Method for producing aerogels and aerogels obtained using said method

Abstract

The present invention relates to a method for producing aerogels using a sol-gel process, wherein first a lyogel is formed from at least two precursor sols and then the lyogel is converted into an aerogel.

Description

[0001] The present invention relates to the technical field of producing aerogels. In particular, the present invention relates to a method for producing aerogels using a sol-gel process.

[0002] Furthermore, the invention relates to aerogels, in particular obtainable by the process according to the invention, and their use, in particular as or in insulating materials.

[0003] Furthermore, the invention relates to an apparatus for producing aerogels.

[0004] Finally, the invention relates to a method for producing a lyogel using a sol-gel process.

[0005] Aerogels are highly porous solids whose volume can consist of up to 99.98% pores. Aerogels usually consist of a dendritic structure with strong branching of partial chains, resulting in many interstices, especially in the form of open pores. The chains have a large number of contact points, resulting in a stable, sponge-like structure. The pore size is usually in the nanometer range and the internal surface area can be up to 1,000 m 2 / g or more. Aerogels can be composed of various materials, such as silica, plastic or carbon, as well as natural organic polymers, such as alginates, or metal oxides.

[0006] Due to their high porosity, aerogels are often used as insulating materials, for example for thermal insulation purposes, or as filter materials. Similarly, aerogels are also used as storage materials, for example for liquids or gases.

[0007] Aerogels are nanostructured, open-pored solids which are usually produced by the sol-gel process.

[0008] Aerogels are usually produced by drying colloidal gels, mainly concentrated silicic acid. Aerogels obtained with silicic acid and similar starting materials, such as silica sols, silane hydrolysates or silicates, include SiO2 structural units and are generally referred to as silica aerogels. Steven Kistler completed the first synthesis of silica aerogels in 1931 / 1932, when he was the first person to develop a method for drying gels, and the gel did not show any shrinkage (Kistler SS, The Journal of Physical Chemistry 1932, 36 (1): Coherent expanded Aerogels, pp.52-64). In the method developed by Kistler, water glass is used as a starting material, and in the first step, a silica hydrogel is obtained from water glass by acidification with a mineral acid. In the next step, the gel is washed without alkali metal ions. The water contained in the hydrogel is then completely exchanged for ethanol or methanol. This is followed by supercritical drying of the resulting alcohol gel in an autoclave.

[0009] At the same time, other methods have been developed, such as the method described in DE 18 11 353 A. DE 1811 353 A discloses a method for producing silica aerogels, in which tetraethoxysilane (TEOS) is hydrolyzed in methanol or ethanol with precisely metered amounts of water and a catalyst. During the hydrolysis, a SiO2 gel in the form of an alcohol gel is formed under the breakup of alcohol and water. The alcohol gel is then supercritically dried in an autoclave. The method can also be used to produce organic aerogels from melamine formaldehyde resins and resorcinol formaldehyde resins. In the supercritical drying technique, the gel to be dried is subjected to temperature and pressure conditions that at least reach the critical point of the solvent used.

[0010] The disadvantages of such supercritical drying techniques are the temperature and pressure conditions and the discontinuous mode of operation, based on the supercritical conditions of the solvents used. For example, when drying a water-containing gel, a temperature of at least 370° C. and a pressure of at least 220 bar are required. When drying a gel containing methanol, a temperature of at least 240° C. and a pressure of at least 81 bar are required.

[0011] An alternative to this supercritical drying process is to use compressed carbon dioxide. For example, a method for drying with supercritical carbon dioxide is disclosed in EP 171 722 A. In this process, the organic solvent is exchanged for liquid carbon dioxide before supercritical drying. The supercritical drying with CO2 then takes place at much lower temperatures, for example at a critical temperature of 31.1°C and a critical pressure of 73.9 bar for carbon dioxide.

[0012] Aerogels are usually produced industrially using the Cabot process. This is described, for example, in DE 19 648 798 A and DE 69 903913 T2. For this purpose, diluted sodium silicate is reacted with hydrochloric acid at 60 to 80° C., wherein the gelation time, i.e. the time required for gel formation, can be set to a few minutes. In order to solidify and mature the gel, the gel is then tempered at 80 to 100° C. The aging time is prescribed as 30 minutes. During or after the aging process, the gel is washed until the washing water is free of electrolytes.

[0013] This is followed by silanization of the hydrogel to enable subcritical drying. Trimethylchlorosilane is used as the silanizing agent. Trimethylchlorosilane reacts primarily with the water present in the hydrogel to form trimethylsilanol and further condenses to form hexamethyldisiloxane, which is incorporated into the pores and partially replaces the water.

[0014] It should be noted here that the silanizing agents used are added in very large amounts. For example, 100 g of hydrogel are reacted with 140 ml of trimethylchlorosilane. Only with this ratio of hydrogel to trimethylchlorosilane can a partial reaction of the hydroxyl groups on the silicon be achieved. As an alternative silanizing agent, hexamethyldisiloxane and hydrochloric acid are used in a gas stream. Here, a partial reverse reaction of hexamethyldisiloxane to trimethylchlorosilane occurs, which can then react with the hydroxyl groups of the silicon.

[0015] Looking at the molar ratio of HCI to hexamethyldisiloxane in the examples of the above-mentioned patents and patent applications, it can be seen that the hexamethyldisiloxane is added in a five- to six-fold excess and only a small portion of the hexamethyldisiloxane used can react to form trimethylchlorosilane. This shows the importance of incorporating the hexamethyldisiloxane in the pores of the lyogel. Only subcritical drying is possible in this way. The drying itself is then carried out in a nitrogen stream at 200°C.

[0016] The importance of the molar ratio of silanizing agent to SiO2 network is discussed in more detail in the Aerogel Handbook (MAAergerter et al., Aerogels Handbook, Advances in Sol-Gel Derived Materials and Technologies, 2011, p. 120). The hydrophobization step, using large amounts of toxic, flammable and corrosive trimethylchlorosilane, represents the most expensive process step in the production of aerogels by the Cabot process.

[0017] In the drying process, it is also often found that solvent exchange, especially from a polar solvent to a less polar solvent, is important for successful drying.

[0018] From the studies of sodium silicate-based aerogels by Kistler it is known that the solvent exchange from water to ethanol does not lead to any significant changes in the pore geometry. This result is independent of whether the solvent exchange is carried out in one step or in several steps with increasing ethanol content. For the direct supercritical drying of SiO2 gels from ethanol implemented there, an ethanol mass fraction of 95 wt.% is sufficient. On the other hand, for supercritical drying with CO2, no values ​​are known.

[0019] Gurikov et al. studied the solvent exchange from water to ethanol under the influence of compressed CO2. The gel used is composed of alginate and uses CO2 induced gelation production. The sample includes a diameter of 10 to 12mm and is positioned in a preheated autoclave and subjected to supercritical CO2 (120 bar, 313K). Then the mixture of water and ethanol is pumped into the autoclave in several stages, and each stage carries out 2.5 hours of solvent exchange, wherein the ethanol content of 30wt.% is realized in the first stage, the ethanol content of 60wt.% is realized in the second stage, and the ethanol content of 90wt.% is realized in the third stage. Then CO2 is rinsed with 25wt.% of ethanol to extract water completely from the hole before supercritical drying gel 3 hours. The progress of the composition analysis solvent exchange calculated by the solvent density is used. For this purpose, 5ml sample is taken out from each autoclave. Under the conditions given, the required time of each solvent exchange step is reduced to 2.5 hours from 12 hours.

[0020] The use of supercritical carbon dioxide during the solvent exchange additionally reduced the required drying time from 6 hours to 3 hours.

[0021] The density of the gel after solvent exchange under the influence of compressed CO2 was 0.021 g / cm 3 According to BET, the specific surface area is 538m 2 / g and a pore volume of 5.96 cm according to BJG 3 The obtained aerogels comprise properties similar to those of reference samples prepared via solvent exchange under ambient conditions. A direct effect of solvent exchange under pressure on the properties of the prepared aerogels cannot be inferred from the available data, since different synthesis conditions were used for different processes.

[0022] In addition to the aforementioned challenges of stabilizing the gel during the drying process, another problem in producing aerogels, especially silica aerogels, is represented by long process times. These make aerogel production more expensive and thus prevent the use of aerogels in a large number of applications for which aerogels would be suitable based on their physical property profile. The respective process times for the individual process steps for producing silica aerogels from tetraethyl orthosilicate (TEOS) are as follows:

[0023] - The hydrolysis and condensation time is at least 8 hours (see AATweij Wesam, Temperature Influence on the Gelation Process of Tetraethylorthosilicate using Sol-Gel Techique, Iraqi Journal of Science 2009).

[0024] - Gel aging time ranges from 6 to 72 hours (see Einarsrud, M.-A., Kirkedelen, MB, Nilsen, E., Mortensen, K., Samseth, J., Structural Development of Silicagels aged in TEOS, Journal of Non-Cryst Solids 231, 1998, pp. 10-16).

[0025] -Supercritical washing time / solvent exchange about 24 hours / each washing cycle (see Kerstin Quarch, Product design on colloidal agglomerates and gels, gelation and fragmentation of inorganic silica, PhD thesis, KIT, 2010).

[0026] - Supercritical drying time strongly depends on previous solvent exchange and sample size

[0027] A one-pot method developed by the Swiss Federal Laboratories for Materials and Testing (EMPA) offers a significant improvement, with the individual steps taking the following time:

[0028] - By using hexamethyldisilazane (HDMSO), ammonia, water, ethanol and TEOS, the gel formation and aging time is about 2 hours.

[0029] - Hydrophobization of the wet gel was performed using a mixture of HCl and HDMSO over a period of 1 hour.

[0030] -Supercritical drying time is about 1 hour.

[0031] Therefore, the total process time is between 4 and 6 hours.

[0032] However, even these process times still pose significant challenges for large-scale industrial production, wherein in particular the hydrophobization also requires the use of large amounts of residual hydrophobizing agent to achieve the necessary hydrophobization for solvent exchange.

[0033] Another common problem for all the above-mentioned methods for producing aerogels is that ill-defined particles are generally obtained without a regular external shape, which are difficult to use for loose filling or even difficult to incorporate into insulating plaster systems. These irregular particles are much less elastic mechanically and form spherical packings with a density lower than would be expected for regular particles, especially spherical particles. For this reason, in practice the effectiveness of the aerogels generally does not reach the calculated values.

[0034] Therefore, the prior art still lacks a system for reproducibly producing aerogels with significantly reduced process times, enabling continuous or quasi-continuous production at reduced costs. Furthermore, it is likewise not possible to produce aerogels with defined geometries on an industrial scale and in a reproducible manner. For many applications, particularly spherical, i.e. spherical, aerogel particles are preferred, since these are likely to comprise significantly higher mechanical load-bearing capacities.

[0035] Similarly, it has not been possible to produce aerogel particles of a preselected size in a directional manner.

[0036] The present invention now aims to eliminate or at least alleviate the disadvantages associated with said prior art.

[0037] In particular, it was an object of the present invention to provide a process for producing aerogel particles which can be carried out with significantly shorter process times and preferably continuously or quasi-continuously.

[0038] A further object of the invention is to be able to produce aerogels with defined properties in a directional manner, in particular also aerogels with a defined shape and a defined particle size.

[0039] Furthermore, a further object of the present invention is to provide an aerogel which is mechanically elastic and particularly suitable for use as an insulating material.

