in accordance with the method ^Determining the Moisture Content or Loss on Drying".
Moisture correction table for dibutyl phthalate absorption (anhydrous)

IR determination
By means of IR spectroscopy it is possible to ascertain the different kinds of SiOH groups (isolated, bridged, + H20). To determine the intensities of the different silanol groups, the silicas are subjected to measurement in the form of powder layers. The absorbance values of the different silanol groups are divided (standardized) by the absorbance value of the SiO combination vibration band at 1870 cm"1.
The IR-spectroscopic determination takes place by means of a Bruker IFS 85 FR-IR spectrometer. Measurement takes place using a transparent NaCl monocrystal disc (round d=25 mm, h=5 mm) from K.Korth, Kiel, Germany, a 0.5 mm Teflon spacer and a mount for the discs. The spacer is placed on one clean, polished transparent NaCl monocrystal disc. The sample material is dusted on between the spacer and is covered with a further clean, polished transparent NaCl monocrystal disc; there must be no air bubbles included. The two transparent NaCl monocrystal discs with the powder layer are clamped into the sample mount. The sample mount is brought into the IR beam path and the sample chamber is closed. Prior to the measurement, the sample chamber is flushed with air cleaned to remove steam and carbon dioxide. In the alignment mode an "Align" is carried out, and measurement is commenced.
Measurement is carried out using the following parameters:
Resolution: 2 cm-1
Scanner'speed: 6; 10.51 Hz
Measuring range: 4500 cm"1 to 100 cm"1

Apodization function: triangular
Number of scans: 128
The spectrum is expressed, in the wavenumber range from 4000 to 1400 cm"1, in continuous wavenumbers.
The SiOHiSOiated absorbance ratio is determined as follows
(Figure 1):
First of all, two baselines are set. This is done by applying two tangents to the absorption plot. The first tangent (1st baseline) touches the absorption plot first in the region from 4000 cm"1 to 3800 cm"1 and secondly in the region from 3000 cm"1 to 2100 cm"1. It should be ensured that the tangent does not intersect the absorption plot either in the region from 4000 cm"1 to 3800 cm"1 or in the region from 3000 cm"1 to 2100 cm-1. The second tangent (2nd baseline) touches the absorption plot first in the region from 2200 cm"1 to 2000 cm"1 and secondly in the region from 1850 cm"1 to 1650 cm"1. It should be ensured that the tangent does not intersect the absorption plot either in the region from 2200 cm"1 to 2000 cm"1 or in the region from 1850 cm"1 to 1650 cm"1.
After the baselines have been set, a perpendicular line is taken down from the maximum of the bands in question (3750 and 1870 cm"1) to the respective baseline, and a measurement is made with the respective heights from the maximum to the baseline, in mm. A ratio is formed as follows:
height from maximum to base line in mm at 3750 cm '"
Absorption ratio (5iOHjM.,t J«-—;—; r ; ; — ; TTT^T.
J height from maximum to base line in mm at 1870 cm -
For each sample six IR spectra are recorded, measurement taking place in each case with new sample

material. Each IR spectrum is evaluated five times in accordance with the procedure described above. The absorbance ratio (SiOiSOiated) is reported, finally, as the average value of all the evaluations.
Determining the contact angle
The contact angle is determined as described in W. T. Yen, R. S • Chahal, T. Salman, Can. Met. Quart. , Vol. 12, No. 3, 1973.
Determining the silanol group density
First of all the moisture content of the silica sample is determined in accordance with the section "Determining the Moisture Content or Loss on Drying". Thereafter 2 - 4 g of the sample (to an accuracy of 1 mg) are transferred to a pressure-tight glass apparatus (glass, flask with dropping funnel) with a pressure measurement means attached. In this apparatus it is dried under reduced pressure (
calculated as follows:
„., 7 j silanol group concentration
Silanol group density = s—
BET surface area
Determining the particle size distribution by means of laser diffraction
The particle distribution is determined in accordance with the principle of laser diffraction on a laser diffractometer (Horiba, LA-920).
First of all the silica sample is dispersed in 100 ml of water without additional dispersing additives in a 150 ml glass beaker (diameter: 6 cm) in such a way as to give a dispersion having a weight fraction of 1% by weight Si02. This dispersion is then dispersed using an ultrasonic probe (Dr. Hielscher UP400s, Sonotrode H7) for a duration of 5 minutes intensely (300 W, without pulsing). For this purpose the ultrasound probe is to be mounted such that its lower end is immersed to a distance of approximately 1 cm above the base of the glass beaker. Immediately following the dispersing operation the particle size distribution of a sample of the ultrasonicated dispersion is determined using a laser diffractometer (Horiba LA-920). For the evaluation, using the standard software supplied with the Horiba LA-920, a refractive index of 1.09 should be selected.
All measurements take place at room temperature. The particle size distribution and also the relevant variables such as, for example, the particle size d90 are automatically calculated and depicted in graph form by the instrument. Attention should be paid to the notes in the operating instructions.

