WO2010022544A1

WO2010022544A1 - Process for preparing precipitated silicas, precipitated silicas and their use

Info

Publication number: WO2010022544A1
Application number: PCT/CN2008/001556
Authority: WO
Inventors: Gottlieb-Georg Lindner; Yihmeng Simon Shen
Original assignee: Evonik Degussa Gmbh; Evonik Degussa (China) Co., Ltd
Priority date: 2008-09-01
Filing date: 2008-09-01
Publication date: 2010-03-04
Also published as: CN102137813A

Abstract

Provided are a process for preparing precipitated silicas, precipitated silicas and their use as a filler in an elastomer. The process comprises the steps of: preparing initial charge; simultaneously metering silicate and acidifying agent into the initial charge and reacidifying the precipitation suspension to a pH of 2.5 to 6.0, wherein the silicate used has metal oxide of 4wt% ~7wt% and silicon dioxide of 14wt% ~23wt% and the acidifying agent is selected from the group consisting of sulphuric acid of 90wt%~98.5wt%, hydrochloric acid of 34wt%~42.7wt%, nitric acid of 60wt%~68wt%, phosphoric acid of 80wt%~100wt%, carbonic acid or CO₂ and sodium hydrogen sulphite or SO₂.

Description

PROCESS FOR PREPARING PRECIPITATED SILICAS, PRECIPITATED SILICAS AND THEIR USE

The invention relates to a new process for preparing precipitated silica, to innovative precipitated silicas, and to their use.

Precipitated silicas are speciality chemicals whose properties can be tailored to the desired fields of application. This diversity and variability has resulted in precipitated silicas now being used in numerous fields of application. Examples thereof are identified in Ullmann's Encyclopedia of Industrial Chemistry, Wiley- VCH Verlag GmbH & Co. KGaA, Online Edition, DOI (Digital Object Identifier): 10.1002/14356007.a23_583.pub3, 2008, section 7.4.

The properties of precipitated silicas are determined by their preparation processes. The preparation process in turn may be subdivided roughly into the steps of precipitation and precipitation work-up.

For the precipitation there are a series of processes that are known. In one version, for example, a mineral acid is added to an initial charge of alkali metal silicate solution. This process can be varied, for example, by adding neutral salts of strong acids and strong bases, such as sodium chloride or sodium sulphate, for instance, to the alkali metal silicate solution.

A feature of other processes, which are practiced predominantly at present, is that the precipitation is carried out by simultaneous addition of both reaction components, i.e. the silicate solution and the acidifying agent, to an initial charge. This initial charge may be composed of water, but also of a portion of the silicate solution and/or electrolyte solutions. These preparation processes, with simultaneous addition of the reaction components, are frequently carried out at a constant pH or at a constant alkali number. Precipitation at constant alkali number means that the concentration of the freely available sodium ions in the reaction solution is held constant. As a result of the acid-base reactions during the precipitation of, for example, waterglass with sulphuric acid, for instance, sodium ions are released in the form of sodium sulphate, but sodium ions are also incorporated into the silicate agglomerates that form. Since these two reactions run independently of one another in kinetic terms, the course of precipitations at constant pH is different from that of alkali number-controlled precipitations. Also known are processes in which neither pH nor alkali number is held constant.

Another version of the precipitation reaction involves the use of growth nuclei. In this case, first of all, in a "pre-precipitation reaction", small silica particles are formed in an initial charge and then the precipitation is carried out by simultaneous addition of acid and silicate. In this case as well the precipitation itself may be controlled by holding the pH or the alkali number constant. Examples thereof are found in EP 0520862 Bl, EP 0670813 Bl, EP 0670814 Bl or EP 0917519 Bl. These processes are very time-consuming, since the "pre- precipititation" constitutes an additional process step.

Further precipitation control possibilities are ageing steps and/or variations of the feed rates of the reactants during the precipitation. Thus, for example, EP 1 764 344 Al describes a process in which different primary particles are formed as a result of different precipitation rates, with the consequence that, at the end of precipitation, a silica suspension is obtained in which the silica unites the properties of two different precipitated silicas with one another.

The process versions discussed above represent only a selection of the multiplicity of known precipitation processes. Besides the versions of the precipitation process there is an equally large number of versions of the work-up process, thereby resulting for the overall preparation process in a virtually innumerable diversity of processes. These processes in the majority of cases lead to highly specific products, but have the disadvantage that the products are so specific that they can be used in each case only for a decidedly limited field of application. This means in turn that the production rates per product fall and, consequently, that it is always necessary at frequent intervals to switch production, which in turn is time-consuming and cost-intensive.

A further disadvantage of many known processes for preparing precipitated silicas having specific properties is the complexity of the precipitation process. In some cases, for instance, two or more precipitations have to be carried out alongside one another, and the suspensions reunited thereafter, thereby adversely affecting the complexity and the cost of the equipment. The incorporation of holding steps, and the use of different precipitation rates, have negative impacts on the space-time yield and/or complicate the effort involved in control, thereby increasing the susceptibility to error.