[0040] The above stated objects are solved according to the invention by the method according to the invention, advantageous further developments and embodiments of the method according to the invention being the respective subject matter of the invention.

[0041] A further subject of the invention according to the second aspect of the invention is an aerogel according to the invention. Further advantageous embodiments of this aspect of the invention are respective subjects of the invention.

[0042] Furthermore, a further subject of the invention according to the third aspect of the invention is the use of the aerogel according to the invention.

[0043] A further subject of the invention according to the fourth aspect of the invention is the use of the aerogel according to the invention.

[0044] Furthermore, another subject of the invention according to the fifth aspect of the invention is a device according to the invention for producing aerogels.

[0045] Finally, a subject of the invention according to a sixth aspect is a method according to the invention for producing a lyogel.

[0046] It goes without saying that, for the purpose of avoiding unnecessary repetition, the specific features, characteristics, embodiments and advantages etc. described below with respect to only one aspect of the present invention are of course correspondingly applicable to other aspects of the present invention without explicit mention.

[0047] Furthermore, it applies that all values ​​or parameters etc. mentioned below can in principle be determined by standardized or well-defined determination methods or determination methods with which the person skilled in the art is familiar.

[0048] Furthermore, it goes without saying that all percentages relating to weight or amount are selected by a person skilled in the art in such a way that the total results in 100%; however, this goes without saying.

[0049] With this proviso made, the present invention will now be described in more detail below.

[0050] According to the present invention First The subject matter of the present invention in this aspect is a method for producing silica aerogels using a sol-gel process, wherein first a lyogel is formed and then the lyogel is converted into an aerogel, wherein, to produce the lyogel, at least two precursor sols, preferably two precursor sols, are mixed with one another, wherein the first precursor sol comprises an acidic pH value or an alkaline pH value and the second precursor sol comprises a pH value different from the first precursor sol.

[0051] For, since the applicant has surprisingly found that, based on the method according to the invention, firstly a particularly homogeneous, uniformly structured and uniformly formed lyogel can be obtained in a particularly time-efficient manner, from which preferably spherical aerogel particles can then be obtained.

[0052] A particular advantage of the present invention is that the production of the lyogel, in particular the rate of formation of the lyogel, can be precisely and specifically influenced based on a precursor sol for producing or forming the lyogel, the precursor sol in particular comprising a specially adjusted pH value. In this regard, it has proven to be very useful if the method according to the invention, in particular the method for the production of the lyogel, is carried out within a specific pH value range, wherein these pH value ranges can be precisely adjusted and controlled by means of the procedure according to the invention. For example, by converting the pH from a neutral to a slightly acidic or alkaline range, it is possible to achieve a relatively slowing down of the gel formation or gelation by a few seconds, so that homogenization and uniform mixing of the precursor sol are possible immediately before the start of gel formation. In contrast, in particular, if the method according to the invention is carried out at a neutral pH, instantaneous gelation occurs. Therefore, the present invention particularly utilizes the pH dependence of the gelation or gel formation rate in order to exert a specific influence on the lyogel formation on this basis, in particular with regard to the shaping and gel particle size. For example, the method according to the invention makes it possible to produce spherical lyogel particles, the diameter of which can be adjusted to several millimeters.

[0053] The present invention is characterized by a simple and easy-to-understand procedure, wherein gel formation can be carried out in particular without pressure or at atmospheric pressure or at a pressure that is only slightly elevated, preferably at a pressure of no more than 40 bars. Up to now, it is impossible to produce spherical silica aerogels or liquid gels from preferably at least part of aqueous-based precursor sols without pressure, at least it is impossible to produce spherical silica aerogels or liquid gels with a method that can be quickly and easily carried out. The only known method is that the sol is dropped into oil at a relatively low pressure so that spherical particles are shaped and then slowly gelled (see Lee, Kyoung-Jin, Fast Synthesis of Spherical Silica Aerogel Powders by EmulsionPolymerization from Water Glass, ChemistrySelect, Vol 3.Issue 4, January 31, 2018). However, in particular, in the context of the present invention, it is unnecessary or intentional to use oil as a solvent or surrounding medium.

[0054] Furthermore, rapid gelation of the precursor sol can be achieved in the process according to the invention, in particular within a few seconds. In this way, short process times can be achieved with low equipment expenditure. In particular, within the scope of the present invention, the process time for producing a silica aerogel from gel formation to drying can be reduced to 1 to 2 hours, in particular to a time of less than 1.5 hours, if all process steps are carried out in the same reaction apparatus.

[0055] The main difficulty or challenge in producing aerogels is in particular the production of uniform, homogeneous and structurally homogeneous particles. This difficulty can be reliably overcome with the method according to the invention. In particular, the controlled metering or mixing of different precursor sols makes it possible to time the solvation in such a way that a uniform, homogeneous mixing or distribution of the precursor sols with one another is ensured. Likewise, within the scope of the present invention, the gel formation and the introduction or incorporation of the precursor sol or the gel formed therefrom can be controlled in such a way that a stable gel can be obtained, in particular in the form of spherical gel particles, wherein these particles can then be further converted into, in particular, spherical aerogel particles.

[0056] Here, in particular, the method according to the invention also makes it possible to particularly influence or adjust the size of the lyogel particles or their particle size distribution. In particular, the gelation time or the period of time for gel formation and the manner in which the precursor sols are mixed with one another and then immediately supplied to the reaction device according to the invention allow the targeted production of lyogel particles of defined shape and size. The particles according to the invention do not lose this shape and size, in particular the spherical or spherical shape and size, even if the lyogel is converted into an aerogel.

[0057] The aerogel particles obtained by the method according to the invention, in particular spherical or cylindrical aerogel particles, are characterized by better flowability, higher strength under uniaxial compressive load and more optimal packing density over known prior art aerogels of predominantly non-uniform shape or cubes, which can be attributed in particular to the uniform spherical or spheroidal shape. The aerogels according to the invention, in particular those having a spherical or spheroidal shape, have not been obtainable by the methods known in the prior art for producing aerogels. Only the procedure or method according to the invention makes it possible to reliably produce aerogel particles having in particular a spherical shape and a circular cross section.

[0058] The spherical aerogel particles obtainable by the process according to the invention are extremely suitable as thermal insulation material, in particular in loose packing, but also for incorporation into insulating plaster systems due to their excellent mechanical properties or electrical resistance and the possibility of producing dense spherical packings. The spherical aerogels according to the invention can preferably be used in powder fills or powder mixtures, such as insulating plasters, due to, in particular, improved flowability, higher strength under uniaxial compression load and higher packing density compared to conventional aerogel powders, which are usually based on amorphous or cubic particles.

[0059] In the context of the present invention, a sol-gel process is understood to be a process in which non-metallic inorganic or organic materials or inorganic-organic hybrid materials are obtained from colloidal dispersions (so-called sols). In the sol-gel process, particles in the nanometer range are usually obtained from the colloidal dispersion sol by aggregation, which subsequently form a gel by further condensation and aggregation, i.e. a three-dimensional network whose pores are filled with a fluid, wherein the fluid is a liquid or a gas.

[0060] In the context of the present invention, gel is understood to be a dimensionally stable dispersed system rich in liquid and / or gas and composed of at least two components, these components are at least solid and fluid, the solid is a colloidal dispersed material with long or extensive branched particles, such as gelatin, silicic acid, montmorillonite, bentonite, polysaccharide, pectin and others, the fluid is particularly gas or liquid, as a dispersant. In this case, the solid substance is coherent, that is, it forms a spatial network in the dispersant, wherein the particles adhere to each other at different points (so-called adhesion points) by secondary or primary valence. If the space between the particles is filled with liquid, liquid gel occurs. If the dispersant is air, gel is referred to as aerogel. For more details about the term gel, please refer to ROEMPP Chemie Lexikon, the 9th expanded and newly revised version, volume 2, 1999, the 1511th page (ROEMPP Chemie Lexikon, 9th expanded and newly processed edition, belt 2, 1999, p.1511) keyword "gel (gels)" entry.

[0061] A lyogel is a gel, i.e. a three-dimensional network whose pores are filled with a liquid. Specific cases of lyogels are hydrogels (in which the liquid is water) or alcogels (in which the liquid is an alcohol, usually ethanol). Lyogels containing organic solvents are also called organogels.

[0062] In the context of the present invention, sol means a solution or a finely divided dispersion, ie a colloidal dispersion.

[0063] A solution in the context of the present invention is understood to be a single-phase mixture in which one substance - the solute - is homogeneously distributed in a second substance - the solvent. In the context of the present invention, a dispersion is understood to be a two-phase mixture in which a first phase (the so-called discontinuous phase) with a dispersed substance is finely distributed, in particular homogeneously distributed, in a second phase (dispersant or continuous phase). The transition from solution to dispersion is fluid and cannot be strictly distinguished from each other; for example, a colloidal solution cannot be clearly assigned to a solution or a dispersion. Even in the case of a "solution" of polymer macromolecules, it is impossible to clearly determine whether a solution or a dispersion exists. In the context of the present invention, therefore, a sol is preferably understood to mean a solution or a finely dispersed, i.e., a colloidal dispersion.

[0064] With regard to carrying out the method according to the invention in detail, it is particularly envisaged that the precursor sols are mixed with one another and supplied to the reaction apparatus. It is further preferred if the precursor sols are mixed and subsequently supplied to the reaction apparatus, in particular immediately after mixing. In this context, it is again further preferred if the precursor sols, in particular immediately after mixing, are supplied to the reaction apparatus in the form of droplets, in particular dropwise.

[0065] Here, it is particularly preferred according to the invention if the in particular mixed precursor sol is sprayed or dropped, in particular dripped, into the reaction apparatus.

[0066] It has further proved to be particularly advantageous if the precursor sol is supplied to the reaction apparatus in the form of droplets, in particular dropwise, with pressure applied to the reaction apparatus, preferably by spraying or dropping the precursor sol, more preferably dripping it into the reaction apparatus.

[0067] Furthermore, in the context of the present invention, it has proven to be advantageous if the precursor sols used to produce the lyogel are mixed continuously with one another, in particular by means of a feed system, preferably by means of a two-substance feed system.

[0068] Based on this procedure according to the invention, it can be achieved in particular that the two precursor sols are mixed with one another uniformly and homogeneously, so that gel formation or gelation occurs in a controlled manner, in particular after a controllable or adjustable duration, preferably a few seconds.

[0069] In this context, the method according to the invention is preferably designed in such a way that gel formation or gelation occurs or starts, in particular when the precursor sols mixed with one another are supplied to the device, preferably in the form of droplets or dropwise, or in particular sprayed or falling, preferably at the moment of dripping, i.e. at the moment when the precursor sol enters the device via an inlet opening, in particular a nozzle. Thus, at the moment of droplet formation, due to the start of gelation, both the shape and the size of the formed lyogel particles can be reliably controlled and regulated. According to the invention, this forms in particular the basis for the production of a lyogel which is homogeneous with respect to its particle size distribution and, starting from its in particular dropwise supply to the reaction device, is preferably configured to be spherical or spherical. On this basis, in particular spherical or spherical aerogel particles can also be produced, which is also preferably provided within the scope of the present invention.