Determining the modified tapped density
With the "conventional" tapped density determination of DIN EN ISO 787-11, the result can be falsified by the fact that the silica has already undergone preliminary compaction in the course, for example, of being packed. In order to rule this out, a "modified tapped density" is determined for the silicas of the invention.
A porcelain suction filter (nominal size 110, diameter = 12 cm, height = 5.5 cm) fitted with a circular filter (e.g. type 598, Schleicher + Schull) is filled loosely with silica to approximately 1 cm from the top edge, and is covered with elastic film (Parafilm®). The shape and dimensions of the elastic film are to be selected such that it finishes very closely or completely flush with the edge of the porcelain suction filter unit. The unit is mounted on a suction bottle and then a vacuum of -0.7 bar is applied for a period of 5 minutes. In the course of this operation, the silica is compacted uniformly by virtue of the film under suction. Then air is cautiously readmitted and the resulting silica plaque is removed from the filter unit by being tipped forcefully into a porcelain dish.
The slightly precomminuted material is redispersed uniformly (in the manner of a silica/air aerosol) via a centrifugal mill (ZM1, Retsch, 0.5 mm screen insert, speed setting 1, without cyclone, without internal funnel insert) with an internal collecting dish (the silica (starting material) is introduced slowly spatula by spatula - into the mill feed; the internal product collection dish should never become completely full) . During this operation the power consumption of the mill should not exceed 3 amperes. This operation is less a conventional grinding than a defined loosening of the silica structure (of air-jet-milled silicas, for example), since the energy input here is substantially

weaker than in the case of jet milling.
5 g of the resulting material are weighed out to an accuracy of 0.1 g into the 250 ml volumetric cylinder of the jolting volumeter (STAV 2003 from Engelsmann). In a method based on DIN ISO 787-11, after jolting 1250 times, the resulting volume of the silica, in ml, is read off on the scale.

The examples below are intended to illustrate the invention without restricting its scope.
The water glass and the sulphuric acid used at various points in the directions of the examples below are characterized as follows:
Water glass: density 1.348 kg/1, 27.0% by weight
Si02, 8.05% by weight Na20
Sulphuric acid: density 1.83 kg/1, 94% by weight
Example 1:
A 2m3 precipitating vessel (diameter 160 cm) with inclined base, MIG inclined-blade stirrer system and Ekato fluid shear turbine is charged with 1679 1 of deionized water and this initial charge is heated to 92°C. After the temperature has been reached, and over a period of 100 minutes, water glass is metered in at a rate of 3.93 kg/min, and sulphuric acid is metered in at a rate of 0.526 kg/min with stirring. The rate of metering of sulphuric acid must be corrected if appropriate so that during the entire precipitation time a pH of 8.5 is maintained. Thereafter the water glass feed is shut off, with retention of the

temperature, and with the same rate of metering of sulphuric acid the precipitation suspension is acidified to a pH of 3. The precipitation suspension has a solids content of 54 g/1.
The resulting suspension is filtered with a membrane filter press and the filtercake is washed with deionized water until the wash water is found to have a conductivity of Before drying by means of a spray drier, the filtercake is redispersed with deionized water to a solids content of 8% - 13%, during which it must be ensured that it is not exposed to any strong shearing forces. The metering of the liquefied filtercake into the spray drier takes place in such a way that the temperature measured at the drier outlet is approximately 150°C.
Subsequently the material is treated in a fluidized-bed reactor (expanded fluidized bed height approximately 1.5 m, fluidized-bed diameter approximately 0,5 m). For this purpose the following conditions should be observed:
First of all, 30 kg of the spray-dried powder are introduced into the fluidized-bed reactor with fluidizing base. The fluidizing base is traversed by a gas mixture comprising dry nitrogen and dry air. These two gases are metered prior to their entry into the reactor in such a way that a resulting oxygen content of 6% by volume is not exceeded and in such a way as to give a fluidization velocity in the reactor of 0.05 m/s. The reactor is then heated from room temperature to 450°C. The flow rates of the fluidizing gas should be regulated during the heating phase such that the fluidization velocity in the reactor remains constant at 0.05 m/s.

