EP2580173A2

EP2580173A2 - Process for producing a silica sol material

Info

Publication number: EP2580173A2
Application number: EP11725672.7A
Authority: EP
Inventors: Ekkehard Barth; Rolf Bachmann; Arne Braun; Maren Heinemann; Sebastian Schmidt
Original assignee: Bayer Innovation GmbH
Current assignee: Bayer Innovation GmbH
Priority date: 2010-06-10
Filing date: 2011-06-06
Publication date: 2013-04-17
Also published as: CA2801970A1; JP2013528154A; US20130145964A1; BR112012031454A2; WO2011154361A2; US9221690B2; WO2011154361A3; MX2012014213A; CA2801970C; RU2012157528A; AU2011263823A1; DE102010023336A1

Abstract

The present invention relates to an improved process for producing a biodegradable and/or bioabsorbable silica sol material, which is advantageous especially with regard to the reproducibility of the material, the rapidity of the synthesis and the possibility of producing the silica sol material on an industrial scale. The invention further relates to a biodegradable and/or bioabsorbable silica sol material which can be produced by the synthesis process according to the invention.

Description

Process for producing a silica sol material

The present invention relates to a process for the preparation of a biodegradable and / or resorbable silica sol material, which is particularly advantageous in terms of the reproducibility of the material, the rate of synthesis and the ability to produce the silica sol material on an industrial scale. Another object of the invention relates to a biodegradable and / or resorbable silica sol material which can be prepared by the inventive synthesis method.

Biologically degradable and / or resorbable silica sol materials and their preparation are described in the prior art. In DE 196 09 551 C1 biodegradable / bioabsorbable fiber structures are described which can be obtained in a sol-gel process by drawing threads from a spinning dope and optionally drying them. The preparation of the sol material comprises a hydrolysis condensation step in which the educts tetraethoxysilane (TEOS), ethanol, water and nitric acid in a molar ratio of 1: 1.26: x: 0.01 (where x = 1.6, 1.7, 1.8, 1.9 or 2.0). DE 196 09 551 C1 describes that the required water / acid mixture is added directly to the silicon compound (TEOS) to be hydrolyzed at room temperature or with slight cooling, and the resulting mixture is subsequently stirred for one to several hours. After the hydrolysis has ended, the solvent is removed from the resulting mixture until the mixture has a viscosity between 0.05 and 50 Pa · s at room temperature and a shear rate of 20 s ^-1 , preferably followed by filtration Matured in a closed vessel at a temperature of 3 ° C and a maturation time of 6 hours to 6 months to silica sol material then the silica sol material can be spun into a fiber.

When adding an aqueous acid to the Si compound present in a solvent, misting may occur in the reaction vessel. The particles that are in such a mist can subsequently be removed only via a filtration step. If the particles are not removed via a filtration step, they can, for example, in the further processing of the silica sol material to a silica gel fiber by means of a spinning system, clog the spinning nozzles of the spinning plant. The spinning process must then be interrupted and the nozzles must be cleaned. On the other hand, an additional filtration step is to be avoided if possible, since the filtration loses synthesis material which can no longer be converted to the silica sol material. , -

WO2008 / 086970A1 relates to a similar silica sol material but in which the hydrolysis-condensation step is carried out over a period of at least 16 hours. The hydrolysis-condensation reaction is preferably carried out batchwise in a stirred tank. The Si compound and the solvent are preferably charged. This is followed by the rapid addition of an acid, preferably in the form of HNO ₃ . It is described that the hydrolysis-condensation reaction proceeds rapidly due to the acid strength and the contents of the container heat up by about 40 ° C. The subsequent removal of the solvent occurs to a viscosity of the mixture of 0.5 to 2 Pa · s at a shear rate of 10 s ^-1 at 4 ° C. Allegedly, filtration is not required, followed by ripening at a temperature of preferably 2 ° C to 4 ° C to effect further condensation under kinetically controlled conditions to suppress formation of a three-dimensional polymeric gel network The product of ripening preferably has a viscosity of 35 to 45 Pa · s (shear rate 10 s ^-1 at 4 ° C) at a loss factor (at 4 ° C, 10 s ^-1 , 1% deformation) of 2.5 to 3.5.