[0070] According to a preferred embodiment of the invention, it has proven to be very good if the production of the lyogel is carried out at atmospheric pressure or at only slightly elevated pressure, in particular in a range of less than 40 bar, in particular less than 30 bar, preferably less than 20 bar, more preferably less than 10 bar, particularly preferably at atmospheric pressure, i.e. at a pressure of about 1 bar. In the context of the present invention, it is even further preferred if the production of the lyogel is carried out at a pressure in the range of 1 to 40 bar, in particular 1 to 30 bar, preferably 1 to 20 bar, more preferably 1 to 10 bar. In the context of the present invention, the above-mentioned preferred pressures or pressure ranges are understood to be absolute pressures or pressure ranges.

[0071] It has therefore proven advantageous within the scope of the present invention if the production of the lyogel is carried out in an apparatus which can be pressurized, in particular an autoclave, for example by supplying the precursor sol to the autoclave.

[0072] In addition, with regard to the lyogel formation, it has proven to be advantageous if the lyogel production is carried out under a controlled atmosphere, in particular a CO 2 , N 2 or Ar atmosphere or an atmosphere consisting of a mixture of these gases, if necessary in combination with further gases or substances. In particular, it has proven to be very good if CO 2 and / or N 2 are used, if necessary in combination with further gases or substances. Usually, CO 2 , a mixture of CO 2 and N 2 or a mixture of N 2 and ammonia (NH 3) is used as process medium or controlled atmosphere. In the context of the present invention, a substance is particularly understood to mean a chemical substance, i.e. a compound or element with specific physical or chemical properties.

[0073] With regard to the provision of the precursor sols, in the context of the present invention it is now generally preferred if the precursor sols are provided separately from one another.

[0074] Furthermore, it has proven to be very advantageous for the method according to the invention if the precursor solutions for producing the lyogel are dosed separately from one another and fed continuously to one another.

[0075] The method according to the invention is therefore characterized in particular in that the precursor sols used, which comprise different pH values ​​from one another, are kept separately from one another and are also metered separately from one another, after which they are then fed continuously to one another in metered form, in particular via a feed system, preferably a two-substance feed, and are mixed with one another during the process or subsequently, in particular in a mixing section. Based on this procedure, a particularly homogeneous and uniform mixing of the precursor sols can be achieved, which can in particular be further positively influenced by the fact that static mixing elements are arranged in the mixing section, which contribute to a thorough swirling and mixing of the precursor solutions in the mixing section.

[0076] In order to form a homogeneous, uniform gel, it has proven to be particularly important in the context of the present invention to control the rate of gel formation based on the pH value which the particularly mixed precursor sol comprises.

[0077] In this context, it has proven to be particularly good if the first precursor sol comprises an acidic pH value and the second precursor sol comprises an alkaline pH value.

[0078] The specific pH value to which the precursor sol is adjusted for this purpose can vary within a wide range of acidic or alkaline pH values. However, in the context of the present invention, particularly good results are obtained if the acidic pH value is in the range of pH 0 to 6, in particular pH 1 to 4, preferably pH 1.5 to 2.5. Likewise, for the process according to the invention, it has proven to be particularly good if the alkaline pH is in the range of pH 7 to 13, in particular pH 8 to 12, preferably pH 9 to 11.

[0079] Ultimately, a decisive factor for the method according to the invention is in particular the pH value of the precursor sols mixed with one another. In this context, it has proven to be particularly good if the precursor sols mixed with one another have a pH value in the range of pH 4.5 to 9.5, in particular pH 5 to pH 9, preferably pH 5.3 to 8.5.

[0080] It is even more preferred if the precursor sols mixed with one another comprise a slightly acidic pH, in particular in the range of pH 4.5 to 6.8, preferably pH 5 to 6.5, or a slightly alkaline pH, in particular in the range of pH 7.5 to 9.5, preferably pH 7.8 to 9.

[0081] Finally, in the context of the method according to the invention, particularly good results are obtained if the precursor sols which are mixed with one another comprise a slightly acidic pH, in particular in the range from pH 4.5 to 6.8, preferably from pH 5 to 6.5.

[0082] In this context, within the scope of the invention, it can additionally be provided that a buffer is added to other precursor sols, in particular precursor sols are mixed with one another. In this context, a buffer is understood to be a mixture of substances, the pH of which changes much less on addition of acid or base than would be the case in an unbuffered system. Thus, by additionally adding a buffer to, in particular, mixed precursor sols, an even more precise control of the pH value or an even more effective stabilization of the pH value can be achieved, which is particularly present after mixing the precursor sols.

[0083] For the pH ranges mentioned above, it can be observed that highly transparent lyogels, or in particular hydrogels, are formed, wherein this particularly includes additional elastic properties within the basic pH range. In contrast, gels formed in the acidic range are preferably characterized by higher strength or reduced elasticity compared to gels formed in the alkaline range. For aerogels obtained from these lyogels, it has been shown that lyogel particles formed in both the alkaline and acidic ranges lead to highly porous gels including exemplary porosities of 95.7% and more. In addition, it can be observed that aerogels, in particular from lyogel particles formed in the acidic state, include a larger specific surface area according to BET and a higher pore volume compared to aerogels from lyogels formed in the alkaline state, wherein a difference of almost 2 times can be determined with respect to the BET surface area, and a difference of 1.3 times can be determined with respect to the pore volume.

[0084] Furthermore, particularly good results are obtained for the process according to the invention if the production of the lyogel is carried out at a temperature above 50° C., in particular 60° C., preferably 70° C., more preferably 80° C.

[0085] At the abovementioned pH values, pressures and temperatures, gel formation can be achieved particularly quickly and also in a controlled manner, whereby, for example, an almost spherical lyogel can be obtained which is dimensionally stable and which can also retain its shape during the further course of the method.

[0086] According to the invention, it has proven to be particularly useful if the production of the lyogel from the mixed precursor sol takes place in less than 60 seconds, in particular less than 30 seconds, preferably less than 20 seconds, more preferably less than 10 seconds, further preferably less than 5 seconds.

[0087] Likewise, it is preferably provided in the context of the present invention that the production of the lyogel from the mixed precursor sol occurs in more than 0.1 second, in particular more than 0.5 second, preferably more than 1 second. It is very particularly advantageous for the method according to the invention that the time period for producing the lyogel is in the range of 1 to 5 seconds.

[0088] According to a preferred embodiment of the invention, it is also preferably provided that the production of the lyogel and the conversion of the lyogel into the aerogel is carried out continuously or quasi-continuously. In fact, based on the method according to the invention, it is particularly possible to shorten the process time, in particular the time of the individual method steps, in such a way that aerogels, in particular silica aerogels, can be produced continuously or at least quasi-continuously. The production can be carried out as a one-pot process, i.e. in a reaction apparatus, in particular in an autoclave, or in a continuous apparatus, in particular in several autoclaves.

[0089] Turning now to the composition and properties of the precursor sol, in the context of the present invention it is generally preferred if the precursor sol is in the form of a solution or dispersion.

[0090] Here, in the context of the present invention, a precursor is understood to be a precursor substance from which the desired target compound, in particular a SiO 2 network, is formed by chemical reaction, in particular for example by hydrolysis or solvolysis and subsequent condensation.

[0091] In principle, therefore, all compounds capable of forming a gel can be used as precursors within the scope of the present invention.

[0092] In this context, it is particularly preferred according to the invention if the precursor sol comprises a silicon-based precursor. With regard to the composition of the precursor sol used according to the invention, it has further proved to be advantageous if the precursor sol comprises silicon in an amount in the range of 3 to 20 wt.%, in particular 4 to 15 wt.%, preferably 5 to 10 wt.%, based on the precursor sol.

[0093] In this sense, it is preferred to use supersaturated precursor solutions in the context of the present invention, since it has been observed that sufficiently fast gel formation can be achieved, in particular when supersaturated solutions are used. For example, the saturation concentration of a suitable precursor such as monomeric silicic acid at pH=7 is about 0.002 mol / l. In contrast, according to the present invention, it is preferred if the silicon concentration in an exemplary silicic acid-based precursor sol is from 0.83 mol / l (5 wt.%) to 1.66 mol / l (10 wt.%). Mathematically, this results in a supersaturation of more than 400 times.

[0094] In the context of the present invention, particularly good results are now obtained if the precursor is selected from the group consisting of silicon dioxide, in particular colloidal silicic acid, silicic acid sols, silicon dioxide sols, silanes, in particular tetraalkoxysilanes, siloxanes, silicates, and mixtures thereof.

[0095] On hydrolysis, the abovementioned compounds give rise to silica networks, optionally organomodified, which are very suitable for the production of silica aerogels.

[0096] In this context, particularly good results are obtained if the precursor is selected from silicic acid, in particular colloidal silicic acid, silica sol and tetraalkoxysilane, preferably tetraethoxysilane and / or tetramethoxysilane. It is particularly preferred if the precursor is silicic acid.

[0097] In a preferred embodiment of the invention, it has proven particularly good if silicic acid is used as precursor for the acid-adjusted precursor sol, which is produced, for example, from sodium silicate using ion exchange and comprises, in particular, a solids content of up to 10 wt.%. The pH value of such a silicic acid solution is pH 3.5 to 2.0. For improved storage stability, the pH value can also be lowered further, for example with hydrochloric acid to a value of about pH 1.

[0098] According to the invention, it is even more preferred for an alkaline precursor sol if the precursor is an alkaline silica solution, which can be produced, for example, from sodium silicate and can then be adjusted to a pH value of 8.5 to 9.5 with a base (e.g. sodium hydroxide solution and / or ammonia). The solids content can preferably be up to 10 wt.%.

[0099] Alternatively, it has also proven to be particularly advantageous if a water glass solution based on sodium and / or potassium water glass, in particular having an SiO 2 content of 10 wt. %, is used as precursor for the alkaline-controlled precursor sol.

[0100] Furthermore, within the scope of the invention it can preferably be provided that for the alkaline-adjusted precursor sol, spherical polysilicic acid molecules with 10 to 40 wt. % SiO2 are used as aqueous colloidal suspension, in particular with a pH in the range of pH 8 to 10.

[0101] Last but not least, it has also proven to be particularly suitable for the process according to the invention if alkoxysilane solutions, in particular prehydrolyzed and partially condensed, are used as precursors for the alkaline precursor sol, preferably with a solids content of 10 to 20 wt.%. Such solutions are in particular partially aqueous solutions, wherein the water content can be adjusted according to the alkoxysilane used and preferably the molar ratio of H2O:number of alkoxy groups is from 1:1 to 2:1. For this preferred embodiment of the invention, it has also been shown to be advantageous if surfactants, such as cetyltrimethylammonium chloride, are added to the alkaline precursor sol, in particular because these can increase the solubility of the partial organoalkoxysilane solution in the acidic precursor sol, such as the acidic aqueous silica solution.

[0102] In the context of the present invention, it is further customary for the precursor sol to comprise at least one solvent or dispersant, respectively.

[0103] In this context, it has proven to be very useful if the solvent or dispersant is selected from alcohols, in particular methanol, ethanol, isopropanol, ethers, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), acetone, propylene carbonate, ethyl acetate, water and mixtures thereof.

[0104] In this context, particularly good results are obtained if the solvent or dispersant consists of an alcohol, in particular methanol, ethanol, isopropanol, water and mixtures thereof. In particular, mixtures of organic solvents and water, in particular ethanol and water, are preferably used in the context of the present invention, since, on the one hand, water causes rapid hydrolysis and condensation of the precursor compounds and, on the other hand, a portion of the organic solvent promotes the removal of the solvent or dispersant from the pores of the lyogel.