After 450°C has been reached, a preheated gas mixture of steam, nitrogen and air is fed into the reactor for a period of 3 hours. The three components are mixed so as to set a steam concentration of 50% and an oxygen content of 3%. Volumes of nitrogen and of air are adapted so that, again, a fluid gas velocity of 0.05 m/s comes about.
Thereafter the addition of steam is interrupted. Nitrogen and air volumes are adapted so as to result, again, in a fluidization velocity of 0.05 m/s and an oxygen content of approximately 6%.
After at least five times the volume of dry process gas has been passed through the fluidized-bed reactor, after the interruption of the steam supply, the product is cooled to room temperature in a dry process gas atmosphere. Cooling takes place with fluidization in a nitrogen/air mixture in which the oxygen content is approximately 6%. In the cooling phase, particular care should be taken to ensure that at this point there is no longer any steam present.
Following the surface treatment in the fluidized bed, the material is ground on a fluid-bed opposed-jet mill AFG 50 from Alpine. The chemicophysical data of Example 1 are listed in Table 1 and an IR spectrum is depicted in Figure 2.
Example 2
A 2m3 precipitating vessel (diameter 160 cm) with inclined base, MIG inclined-blade stirrer system and Ekato fluid shear turbine is charged with 1680 1 of deionized water and this initial charge is heated to 92°C. After the temperature has been reached, and over a period of 100 minutes, waterglass is metered in at a

rate of 3-93 kg/min, and sulphuric acid is metered in, at a rate of 0.526 kg/min with stirring. The rate of metering of sulphuric ,acid must be corrected if appropriate so that during the entire precipitation time a pH of 8.5 is maintained. Thereafter the waterglass feed is shut off, and with the same rate of metering of sulphuric acid, and still at 92°C, the precipitation suspension is acidifed to a pH of 3. The precipitation suspension at this point has a solids content of 54 g/1.
The resulting suspension is filtered with a membrane filter press and the filtercake is washed with deionized water until the wash water is found to have a conductivity of Before drying by means of a spray drier, the filtercake is redispersed with deionized water to a solids content of 8%-13%, during which it must be ensured that it is not exposed to any shearing forces. The metering of the liquefied filtercake into the spray drier takes place in such a way that the temperature measured at the drier outlet is approximately 150°C.
The spray-dried material is subjected to preliminary grinding via a mechanical beater mill to an average particle size of 10-12 μm. After this preliminary grinding, the material is subjected to ultrafine grinding on a steam-operated fluid-bed opposed-jet mill at a superatmospheric pressure of 38 bar. Details of the grinding system (mill) and of the grinding method used can be found in DE 10 2006 048 850.4 and also in the description which follows. The entire content of the three cited patent applications is hereby incorporated explicitly to become part of the present specification.

In preparation for actual grinding with superheated steam, a fluid-bed opposed-jet mill as shown in Figure 7, with an integrated dynamic pneumatic classifier as shown in Figures 8a and 8b, is first heated via the two heating nozzles 5a (of which only one is shown in Figure 7) , which are charged with hot compressed air at 10 bar and 160°C, until the mill exit temperature is approximately 105°C.
For the separation of the ground material, a filter system is connected downstream of the mill (but not shown in Figure 7), its filter housing being heated in the lower third indirectly via attached heating coils by means of 6 bar saturated steam, likewise for the purpose of preventing condensation. All of the apparatus surfaces in the region of the mill, the separating filter, and the supply lines for steam and hot compressed air have special insulation.
After the heating temperature has been reached, the supply of hot compressed air to the heating nozzles is shut off and the charging of the three grinding nozzles with the grinding medium of superheated steam (37.9 bar (abs), 325°C) is commenced.
In order to protect the filter means used in the separating filter, and also in order to set a defined level of residual water in the ground material (see Table 1), water is introduced in the starting phase and during grinding, into the grinding chamber of the mill, via a two-fluid nozzle operated with compressed air, as a function of the mill exit temperature.
The following mill configurations and operating parameters are used: grinding nozzle diameters 2.5 mm, nozzle type = Laval, number of nozzles 3 units; internal mill pressure = 1.306 bar (abs.), grinding medium entry pressure = 37.9 bar (abs.),