WO2008 / 148384A1 relates to a similar silica sol material. During the production of this material, a gas-diffusion-tight container, preferably a rotary evaporator, is used during evaporation. The hydrolysis-condensation step involves the immediate addition of a mixture of H ₂ O and HNO ₃ to a mixture of TEOS and ethanol. The kinetically controlled ripening is carried out in particular by vibration-free storage of the reaction mixture in a closed gas-diffusion-tight vessel.

WO2009 / 077104A relates to a similar silica sol material, in which, compared to WO2008 / 148384A1, evaporation in a closed apparatus is optionally carried out by continuously supplying a chemically inert drag gas stream. All of the above-mentioned prior art processes comprise at least three steps: hydrolysis-condensation, solvent-removal and maturation. The steps are obviously carried out in different vessels. The times for the individual steps sometimes vary considerably. The individual steps are completed when a preferred viscosity has been set; that is, it is obviously necessary to follow the viscosity of the reaction mixtures in order to complete one step in the presence of the desired (intermediate) product.

In the case of the cited prior art processes, the hydrolysis-condensation reaction takes place with rapid combination of the silicon compound to be hydrolyzed and the acid. As described above, there is a considerable amount due to the exothermic nature of the reaction Temperature increase. Such an increase in temperature is to be avoided, in particular for an upscaling to an industrial production on an industrial scale, inter alia, for safety reasons. In addition, as a result of temperature inhomogeneities in the reaction mixture, product inhomogeneities can occur, especially on an industrial scale. According to the state of the art, ripening is preferably carried out vibration-free at a temperature of -20 ° C to 10 ° C and it can take 6 hours to 6 months. Such a method is not scalable and therefore not suitable for large-scale production.

Due to the insufficient controllability of the processes described in the prior art, the end of the silica sol material production process at the beginning of the preparation can only be predicted within a range of ± 2 days. Especially with regard to the biodegradable or bioresorbable properties of the materials, which may allow only short storage times, the above-described imponderables counteract a meaningful economic planning and production.

The object of the present invention was therefore to address the disadvantages of the prior art and to provide an improved process for the preparation of silica sol materials. The method should allow a reproducible production of silica sol materials. The method should be scalable to an industrial scale and allow a more accurate prediction of the end of the reaction. The change of reaction vessels between the individual steps should be avoided. The object is achieved by a method for producing a silica sol material, which comprises the following steps:

(a) controlled combining of an aqueous acid solution and a hydrolyzable silicon compound,

(b) subsequently evaporating to a single phase solution having a viscosity in the range of 0.5 to 30 Pa · s at a shear rate of 10 s ^-1 at 4 ° C, and

(c) ripening the single-phase solution obtained in step (b) into a silica sol material having a viscosity between 30 and 100 Pa · s at a shear rate of 10 s ^-1 at 4 ° C and a loss factor of 2 to 5.

Instead of the rapid addition described in the prior art (WO 2008/086970 p. 4 Z. 24) of an aqueous acid to the hydrolyzable silicon compound, a controlled combination of these components takes place in the present invention. Under a controlled cooperation It is understood here that the merger is not carried out quickly, not quickly, but over a predetermined, longer period of time. The merging is done so that the temperature of the mixture remains within a predetermined temperature range.

The combination preferably takes place over a period of at least 15 minutes, more preferably at least 30 minutes, and even more preferably at least 1 hour. The shorter the period of time for metering, the more likely additional equipment will be required to remove the heat generated during the reaction and maintain the temperature of the reaction mixture within a predetermined range.