An objective of the present invention, therefore, was to provide a simple process for preparing precipitated silicas that nevertheless allows the preparation of precipitated silicas which exhibit outstanding application properties across a broad application spectrum. In one specific object of the present invention the intention is that the precipitated silicas obtained by the new process ought to have good properties not only as a filler for tyres but also as a filler in mechanical rubber goods such as footwear soles, for example. Further objects, not stated explicitly, will become apparent from the overall context of the following description, examples and claims.

It is readily apparent that the requirements imposed on precipitated silicas to be used as a filler in car tyres are different from those imposed on precipitated silicas which are incorporated into footwear soles. For instance, the precipitated silicas in car tyres ought indeed to ensure effective reinforcement, but ought also to ensure a low rolling resistance, thereby allowing the fuel consumption of the vehicles to be lowered. In footwear soles, in turn, the precipitated silicas must exhibit primarily reinforcing properties, and must also guarantee the durability of the soles (abrasion). These properties are indeed similar to the requirements affecting the tyres, but because of the different compositions of the rubber compounds an equally good effect in both systems is not a given.

Following intensive research efforts it has now been found, surprisingly, that through deliberate selection of the reactants and deliberate control of the precipitation reaction it is possible to obtain precipitated silicas which achieve the objects identified above.

The present invention accordingly provides a process for preparing precipitated silicas which comprises the following steps: a) preparing an initial charge of water or an aqueous solution of an alkali metal silicate and/or alkaline earth metal silicate, b) simultaneously metering alkali metal silicate and/or alkaline earth metal silicate and acidifying agent into this initial charge with stirring at 80 to 100⁰C, c) reacidifying the precipitation suspension to a pH of 2.5 to 6.0, d) filtering, washing and drying, and which is characterized in that the alkali metal silicate and/or alkaline earth metal silicate used in steps a) and/or b) has an alkali metal oxide and/or alkaline earth metal oxide content in the range from 4% to 7% by weight and a silicon dioxide content in the range from 14% to 23% by weight, in that the acidifying agent used in step b) and/or c) is an acidifying agent selected from the group consisting of sulphuric acid having a concentration of 90% to 98.5% by weight, hydrochloric acid having a concentration of 34% to 42.7% by weight, nitric acid having a concentration of 60% to 68% by weight, phosphoric acid having a concentration of 80% to 100% by weight and also carbonic acid (or CO₂ gas) and sodium hydrogen sulphite (or SO₂ gas) in the correspondingly possible concentrations, and in that the pH of the precipitation suspension (measured at 60°C) drops in the course of precipitation by 1 % to 20%, based on the pH at the start of precipitation (measured at 6O⁰C).

Optionally it is possible between steps c) and d) for the resulting suspension to be afterstirred at 60 to 100°C for 1 to 90 minutes. Step b) may likewise be optionally interrupted for 1 to 60 minutes to allow the silica particles obtained to age, but typically this is not necessary.

The present invention further provides precipitated silicas obtainable by the process of the invention.

A final subject of the present invention is the use of the precipitated silicas of the invention as a filler in pneumatic tyres, tyre treads for summer tyres, winter tyres and all-year tyres, car tyres, tyres for utility vehicles, motorcycle tyres, tyre body parts, cable sheathing, hoses, drive belts, conveyor belts, roll coverings, footwear soles, gasket rings and damping elements, and also, more generally, MRG (mechanical rubber goods). The subject matter of the present invention is described below in detail. The terms "silicate solution" and also "alkali metal silicate and/or alkaline earth metal silicate solutions" are used synonymously.

In contrast to known processes such as in EP 1764344 Al, for example, where not only concentrated silicate solutions but also a concentrated acidifying agent are used, the process of the invention uses a silicate solution which - in comparison to that used in EP 1764344 Al - is more highly diluted by at least 10%, preferably at least 15%, more preferably at least 20%. The alkali metal oxide and/or alkaline earth metal oxide content of the silicate solution used in accordance with the invention is in the range from 4% to 7% by weight, preferably in the range from 5% to 6.5% by weight, more preferably from 5.5% to 6.5% by weight. The silicate solution used in the process of the invention is especially sodium silicate solution (waterglass) and/or potassium silicate solution.

The silicon dioxide content of the silicate solution used in accordance with the invention is 14% to 23% by weight, preferably 18% to 22% by weight, more preferably 20% to 22% by weight, and is therefore likewise lower by at least 10%, preferably at least 15%, more preferably at least 20% than in conventional processes. The modulus, i.e. the weight ratio of silicon dioxide to alkali metal oxide and/or alkaline earth metal oxide, in the silicate solution used in accordance with the invention is preferably 2.0 to 5.75, more preferably 2.5 to 4.5, very preferably 3 to 4, and with more particular preference 3.2 to 3.7.