grinding medium entry temperature = 325°C, grinding medium mill exit temperature = 149.8°C, classifier speed = 3500 min"1, classifier flow = 54.5 A%, outlet port diameter (immersed pipe diameter) = 100 mm.
Product feed is commenced when the abovementioned operational parameters are constant. The feed quantity is regulated as a function of the classifier flow which comes about. The classifier flow regulates the feed quantity such that it is not possible to exceed approximately 70% of the nominal flow.
The introduction member (4) which functions here is a speed-regulated bucket wheel which meters the feed material from a reservoir container via a cyclical lock, which serves as a barometric endpoint, into the grinding chamber, which is at superatmospheric pressure.
The coarse material is comminuted in the expanding steam jets (grinding gas). Together with the depressurized grinding gas, the product particles ascend in the centre of the mill vessel to the classifying wheel. Depending on the classifier speed and grinding steam quantity which have been set, the particles whose fineness is sufficient enter along with the grinding steam into the fines exit, and from there they pass into the downstream separating system, while particles which are too coarse pass back into the grinding zone and are subjected to a repeat comminution. The discharge of the separated fines from the separation filter into the subsequent silo storage and bagging operation takes place by means of a bucket-wheel lock.
The grinding pressure of the grinding gas that obtains at the grinding nozzles, and the resulting volume of grinding gas, in conjunction with the speed of the

dynamic paddle wheel classifier, determine the fineness of the particle-size distribution function and also the upper particle-size limit.
The material is ground to the particle size defined in Table 1 by the d9o value and by the fraction of particles Subsequently the material is treated in a fluidized-bed reactor. For this purpose the following conditions should be observed:
First of all, 5 kg of the powder are introduced into the fluidized-bed reactor with fluidizing base. The fluidizing base is traversed by a gas mixture comprising dry nitrogen and dry air. These two gases are metered prior to their entry into the reactor in such a way that a resulting oxygen content of 6% by volume is not exceeded and in such a way as to give a fluidization velocity in the reactor of 0.05 m/s. The reactor is then heated from room temperature to 600°C. The flows of the fluidizing gas should be regulated in the heating phase such that the fluidization velocity in the reactor remains constant at 0.05 m/s.
After 600°C has been reached, a preheated gas mixture of steam and nitrogen is fed into the reactor for a period of 2 hours. The two components are mixed so as to set a steam concentration of 90% and a nitrogen content of 10%. The gas volumes are adapted so that, again, a fluid gas velocity of 0.05 m/s comes about.
Thereafter the addition of steam is interrupted, and for 30 minutes pure nitrogen at 600°C is passed through the fluidized-bed reactor.
The material is then cooled to room temperature in a stream of dry nitrogen and is discharged from the

reactor. In the cooling phase, particular care should be taken to ensure that at this point there is no longer any steam present.
The chemicophysical data of Example 2 are listed in Table 1 and an IR spectrum is depicted in Figure 6.
Comparative Examples 1-2
The following commercially available silicas were analysed (see Table 1) and incorporated into sealants in Example 4:
Comparative Example 1: Siloa™ 72 X (Rhodia AG)
Comparative Example 2: Ultrasil® VN 3 (Degussa AG)

Example 3: Performance -bests
3.1 Preparation of acetate-crosslinking RTV-1K
silicone sealants with precipitated silicas
The amounts required for preparing the formulation below are indicated in Table 2. In the course of preparation, cooling with mains water should be carried out so that the formulation does not undergo warming substantially beyond room temperature. Preparation takes place at room temperature and at a relative humidity of 40% to 60%.
A planetary dissolver (from H. Linden, type LPMD 2SP) equipped with a 2 1 stirring vessel with jacket, cooling water connection and independently controllable planetary drive and a dissolver drive is charged with silicone polymer, plasticizer (silicone oil) and crosslinker and this initial charge, is homogenized for 1 minute at a speed of 50 min"1 (planetary drive) and 500 min"1 (dissolver drive) . Then the catalyst is added and the batch is homogenized for 15 minutes under an N2 atmosphere with the same planetary and dissolver drive speeds. Thereafter the stabilizer and the silica are incorporated, again at the same speeds. As soon as the silica is fully wetted a vacuum of approximately 200 mbar is applied and dispersion takes place for 10 minutes at 100 min"1 of the planetary stirrer mechanism and 2000 min-1 of the dissolver. Immediately after the end of the dispersing operation the stirring vessel is flushed with nitrogen. Using a drum press, the sealant is dispensed as quickly as possible into aluminium tubes (cartridges).
3.2 Production of vulcanizates from RTV-1K
In order to examine the performance properties of RTV-1K silicone sealants in which the silicas of the invention are used it is necessary to produce

vulcanizates from the sealants prepared above. These vulcanizates are processed into test specimens. For this purpose, first of all the silicone sealant is applied to a smooth support plate in a sufficient amount and is coated with a coating bar (slot height: 2 mm) to form a strip 2 mm in height, approximately 80 mm in width and with a length of approximately 300 mm. Care should be taken to ensure that no air bubbles are formed. The shapes needed for the respective test are then punched out from this silicone strip. The support plate ought to be made from polypropylene, polyethylene, Teflon or another plastic from which the vulcanized sealant is readily detachable.
The silicone strips are stored for 7 days for complete curing and then for at least 2 days under standard conditions (23°C, 50% relative humidity).