It is surprising that, as a result of the combination of hydrolyzable silicon compound and aqueous acid solution, which has been extended over a relatively long period of time compared with the prior art, there is no noticeable change in the mass distribution of the polymers. If, for example, the synthesis method described in WO2008 / 086970A1 is modified according to the invention so that the addition of the aqueous acid solution to the hydrolyzable silicon compound is not rapid but over a period of 1 hour, this has no appreciable effect on the end of the process (after maturation) resulting polymer distribution, for example as measured by gel permeation chromatography.

However, controlled pooling has significant advantages, particularly with regard to a process operated on an industrial scale, for example in the compliance with safety regulations, in the process control and in the reproducibility. It is conceivable to carry out the combination in step (a) of the method according to the invention at a constant rate. It is also conceivable to carry out the combination in such a way that the temperature of the reaction mixture develops within a predetermined range. In the latter case, therefore, there is a control loop in which the combination of the components is controlled by the temperature and / or temperature change in the reaction mixture. In a preferred embodiment of the process according to the invention, the combination in step (a) takes place under quasi-isothermal conditions. By the term "quasi-isothermal conditions" it is meant that a chemical reaction is carried out at as constant a temperature as possible In the present process at step (a), the reaction under quasi-isothermal conditions would preferably be within a sump temperature range (ie, measured within the reaction mixture) of ± 5 ° C, preferably from ± 2 ° C, particularly preferably run from ± 0.5 ° C. In a further preferred embodiment of the process according to the invention, the components are combined in step (a) so that the heat released by the hydrolysis-condensation reaction is used to heat up the synthesis approach. It can thus be avoided overheating of the synthesis approach, for example, to low-performance or inert heat exchangers and the hydrolysis-condensation reaction can be performed in a controlled manner at the desired temperature.

The hydrolysis-condensation reaction in step (a) is preferably carried out with stirring.

The term "hydrolyzable silicon compound" preferably refers to a Si compound of the formula (I) SA, (I) wherein the radicals X are the same or different and are hydroxy, hydrogen, halogen, amino, alkoxy, acyloxy, acylcarbonyl and or alkoxycarbonyl and derived from alkyl radicals which optionally represent substituted straight-chain, branched or cyclic radicals having 1 to 20 carbon atoms, preferably having 1 to 10 carbon atoms, and by oxygen or sulfur atoms or by amino In a preferred embodiment of the invention, X in the formula (I) is an optionally substituted straight-chain, branched and / or cyclic alkoxy radical having 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms X in the formula (I) is an optionally substituted straight-chain and / or branched C 1 -C ₅ -alkoxy radical preferred are substituted, but preferably unsubstituted, straight-chain and / or branched C ₂ -C 3 -alkoxy radicals, such as, for example, ethoxy, N-propoxy and / or isopropoxy. Very particular preference according to the invention is given to using tetraethoxysilane (TEOS) as hydrolyzable Si compound in the process according to the invention.

The term "aqueous acid solution" describes mixtures and / or solutions which have a pH of from 0 to <7, preferably from 0 to 2. In addition to water and a proton donor, the aqueous acid solution may comprise one or more further substances which Preferably, a water-soluble solvent is added, particularly preferably ethanol, The aqueous acid solution preferably comprises water and ethanol in a molar ratio of 1 to 1.27 to 1 to 1.59, particularly preferably in a molar ratio of 1 to 1.41.

Nitric acid is preferably used as proton donor. - -

In addition to a preferably ethanolic aqueous salaric acid solution, aqueous or alcoholic (preferably an aqueous dilute ethanolic) solution of a physiologically tolerated acid (eg citric, succinic, acetic or ascorbic acid) and at least one essential (eg L-arginine), particularly preferred; L-valine, L-leucine, L-isoleucine, L-phenylalanine, L-thyroxine, L-methionine, L-lycin or L-tryptophan) or non-essential amino acid (eg L-glutamine, L-glutamic acid, L-asparagine, L-aspartic acid, L-cysteine, L-glycine, L-alanine, L-proline, L-histidine, L-tyrosine). Such mixtures and / or solutions form in a physiological environment with molecular oxygen enzymatically (by means of a nitroxide synthase, NOS) nitric oxide (NO). In addition, organic nitrates or nitrate esters (so-called NO donors) such as. Ethyl nitrate, which form NO with the help of an organic nitrate reductase, are used. Thiol groups (cysteine) are required for this enzymatic release of NO.