At this point attention may be drawn to the fact that the literature also includes descriptions of processes in which dilute waterglass is used, examples being EP 0520862 Bl and EP 0670813 Bl. In those processes, however, both reactants, i.e. both the silicate solution and the acidifying agent are employed in the form of a dilute solution. Processes of that kind have the disadvantage that, owing to the generally higher dilution factor, they exhibit a lower space- time yield and a higher energy consumption - in comparative terms, the amount of water that must be heated is much greater. Furthermore, on the basis of that mode of preparation it is only possible to prepare silicas situated within a defined/narrow parameter band. In the process of the invention, in contrast, only the silicate solution is employed as a dilute solution, while the acidifying agent is used as a concentrated solution. As a result of this it is possible to realise a higher space-time yield and a lower energy consumption - in comparative terms, the amount of water that must be heated is less. Moreover, on the basis of this mode of preparation, a greater span of silicas can be prepared. Without being tied to any one theory, the inventors are of the view that when the acid (acids) is diluted, a dissociation equilibrium comes about in general within a very short time. The attainment of equilibrium in the case of a sodium silicate solution, in contrast, occurs over the course of several hours. As a result, and owing also to the different compositions of the waterglasses, different products are formed.

As acidifying agents it is preferred in the process of the invention to use concentrated mineral acids such as hydrochloric acid, sulphuric acid, nitric acid or phosphoric acid or CO₂. Concentrated acid means, in the case of hydrochloric acid, a concentration of 34% to 42.7% by weight, preferably 36% to 40% by weight; in the case of sulphuric acid, a concentration of 90% to 98.5% by weight, preferably 93% to 98.5% by weight and very preferably 96% to 98% by weight; in the case of nitric acid, a concentration of 60% to 68% by weight; and, in the case of phosphoric acid, a concentration of 80% to 100% by weight, preferably 80% to 90% by weight, more preferably 80% to 85% by weight.

The inventors have found, furthermore, that for the process of the invention it is essential that the pH of the precipitation suspension (measured at 6O⁰C) must fall in the course of precipitation by 1% to 20%, preferably 2% to 15%, more preferably 3% to 10%, very preferably 5% to 10%, based on the pH at the start of precipitation, i.e. at the beginning of the simultaneous addition of silicate solution and acid. "In the course of precipitation" means that the start point is defined by the beginning of the simultaneous addition of silicate solution and acid, and the end point is defined by the ending of the simultaneous addition of silicate solution and acid. Where ageing/maturation steps are to be carried out in the process of the invention, then the end point is taken as the point in time at which silicate solution and acid are added simultaneous for the last time in the process as a whole.

The starting pH of the precipitation is adjusted preferably in the range from 8 to 12, more preferably from 9 to 11.5, very preferably in the range from 10 tol l. The pH at the end of precipitation is preferably 6.5 to 11.5, more preferably 7 to 1 1, very preferably 8 to 10.5, with special preference 9 to 10, and with very special preference 9.5 to 10.

The concentration of alkali metal ions in the reaction solution - expressed by the Y value - may remain constant during precipitation or may change in the course of precipitation. The Y value reflects the chemical reactions during the precipitation, more particularly the incorporation of ions into the silica framework. From this value it is possible to draw conclusions concerning the underlying structure of the silica and, accordingly, even prior to the physicochemical analysis of the end product, to predict the quality and reproducibility of the product in question. In one preferred version of the present invention, therefore, precipitation is carried out such that the Y value during precipitation is held in the range between 4 and 8. In a first particularly preferred version, the Y value during the precipitation is held in a range between 3 and 6, with more particular preference 3.5 to 5.5. In a second particularly preferred version of the process of the invention, the Y value during precipitation is held constant in the range from 6 to 8, with particular preference 6 to 7.5. In one specific embodiment it has proven to be advantageous if the Y value during precipitation falls by up to 25%, more preferably 5% to 20%, with special preference 10 to 15%, based on the Y value at the start of precipitation.

The precipitation is carried out preferably at a temperature of 80 to 95⁰C. The pure precipitation time, i.e. the duration of the simultaneous addition of silicate solution and acidifying agent, may in one preferred version of the present invention - without consideration of interruption times - be 50 to 80 min, in another preferred version of the present invention 80 to 120 min, more preferably 80 to 100 min. The feed rates of the acidifying agent and of the silicate solution are chosen such that the desired precipitation time - but also, at the same time, the desired pH profile of the precipitation suspension - can be maintained.

It is possible to add an electrolyte prior to or during the simultaneous addition of silicate solution and acidifying agent. Electrolytes for the purpose of the present invention are metal salts or their aqueous solutions which are not incorporated into the amorphous SiO₂ framework, such as, for example, Na, K, Rb, Ba, in each case as sulphate, acetate, halide or carbonate. The fraction of the electrolyte is 0.01% - 26% by weight (calculated as metal ion).