[0105] The use of organic solvents such as ethanol, acetone, dimethyl sulfoxide for the gel synthesis also offers the possibility to also use hydrophobic agents such as trimethylsilanol, methyltriethoxysilane, diphenylsilanediol, hexamethyldisilazane etc. directly during the gelation process.

[0106] According to the invention, in order to produce a silica aerogel or lyogel, in particular, a precursor solution based on preferably silica sol, colloidal silicic acid and tetraethyl silicate is first prepared and provided. In the case of silica sol and silicic acid, the precursor solution is pre-siliconized water glass (polysilicic acid) with different degrees of silicification and reduced alkali content. Due to the condensation process, the monosilicic acid, which is usually prepared using ion exchange, is present mainly as disilicate and trisilicic acid.

[0107] On the other hand, silica sols generally include a much higher degree of silicification and generally have a primary particle size of 5 to 40 nm. Compared with tetraethyl silicate (TMOS, TEOS) and potassium silicate generally used in aerogel production, the use of silica sols and silicic acid offers the possibility of selectively controlling the gelation of lyogels or in particular hydrogels and the subsequent aging process. In silica sols and silicic acid, the silica nanoparticles are generally present in a solution stabilized by ionic charge.

[0108] One way to obtain polysilicates with a low water content and a higher proportion of organic solvent is to use tetraethyl silicate, but these must first be prehydrolyzed to ensure sufficiently rapid polycondensation of the monosilicic acid formed. In order to increase the amount of monosilicic acid in the precursor solution or sol, an aqueous solution of silicon dioxide can be added after the hydrolysis of the tetraethyl silicate, so that gel formation can then be initiated to produce an organogel with a low water content.

[0109] As already indicated, it is preferred according to the invention if the precursor sol is a supersaturated sol. In this context, it has further been found to be advantageous if the precursor sol, in particular a supersaturated sol, comprises a certain solid content such that a dimensionally stable gel is configured. The solid content of the sol is understood to be the proportion of the sol that remains after removal of all liquid components.

[0110] In the context of the present invention, it has proven to be good if the precursor sols, in particular individually or independently of one another, comprise a solids content of at least 2 wt.%, in particular 2.5 wt.%, preferably 3 wt.%, more preferably 4 wt.%, in particular 5 wt.%, based on each sol.

[0111] According to a preferred embodiment of the present invention, it is provided in this context that the precursor sols, in particular individually or independently of each other, comprise a solids content in the range of 2 to 30 wt.%, in particular 2.5 to 20 wt.%, preferably 3 to 15 wt.%, preferably 4 to 10 wt.%, particularly preferably 5 to 9 wt.%, based on each sol.

[0112] With a solids content in the abovementioned range, a dimensionally stable lyogel can be obtained particularly quickly which also comprises a desired high pore content.

[0113] In the context of the invention, it can be provided that the precursor sol or in particular at least one of the precursor sols contains a hydrophobizing agent, in particular a silanizing agent. The use of a hydrophobizing agent, in particular a silanizing agent, in one or more precursor sols leads in particular to the incorporation of hydrophobic groups into the framework of the lyogel. This in turn leads to a more elastic gel structure, which is significantly more elastic than, for example, a pure SiO2 structure, for example, during solvent exchange or also during drying to form an aerogel.

[0114] In the context of the present invention, it is preferred if the hydrophobizing agent is selected from organosilanes, in particular monoorganosilanes, diorganosilanes, triorganosilanes, silazanes, silanols, in particular monoorganosilanols, diorganosilanols, and mixtures thereof. In the context of the present invention, organosilanes or organosilanols are understood to mean silanes or silanols having an organic group, in particular a hydrophobic organic group, such as an alkyl, alkenyl or aryl group.

[0115] If silanes are used as hydrophobizing agents in the context of the present invention, their chemical nature can likewise vary within wide ranges. However, particularly good results are obtained if silanes of the general formula I are used. 1 nSr 2 4-n (I)

[0116] in

[0117] n=1 to 3, especially 1 or 2, preferably 1;

[0118] R 1 =C1-to-C 30 -alkyl and / or C6- to C 30 - aryl,

[0119] Especially C2- to C 20-alkyl and / or C6- to C 20 - aryl,

[0120] Preferred C3- to C 20 -alkyl and / or C6- to C 20 - aryl,

[0121] More preferably C4-C 15 -alkyl and / or C6-C 15 - aryl,

[0122] Even more preferably C5-C 12 -alkyl and / or C6-C 12 - aryl,

[0123] C5-C is particularly preferred 12 -alkyl;

[0124] R 2 =halides, in particular chloride, bromide and / or iodide,

[0125] OX, where X = hydrogen, alkyl, aryl, polyether and / or carboxylic acid derivatives,

[0126] In particular, alkyl groups, preferably C1- to C8-alkyl groups, preferably C2- to C4-alkyl groups.

[0127] In the context of the present invention, particularly good results are obtained if the hydrophobic agent is selected from organochlorosilanes, in particular monoorganochlorosilanes, diorganochlorosilanes, triorganochlorosilanes, methoxyorganosilanes, in particular trimethoxyorganosilane, dimethoxydiorganosilane, methoxytriorganosilane, ethoxyorganosilane, in particular triethoxyorganosilane, diethoxydiorganosilane, ethoxytriorganosilane, hexamethylenedisilazane, trimethylsilanol, diphenylsilanediol, phenyltriethoxysilane, trimethylisopropenoxysilane and mixtures thereof. The early use of hydrophobic agents, in particular silanizing agents, before gel formation can influence the network structure formed and control the configured pore size. In addition, the elasticization of the gel network can be achieved by incorporating monofunctional and difunctional silanizing agents. For example, both can be used to accelerate the subsequent solvent exchange of the produced hydrogel.

[0128] In the context of the present invention, as already mentioned, it is preferred if the in particular mixed precursor sol is supplied in the form of droplets into a device, in particular a device to which pressure can be applied, in particular sprayed or dropped in, preferably dropped in.

[0129] By supplying the precursors in the form of droplets, for example by dropping or spraying them into a particularly pressurizable apparatus, such as an autoclave, aerogels having an almost circular cross section can be synthesized. Depending on the regulation of the dropping rate, i.e. the dosage of the precursor sol, or the conditions under which the particularly mixed precursor sol is supplied into the apparatus, almost spherical and / or otherwise cylindrical particles can be obtained. The nozzle can be designed, for example, in the form of a slotted nozzle or a capillary, and the particularly mixed precursor sol can be supplied to the apparatus by a pump, in particular a high-pressure pump.

[0130] The droplet size in this case can be controlled in particular by the selected nozzle and / or gelation speed and is typically in the range of 0.5 to 5 mm when a 2 mm nozzle is used. By selecting a smaller nozzle, the gel particle size can be further reduced. Preferably, the particles formed comprise a spherical shape and retain this shape during the subsequent method steps.

[0131] Thus, the supply of the mixed sol in the form of droplets, in particular, to a pressurizable device, in particular, makes it possible to obtain almost spherical lyogel particles which also remain dimensionally stable during further procedures. This makes it possible to obtain spherical aerogels which, compared with the prior art, comprise improved mechanical properties and can form denser spherical packings and are therefore more suitable as thermal insulation material in loose fill and, for example, for incorporation into insulating plaster systems.

[0132] According to a specific embodiment of the present invention, it can be provided that the precursor sol is pregelled before being supplied to a particularly pressurizable device. Pregelation or precondensation is understood to mean the generation of a larger network structure and aggregates, wherein however, a continuous spatial network is not yet obtained. Pregelled, particularly sols mixed with each other are still flowable, and therefore can be sprayed or dropped into the device. Pregelation can be achieved, for example, by adjusting the path or mixing distance until the precursor sol is supplied to the reaction device. The duration of pregelation depends on the type and concentration of the precursor, the pH value and / or the size and shape of the liquid gel or aerogel particles to be formed, etc.

[0133] In addition to the gelation, which is preferably controlled or initiated by pH regulation according to the invention, the gelation of the precursor sol can also be further influenced, in particular also pre-gelation, and / or the hydrolysis and condensation rates of the silica sol can be accelerated in particular by electrolyte additives such as polyvalent metal salts and denaturing solvents such as ethanol and acetone. The polycondensation ability of the precursor, such as in particular silica, represents here the rate-determining step in the formation of a dimensionally stable three-dimensional network. It has been shown that the use of ethanol and / or electrolytes makes it possible to selectively gelate the precursor sol, or in particular silicic acid and / or silica sol. In this way, organic gels with an ethanol content of 66 vol% can be synthesized. These are characterized by high hydrolysis and condensation rates and the production of a dimensionally stable organic gel network.

[0134] As mentioned above, preferred tetraethyl silicates, such as tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS), offer the possibility of producing organogels with low water content, for example, which can significantly accelerate subsequent solvent exchange. In order to accelerate the gelation rate of these precursor sols, a prehydrolysis of the metal alkoxide can be performed, which can be performed in both acidic and alkaline pH ranges, wherein the configuration of the three-dimensional network is favored in the acidic pH range.

[0135] Mineral acids such as hydrochloric acid can be used as catalysts for pregelation or precondensation. In particular, precondensation can be accelerated by using catalysts such as organic acids (especially acetic acid), inorganic acids (such as hydrochloric acid) or Lewis acids (such as titanium tetrabutyrate).

[0136] The upstream hydrolysis of the silicate esters can be carried out in an alkaline pH range, for example using ammonia at pH 9 to accelerate the downstream hydrolysis.

[0137] Precondensation with acetic acid at a pH of 3.5 to 4.5 and a stoichiometric content of water to tetraethyl orthosilicate of 2.5 to 3.5 produces a precursor sol within a few hours, which can be gelled by pH change and addition of water. In addition, the pH of these precondensed tetraethyl orthosilicate solutions or sols can be changed to the alkaline range.

[0138] According to a preferred embodiment of the present invention, the present invention relates to the method described previously, wherein

[0139] (a) in a first method step, at least two precursor sols, preferably two precursor sols, are mixed with one another, wherein the first precursor sol comprises an acidic pH or an alkaline pH and the second precursor sol comprises a pH value different from the first precursor sol, and

[0140] (b) in a second process step following the first process step (a), are mixed with one another and, in particular immediately after mixing, supplied to a reaction apparatus, preferably in the form of droplets, more preferably dropwise, wherein a microgel is obtained.

[0141] For this particular embodiment of the method according to the invention, all advantages and properties and features mentioned above apply equally.

[0142] In the context of the present invention, it can further be provided that the lyogel is aged after its production. If the lyogel is aged, it is preferred if the lyogel is aged for a period of 1 minute to 1 hour, in particular 5 to 50 minutes, preferably 10 to 45 minutes, more preferably 15 to 40 minutes. Aging the lyogel in particular solidifies the gel structure, making it significantly more stable and resistant to subsequent drying processes.

[0143] Preferably, the aging of the lyogel is carried out at the temperature at which the lyogel is produced. In this context, it is preferred if the aging of the lyogel is carried out at a temperature above 50° C., in particular 60° C., preferably 70° C., more preferably 80° C. According to the invention, particularly good results are obtained here if the aging of the lyogel is carried out in a temperature range of 50 to 150° C., in particular 60 to 140° C., preferably 70 to 130° C.

[0144] The pressure at which the aging process is carried out can vary within wide ranges. However, in the context of the present invention, it is particularly preferred if the aging of the lyogel is carried out at the same pressure as used in the production of the lyogel.