For the hydrolysis of the silicon compound, such a large amount of water is used that the molar ratio of S1X ₄ : water in the range of 1: 1, 5 to 1: 2.5, preferably in the range of 1: 1, 7 to 1 : 1, 9, more preferably in the range of 1: 1, 7 to 1: 1.8.

The combination of the components in step (a) can take place in various ways. It is conceivable to introduce the hydrolyzable silicon compound in the reactor and to add the aqueous acid solution. It is conceivable to present the hydrolyzable silicon compound in a suitable solvent (for example ethanol in the case of TEOS). A further preferred embodiment relates to a process for producing a silica sol material in which both the acid and the hydrolyzable silicon compound are added in a controlled manner to a solvent in parallel. Preferably, the hydrolyzable silicon compound is pre-mixed with a portion of the solvent, preferably 35 to 38% of the solvent. Overall, the amount of solvent thus preferably does not change. However, at the beginning there is proportionately less solvent in the reaction vessel to which the acid and the described mixture of a part of the solvent and the hydrolyzable silicon compound are metered in in a controlled manner. This embodiment is particularly important in terms of a continuous manufacturing process (on a large scale) of importance. In this embodiment, the acid and the hydrolyzable silicon compound can be metered in independently of each other (for example via different reaction vessel openings and pump systems) over different periods of time. Preferably, however, the same period of time for metering is selected in proportion to the volume flow. The added hydrolyzable silicon compound quantity or the added amount of acid is preferably constant per unit time. This embodiment of the invention is preferably carried out under quasi-isothermal conditions. - - leads. Preferably, the hydrolyzable silicon compound or acid is metered in over a period of at least 15 minutes, preferably at least 30 minutes and more preferably at least 1 hour.

Another object of the present invention is directed to a process for the preparation of a silica sol material in which a hydrolyzable silicon compound is added to an acid present in a solvent. This controlled combination, also referred to below as "inverse dosing", surprisingly leads to a novel sol which is reproducible and can be prepared in a controlled manner but whose physical properties differ from those described in the prior art Sol can be spun into a fiber and is also biodegradable and / or resorbable.This sol has a lower viscosity at the same loss factor as compared to those described in the prior art.

The loss factor is the quotient of the viscous to elastic portion of the dynamic viscosity. Too low a loss factor means too much elasticity of the material, e.g. prevents the formation of a stable thread during spinning (gelling, tearing of the thread). If the loss factor is too high, the material is so fluid that stable thread formation is not possible (drop formation).

Therefore, the loss factor is an important parameter for determining the quality of the silica sol material for its further use (see below for more details). If the viscosity at the same loss factor is lower than in the case of the silica sol material resulting from the inverse addition, such a material is easier to process and is accordingly advantageous.

In the inverse dosage, the hydrolyzable silicon compound is preferably not previously or only in a small part of a solvent, preferably 0 to 5%, dissolved. Overall, the amount of solvent thus preferably does not change. However, at the beginning of the reaction, there is no or proportionately less solvent in the reaction vessel to which the silicon compound or the mixture of a part of the solvent and the hydrolyzable silicon compound is metered in in a controlled manner. The inverse dosage is preferably carried out under quasi-isothermal conditions. The amount of hydrolyzable silicon compound added per unit time is preferably constant or approximately constant. Preferably, the hydrolyzable silicon compound is metered in over a period of at least 15 minutes, preferably at least 30 minutes and more preferably at least 1 hour. Surprisingly, in the inverse dosage compared to the other in the prior art and the , - Hitherto described embodiments also found that the reaction accelerates overall. At a temperature of 37 ° C and otherwise the same conditions, the reaction lasts only 4 hours instead of 18 for the other controlled dosing methods. Even with the inverse dosage, the reaction also accelerates at higher temperature. Another object of the invention is directed to a silica sol material prepared by a controlled metered addition of a hydrolyzable silicon compound to an in-solvent acid, followed by evaporation to a single-phase solution having a viscosity in the range 0.5 to 1.5 Pa · s at a shear rate of 10 s ^-1 at 4 ° C, and ripening of this single-phase solution to a silica sol material having a viscosity of 30 Pa · s at a shear rate of 10 s ^-1 at 4 ° C, a loss factor of third