It is also possible to add metal salts or their solutions to the precipitation mixture that are incorporated into the SiO₂ framework, thus giving silicates. The fraction of these metal ions may be between 0.5% and 50% by weight, preferably 1% to 10% by weight; common ions are Al, Zr, Ti, Fe, Ca and Mg.

The precipitated silica suspensions prepared by the process of the invention are filtered in step d) and the filter cake is washed with water.

The filtration, liquefaction (e.g. in accordance with DE 2447613) and long or short drying of the silicas of the invention are familiar to the skilled person and can be read, for example, in the documents cited in this description. The filtration and the washing of the silica take place preferably in such a way that the conductivity of the end product is < 2000 μS/cm and particularly < 1300 μS/cm.

The silica of the invention is preferably dried in a pneumatic dryer, spray dryer, staged dryer, belt dryer, Bϋttner dryer, rotary tube dryer, flash dryer, spin-flash dryer or nozzle tower dryer. These drying variants include operation with an atomizer, a single-fluid or two-fluid nozzle or an integrated fluid bed. Spray drying may be carried out, for example, in accordance with US 4094771. Nozzle tower drying may be carried out, for example, as described in EP 0937755. The contents of US 4094771 and of EP 0937755 are hereby explicitly incorporated into the content of the present specification.

The precipitated silicas of the invention may be present in the form of a powder having a particle size d₅₀ of 1 to 80 μm as determined by means of laser diffraction. The powder particles may have an irregular or else a regular external form, i.e. they may also be substantially spherical, for example. Preferably the precipitated silicas of the invention are in the form of substantially spherical particles (microgranules) having a particle size d₅o of 80 μm to 1000 μm as determined by means of sieve residue analysis (Alpine). In the latter case the silicas of the invention are prepared preferably by means of nozzle tower drying, as described in EP 0937755, and exhibit an external form that is characteristic of this drying method (see figures in EP 0937755). The content of EP 0937755 is hereby explicitly incorporated into the content of the present specification. With particular preference the precipitated silicas of the invention are in the form of granules (d₅₀ > 1000 μm (Alpine sieve residue)), and following granulation have a particle size distribution such that by means of sieve residue analysis (Ro-Tap) at least 80% by weight of the particles are larger than 300 μm and not more than 10% by weight are smaller than 75 μm.

Granulation may be carried out using, for example, a roll press from Alexanderwerk AG, Remscheid. In that case preferably the powder product is deaerated by a vacuum system, without further addition of binders or liquids, via a horizontal feed system with single or double screw, and is introduced uniformly between the double-sidedly mounted, vertically disposed rolls. This presses the powder to a flake product, which is brought to the desired maximum granule size by means of a crusher.

In one specific embodiment of the present invention the precipitated silicas of the invention can be ground. The techniques for optional grinding of the silicas of the invention are known to the skilled person and can be read for example in Ullmann, 5¹ edition, B2, 5-20. For the grinding of the silicas of the invention it is preferred to use impact mills or opposed-jet mills. The milling parameters are preferably chosen such that the ground product has a d₅₀ of the volume-based particle distribution curve, determined by means of laser diffraction, of between 1 and 15 μm, preferably 3 to 10 μm, more preferably 4 to 10 μm. The products thus ground, but also the unground products, can also be employed in non-rubber applications, such as for support material, for example.

In another specific embodiment the particle size of the powders of the invention is 15 to 80 μm. These powders are suitable with particular preference for applications for reinforcement of rubber products. The precipitated silicas obtained by the process of the invention can be used as a filler in pneumatic tyres, tyre treads for summer tyres, winter tyres and all-year tyres, car tyres, tyres for utility vehicles, motorcycle tyres, tyre body parts, cable sheathing, hoses, drive belts, conveyor belts, roll coverings, footwear soles, gasket rings and damping elements.

The examples and measurement procedures which follow are intended to elucidate the invention in more detail without restricting its scope.

Determination of the Y value:

The Y value is determined using sulphuric acid as standard solution and phenolphthalein as indicator.

Reagents

Phenolphthalein solution in ethanol with a concentration of 10 g/1 Sulphuric acid solution with a concentration of 1 mol/1

Procedure 100 ml of the sample under analysis, a precipitation suspension for example, are placed in a 500 ml glass beaker and about 10 ml of water and also 3-5 drops of phenolphthalein solution are added. The mixture is stirred using a magnetic stirrer and the sulphuric acid solution is added using a burette until the phenolphthalein colour change from red to pale pink occurs. 3-5 additional drops of phenolphthalein are added and titration is continued. This process is repeated until no further colour change can be observed.

The Y value is calculated as follows:

Y = V ⁺ N where: V = volume of sulphuric acid consumed in the titration, in ml N = normality of the acid Determination of the pH of the initial charge/precipitation suspension

A sample of 50-100 ml of the initial charge or precipitation suspension is taken and the pH is determined at 60⁰C according to known processes.