3.3 Determining the rheological properties and the storage stability of RTV-1K sealants
The sealants prepared in accordance with Example 3, section 1, "Preparation of Acetate-Crosslinking RTV-1K Silicone Sealants with Precipitated Silicas" are stored prior to testing for at least 24 hours in a controlled-climate chamber at 23°C/50% relative humidity.
To test the storage stability of the sealants, two tubes are stored for 35 days in a cont rolled-climate chamber at 23°C at 50% relative humidity and are tested after storage periods respectively of 1, 7, 14, 21, 28 and 35 days. Additionally, two further tubes are stored in a forced-air oven at 50°C for 35 days and likewise tested after 1, 7, 14, 21, 28 and 35 days of storage.
The rheological properties are determined using a Haake RheoStress 1 rheometer (controlled via PC using the RheoWin Pro program). The operation of the instrument and of the software is described in detail in the Haake operating instructions. For the measurement it is necessary to use a die having a diameter of 35 mm, and the measuring-plate attachment MPC 35. Measurement is conducted under the following conditions:
Slot distance between die and
measuring-plate attachment: 0.5 mm
Measurement temperature: 23°C
Measurement range (shear rate) 0-10 1/s
Number of measurement points: 400
The measurement points are plotted in a diagram which shows the shear rate y on the x axis and the shear stress x on the y axis. At a shear rate of 10 1/s the shear stress is read off and from this figure the viscosity r| at 10 1/s is calculated using r| = x/y. Two tubes are measured, with at least three measurements

being carried out per tube. From the six individual results the highest and lowest values are discarded. The remaining four results are used to calculate the average value.
For the determination of the yield point the Casson model is used. The data basis for calculating the Casson flow curve is the range from 0.2 to 2 1/s from the shear rate/shear stress diagram. The following relationship is defined:

The value on the y axis, at which it intersects the flow curve calculated by the method of Casson is reported as the Casson yield point.
The determination both of the viscosity at 10 1/s and
of the Casson yield point is made automatically under
the conditions indicated above by the RheoWin Pro
software.
3.4 Determining the tensile strength and the breaking extension of vulcanized silicone rubber
This determination is made by a method based on DIN 53504 and is used to determine the tensile strength and the breaking elongation of specimens of defined shape made from elastomers, when the specimens are extended at a constant speed until they rupture. The tensile strength and the breaking extension in this case are defined as follows:
The tensile strength 5max is the ratio of the measured maximum force Fmax to the initial cross-section A 0 of the specimen.

The breaking elongation sR is the ratio of the length change L A measured at the moment of rupture to the original measurement length L 0 of the specimen.
Measurement is carried out on a tensile testing machine (Zwick/Roell, type Z010), ensuring that the preselected maximum force is variable, that the clamping device holds the test specimen firmly without mechanical damage even at high stretch, and holds the centre piece of the test specimen at the set measurement length L 0, without mechanical damage, even at high stretch, and that the spring pressure on the clamping jaws of the fine extension gauge is adjustable.
The standard dumbbells S 1 described in Figure 4 are to be used.
The corresponding test specimens are punched from the vulcanized strip 2 mm thick using a punching iron for S 1 standard dumbbells, and these test specimens are stored for at least 24 h under standard conditions (23°C, 50% relative humidity) before testing.
4-6 specimens are to be tested at a temperature of (23 ± 2)°C. Prior to the tensile test, a measurement should be made of the thickness d and the width b of the specimens. On clamping, the standard dumbbells should be clamped in centrally between the two clamping jaws. The distance between the clamping jaws is L = 50 mm. The fine extension gauge should be set to a measurement length L 0 of 25 mm and should be fixed centrally between the clamping jaws. The displays should stand at zero. The rate of advance of the pulling bracket is v = 500 mm/min. The force Fmax and the length change L A at rupture are recorded. From these figures the tensile strength and breaking extension are calculated as shown below and are reported as the average value of the individual