After combining the components in step (a) and before evaporation in step (b), the reaction mixture is preferably stirred for some time until a dynamic equilibrium has been established.

The duration of the reaction depends on the selected temperature and the period of the controlled metered addition. For example, the reaction, at a temperature of 55 ° C and a controlled metered addition of the acid to the hydrolyzable silicon compound present in the solvent over a period of one hour, takes a total of about 5 hours on a laboratory scale. At a temperature of 37 ° C and otherwise the same conditions, the reaction lasts 18 hours. The hydrolysis condensation is preferably carried out without pressure (i.e., without overpressure at about 101325 Pa), preferably at a temperature of 0 ° C to 78 ° C. By appropriate pressure regulation, it is also possible to carry out the reaction at temperatures above the boiling point of ethanol (i.e., 78 ° C).

Reactive evaporation in step (b) to give a single-phase solution is carried out as described in the prior art up to a dynamic viscosity (η) of the mixture at 0.5 to 30 Pa · s at a shear rate of 10 s ^-1 at 4 ° C , preferably 0.5 to 2 Pa · s at a shear rate of 10 s ^-1 at 4 ° C., more preferably 1 Pa · s at a shear rate of 10 s ^-1 at 4 ° C.

In a preferred embodiment of the invention, step (b) is carried out in a closed apparatus in which mixing is possible and at the same time the solvent (ie, water, ethanol) can be evaporated. Preferably, the bottom temperature is maintained by pressure regulation (temporally variable adjustment between preferably 500 to 120 mbar) constant (ie ± 5 ° C, preferably of ± 2 ° C), so that continuously solvent under , , light boiling from the batch to the above viscosity is removed. The reaction temperature may be selected as described in the prior art, ie preferably between 30 ° C to 78 ° C, and more preferably between 60 ° C and 75 ° C. Preferably, step (b) is carried out with careful mixing of the reaction system. The single-phase solution resulting from step (b) is subjected to ripening in step (c). In contrast to the prior art (WO2008 / 148384A1 p. 9 Z. 31), the single-phase solution is preferably stirred in this maturation process. Mixing the system with agitation causes the ripening to be slightly accelerated. In addition, the mixing of the single-phase solution leads to a degradation of temperature gradients, which in turn causes a better temperature control and thus an easier scalability of the process. It is preferable to use a stirrer which does not cause bubbles in the single-phase solution. In this case, those based on the principle of a helix, have proven to be particularly suitable. Also, the speed of the stirrer is chosen so that no bubbles can arise in the single-phase solution. Speeds of 4 to 50 rpm, especially of less than 25 rpm, in particular those of less than 10 rpm, have proven to be expedient.

By mixing the single-phase solution during the ripening, it is now possible to introduce pharmaceutical active ingredients homogeneously into the sol. In particular, the introduction of temperature-sensitive active substances is suitable for this because the reactive-reactive step (combined with the higher reaction temperatures required for evaporation) has already been completed. For the purposes of the invention, "active substances" are defined as substances which produce a specific action, a reaction, in a small dose in an organism Temperature-sensitive active substances or pharmaceutical substances are those whose degradation is markedly accelerated at temperatures below 8 ° C., preferably below 2 ° C.