Determination of the solids content of filter cakes

This method is used to determine the solids content of filter cakes by removal of the volatile fractions at 105°C.

It involves weighing out 100.00 g of the filter cake (initial mass E) into a dry, teared porcelain boat (diameter 20 cm). Where appropriate the filter cake is broken up with a spatula to give loose crumbs 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 as SC = A/E * 100%, where A = final mass in g and E = initial mass in g.

Determination of the solids content of the silica feed

10 g of sample (initial mass E) are dried to constant weight in a porcelain boat (45 mm in diameter) under an infra-red drying lamp at 120-140°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 as

SC = A/E * 100%, where A = final mass in g and E = initial mass in g.

Determination of the pH of the silica

The pH of the silica is determined in the form of a 5% suspension in water at room temperature in a modified version of DIN EN ISO 787-9. Relative to the specifications of that standard, the initial masses were changed (5.00 g of silica per 100 ml of deionized water). Determination of the moisture content

The moisture content of silica is determined in accordance with ISO 787-2 following 2-hour drying in a forced-air drying cabinet at 105°C. This loss of drying is composed predominantly of water moisture.

Determination of the BET surface area

The specific nitrogen surface area (referred to below as BET surface area) of the powder, sphere or granule silica is determined in accordance with ISO 5794-1 /Annex D using an AREA-meter (Strohlein, JUWE).

Determination of 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). The adsorption of CTAB takes place in aqueous solution with stirring and ultrasound treatment. Excess, unadsorbed CTAB is determined by back-titration with NDSS (dioctylsodium sulphosuccinate solution, "Aerosol OT" solution) using a titroprocessor, the end point being indicated by the maximum clouding 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 equation:

(C₂₀H₃₇O₄)SO₃Na + BrN(CH₃MC₁₆H₃₃) ^> (C₂₀H₃₇O₄)SO₃N(CH₃MC₁₆H₃₃) + NaBr NDSS CTAB

Apparatus METTLER Toledo DL 55 titroprocessor and METTLER Toledo DL 70 titroprocessor, each equipped with: pH electrode, Mettler, type DG 111, and phototrode, Mettler, type DP 550

100 ml polypropylene titration beaker

150 ml glass titration vessel with lid

100 ml capacity pressure filtration device Cellulose nitrate membrane filter, pore size 0.1 μm, 47 mm 0, e.g. Whatman (order No. 7181-004)

Reagents

The solutions of CTAB (C_CTAB ⁼ 0.015 mol/1 in deionized water) and NDSS (concentration = 0.00423 mol/1 in deionized water) are purchased in ready-to-use form (Bernd Kraft GmbH, 47167 Duisburg: order No. 6056.4700 CTAB solution of concentration 0.015 mol/1; order No. 6057.4700 NDSS solution 0.00423 mol/1), stored at 25°C and used within one month.

Procedure

1. Blank titration

The consumption of NDSS solution for titrating 5 ml of CTAB solution should be checked 1 x daily prior to 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 NDSS solution is carried out with stirring, by the measurement method familiar to the skilled person, using the DL 55 titro processor, until the solution reaches maximum turbidity. The consumption V_A of NDSS solution, in ml, is ascertained. Each titration should be performed in triplicate.

2. Adsorption

10.0 g of the powder, bead or granule silica with a moisture content of 5 ± 2% (if appropriate, the moisture content is adjusted by drying at 105°C in a drying cabinet or by uniform wetting) are size-reduced for 30 seconds using a mill (Krups, Model KM 75, Article No. 2030-70). Precisely 500.0 mg of the size-reduced 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 the contents are stirred with an Ultra Turrax T 25 stirrer (stirrer shaft KV- 18G, 18 mm diameter) at 18 000 rpm for a maximum of 1 minute until wetting is complete. The titration vessel is screwed onto the DL 70 titroprocessor and the pH of the suspension is adjusted with KOH (0.1 mol/1) to a figure of 9 ± 0.05.

The suspension is sonicated in the titration vessel for 4 minutes in a ultrasound bath (Bandelin, Sonorex RK 106 S, 35 kHz, 100 W effective or 200 W peak power) 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 onto the DL 55 titroprocessor and titrated with NDSS solution, with stirring, until maximum turbidity is reached. The consumption V_B of NDSS solution, in ml, is ascertained. Each titration should be performed in triplicate.