This determination is carried out by a method based on ASTM D 624 B. The tear propagation test on elastomers is used to determine the resistance presented by an incised sample to the continuation of a tear.
The tear propagation resistance of an elastomer is dependent on its formulating constituents and on its processing, on the vulcanization, and on the testing speed. The effect tested is the influence of the reinforcing filler on the tear propagation resistance of the vulcanizates. The measurement is carried out on

a tensile testing machine (from Zwick/Roell, type Z010), ensuring that the preselected maximum force is variable and that the clamping device holds the test specimen firmly, without mechanical damage, even at high extension.
Test specimens (Figure 5) are cut from the vulcanized silicone strips 2 mm thick, using a punching iron compliant with ASTM D 624 B and are stored for at least 24 h under standard conditions (23°C, 50% relative humidity) before testing.
At the vertex of the inside radius, the punching iron has a blade with which a slot 0.5 mm ±0.05 mm in depth is incised at this point in the course of punching.
4-6 specimens should be tested, at a temperature of (23 ± 2) °C. The specimens must be stored at the test temperature for 3 hours prior to testing. The thickness a of the specimens should be determined to ± 0.01 mm prior to testing, using a thickness gauge. The sample is clamped into the clamping brackets of the tensile testing machine and ruptured at a rate of advance of v = 500 mm/min, ensuring that the slot is on the left-hand side from the tester.
The two test specimens with the highest and lowest values are disregarded for the evaluation. The tear propagation resistance 5wl in [N/mm] is calculated from the variables Fmaxl (maximum force in [N] ) and also al (thickness in [mm] ) and is reported as the average value of the remaining individual measurements:
Tear propagation resistance 8w} =

3.6 Assessing the results
The consistency of silicone rubber formulations into which the silicas of the invention have been incorporated is assessed using the measurement results for the Casson yield point and for the viscosity at a shear rate of 1/10 (Table 3).
By consistency is meant the rheological behaviour of an RTV-IK silicone sealant. Consistency is said to be good if silicone rubber applied to a vertical surface adheres to that surface without running over 24 h in the course of curing. Adequate consistency can be recognized from a viscosity of > 100 Pas and a yield point of > 90 Pa. The values for the Casson yield point for the silicone rubber formulations of Examples 3a to 3d clearly show that the yield point of silicone rubber formulation comprising the inventive silica of Example 1, with a figure of 100 Pas, is much higher than in the case of the formulations with the comparison silicas. In the case of the silicone rubber formulation 3b with the inventive silica of Example 2, with a Casson yield point of 260, this effect is very much more pronounced. In other words, silicone rubber formulations into which the inventive silicas have been incorporated have a better consistency on the basis of the particular properties of the inventive silicas. Formulations of this kind remain in the form in which they have been applied without showing any tendency to run. This is also confirmed by the viscosity values. Thus, the inventive silicas display equal or in Example 2 markedly improved, i.e. higher viscosity than the comparative examples.
The mechanical stability (tensile strength and tear
propagation resistance) and also the flexibility
(breaking extension) of the cured silicone rubber
formulations 3a to 3d can be assessed through their

behaviour under tensile load under different conditions (Table 4) .
The results of measurement for the mechanical properties can be interpreted as follows: for the silicone rubber formulation 3d containing the silica of comparative example 2 it is not possible at all to produce silicone vulcanizates (complete curing directly after preparation) , and accordingly it is not possible to measure the mechanical properties. The measurements for the inventive silica show that the minimum requirements concerning the mechanical stability (tensile strength and tear propagation resistance) and also the flexibility (breaking extension) of the cured silicone rubber formulations are met. In comparison to comparative Example 1 (silicone rubber formulation 3c), the inventive silicas of Examples 1 and 2 in the silicone rubber formulations 3a and 3b leads to better mechanical stability (higher values for tensile strength and tear propagation resistance) and also the same or better flexibility (equal or higher values for breaking extension).
The storage stability, i.e. the change in rheological properties such as yield point and viscosity and also the negative cure behaviour in the tube over time, is shown in Tables 5 and 6. Consideration was given here both to the storage at room temperature and to the storage at elevated temperature (50°C). It is clearly apparent that the silicone rubber formulations 3a and 3b with the inventive silicas of Examples 1 and 2 undergo no change, or no substantial change, in their rheological properties, viscosity of 1/10 shear rate and Casson yield point over the storage period under investigation (namely 35 days). In other words, the effective thickening and processing properties (such as extrudability, for example) are still present even after storage under the stated conditions, without the

sealant having undergone preliminary crossiinking or full vulcanization while still in the tube.
In contrast, for Comparative Example 1, a negative change in the rheological properties is observed within the first 21 days. In comparison to the initial values, the viscosity and yield point increase significantly, which points to premature crosslinking of the sealant. Between day 21 and day 28, the sample cures while still in the tube, and hence is no longer processable. The situation is similar after storage at elevated temperature. There, the premature curing of Comparative Example 1 occurs as early as between day 2 and day 7. Even worse is the precipitated silica Ultrasil® VN 3. Here, the silicone rubber has cured immediately after preparation and can no longer even be dispensed into the cartridge (tube) and this silicone rubber compound is not suitable for RTV-1K applications.

