Method for producing aerogels and aerogels obtained using said method

Abstract

The invention relates to a method for producing an aerogel under increased pressure, to the aerogels obtained using said method and to their use.

Description

[0001] Methods for the production of aerogels and aerogels obtainable with this method

[0002] The present invention relates to the technical field of aerogel production. In particular, the present invention relates to a method for producing an aerogel using a sol-gel process.

[0003] Furthermore, the present invention relates to aerogels, which are obtainable in particular by the method according to the invention, and their use, in particular as or in insulating materials.

[0004] Furthermore, the present invention relates to a device for the production of aerogels. Finally, the present invention relates to a method for the production of a lyogel using a sol-gel process.

[0005] Aerogels are highly porous solids whose volume can consist of up to 99.98% pores. Aerogels typically exhibit dendritic structures with extensive branching of the partial chains, resulting in numerous interstitial spaces, particularly in the form of open pores. The chains possess a multitude of contact points, forming a stable, sponge-like structure. The pore size is typically in the nanometer range, and the internal surface area can reach up to 1,000 m². 2 aerogels can consist of a variety of materials, such as silicon dioxide, 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, or as filter materials. Aerogels are also used as storage materials, for example for liquids or gases.

[0007] Aerogels are nanostructured, open-pored solids that are usually produced using a sol-gel process.

[0008] Aerogels are generally produced by drying a gel-like gel, usually condensed silica. Aerogels obtained with silica and similar starting materials such as silica sols, silane hydrolysates, or silicates exhibit SiC structural units and are often referred to as silica aerogels. The first synthesis of silica aerogels was achieved by Steven Kistler in 1931 / 1932. He was the first to develop a method for drying gels without causing them to shrink (Kistler SS, The Journal of Physical Chemistry 1932, 36(1): Coherent expanded Aerogels, pp. 52-64). In Kistler's method, water glass is used as the starting material, from which a silica hydrogel is obtained in a first step by acidification with a mineral acid. This gel is then freed from alkali metal ions by washing.The water contained in the hydrogel is then completely replaced with ethanol or methanol. The resulting alcogel is then supercritically dried in an autoclave. Meanwhile, other processes have been developed, such as the one described in DE 18 11 353 A. DE 18 11 353 A discloses a process for producing silica aerogels, in which tetraethoxysilane (TEOS) is hydrolyzed in methanol or ethanol with a precisely measured amount of water and a catalyst. During hydrolysis, a SiO₂ gel in the form of an alcogel is formed with the elimination of alcohol and water. The alcogel is then supercritically dried in an autoclave. Organic aerogels made from melamine formaldehyde resins and resorcinol formaldehyde resins can also be produced using this process.In supercritical drying techniques, the gel to be dried is exposed to temperature and pressure conditions at which at least the critical point of the solvent used is reached.

[0009] The disadvantages of such supercritical drying processes, which rely on supercritical conditions of the solvent used, are the temperature and pressure requirements, as well as the discontinuous operation. For example, drying aqueous gels requires temperatures of at least 370 °C and pressures of at least 220 bar. Drying methanol-containing gels requires temperatures of at least 240 °C and pressures of at least 81 bar. An alternative to this supercritical drying process is the use of compressed carbon dioxide. A process for drying with supercritical carbon dioxide is disclosed, for example, in EP 171 722 A. In this process, the organic solvent is replaced with liquid carbon dioxide before supercritical drying.Supercritical drying with CO2 then takes place at significantly lower temperatures, for example, at the critical temperature of 31.1 °C and the critical pressure of 73.9 bar of carbon dioxide. Subcritical drying processes are also known. In subcritical drying, the gel to be dried is subjected to temperature and pressure conditions below the critical point of the solvent used, preferably at atmospheric pressure. The disadvantages of subcritical drying at atmospheric pressure with heat input by contact or convection are that the resulting capillary forces can cause the gel to collapse. This risk is particularly high with hydrogels or lyogels with a low solids content, as is known, for example, from DE 43 16 540 A.Due to the low equipment and energy requirements, methods for subcritical drying of silica aerogels have already been developed on an industrial scale. However, this usually requires chemical modification of the gel to reduce the capillary forces that occur during drying and to prevent gel collapse.