Calculation

_nτ . _Ω . . . VA - VB CCTAB * MCTAB * TI * P

CTABf non - moisture - corrected) =

VA ⁼ consumption of NDSS solution in ml for titrating the blank sample

VB = consumption of NDSS 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 = occupancy of CTAB = 578.435 m7g

I = initial mass of silica The CTAB surface area is based on the anhydrous silica, and so the following correction is made:

CTAB(non - moisture - corrected) in m?g * 100 %

100 % - moisture content in %

The moisture content of the silica is determined in accordance with the described methods "Determination of the moisture content". Determination of the DBP absorption

The DBP absorption (DBP number) which is a measure of the absorbency of the precipitated silica, is determined in a method based on standard DIN 53601, as follows: 12.50 g of powder or bead silica of 0-10% moisture content (the moisture content is adjusted if appropriate by drying in a drying cabinet at 105°C) are introduced into the kneader chamber (article number 279061) of the Brabender Absorptometer "E" (without damping of the outlet filter of the torque transducer). 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 through the sieve with a pore size of 3.15 mm using a plastic spatula). With continuous mixing (kneader paddles rotating at a speed of 125 rpm), dibutyl phthalate is added dropwise at room temperature to the mixture at a rate of 4 ml/min using the Brabender T 90/50 Dosimat. Its incorporation 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. When the display shows 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:

V * D * 100 s DBP = -^- — * — ≤— + C

/ WO g where DBP = DBP absorption in g/(100 g) V = consumption of DBP in ml D = density of DBP in g/ml (1.047 g/ml at 20°C) / = initial mass of silica in g C = correction value from moisture correction table, in g/( 100 g)

The DBP absorption is defined for the anhydrous, dried silica. When moist precipitated silicas are used, it is necessary to take account of the correction value C for the calculation of the

DBP absorption. This value can be determined from the correction table below; for example, a silica water content of 5.8% would imply an add-on of 33 g/(100 g) for the DBP absorption. The moisture content of the silica is determined in accordance with the "Determination of the moisture content or loss on drying" method.

Moisture correction table for dibutyl phthalate absorption (anhydrous)

Determination of the sieve residue (Ro-Tap)

This method is for the determination of the fractions of relatively coarse particles (> 300 μm) and the fraction of relatively fine particles (< 75 μm) which are retained on a predetermined sieve. For the sieve residue analysis, test sieves - analytical sieves with a metal sieve fabric (DIN ISO 565 T.2) in different nominal mesh sizes with a sieve diameter of 200 mm in each case - are stacked atop one another in a sieve tower in the following order:

Sieve tray, 75 μm, 150 μm, 300 μm The sieve tower is introduced in the order stated into a Tyler Ro-Tap B 8260 analytical sieving machine with timer, and a homogeneous sample quantity of 100.00 g of the silica granules is transferred to the topmost sieve. To homogenise the sample, it is mixed carefully in a plastic bag and weighed to 100 ± 1 g. The sieve cover and the taper are mounted, and sieving takes place with a circular motion and tapping motion for 5 minutes. After 5 minutes of vibration sieving, the residues on the various sieves are weighed to an accuracy of 0.1 g. The results are calculated using the following formula:

The sieve residues (Ro-Tap) are determined as follows:

Sieve fraction (Ro-Tap, < 75 μm) in % = (A_s/E) * 100%, and Sieve residue (Ro-Tap, > 300 μm) in % = (A₃₀₀/E) * 100%, where

E = initial mass of granules in g. A₈ = final mass on sieve tray in g.

A₃oo - final mass on 300 μm test sieve in g.

Determination of the particle size by means of laser diffraction

The use of laser diffraction to determine particle sizes of powders is based on the phenomenon whereby particles scatter monochromatic light with a different intensity pattern in all directions. This scattering is dependent on the particle size. The smaller the particles, the greater the scattering angles. Sample preparation and measurement (rinsing of the module, etc.) take place with fully deionized (DI) water in the case of hydrophilic precipitated silica, or with pure ethanol in the case of precipitated silica which is not sufficiently wettable with water. Prior to the beginning of the measurement, the laser diffraction instrument LS 230 (Coulter) and the liquid module (small volume module plus, 120 ml, Coulter) are warmed up for 2 h, the module is rinsed three times with DI water and calibrated, and in the case of hydrophobic precipitated silicas it is rinsed three times with ethanol.

In the control bar of the instrument software, the file window "Calculate Opt. Model" is selected via the menu item "Measurement" and the refractive indices are defined in an .rfd file: liquid refractive index B.I. real = 1.332 (1.359 for ethanol); material refractive index real = 1.46; imaginary = 0.1 ; shape factor 1. Additionally, in this file window, the following items are selected: offset measurement, adjustment, background measurement, set measurement cone, input sample info, input measurement info, measuring time 60 s, number of measurements 1, no PIDS data, size distribution. The pump speed is set at 30% on the instrument.

The homogeneous suspension of 1 g of silica in 40 ml of DI water is added, using a 2 ml single-use pipette, to the liquid module of the instrument, in such a way that a constant concentration with a light absorption of 8% to 12% is achieved and the instrument reports "OK". Measurement takes place at room temperature. From the raw data plot, the software calculates the particle size distribution and the d₅₀ figure (median value), on the basis of the volume distribution, taking into account the Mie theory and the optical model parameters (.rfd file).