An important factor influencing maturation (step (c)) is the temperature. In principle, the ripening can be carried out at temperatures of up to -80 ° C to 78 ° C and with regulation of the pressure also above it. At all temperatures, a sol known in the art may be prepared. However, it has proved to be particularly advantageous when the maturation is carried out at temperatures of 25 ° C to 35 ° C. On the one hand, the production time at these temperatures is significantly shortened (from 2 to 3 weeks when ripening is carried out at 4 ° C compared to 2 days when the reaction is carried out at 31 ° C). On the other hand, the maturation should ideally not exceed a temperature of 45 ° C, otherwise the ripening of the maturation - can not be guaranteed at the below target values for viscosity and loss factor, the reaction continues to run and a material is obtained, which is not desirable, that is no longer biodegradable or exceeds the gel point and is no longer spinnable.

According to the invention, the silica sol material obtained in step (c) preferably has a viscosity between 30 and 100 Pa · s (shear rate 10 s ^-1 at 4 ° C.), preferably from 35 to 70 Pa · s (shear rate 10 s ^-1 at 4 ° C) with a loss factor (at 4 ° C, shear rate 10 s ^-1 , 1% deformation) of 2 to 5, preferably from 2.5 to 3.5, and more preferably from 2.8 to 3.2 These conditions for the Ripening is particularly preferred when the silica sol material is to be spun into a fiber thereafter For powders and monoliths, viscosities of from 60 Pa.s at a shear rate of 10 s ^-1 at 4 ° C. are preferred.

The silica sol material produced by the process according to the invention can be further processed into a fiber, a fleece, a powder, monolith and / or a coating. Further processing is known to the person skilled in the art.

Spinning processes of such silica sol materials into fibers or nonwovens have been described, for example, in DE 196 09 551 C1 and DE 10 2004 063 599 A1. The production of powder, monolith and / or a coating starting from the silica sol material according to the invention has been described, for example, in WO2008 / 086970A1, WO2008 / 148384A1 and WO2009 / 077104.

Preferably, all preparation steps (a) to (c) are carried out in one and the same reaction vessel. Preferably, all preparation steps (a) to (c) are carried out with moderate stirring. The reaction vessel is preferably a stirred tank, which comprises the following features: 1) It is closable and pressure-resistant to at least 10 bar and at least from -20 ° C to 80 ° C tempered. The pressure and temperature are recorded, displayed and can be regulated. 2) It has an access to the metering of the respective liquid components, a bottom outlet valve for taking out the product and a gas access for pressure application or to remove part of the gaseous alcohol and the aqueous acid by distillation from the reaction mixture.

It is conceivable to equip the reaction vessel with probes for measuring the viscosity of the reaction mixture. Other probes, such as IR or Raman probes, can be used to monitor the concentrations of reaction components.

It has surprisingly been found that the individual reaction steps (a) to (c) obey a simple Arrhenius equation. The pre-exponential factor and the activation energy can be - - NEN be determined empirically by methods known in the art. Thus, it is possible to either predict the time for a given reaction temperature, when the reaction will be completed or to determine the required reaction temperature for a given reaction time. In contrast to the prediction methods described in the prior art, which allow only a very inaccurate prediction (prediction within a range of ± 2 days), it is possible with the present method to determine the end of the production of the specified silica sol material in a range of ± 2 hours to predict.

The method according to the invention overcomes the disadvantages of the prior art. Controlled merging of the starting materials makes the production process overall easier to control. The reproducibility is increased and the manufacturing process becomes scalable. Further improvements result from the additional measure of stirring the single-phase solution in the ripening step (c), preferably at a temperature of 25 ° C to 35 ° C. In particular, carrying out the ripening at the described preferred temperatures also causes a considerable acceleration of the entire production process without having to accept the required product properties of the silica sol material. By the influencing variables described in the invention, the total synthesis time could be reduced by almost 90%. A prediction method now also makes it possible, among other things due to the improved manufacturing process, to predict the end of the silica sol material production process at the beginning of production within a range of ± 2 hours.

The invention is explained in more detail below by examples, without, however, limiting them to them.