[0145] Thus, in the context of the present invention, the aging time of lyogels, in particular hydrogels, which usually requires at least 2 hours, can be reduced to about 30 minutes.

[0146] Within the scope of the invention, it can be provided that after the production of the lyogel, in particular after process step (b), a solvent exchange is carried out, in particular in the third process step (c). A solvent exchange may be necessary, in particular, to facilitate the subsequent drying of the lyogel to form an aerogel.

[0147] In particular, the water added to the precursor sol during the drying process is difficult to remove from the usually hydrophilic network of the lyogel, in particular the SiO2 network, by adding thermal energy. This is also true if the lyogel has been hydrophobized. The lyogel particles, in particular hydrogel particles, which are produced and have a circular cross section in particular, therefore generally have a water content that makes drying difficult. However, it has been shown that a reduction in the water in the initially used precursor sol, in particular the silicic acid solution, can significantly accelerate the drying rate of the lyogel particles, depending on the organic solvent added. Alternatively or additionally, an exchange of the solvent, in particular water, with a more volatile solvent can then be used to facilitate the subsequent drying process.

[0148] Thus, in particular in order to reduce the water content of a previously prepared lyogel, in particular a hydrogel or organogel, before the actual drying step, it may be necessary to subject the gel to a solvent exchange, for example to coat the particles with an organic solvent.

[0149] In this context, it is preferred if the lyogel is contacted with a liquid or gaseous organic solvent to effect a solvent exchange.

[0150] The organic solvent can be supplied to the reaction chamber in gaseous form and then displace the water or other organic solvent stored in the pores of the lyogel. Similarly, the lyogel can also be brought into contact with a liquid solvent, in particular dispersed therein or covered by it, and thus achieve extensive solvent exchange, for example, by multiple coverage with the solvent and removal of a mixture of water and / or organic solvents. Preferably, the solvent with which the solvent exchange is carried out is soluble in the drying gas, in particular carbon dioxide. In this way, for example, supercritical drying with carbon dioxide can be carried out much faster and more gently.

[0151] In the context of the present invention, it is also preferred if the solvent exchange reduces the water content of the lyogel in particular to values ​​of less than 30 wt.%, in particular less than 20 wt.%, preferably less than 15 wt.%, more preferably less than 10 wt.%, based on the lyogel. By reducing the proportion of water in particular in the lyogel, targeted and gentle drying with carbon dioxide in the supercritical range becomes possible.

[0152] In the context of the present invention, it is preferably provided that the solvent exchange, in particular the contacting of the lyogel with a solvent, is carried out at atmospheric pressure or at a moderately elevated pressure, in particular in the range from 1 to 40 bar. Surprisingly, it has been shown that, at temperatures in particular above 80° C., a pressure just above the vapor pressure of the solvent used is already sufficient to achieve a solvent exchange. Preferably, in the context of the present invention, during the solvent exchange, the liquid solvent or the mixture of water and organic solvent is removed from the apparatus, or the gas phase contaminated with water is at least partially removed from the reactor, and the new solvent is supplied to the reactor in the gaseous state in order to obtain a solvent exchange that is as complete as possible.

[0153] In the context of the present invention, therefore, particularly good results are obtained if the solvent exchange, in particular the contacting of the lyogel with a solvent, is carried out at atmospheric pressure or at a moderately elevated pressure, in particular at a pressure in the range of 0 to 40 bar, preferably 0 to 30 bar. In this context, it has further proved to be very good if the solvent exchange is carried out under a controlled atmosphere, in particular under a CO 2 , N 2 or Ar atmosphere or an atmosphere consisting of a mixture of these gases, since this also particularly preferably provides for the lyogel formation.

[0154] Now, with regard to the temperature range in which the solvent exchange is carried out, it has proven to be very good if the solvent exchange is carried out at elevated temperature. In this context, particularly good results are obtained if the solvent exchange, in particular the contacting of the lyogel with the solvent, is carried out at a temperature above 70° C., in particular above 80° C., preferably above 90° C., more preferably above 100° C., particularly preferably above 110° C. By means of high temperatures, especially under preferably applied pressure, surprisingly the most rapid and complete solvent exchange possible can be achieved.

[0155] In this context, it is likewise conceivable that the solvent exchange, in particular contacting the lyogel with a solvent, is carried out at a temperature in the range of 70 to 180° C., in particular 80 to 160° C., preferably 90 to 150° C., more preferably 100 to 140° C., preferably 110 to 130° C.

[0156] Now, as far as the organic solvent used in the solvent exchange process is concerned, it has proven to be good if the solvent is selected from the group of hydrophilic organic solvents, hydrophobic organic solvents and mixtures thereof. In the context of the present invention, it is particularly preferred if the organic solvent is soluble in carbon dioxide.

[0157] In the context of the present invention, an organic solvent is understood to be a solvent or dispersant which comprises an organic group.

[0158] Now, as far as the organic solvent is concerned, it has proven to be very good if the organic solvent is selected from the group of alcohols, ethers, dimethyl sulfoxide, N,N-dimethylformamide, C5 to C8 alkanes and mixtures thereof. In the context of the present invention, particularly good results are obtained if the organic solvent is selected from the group of methanol, ethanol, isopropanol, dimethyl sulfoxide, n-pentane, n-hexane, n-heptane, cyclohexane and mixtures thereof. The above solvents not only allow solvent exchange but also facilitate subsequent drying. The solvent is also ideal for contacting the lyogel with the modifying agent.

[0159] In particular, in the context of the present invention, it can also be provided that the organic solvent is brought into contact with the lyogel together with a hydrophobizing agent, in particular a silanizing agent. Therefore, within the scope of the present invention, the lyogel can also be hydrophobized, in particular silanized, during the solvent exchange, so that the hydrogel can subsequently be simply dried and converted into an aerogel. In order to achieve a particularly effective hydrophobization, in particular a silanization, it is advantageous if the water content of the lyogel is at least 50 wt.%, in particular at least 60 wt.%, preferably at least 70 wt.% at the beginning of the contact of the organic solvent and the hydrophobizing agent with the lyogel. In this way, a rapid hydrolysis and reaction of the reactive groups of the hydrophobizing agent, in particular the silanizing agent, is provided.

[0160] Now, as far as the chemical nature of the hydrophobizing agent is concerned, it has proven to be very good if the hydrophobizing agent is selected from organosilanes, in particular monoorganosilanes, diorganosilanes, triorganosilanes, silazanes, silanols, in particular monoorganosilanols, diorganosilanols, and mixtures thereof.

[0161] If silanes are used as hydrophobizing agents in the context of the present invention, their chemical nature can vary within wide ranges. However, particularly good results are obtained if silanes of the general formula I are used.

[0162] R 1 nSr 2 4-n (I)

[0163] in

[0164] n=1 to 3, especially 1 or 2, preferably 1;

[0165] R 1 =C1-to-C 30 -alkyl and / or C6- to C 30 - aryl,

[0166] Especially C2- to C 20 -alkyl and / or C6- to C 20 - aryl,

[0167] Preferred C3- to C 20 -alkyl and / or C6- to C 20 - aryl,

[0168] More preferably C4-C 15 -alkyl and / or C6-C 15 - aryl,

[0169] Even more preferably C5-C 12 -alkyl and / or C6-C 12 - aryl,

[0170] C5-C is particularly preferred 12 -alkyl;

[0171] R 2 =halides, in particular chloride, bromide and / or iodide,

[0172] OX, where X = hydrogen, alkyl, aryl, polyether and / or carboxylic acid derivatives,

[0173] In particular, alkyl groups, preferably C1- to C8-alkyl groups, preferably C2- to C4-alkyl groups.

[0174] In this context, particularly good results are obtained if the hydrophobizing agent is selected from the group consisting of organochlorosilanes, in particular monoorganochlorosilanes, diorganochlorosilanes, triorganochlorosilanes, methoxyorganosilanes, in particular trimethoxyorganosilane, dimethoxydiorganosilane, methoxytriorganosilane, ethoxyorganosilanes, in particular triethoxyorganosilane, diethoxydiorganosilane, ethoxytriorganosilane, hexamethylenedisilazane, trimethylsilanol, diphenylsilanediol, phenyltriethoxysilane, trimethylisopropenoxysilane and mixtures thereof.

[0175] The hydrophobizing agent preferably used during the solvent exchange therefore corresponds to the hydrophobizing agent also used during the hydrophobizing or silanization of the particularly mixed precursor sol. In the context of the present invention, it is particularly preferred if a hydrophobizing agent, particularly a silanizing agent, is both added to the precursor sol and further hydrophobizing is carried out after the lyogel has formed.

[0176] The hydrophobization after the production of the lyogel, in particular as part of a solvent exchange or as a separate process step, results in the hydrophobization of the pores of the lyogel. During the solvent exchange, the use of additional hydrophobizing agents, in particular silanizing agents, can achieve a hydrophobization of the pores, in particular a silanization of the pores. In this context, it has been found in particular that the use of additional hydrophobizing agents, such as hexamethyldisilazane, can significantly accelerate the required solvent exchange step. For a successful silanization, the residual water content of the lyogel should be sufficiently high, preferably above 50 wt.%, based on the weight of the lyogel.

[0177] The pH value of the solution or dispersion of the hydrophobizing agent, in particular the silanizing solution, can vary depending on the hydrophobizing agent, in particular the silanizing agent, used. When trimethylsilanol, diphenylsilanediol, hexamethyldisilazane and hexamethyldisiloxane and other silanols or substances forming silanols are used, it has been shown that pH values ​​greater than 8 are advantageous. Organic solvents to which the aforementioned hydrophobizing agent, in particular the silanizing agent, is added, such as non-polar alkanes (hexane), aprotic solvents or alcohol solvents, such as methanol, ethanol, isopropanol, etc., can be used as silanizing solutions. The lyogel can be immersed in or covered with a solution or dispersion containing a hydrophobizing agent, wherein the contact time is preferably up to 30 minutes.

[0178] Alternatively, the hydrophobic agent, in particular the silanizing agent, can also be used in a compressed phase saturated or partially saturated with an organic solvent, in particular a CO2, N2 and / or Ar atmosphere, preferably a CO2 phase, wherein the phase can be both a subcritical gas phase and a supercritical phase. Suitable organic solvents include non-polar solvents such as hexane, aprotic solvents such as dimethyl sulfoxide, or alcohol solvents such as ethanol. The solvent used can increase the solubility of the hydrophobic agent, in particular the silanizing agent, in the compressed CO2 phase. If the solubility of the hydrophobic agent, in particular the silanizing agent, in the process medium, i.e. the above-mentioned gas forming the reaction atmosphere, is sufficient, in particular the solubility in compressed CO2 is sufficient, the use of an organic solvent can be omitted.

[0179] In the context of the present invention, it can be provided that the solvent exchange is carried out in several process stages, in particular in 2 to 15, preferably 3 to 10, more preferably 3 to 4 process stages. In this context, it can be provided that the lyogel is contacted with the organic solvent several times. Preferably, it is provided that in each process stage, at least a portion of the mixture of solvent and water or the solvent to be replaced is removed from the reactor and new organic solvent is supplied.

[0180] In the context of the present invention, it is particularly preferred if the solvent exchange reduces the water content of the lyogel to less than 20% by volume, preferably to less than 15% by volume, preferably to less than 10% by volume, based on the total volume of solvent or dispersant.