List of reference numerals for Figures 7, 8a and 8b
1 jet mill
2 cylindrical housing
3 grinding chamber
4 feed material for grinding
5 grinding jet inlet
5a heating nozzles
6 product outlet
7 pneumatic classifier
8 classifying wheel
9 Inlet opening or inlet nozzle
10 grinding jet
11 heat source
12 heat source
13 supply pipe
14 thermal insulation jacket
15 Inlet
16 outlet
17 centre of grinding chamber
18 reservoir or generating means
19 pipe installations
20 exit port (immersed pipe)
21 classifier housing
22 top housing part
23 Bottom housing part
24 circumferential flange
25 circumferential flange
26 articulated joint
27 Arrow
28 inspection chamber housing
28a carrying arms
29 discharge cone
30 Flange
31 Flange
32 coverplate
33 coverplate
34 Paddle
35 classifying wheel shaft

35a pivot bearing
36 top machined plates
37 Bottom machined plate
38 end section of housing
39 product feed port
40 axis of rotation
41 exit chamber
42 top coverplate
4 3 removable lid
4 4 carrying arms
45 conical annular housing
46 intake filter
47 perforated plate
48 fines discharge pipe
4 9 deflection cone
50 classifying air entry coil
51 coarse product charge
52 Flange
53 Flange
54 dispersion zone
55 flange machined (bevelled) at the inside edge, and lining
56 replaceable protection pipe
57 replaceable protection pipe
58 fines exit/outlet
59 ring of paddles
60

Claims:
1. Precipitated silica characterized in that it has an SiOH1soilated absorbance ratio of greater than or equal to 1.
3. Precipitated silica according to Claim 1, characterized in that the silanol group density is 0.5 to 3.5 SiOH/nm2.
3. Precipitated silica according to Claim 1 or 2, characterized in that the modified tapped density is less than or equal to 70 g/1.
4. Precipitated silica according to either of Claims 1 and 3, characterized in that it has the following properties:
BET surface area 50 - 600 m2/g
CTAB surface area 50 - 350 m2/g
DBP (anhydrous) 150 - 400 g/100 g
5. Precipitated silica according to any one of Claims 1 to 4, characterized in that 5% to 100% of the particles in the volume-based particle distribution curve are 6. Precipitated silica according to any one of Claims 1 to 5, characterized in that the d90 value is not greater than 0.001 - 10 jam.
7. Precipitated silica according to any one of Claims 1 to 6, characterized in that the particle distribution curve is bimodal.
8. Precipitated silica according to any one of Claims 1 to 1, characterized in that the loss on ignition is 0.1% - 3.0% by weight.

Precipitated silica according to any one of Claims 1 to 8, characterized in that the loss on drying is 0.1% - 3.0% by weight.
Precipitated silica according to any one of Claims 1 to 9, characterized in that the pH is 4-8.
Precipitated silica according to any one of Claims 1 to 10, characterized in that it is a hydrophilic precipitated silica.
Process for preparing a silica according to any one of Claims 1 to 11, characterized in that the process comprises the following steps:
1. reacting at least one silicate with at least one acidifier
2. filtering and washing the resulting silica
3. drying the resulting silica or filtercake
4. heat-treating the dried silica.
Process according to Claim 12, characterized in that step 1 comprises the following substeps:
la preparing an initial charge of water or of water and at least one silicate and/or a silicate solution, the pH of the resulting initial charge being preferably between 5 and 10 and the temperature of the initial charge being preferably between 80 and 100°C.
lb metering at least one silicate and/or a

silicate solution and at least one acidifier into the initial charge from substep la) with stirring at 80 to 100°C until the solids content of the precipitation suspension reaches a level which leads to the solids content which is to be reached in substep lc).
1c adding an acidifier at a temperature of the precipitation suspension of 80 to 100°C, so that the pH of the precipitation suspension is lowered to 2 to 6 and the solids content of the precipitation suspension at the end of this substep is between 30 and 70 g/1.
Process according to Claim 13, characterized in that for the period of substep lb) the pH is held constantly at a level between 7 and 10.
Process according to any one of Claims 12 to 14, characterized in that the silicas are ground in a step 3a, i.e. between steps 3 and 4, or in a step 5, i.e. after step 4, or both in step 3a, i.e. between steps 3 and 4, and in step 5, i.e. after step 4.
Process according to Claim 15, characterized in that the grinding parameters are selected such that the ground product in the range Process according to Claim 15 or 16, characterized in that grinding is carried out using a jet mill, preferably a fluid-bed opposed-jet mill.