- -

Examples

Figure 1 shows the schematic structure of a synthesis boiler and its immediate periphery for the production of spinnable and biodegradable silica sol material.

EXAMPLE 1 As starting materials for the hydrolysis condensation, 5.4 mol of TEOS (tetraethoxysiloxane) in ethanol were initially charged with 6.8 mol in a closed reaction vessel. 9.6 mol of water in the form of a 0.006 N HN0 ₃ solution were mixed in advance and then metered into the ethanol / TEOS mixture over a period of 1 hour at a constant temperature (isothermal procedure) of 37 ° C. The reaction solution is stirred for 17 hours until reaching an ethanol concentration of about 68% by weight. Thereafter, the single-phase solution was successively evaporated at a constant bottom temperature of 62 ° C. and a pressure of 500 to 120 mbar to a viscosity of 1 Pa.s at a shear rate of 10 s ^-1 at 4 ° C. The ripening of the silica sol The material was stirred at a temperature of 28.1 ° C to a viscosity of 55 Pa · s at a shear rate of 10 s ^"1 at 4 ° C and a loss factor of 3. 2. Embodiment

Ethanol (2.6 mol, 100%) was placed in a closed reaction vessel. The remaining ethanol (4.2 mol, 100%) was metered in together with 5.4 mol of TEOS via an access to the ethanol in the reaction vessel over a period of one hour in a controlled manner. At the same time 9.6 mol of water in the form of a 0.006 N HN0 ₃ solution over a period of one hour were added via another access to the reaction vessel. The reaction was carried out so that the bottom temperature in the reaction vessel during the entire reaction is 37 ° C (isothermal mode). The further process steps were carried out as described in the first embodiment, with the exception of the ripening temperature. The ripening temperature was 4 ° C.

3. EMBODIMENT Ethanol (6.8 mol, 100%) was charged together with 9.6 mol of water in the form of a 0.006 N HNO ₃ solution in a closed reaction vessel. 5.4 mol of TEOS (tetraethoxysiloxane) were added to the mixture in the reaction vessel in a controlled manner over a period of one hour. The reaction was carried out so that the bottom temperature in the reaction vessel during the entire reaction was 37 ° C (isothermal mode). The further procedural steps were carried out as described in the first exemplary embodiment. The maturation of the - -

Silica sol material was stirred at a temperature of 7 ° C to a viscosity of 30 Pa · s at a shear rate of 10 s ^-1 at 4 ° C and a loss factor of 3.1.

Claims

claims

1. A method for producing a silica sol material by

(a) Controlled combining of an aqueous acid solution and a hydrolyzable silicon compound, (b) subsequent evaporation to a single phase solution having a viscosity in the range of 0.5 to 30 Pa · s at a shear rate of 10 s ^-1 at 4 ° C, and

2. The method according to claim 1, characterized in that the controlled addition in step a) takes place over a period of at least 15 minutes.

3. The method according to any one of the preceding claims, characterized in that the step a) is carried out under isothermal conditions.

4. The method according to any one of the preceding claims, characterized in that the bottom temperature is kept constant at step b) by pressure regulation.

5. The method according to any one of the preceding claims, characterized in that the maturation in step c) is carried out with stirring the single-phase solution.

6. The method according to any one of the preceding claims, characterized in that the ripening in step c) is carried out at a temperature of -25 ° C to 78 ° C under a normal pressure of 1 atm or at temperatures> 78 ° C at overpressure ,

7. The method according to any one of the preceding claims, characterized in that step a) to step c) is carried out in the same reaction vessel.

8. Silica sol material prepared by the controlled addition of a hydrolyzable silicon compound to an acid in a solvent, followed by evaporation to a single-phase solution with a viscosity in the range of 0.5 to

2 Pa.s at a shear rate of 10 s ^-1 at 4 ° C., and ripening of this single-phase solution to a silica sol material having a viscosity of 30 Pa.s at a shear rate of 10 s ^-1 at 4 ° C. and a loss factor from 3.1.