[0181] According to a preferred embodiment, the solvent exchange can be carried out by using a water-miscible solvent, such as ethanol, methanol, isopropanol and dimethyl sulfoxide. Here, it is shown that the residual water content in the spherical liquid gel particles should preferably be reduced to less than 10% by volume before the downstream drying begins. Alternatively and also preferably, a hydrophobic organic solvent can also be used for this process step, such as hexane, pentane or cyclohexane, which can displace the water stored in the pores from the liquid gel if fully pre-silanized. The solvent exchange is preferably carried out in compressed carbon dioxide. Here, the solvent is metered into the reaction device. Surprisingly, the result is that even if the solvent does not contact the gel particles in liquid form, the solvent exchange can be successfully carried out. On the contrary, it is sufficient if the solvent dissolves in the compressed CO2 and thus penetrates the gel and displaces water from the pores.

[0182] According to a preferred embodiment of the present invention, the present invention relates to a method for producing an aerogel as previously described, wherein

[0183] (a) in a first method step, at least two precursor sols, preferably two precursor sols, are mixed with one another, wherein the first precursor sol comprises an acidic pH or an alkaline pH and the second precursor sol comprises a pH value different from the first precursor sol, and

[0184] (b) in a second process step after the first process step (a), are mixed with one another and, in particular immediately after mixing, are supplied to a reaction apparatus, preferably in the form of droplets, more preferably dropwise, wherein a microparticulate lyogel is obtained, and

[0185] (c) In a third method step, following the second method step (b), the lyogel is solvent exchanged and / or hydrophobized.

[0186] The solvent exchange in process step (c) can be carried out in a period of up to 50 minutes, in particular up to 40 minutes, preferably up to 30 minutes. In particular, in the context of the present invention, it is preferred if the solvent exchange is carried out in a period of 10 to 50 minutes, in particular 20 to 40 minutes, preferably 20 to 30 minutes.

[0187] For the above-described embodiments of the method according to the invention, all further embodiments, features and special features described above apply.

[0188] In the context of the present invention, it is generally provided that the lyogel is converted into an aerogel by removing the solvent or dispersant, in particular in a subsequent process step (d).

[0189] In this context, it can be provided that after solvent exchange and / or hydrophobization of the lyogel, in particular after process step (c), the lyogel is converted into an aerogel. In the context of the present invention, it is preferred if the solvent removal is carried out under elevated pressure.

[0190] In general, it is envisaged that in order to convert the lyogel into an aerogel, the lyogel is contacted with a drying medium, in particular a drying gas or a supercritical medium. Preferably, the drying medium is carbon dioxide. In this context, it can be provided that the lyogel is contacted with a drying medium, in particular a drying gas or a supercritical medium, in a continuous or discontinuous manner. In the case of discontinuous contact, the lyogel is contacted with a predetermined amount of drying medium in an apparatus for a preselected period of time. The drying medium contaminated with the solvent is then removed and, if necessary, replaced with fresh drying medium until the desired degree of dryness is reached. In the case of continuous contact of the lyogel with the drying medium, also referred to as continuous drying, the lyogel is swept or flowed through the drying medium in the apparatus until the desired degree of dryness is reached.

[0191] In this context, particularly good results are obtained if the removal of the solvent is carried out at a pressure of more than 50 bar, in particular more than 60 bar, preferably more than 80 bar, more preferably more than 100 bar. Similarly, it is conceivable that the removal of the solvent is carried out in the range of 50 to 180 bar, in particular 80 to 175 bar, preferably 100 to 170 bar, more preferably 110 to 165 bar, particularly preferably 120 to 160 bar.

[0192] Now, with regard to the temperature at which the removal of the solvent is carried out, it has proven to be good if the removal of the solvent is carried out at an elevated temperature.

[0193] Typically, the removal of the solvent is carried out at a temperature above 50°C, in particular above 55°C, preferably above 60°C.

[0194] In this context, it can likewise be provided that the removal of the solvent is carried out at a temperature in the range of 50 to 160°C, in particular 70 to 160°C, preferably 90 to 150°C, more preferably 100 to 140°C, particularly preferably 110 to 130°C.

[0195] By removing the solvent at the above-mentioned pressures and temperatures, aerogels can be obtained particularly quickly, in particular by supercritical drying using CO 2. Generally, in the context of the present invention, it is envisaged to remove the solvent from the lyogel within 10 to 50 minutes, preferably 20 to 30 minutes.

[0196] The subject of the present invention is preferably a method for producing an aerogel as described above, wherein

[0197] (a) in a first method step, at least two precursor sols, preferably two precursor sols, are mixed with one another, wherein the first precursor sol comprises an acidic pH or an alkaline pH and the second precursor sol comprises a pH value different from the first precursor sol, and

[0198] (b) in a second process step after the first process step (a), are mixed with one another and, in particular immediately after mixing, are supplied to a reaction apparatus, preferably in the form of droplets, more preferably dropwise, wherein a microparticulate lyogel is obtained, and

[0199] (c) in a third method step after the second method step (b), a solvent exchange and / or hydrophobization of the lyogel is carried out, and

[0200] (d) In a fourth method step following the third method step (c), the lyogel is converted into an aerogel using removal of the solvent or dispersant.

[0201] In this specific and preferred embodiment of the invention, all previously mentioned technical features and embodiments may be read without restriction, in particular also the advantages and special features.

[0202] Now, with regard to the total duration of the method described above, the method according to the invention is usually carried out with a total duration of method steps (a) to (d), wherein method step (c) is carried out over a period of 1 to 2 hours, preferably 1 to 1.5 hours.

[0203] In this context, the method according to the invention can be carried out as a one-pot synthesis or process, i.e. in an autoclave. However, it is likewise possible to carry out the individual steps in a plurality of apparatuses connected in series, in particular autoclaves. The method according to the invention can be carried out under a particular atmosphere, in particular a CO2 atmosphere, and optionally under elevated pressure, in particular from the supply of the mixed precursor sol to the reaction apparatus. However, it is preferred if the lyogel formation is carried out at least only under low pressure, preferably at atmospheric pressure.

[0204] Preferably, the drying of the particles is carried out in supercritical CO 2. Hydrophobization with lyogel By the process according to the invention, the drying time of the spherical gel particles obtained with a size of 0.5 to 5 mm can be reduced to 10 to 60 minutes.

[0205] In particular, by feeding compressed carbon dioxide as drying medium, the gas flow can be used for directional continuous drying of the organogel and can ensure a single-stage aerogel particle production, ie in the reactor vessel or reactor.

[0206] Due to the spherical particle shape and typical particle diameters of 0.5 to 5 mm, supercritical drying can be performed at a pressure of 120 bar and a temperature of 60 to 120° C. within a time window of up to 30 minutes.

[0207] The only attached picture Figure 1 The description shows a section through a device according to the invention for carrying out the method according to the invention.

[0208] According to the present invention second A further subject of the invention of this aspect is an aerogel, in particular an aerogel obtainable according to the aforementioned process, wherein the aerogel is in the form of particles having in particular a substantially circular cross section.

[0209] As mentioned previously, the aerogel according to the invention is characterized in particular by a circular cross-section, which significantly increases the mechanical load-bearing capacity on the one hand and the ability to produce dense spherical packings on the other hand.

[0210] In the context of the present invention, it is generally provided that the aerogel particles are spherical or cylindrical.

[0211] Due to their shape, the aerogels according to the invention offer advantages in processing. For example, spherical aerogels are easier to mix into powder mixtures. Due to their improved flowability, higher strength under uniaxial compressive loading and higher packing density compared to conventional aerogel powders based on amorphous or cubic particles, the preferably spherical aerogels according to the invention can be preferably used in powder blends or powder mixtures, such as insulating plasters.

[0212] As far as the particle sizes of the aerogel particles concerned are concerned, these can naturally vary within wide ranges. However, it has proven to be very good if the aerogel comprises a particle size in the range of 0.1 to 10 mm, in particular 0.2 to 8 mm, preferably 0.3 to 7 mm, more preferably 0.5 to 5 mm. For the determination of the particle size, it is particularly suitable to analyze the particles using a sieve and, for smaller particles in the range below 1 mm, to use an optical microscope.

[0213] Similarly, it may be provided within the scope of the present invention that the aerogel particles comprise a monodisperse particle size distribution.

[0214] However, it is also possible within the scope of the present invention that the aerogel particles comprise a polydisperse particle size distribution. In particular, the particle size distribution can be selectively controlled by varying the conditions of spraying or dropping into the reactor.

[0215] The aerogel particles according to the invention are highly porous solids. Typically, the aerogel comprises a porosity greater than 90%, in particular greater than 91%, preferably greater than 93%.

[0216] Similarly, it is envisaged that the aerogel comprises a porosity of 90 to 96%, in particular 91 to 95%, preferably 93 to 94%.The porosity of the aerogel according to the invention is preferably determined using mercury porosimetry.

[0217] Furthermore, the aerogel according to the invention comprises a high internal surface area. Thus, it may be provided that the aerogel comprises at least 500 m 2 / g, especially 600m 2 / g, preferably 650m 2 / g, more preferably 700m 2 / g, particularly preferably 800m 2 / g of BET surface area.

[0218] Similarly, it may be provided that the aerogel comprises a 2 / g, especially 600 to 1,050m 2 / g, preferably 650 to 1,000 m 2 / g, more preferably 700 to 950m 2 / g, particularly preferably 800 to 900 m 2 In order to determine or calculate the BET surface area, the nitrogen adsorption of the aerogel particles is studied and the results are used in the BET calculation.

[0219] Now, as far as the thermal conductivity of the aerogel is concerned, it can vary within a wide range. However, generally, in the context of the present invention, the aerogel comprises a very low thermal conductivity. Particularly good results are obtained if the aerogel comprises a thermal conductivity of at most 0.025 W / mK, in particular at most 0.022 W / mK, preferably 0.020 W / mK, more preferably 0.019 W / mK.

[0220] Typically, the aerogel comprises a thermal conductivity in the range of 0.012 to 0.025 W / mK, in particular 0.013 to 0.022 W / mK, preferably 0.014 to 0.020 W / mK, more preferably 0.015 to 0.019 W / mK.

[0221] Furthermore, it can be provided in the context of the present invention that the aerogel comprises 0.01 to 0.60 g / cm 3 , especially 0.11 to 0.55 g / cm 3 , preferably 0.12 to 0.50 g / cm 3 , more preferably 0.13 to 0.50 g / cm 3 For determining the thermal conductivity, preferably an instrument of the "C3 Prozess und Analysetechnik" GmbH type Hot Disk is used, with a sensitivity of up to 0.005 W / m*K.

[0222] For further details of the aerogel according to the invention, reference is made to the above explanations of the method according to the invention, which apply to the aerogel according to the invention.

[0223] According to the present invention third A further subject matter of the present invention is the use of the above-described aerogel for insulation purposes, in particular for sound insulation, electrical insulation or thermal insulation, in particular for thermal insulation purposes.

[0224] For further details of the use according to the invention, reference is made to the explanation of the further aspects of the invention which are suitable according to the invention for the use according to the invention.

[0225] In addition, according to the present invention fourth A further subject of the invention of this aspect is the use of an aerogel as described above for insulation purposes, in particular as or in a thermal insulation material.

[0226] In this context, it is conceivable that the aerogels are used in loose fills, powder mixes or insulating compositions, such as insulating plasters.