[0010] One way to modify the silica gel network for drying under normal conditions is to silanize the Si-OH groups in the pore surfaces. Silanizing agents used for this purpose include chlorotrimethylsilane and hexamethyldisilazane. This prevents the formation of Si-O-Si bridges between the approaching pore walls during drying. Thus, the shrinkage that occurs is partially reversible. Additionally, the forces on the gel network can be minimized by selecting a suitable solvent with low surface tension, such as pentane. This drying method was first described by Desphande et al. in 1992. developed (see DM Smith, R. Desphande, CJ Brinker in: Ishizaki, K., Sheppard, L., Okada, S., Hamesaki, T., Huybrechts, B. (ed.), Porous Materials, Vol. 31, American Ceramic Societey, Westerville, 1993, pp. 71 -80).A variant of this process is used by Cabot Corporation for the industrial production of hydrophobic aerogel granules. The lyogel, i.e., the gel filled with liquid, specifically solvent or water, is not produced from silicon alkoxide, but rather by gelation of an aqueous alkali silicate solution ("water glass"). To enable drying under normal conditions, the process by Einarsrud et al. aims for mechanical stabilization of the gel network (see M.-A. Einarsrud, L.E. Farbrodt, S. Haereid, in: Hench, L.L., West, J.K. (eds.), Chemical Processing of Advanced Materials, Wiley, Chichester, 1992, pp. 355-361). For this purpose, the wet gel or lyogel is aged in a tetraalkoxysilane solution, such as TEOS. During aging, the tetraalkoxysilane condenses in the pores of the gel and fills them with silicon dioxide.This makes the network more resilient, but also results in a loss of some porosity (see T. Kornprobst, Aerogels and photocatalysts as an example of innovative building materials, Dissertation TU Munich, 2013).

[0011] The EU-funded Hipin project, completed in March 2015, pursued another process to increase the stability of silica gels. Pre-hydrolysis and pre-condensation of TEOS increases the formal SiO2 content, resulting in more stable gels. Aerogels with typical specific surface areas can be increased by 1000 m². 2 / g can be obtained, however, the drying process takes place under supercritical conditions (see S. Naik, High Performance Insulation based on Nanostructure Encapsulation of Air, http: / / www.hipin.eu (accessed September 23, 2015)). DE 43 16 540 A discloses that aerogels can be obtained by drying inorganic and organic lyogels while preserving their structures by drying the lyogel using dielectric drying processes. Here, dielectric drying processes are understood to be those in which energy is supplied by electromagnetic waves, e.g., microwave drying, high-frequency drying, or radiation. In high-frequency drying with radio waves, frequencies between 1 MHz and 1000 MHz are used, while in microwave drying, frequencies between 10 3 MHz and 10 6MHz gear worket. In this type of drying, the selection of the gel used, the solvent, and the sample geometry must be precisely matched to the applied energy so that an equilibrium can be established between the capillary forces and the solvent evaporating within the gel. However, the teaching described in DE 43 16 540 A from 1993 has not yet led to the industrial implementation of dielectric drying. In general, gels produced using subcritical drying methods are inferior in their properties to those produced using supercritical drying.

[0012] Aerogels are often produced industrially using a method similar to the Cabot process. This process is described, for example, in DE 19 648 798 A and DE 69 903 913 T2. In this process, dilute sodium silicate is reacted with hydrochloric acid at 60 to 80 °C, whereby the gelation time, i.e., the time until gel formation, can be adjusted to a few minutes. For solidification and maturation of the gel, it is then tempered at 80 to 100 °C. The maturation time is specified as 30 minutes. During or after the maturation process, the gel is washed until the wash water is electrolyte-free.

[0013] The hydrogel is then silanized to enable subcritical drying. Trimethylchlorosilane is used as the silanizing agent. Trimethylchlorosilane reacts largely with the water present in the hydrogel to form trimethylsilanol, which then condenses further to hexamethyldisiloxane. This hexamethyldisiloxane is incorporated into the pores and partially displaces the water.

[0014] It is important to note that the silanizing agent used is added in very large quantities. For example, 100 g of hydrogel are reacted with 140 ml of trimethylchlorosilane. Only at this ratio of hydrogel to trimethylchlorosilane is partial conversion of the hydroxide groups on the silicon achieved. Hexamethyldisiloxane and hydrochloric acid are used as alternative silanizing agents in a gas stream. In this case, a partial reverse reaction of the hexamethyldisiloxane to trimethylchlorosilane occurs, which can then react with the hydroxyl groups of the silicon.

[0015] Considering the modular ratios of HCl and hexamethyldisiloxane in the examples of the aforementioned patents and patent applications, it becomes clear that 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 demonstrates the importance of embedding the hexamethyldisiloxane in the pores of the lyogel. Only in this way can subcritical drying be achieved. The drying itself then takes place in a nitrogen stream at 200 °C.