Process according to Claim 17, characterized in that the fluid-bed opposed-jet mill is operated with steam as operational medium.
Process according to any one of Claims 15 to 18, characterized in that grinding is carried out by means of a grinding system (grinding apparatus), preferably a grinding system comprising a jet mill, and in that in the grinding phase the mill is operated with an operational medium selected from the group consisting of gas and/or vapour, preferably steam, and/or a gas comprising steam, and in that in a heating phase, i.e. before the actual operation with the operational medium, the grinding chamber is heated such that the temperature in the grinding chamber and/or at the mill outlet is higher than the dew point of the vapour and/or operational medium.
Process according to any one of Claims 12 to 19, characterized in that the heat treatment of the silica in step 4 is carried out in a fluidized-bed, fluid-bed or rotary-tube reactor.
Process according to Claim 20, characterized in that a fluidized-bed reactor is used and in that 'the following substeps are carried out:
4a introducing the silica into the fluidized-bed reactor,
4b preheating the reactor to 300 to 800°C, the
reactor being traversed at the same time by
a flow of inert gas and/or nitrogen/air
mixture in such a way as to produce a
•fluidization velocity of 0.02 to 0.06 m/s,
4c feeding in a gas mixture I comprising steam

and an inert gas, or a gas mixture II comprising steam, an inert gas and air, at 300 to 800°C for a period of 0.25 to 6 h, the gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and the gas mixtures I and II having a steam concentration of 10 to 95% by volume and in the case of gas mixture II an oxygen content of 0.01% to 21% by volume,
4d interrupting the addition of steam and expelling the steam by means of an inert gas, nitrogen for example, and/or of an inert gas/air mixture at 300 to 800°C, the gas or gas mixture traversing the reactor with a fluidization velocity of 0.02 to 0.06 m/s, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume,
4e cooling the heat-treated silica to room temperature in a dry process atmosphere, and, if using an inert gas/air mixture, said mixture having an oxygen content of 0.01% to 21% by volume.
22. Precipitated silica characterized in that it is
obtainable by the process according to any one of
Claims 12 to 21.
23. Use of precipitated silicas according to any one of Claims 1 to 11 or 22 for producing sealants.
24. Use according to Claim 23, characterized in that the sealant is RTV-1K silicone rubber or a silicone sealant of the various crosslinking systems (acetoxy-crossiinking, alkoxy-crossiinking

and/or oxime-crosslinking).
25. Sealant comprising at least one precipitated
silica according to any one of Claims 1 to 11 or
22.
26. Sealant according to Claim 25, characterized in
that the sealant is RTV-1K silicone rubber or a
silicone sealant of the various crosslinking
systems (acetoxy-crosslinking, alkoxy-crosslinking
and/or oxime-crosslinking).
27. Use of the sealant according to Claim 25 or 26, in
the building industry as a joint-sealant, in the
automotive industry as an adhesive and sealant
and/or as a coating material for textile fabric.

Documents:

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Patent Number

279096

Indian Patent Application Number

1098/CHE/2007

PG Journal Number

02/2017

Publication Date

13-Jan-2017

Grant Date

11-Jan-2017

Date of Filing

25-May-2007

Name of Patentee

EVONIK DEGUSSA GMBH

Applicant Address

RELLINGHAUSER STRASSE 1-11, 45128 ESSEN

Inventors:

#	Inventor's Name	Inventor's Address
1	PANZ, CHRISTIAN	WESER STRASSE 39, 50389 WESSELING-BERZDORF, GERMANY
2	OBLADEN, HELGA	BITTERFELDER STRASSE 28, 50321 BRUHL, GERMANY
3	ALLERDISSE, RENE	RUTTERSWEG 17, 53332 BORNHEIM, GERMANY
4	MEIER, KARL	ROISDORFER WEG 5, 53347 ALFTER, GERMANY
5	RUF, MARKUS	GELTORFSTRASSE 44A, 53347 ALFTER-WITTERSCHLICK, GERMANY
6	KEMPF, MICHAEL	KORNBLUMENWEG 9, 63517 RODENBACH, GERMANY
7	SCHOLZ, MARIO	WEINBERGSTRASSE 35, 63584 GRUNDAU, GERMANY
8	KUHN, DIETER	SUDRING 22, 63517 RODENBACH, GERMANY

PCT International Classification Number

C01B33/193

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	102006024591.1	2006-05-26	Germany
2	102007004757.8	2007-01-31	Germany