[0227] For further details of the use according to the invention, reference is made to the above explanations of the further aspects of the invention, which apply to the use according to the invention.

[0228] According to the present invention fifth Yet another subject of the invention is a device for producing aerogels, wherein the device comprises

[0229] (a) at least one reactor,

[0230] (b) at least one inlet opening, in particular a nozzle, arranged on the reactor for supplying a fluid, in particular a liquid, to the reactor,

[0231] (c) at least two feeds connected to the inlet opening, in particular via a mixing device, and

[0232] (d) at least one outlet opening, in particular a sluice, arranged on the reactor for removing liquids or solids from the reactor.

[0233] Within the scope of the present invention, it can be provided in particular that via at least two feeders connected to the inlet opening, in particular via a mixing device, for example in the form of a mixing section, at least two precursor sols, preferably two precursor sols, for producing the lyogel are firstly metered, in particular separately from one another, and then mixed with one another in the mixing device, and the in particular mixed precursor sols are then immediately supplied, in particular sprayed or dropped into the reactor.

[0234] Preferably, the reactor comprises not only one but several inlet openings for supplying fluids, in particular liquids, i.e. at least one nozzle for supplying a particularly mixed precursor sol to the reactor, and at least one nozzle for supplying an additional solvent, in particular in liquid and / or gaseous form.

[0235] The outlet opening of the reactor is preferably configured in the form of a lock in order to be able to quickly remove the lyogel or aerogel from the reactor or also to ensure multiple solvent exchanges by covering and then draining the contaminated solvent from the reactor.

[0236] Preferably, it is also provided that the reactor can be pressurized, in particular with a pressure in the range from 1 to 40 bar, preferably from 1 to 30 bar, more preferably from 1 to 20 bar.

[0237] According to a preferred embodiment of the present invention, it is provided that the device comprises at least one inlet and / or outlet opening arranged on the reactor for supplying and / or removing gas to and / or from the reactor.

[0238] Preferably, the pressure in the reactor is regulated by the amount of substance, in particular the amount of substance in the gas phase and / or supercritical phase, and / or the temperature. For example, the pressure regulation can be performed so that gas is supplied to the reactor or removed from the reactor.

[0239] Furthermore, in the context of the present invention, it is generally provided that the device comprises means for temperature regulation. Temperature regulation can also be used to particularly influence and control the process in the reactor and thus the overall device. In particular, the reactor can be heated or cooled.

[0240] Furthermore, it is preferably provided that the device comprises at least one device for measuring the pH value, in particular at least two devices. The device for measuring the pH value can be arranged on the feed line, the mixing device and / or the reactor. In particular, it has proven to be good if the device for measuring the pH value is arranged on the feed inlet and / or the mixing device. Furthermore, it has proven to be advantageous in particular if the device for measuring the pH value is arranged in such a way that the pH value of the precursor sol is measured individually and / or after the mixing of the precursor sols, in particular continuously, preferably wherein the metering of the precursor sol is carried out according to the measured pH value and / or in accordance with a predetermined target pH value, preferably for precursor sols that are mixed with each other.

[0241] In this context, it is further preferred that the device comprises at least one, in particular two, means for metering the precursor sol. Preferably, the means for metering the precursor sol is a pump. It has proven to be particularly useful if the means for metering the precursor sol can be adjusted as a function of the pH value of the precursor sol, in particular of the precursor sols mixed with one another.

[0242] Typically, the apparatus has control or regulation devices for this purpose, in particular control or regulation devices for controlling or regulating the pressure, the pH value and / or the temperature in the reactor and / or inlet opening or mixing section.

[0243] The apparatus according to the invention can comprise one reactor or, however, also several reactors, in particular continuous reactors and / or reactors which are connected to one another so that the individual process steps of the process according to the invention are each carried out in a separate reactor. In this way, continuous aerogel production can be carried out.

[0244] For further details of the device according to the invention, reference is made to the above explanations of the further aspects of the invention, which apply correspondingly to the device according to the invention.

[0245] Finally, according to the present invention sixth A further subject matter of the present invention in this regard is a method for producing a lyogel using a sol-gel process, wherein, in order to produce the lyogel, at least two precursor sols, preferably two precursor sols, are mixed with one another, wherein the first precursor sol comprises an acidic pH or an alkaline pH and the second precursor sol comprises a pH value different from the first precursor sol.

[0246] With regard to the production of the lyogel, all advantages, features and embodiments mentioned above with regard to the lyogel in the process for producing an aerogel apply correspondingly.

[0247] For further details of the method according to the invention for producing a lyogel, reference is made to the above explanations of the further aspects of the invention, which apply correspondingly to the method according to the invention for producing a lyogel.

[0248] The subject matter of the invention will be described hereinafter in a non-limiting manner and in an illustrative and non-limiting manner with reference to the only accompanying drawing. Figure 1 The representations and embodiments are described as examples.

[0249] Figure 1 A cross section of an apparatus 1 for carrying out the method according to the invention is shown. The apparatus 1 comprises a reactor 2 in which the lyogel or aerogel formation takes place.

[0250] In order to carry out the method according to the invention, the two precursor sols 3 and 4 are preferably mixed with one another, wherein the first precursor sol comprises an acidic pH or an alkaline pH and the second precursor sol comprises a pH value different from the first precursor sol. In particular, in this context, it is preferred if one of the two precursor sols 3 and 4, in particular the first precursor sol 3, comprises an acidic pH and the other precursor sol, in particular the second precursor sol, comprises an alkaline pH. Therefore, it is further provided in particular that the precursor sols 3 and 4 are provided separately from one another. The precursors preferably used according to the invention are in particular preferably aqueous solutions of at least partly silicic acid, silica sols or silane hydrolysates.

[0251] In a preferred embodiment of the present invention, the acid-adjusted precursor sol 3 comprises a pH value in the range of pH 0 to 6, in particular pH 1 to 4, preferably pH 1.5 to 2.5, and the alkaline-adjusted precursor sol 4 comprises a pH value in the range of pH 7 to 13, in particular pH 8 to 12, preferably pH 9 to 11.

[0252] The precursor sols 3 and 4 are continuously mixed with one another via a feed system, in particular two feeders 5. Here, the feeders 5 can be regulated or opened via valves 6. Furthermore, by means of a metering device 7, in particular by means of a pump, the amount of the precursor sols 3 and 4 fed or mixed with one another for the production or formation of the lyogel can be controlled or metered in particular.

[0253] In a further preferred embodiment of the invention, the metering of the precursor feeds is carried out as a function of the pH value of the mixed precursor sols 3 and 4, in particular. In this respect, it has proven to be very good according to the invention if the mixed precursor sols 3 and 4 comprise a pH value in the range of pH 4.5 to 9.5, in particular pH 5 to pH 9, preferably pH 5.3 to 8.5. Particularly good results can be achieved within the scope of the invention, in particular a particularly precise control and regulation of the lyogel formation, if the mixed precursor sols 3 and 4 comprise a slightly acidic pH, in particular in the range of pH 4.5 to 6.8, preferably pH 5 to 6.5, or a slightly alkaline pH, in particular in the range of pH 7.5 to 9.5, preferably pH 7.8 to 9. In this respect, it is even more preferred for the method according to the invention, in particular with regard to the properties of the aerogel formed in the finally obtained aerogel, if the mixed precursor sols 3 and 4 comprise a slightly acidic pH value in the above-mentioned range.

[0254] According to the invention, the device 1 preferably comprises a mixing device 8. Figure 1 In the representation, the mixing device is exemplarily represented in the form of a mixing section, which in particular comprises a static mixing element 9. The mixing device is preferably incorporated into or comprises an inlet opening, for example in the form of a nozzle in particular. In this context, the mixing device can be a nozzle, i.e. the precursor sols 3 and 4 are mixed in the nozzle immediately before being supplied to the reactor 2. However, it is preferred if the mixed precursor sols 3 and 4 are kept in the mixing device 8 for a certain time so as to ensure complete mixing of the precursor sols 3 and 4 and, if necessary, to start the desired pregelation. The specific mixing parameters of the mixing device 8 depend in particular on the geometry of the mixing device 8, the chemical and physical properties of the precursor sols 3 and 4 and the shape and properties of the aerogel particles to be produced.

[0255] Preferably, the feeder 5 can also be connected to the mixing device 8 via a T-piece. In the mixing device 8, the mixing of the precursor sols 3 and 4 takes place, which are provided separately and metered separately from one another, in particular continuously, wherein preferably a homogeneous and uniform mixing can be achieved using an integrated static mixing element 9. Immediately after the mixing of the precursor sols 3 and 4, the gel formation and the preferably dropwise supply, preferably injection, of the precursor sols 3 and 4 into the reactor 2 via the inlet opening 10, in particular a nozzle, take place simultaneously. Here, starting from the geometry of the inlet opening 10, in particular the nozzle, the shape or geometry of the lyogel particles formed can also be controlled in particular.

[0256] The lyogel formation is initiated by the mixing device 8 and the static mixing element 9, preferably within a time span of less than 60 seconds, in particular less than 30 seconds, preferably less than 20 seconds, and more than 0.1 seconds, in particular more than 0.5 seconds, preferably more than 1 second, in accordance with a particularly predetermined pH value of the precursor solution for mixing in coordination with the respective amounts of the precursor sols 3 and 4 metered into the mixing device 8. In this context, it is particularly decisive that, based on the target pH value adjustment and the uniform and controlled homogeneous mixing of the precursor sols 3 and 4 in the mixing device 8, in particular spherical lyogel particles 11 with a uniform size distribution and also a particularly adjustable size can be obtained reliably and process-safely.

[0257] In the context of the present invention, it is further preferred here to produce the lyogel particles 11 at a pressure of less than 40 bar, in particular less than 30 bar, preferably less than 20 bar, more preferably at atmospheric pressure, i.e. about 1 bar, and at a temperature of more than 50° C., in particular 60° C., preferably 70° C., more preferably 80° C. Furthermore, it has proven to be advantageous if a controlled gas atmosphere, in particular a CO 2 , N 2 or Ar atmosphere or an atmosphere consisting of a mixture of these gases, is provided in the reactor 2.

[0258] The formed lyogel particles 11 are collected at the bottom of the reactor 2, wherein, however, in particular a spherical shape is obtained in particular without damage. The lyogel particles 11 can now be removed from the reactor 2 or can be further processed in the reactor. Preferably, after the production of the lyogel particles 11, a solvent exchange and simultaneous hydrophobization of the lyogel particles 11 are carried out using a suitable organic solvent and a hydrophobizing agent, in particular a silanizing agent.

[0259] Solvent and hydrophobic agent are supplied to the reactor 2 via inlet openings 12 and 13, respectively. Here, it is preferred if the organic solvent is soluble in CO2 to enable closed supercritical drying with CO2. Therefore, a gas, such as CO2, can also be supplied to the reactor via inlet openings 12 or 13 in particular and removed again if necessary. After the solvent exchange has taken place, the lyogel particles 11 are dried, in particular by first draining off the solvent via outlet openings 14 and then supercritically drying the lyogel with CO2, so that an aerogel is obtained.

[0260] Finally, with regard to the control or regulation of the apparatus 1 according to the invention for carrying out the method according to the invention, it is preferred if the apparatus 1 comprises measuring devices 15 , in particular wherein these are preferably suitable for measuring pressure, temperature and / or pH.