Determination of the sieve residue (Alpine)

This determination of sieve residue is an air-jet sieving method based on DIN ISO 8130-1, using an Alpine S 200 air-jet sieve instrument. To determine the d5Q values of microgranules and granules, sieves whose mesh size is > 300 μm are included. To determine the d5Q value, the sieves must be chosen such that they yield a particle size distribution from which the d5Q value can be determined in accordance with Figure 2. Graphical representation and evaluation take place in the same way as in ISO 2591-1, section 8.2.

The d50 value is that particle diameter in the cumulative particle size distribution at which the particle diameter of 50% of the particles is less than or equal to that of the particles whose particle diameter is the d5Q value.

The examples which follow are intended to elucidate the invention in more detail, without restricting its scope. Examples

Example 1

A precipitation vessel with a capacity of 90 m³ is charged with 42 m³ of water. 0.95 m³ of waterglass (Na₂O content 6.1% by weight, SiO₂ content 20.7% by weight) is added. The initial charge is subsequently heated to 91.8⁰C. The pH of the initial charge at the start of precipitation, i.e. of simultaneous addition of waterglass and sulphuric acid (about 98.0 ± 0.5% by weight) to the initial charge, is 10.3. The Y value at the start of precipitation is 5.3. Thereafter, over the course of 72 minutes and with the temperature held constant, waterglass (as specified above) and sulphuric acid (as specified above) are added such that the pH at the end of precipitation is 9.99 and the Y value at the end of precipitation is 4.85. After 72 minutes the addition of waterglass is stopped, and sulphuric acid is added further until a pH of 4.7 is reached. Thereafter the suspension is stirred at this pH for 20 minutes. The resulting suspension is filtered using a membrane filter press, and the filter cake is washed with water. The filter cake, with a solids content of about 20% by weight, is then liquefied in a dissolver. The silica feed, with a solids content of about 20% by weight and a pH of about 5.8, is subsequently spray-dried such that the end product has a pH of 6.2, measured in the form of a 5% suspension. The spray-dried product is then granulated by means of a roll granulator. Roll granulation takes place by means of two shaping rolls pressed together. The powder product, without further addition of binders or liquids, is deaerated by means of a vacuum system (underpressure 0.08 bar) and introduced uniformly between the double-sidedly mounted, vertically arranged rolls. At a rotary speed of 18-20 rpm and a pressure of 70-80 bar, the pressed powder is comminuted by means of a crusher (mesh size 10 mm). The fine fraction is sieved off with a vibration sieve (mesh size 1 x 10 mm) and returned to the powder feed.

The physicochemical data of a representative sample of the resulting powder product (Example Ia) and granulated product (Example Ib) are listed in Table 1. Example 2

A precipitation vessel with a capacity of 90 m³ is charged with 40 m³ of water. 1.27 m³ of waterglass (Na₂O content 6.1%, SiO₂ content 20.5%) are added. The initial charge is subsequently heated to 87⁰C. The pH of the initial charge at the start of precipitation, i.e. of simultaneous addition of waterglass and sulphuric acid (about 98.0 ± 0.5% by weight) to the initial charge, is 10.5. The Y value at the start of precipitation is 6.6. Thereafter, over the course of 90 minutes and with the temperature held constant, waterglass (as specified above) and sulphuric acid (as specified above) are added such that the pH at the end of precipitation is 9.9 and the Y value at the end of precipitation is 6.6. After 90 minutes the addition of waterglass is stopped, and sulphuric acid is added further until a pH of 4.5 is reached. Thereafter the suspension is stirred at this pH for 30 minutes.

The resulting suspension is filtered using a membrane filter press, and the filter cake is washed with water. The filter cake, with a solids content of about 20% by weight, is then liquefied in a dissolver. The silica feed, with a solids content of about 20% by weight and a pH of about 5.8, is subsequently spray-dried in such a way, by a drying operation with metered addition of ammonia, that the end product has a pH of 6.2, measured in the form of a 5% suspension. The spray-dried product is then granulated by means of a roll granulator. Roll granulation takes place by means of two shaping rolls pressed together. The powder product, without further addition of binders or liquids, is deaerated by means of a vacuum system (underpressure 0.08 bar) and introduced uniformly between the double-sidedly mounted, vertically arranged rolls. At a rotary speed of 18-20 rpm and a pressure of 70-80 bar, the pressed powder is comminuted by means of a crusher (mesh size 10 mm). The fine fraction is sieved off with a vibration sieve (mesh size 1 x 10 mm) and returned to the powder feed.

The physicochemical data of a representative sample of the resulting powder product (Example 2a) and granulated product (Example 2b) are listed in Table 1. Example 3

A precipitation vessel with a capacity of 90 m³ is charged with 40 m³ of water. 1.26 m³ of waterglass (Na₂O content 6.3, SiO₂ content 21.4) are added. The initial charge is subsequently heated to 84°C. The pH of the initial charge at the start of precipitation, i.e. of simultaneous addition of waterglass and sulphuric acid (about 98.0 ± 0.5% by weight) to the initial charge, is 10.5. The Y value at the start of precipitation is 6.6. Thereafter, over the course of 90 minutes and with the temperature held constant, waterglass (as specified above) and sulphuric acid (as specified above) are added such that the pH at the end of precipitation is 9.9 and the Y value at the end of precipitation is 6.2. After 90 minutes the addition of waterglass is stopped, and sulphuric acid is added further until a pH of 4.0 is reached. Thereafter the suspension is stirred at this pH for 30 minutes.

The resulting suspension is filtered using a membrane filter press, and the filter cake is washed with water. The filter cake, with a solids content of about 20% by weight, is then liquefied in a dissolver. The silica feed, with a solids content of about 20% by weight and a pH of about 5.8, is subsequently spray-dried such that the end product has a pH of 6.2, measured in the form of a 5% suspension. The spray-dried product is then granulated by means of a roll granulator. Roll granulation takes place by means of two shaping rolls pressed together. The powder product, without further addition of binders or liquids, is deaerated by means of a vacuum system (underpressure 0.08 bar) and introduced uniformly between the double-sidedly mounted, vertically arranged rolls. At a rotary speed of 18-20 rpm and a pressure of 70-80 bar, the pressed powder is comminuted by means of a crusher (mesh size 10 mm). The fine fraction is sieved off with a vibration sieve (mesh size 1 x 10 mm) and returned to the powder feed.

The physicochemical data of a representative sample of the resulting powder product (Example 3a) and granulated product (Example 3b) are listed in Table 1. Table 1

Claims

Claims:

1. Process for preparing precipitated silicas, comprising the steps of: a) preparing an initial charge of water or an aqueous solution of an alkali metal silicate and/or alkaline earth metal silicate, b) simultaneously metering alkali metal silicate and/or alkaline earth metal silicate and acidifying agent into this initial charge with stirring at 80 to 100°C, c) reacidifying the precipitation suspension to a pH of 2.5 to 6.0, d) filtering, washing and drying, characterized in that the alkali metal silicate and/or alkaline earth metal silicate used in steps a) and/or b) has an alkali metal oxide and/or alkaline earth metal oxide content in the range from 4% to 7% by weight and a silicon dioxide content in the range from 14% to 23% by weight, in that the acidifying agent used in step b) and/or c) is an acidifying agent selected from the group consisting of sulphuric acid having a concentration of 90% to 98.5% by weight, hydrochloric acid having a concentration of 34% to 42.7% by weight, nitric acid having a concentration of 60% to 68% by weight, phosphoric acid having a concentration of 80% to 100% by weight, carbonic acid or CO₂, and sodium hydrogen sulphite or SO₂, and in that the pH of the precipitation suspension (measured at 6O⁰C) drops in the course of precipitation by 1% to 20%, based on the pH at the start of precipitation (measured at 60°C).

2. Process according to Claim 1, characterized in that between steps c) and d) the resulting suspension is afterstirred at 60 to 100°C for 1 to 90 minutes.

3. Process according to Claim 1 or 2, characterized in that the alkali metal silicate and/or alkaline earth metal silicate has a modulus of 2.5 to 4.5.

4. Process according to any one of Claims 1 to 3, characterized in that the alkali metal silicate is sodium silicate or potassium silicate.

5. Process according to any one of Claims 1 to 4, characterized in that the pH falls in the course of precipitation by 3% to 10%.

6. Process according to any one of Claims 1 to 5, characterized in that the pH at the start of precipitation is 8 to 12 and at the end of precipitation is 6.5 to 1 1.

7. Process according to any one of Claims 1 to 6, characterized in that the concentration of alkali metal ions in the reaction solution - expressed by the Y value

- falls during precipitation by up to 25%, based on the Y value at the start of precipitation.

8. Process according to Claim 7, characterized in that the concentration of alkali metal ions in the reaction solution - expressed by the Y value

- is in the range from 3 to 6 at the start of precipitation.

9. Process according to Claim 7, characterized in that the concentration of alkali metal ions in the reaction solution - expressed by the Y value

- is in the range from 6 to 8 at the start of precipitation.

10. Process according to any one of Claims 1 to 9, characterized in that the duration of the simultaneous addition of silicate solution and acidifying agent is 50 to

80 min or 80 to 120 min.

11. Process according to any one of Claims 1 to 10, characterized in that an electrolyte is added before or during the simultaneous addition of silicate solution and acidifying agent.

12. Precipitated silica obtainable by the process according to any one of Claims 1 to 11.

13. Use of the precipitated silica obtainable by the process according to any one of Claims 1 to 11 as a filler in pneumatic tyres, tyre treads for summer tyres, winter tyres and all-year tyres, car tyres, tyres for utility vehicles, motorcycle tyres, tyre body parts, cable sheathing, hoses, drive belts, conveyor belts, roll coverings, footwear soles, gasket rings and damping elements.