[0261] The subject matter of the invention is explained below in a non-limiting manner by means of working examples:

[0262] Working Example

[0263] Silica aerogels were produced using the process according to the invention from silicic acid as precursor and the properties of the aerogel particles obtained were investigated:

[0264] 1. Aerogel production

[0265] Production of acidic precursor sol:

[0266] To produce the acid-adjusted precursor sol, silicic acid is prepared from sodium silicate using ion exchange for protons. The solids content is adjusted to 5 to 10 wt.%, preferably 7 to 8 wt.%.

[0267] The silicic acid thus produced comprises a pH of 1.8 to 2.4, preferably 2.0. For storage of the silicic acid, it can be stabilized to a pH of 1 to 1.5 using HCl.

[0268] Production of alkaline precursor sol:

[0269] To produce the alkaline precursor sol, silicic acid is first produced from sodium silicate, again using ion exchange. The solids content is adjusted to 5 to 10 wt.%, preferably 7 to 8 wt.%.

[0270] A few minutes before use by using NH3 (about 25%, about 4.5 g / 100 g solution), the pH of the silicic acid is adjusted to a pH of 9 to 11, preferably 10.5. Very rapid addition and very rapid mixing of the aqueous NH3 solution are essential to prevent immediate gelation.

[0271] The ammonia addition is 7.0 ml of 25 wt.% ammonia per 100 g of silicic acid solution. The storage time of such a solution is usually only a few hours, preferably a maximum of 2 hours.

[0272] Alternatively, commercial silica sols may be used as the alkaline solution. For example, Ludox SM having a solid content of 30 wt.% and a pH of about 10, or LUDOX AS-40 comprising a lower sodium content, or mixtures of these are suitable.

[0273] Mixing ratio of precursor sols for lyogel formation:

[0274] The following table lists the mixing ratios of the above-mentioned precursor sols used in the method of the present invention for producing a lyogel.

[0275] Table 1: Mass balance and corresponding pH values ​​of the silicic acid solutions to be mixed

[0276]

[0277] Procedures for the formation of lyogels and aerogels

[0278] The silicic acid solutions prepared as described above are mixed in the apparatus according to the invention, preferably via a two-substance feed using a T-tube and subsequently a static mixer in the form of a tube insert and using two pumps. In particular, models of low-viscosity systems are suitable for static mixers, for example, according to the "Kenics" design, with a diameter:length ratio of 1:5. Preferably, gelation occurs immediately after the mixing of the two solutions, so that the mixed precursor solution can be dripped in with a short gelation time at a resulting pH value close to the neutral range. The formed lyogel particles, in particular hydrogel particles, are preferably collected at the bottom of the apparatus in the form of spherical particles.

[0279] Since the water present in the lyogel or in particular the hydrogel can interfere with the drying process for conversion into an aerogel, it is preferably exchanged for a suitable, in particular CO2-soluble solvent, such as ethanol. For this purpose, the solvent exchange preferably takes place at a pressure of about 1 to 30 bar and a temperature of 30° C. to 150° C. The gel stored in the reaction apparatus according to the invention, in particular the autoclave, is covered with the solvent in liquid form, such as ethanol. For this purpose, the autoclave is pressurized, in particular to prevent boiling of the solvent, wherein for ethanol a vapor pressure of 5.6 bar is at 130° C., so that a pressurization of 10 bar is suitable here.

[0280] In addition, the hydrophobization of the lyogel can also be carried out as part of a solvent exchange. For this purpose, the gel is covered with a liquid mixture of ethanol and hexamethyldisilazane (HDMZ), wherein the simultaneous addition of HDMZ leads to the hydrophobization of the gel. Alternatively, the exclusive contact of the gel with a gas phase saturated with ethanol can also lead to a sufficient solvent exchange.

[0281] After a residence time of 30 min, the liquid ethanol is drained from the container. Further washing cycles may then be followed by additions of ethanol to the container, wherein the aim is to bring the water content of the gel to less than 10% by volume. In this case, the ethanol phase is replaced after every 20 min. It has proven particularly advantageous if the first solvent exchange is carried out in such a way that the gel is covered with the liquid ethanol phase.

[0282] After the solvent exchange is complete, the gel particles can be supercritically dried, especially if the gel contains less than 5 wt.% of water. For this purpose, compressed carbon dioxide is fed as drying fluid, wherein the gas stream can be used for directional continuous drying of the organogel and can ensure single-stage aerogel particle formation. Based on the spherical particle shape and a typical particle diameter of 1 to 6 mm, supercritical drying can be carried out at a pressure in the range of 120 to 160 bar and a temperature in the range of 60 to 120° C. within a time window of 10 to 60 min.

[0283] 2. Aerosol Characterization

[0284] The characterization of the aerogel obtained according to the invention was carried out as follows:

[0285] granularity:

[0286] Sieve analysis was used to determine the size of the aerogel particles obtained according to the invention. Smaller particles in the size range of less than 1 mm were additionally measured by optical microscopy.

[0287] Particle size analysis shows that the aerogel particles according to the invention comprise particle sizes in the range from 0.1 to 10 mm, in particular from 0.2 to 8 mm, preferably from 0.3 to 7 mm.

[0288] Thermal Conductivity:

[0289] To determine the thermal conductivity, an instrument of the Hot Disk type from C3 Prozess und Analysetechnik GmbH was used, with a sensitivity of up to 0.005 W / m*K. The Hot Disk sensor here consists of a nickel double helix, which serves both as a heat source and for measuring the temperature rise during the measurement.

[0290] For the aerogels according to the invention, the thermal conductivity is measured in the range of 0.012 to 0.025 W / mK, in particular 0.013 to 0.022 W / mK, preferably 0.014 to 0.020 W / mK.

[0291] Pore ​​volume and density:

[0292] To determine the density and pore volume, studies were carried out using mercury porosimetry. Here, the samples were subjected to pressures of up to 400 MPa, which destroyed the samples but thus also allowed a complete examination of the internal pore volume.

[0293] Commercially available subcritically dried and hydrophobized aerogel Enova P300 (Cabot Corporation, average density according to the data sheet is about 150 kg / m 3 ) and aerogel Enova 3110 (Cabot Corporation) were used as references.

[0294] According to the results of mercury porosimetry, the density of the aerogel particles according to the present invention is between 0.01 and 0.60 g / cm 3 , especially 0.11 to 0.55 g / cm 3 , preferably 0.12 to 0.50 g / cm 3 within the range of

[0295] Porosity, BET surface area and average pore radius:

[0296] In order to determine the porosity, BET surface area and average pore radius of the aerogel according to the invention, the nitrogen adsorption of the aerogel is measured and / or determined using the BET method. For this purpose, the measurement is usually carried out in accordance with DIN ISO 9277:2003-05 ("Determination of the specific surface area of ​​solids by gas adsorption using the BET method").

[0297] According to the above method, for the aerogel according to the invention, porosity values ​​of 94 to 99.5%, in particular 95 to 99%, preferably 96 to 98% are obtained. In addition, the aerogel comprises a porosity of 500 to 1,000 m 2 / g, especially 600 to 1,050m 2 / g, preferably 650 to 1,000 m 2 The BET surface area is in the range of 1000 Å / g.

[0298] Reference numerals:

[0299] 1 Equipment 9. Static mixing element

[0300] 2 Reactor 10. Inlet opening

[0301] 3 Precursor sol 11. Liquid gel particles

[0302] 4. Precursor sol 12. Inlet opening

[0303] 5. Feeder 13. Inlet opening

[0304] 6. Valve 14. Outlet opening

[0305] 7. Measuring device 15. Measuring device.

[0306] 8. Mixing device

Claims

1. A method for producing a silica aerogel using a sol-gel process, wherein a lyogel is first formed and then the lyogel is converted into an aerogel, It is characterized in that In order to produce the lyogel, two precursor sols provided separately from each other are mixed with each other, wherein the first precursor sol includes an acidic pH value in the range of pH 0 to 6, and the second precursor sol includes an alkaline pH value in the range of pH 7 to 13, and wherein the precursor sols mixed with each other include a pH value in the range of pH 4.5 to 9.

5.

2. The method according to claim 1, characterized in that: The precursor sols are mixed with each other and supplied to a reaction device in the form of droplets.

3. The method according to claim 1, characterized in that The precursor sols for producing the lyogel are continuously mixed with one another.

4. The method according to claim 1, characterized in that: The first precursor sol comprises an acidic pH and the second precursor sol comprises an alkaline pH.

5. The method according to claim 1, characterized in that: The acidic pH is in the range of pH 1.5 to 2.

5.

6. The method according to claim 1, characterized in that The alkaline pH is in the range of pH 9 to 11.

7. The method according to claim 1, characterized in that The precursor sols mixed with each other include a pH value in the range of pH 5 to pH 9.

8. The method according to claim 7, characterized in that The precursor sols mixed with each other include a weakly acidic pH in the range of pH 5 to 6.5 or a weakly alkaline pH in the range of pH 7.8 to 9.

9. The method according to claim 1, characterized in that: The production of the lyogel is carried out at a pressure of less than 10 bar.

10. The method according to claim 1, characterized in that Production of the lyogel from the mixed precursor sol occurs in less than 10 seconds.

11. The method according to claim 1, characterized in that The precursor sol includes a silicon-based precursor.

12. The method according to claim 11, characterized in that The precursor sol includes silicon in an amount ranging from 3 to 20 wt. % based on the precursor sol.

13. The method according to claim 11, characterized in that The precursor is selected from the group consisting of silicon dioxide, silanes, siloxanes, silicates, and mixtures thereof.

14. The method according to claim 11, characterized in that The precursor is selected from the group consisting of colloidal silicic acid, silica sol, silicic acid sol, tetraalkoxysilane, and mixtures thereof.

15. Silica aerogel obtainable according to claim 1, characterized in that The silica aerogel is in the form of particles having an at least substantially circular cross-section and the silica aerogel comprises a particle size in the range of 0.1 to 10 mm.

16. The silica aerogel according to claim 15, characterized in that The silica aerogel comprises a particle size in the range of 0.5 to 5 mm.

17. Use of the silica aerogel according to claim 15 for sound insulation, electrical insulation or thermal insulation, or as a carrier material, as an absorbent or as an adsorbent.

18. Use of the silica aerogel according to claim 15 as a thermal insulation material or in a thermal insulation material.

19. Apparatus (1) for producing an aerogel according to claim 15 or an aerogel obtainable by a method according to claim 1, It is characterized in that The device comprises (a) at least one reactor (2), (b) at least one inlet opening (10) arranged on the reactor (2) for supplying a fluid to the reactor, (c) at least two feeders (5), connected to said inlet opening (10), via a mixing device (8), and (d) at least one outlet opening (14) arranged on the reactor (2) for removing liquids or solids from the reactor.

20. The device (1) according to claim 19, characterized in that The inlet opening (10) is a nozzle.

21. The device (1) according to claim 19, characterized in that The fluid is a liquid.

22. The device (1) according to claim 19, characterized in that The outlet opening (14) is a gate.

23. A method for producing a silica lyogel using a sol-gel process, It is characterized in that In order to produce the lyogel, two precursor sols provided separately from each other are mixed with each other, wherein the first precursor sol includes an acidic pH in the range of pH 0 to 6, and the second precursor sol includes an alkaline pH value in the range of pH 7 to 13, and wherein the precursor sols mixed with each other include a pH value in the range of pH 4.5 to 9.5.

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