[0016] The Aerogel Handbook (MA Aergerter et al., Aerogels Handbook, Advances in Sol-Gel Derived Materials and Technologies, 2011, p. 120) discusses the significance of the molar ratio of silanizing agents to SiO2 network in more detail. The hydrophobization step, using a large amount of trimethylchlorosilane, which is toxic, flammable, and corrosive, is the most complex process step in the production of aerogels using the Cabot process. Furthermore, drying processes often demonstrate that a solvent exchange, particularly from polar to less polar solvents, is crucial for successful drying.

[0017] Subrahmanyam et al. investigated the influence of different solvents on the structural changes during solvent exchange and supercritical drying of biopolymer-based hydrogels (see Subrahmanyam, R., Gurikov, P., Dieringer, P., Sun, M., Smirnova, I., Gels, 2015, 1 (2): On the Road to Biopolymer Aerogels - Dealing with the Solvent, pp. 291 to 313). The study revealed significant differences between various solvents with respect to changes in pore geometry. The pore geometry decreases when solvent exchange is performed in multiple steps rather than a single step. The influence of solvent exchange on pore geometry can be estimated using Hansen's solubility parameters, thus contributing to the selection of a suitable solvent.

[0018] Kistler's investigations of sodium silicate-based aerogels, however, have shown that solvent exchange from water to ethanol does not cause a significant change in pore geometry. This result is independent of whether the solvent exchange is carried out in one step or in several stages with increasing ethanol content. For the direct supercritical drying of SiO2 gels from ethanol used there, a mass fraction of ethanol of 95 wt% is sufficient. No value is known for supercritical drying using CO2. The biopolymers investigated by Subrahmanyam require a mass fraction of 93 wt% for supercritical drying without a significant reduction in specific surface area. For a mass fraction of 90%, 90 wt% of the specific surface area is retained in the case of the biopolymers.

[0019] The solvent exchange of water to ethanol is investigated by Gurikov et al. under the influence of supercritical CO2. The gels used consist of alginate and are prepared by CO2-induced gelation. The samples have a diameter of 10 to 12 mm and are positioned in a preheated autoclave and surrounded by supercritical CO2 (120 bar, 313 K). Subsequently, mixtures of water and ethanol are pumped into the autoclave in several stages, and the solvent exchange is carried out for 2.5 hours per stage, achieving an ethanol concentration of 30 wt% in the first stage, 60 wt% in the second stage, and 90 wt% in the third stage. The gels are then rinsed with 25 wt% ethanol in CO2 to completely extract the water from the pores before being supercritically dried for 3 hours.The progress of the solvent exchange is analyzed based on the composition calculated from the solvent's density. For this purpose, 5 ml samples are taken from the autoclave at each stage. Under the given conditions, the time required for each stage of the solvent exchange was reduced from 12 hours to 2.5 hours.

[0020] By using supercritical carbon dioxide during solvent exchange, the required drying time is further reduced from 6 hours to 3 hours. The density of the gels after solvent exchange under the influence of supercritical CO2 is 0.021 g / cm³. 3 , the specific surface area according to BET at 538 m 2 / g and the pore volume according to BJG at 5.96 cm³ 3 / g. The obtained aerogels exhibit similar properties to the reference samples produced via solvent exchange under ambient conditions. A direct influence of solvent exchange under pressure on the properties of the produced aerogels cannot be deduced from the available data, as different synthesis conditions are used for the different processes.

[0021] The same authors also perform solvent exchange in alginate-based biopolymers using compressed CO2 at ambient temperature. An acceleration of mass transport is measured at 50 bar and ambient temperature. Changes in solvent concentration are also quantified using the pseudo-second-order kinetic model.

[0022] In addition to the previously described challenge of stabilizing the gel during the drying process, another problem in the production of aerogels, especially silica aerogels, is the long processing times. These increase the cost of aerogel production and thus prevent the use of aerogels in a variety of applications for which they would be suitable due to their physical properties. For example, the respective processing times for the individual steps in the formation of silica aerogels from tetraethyl orthosilicate (TEOS) are: - Hydrolysis and condensation times at least 8 hours (see AA Tweij Wesam, Temperature Influence on the Gelation Process of Tetraethyl Orthosilicate using Sol-Gel Techique, Iraqi Journal of Science 2009)

[0023] - Aging times of the gel range between 6 and 72 hours (cf. Einars-rud, 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)

[0024] - Supercritical washing times / solvent exchange approx. 24 hours per washing cycle (cf. Kerstin Quarch, Product design on colloidal agglomerates and gels, gelation and fragmentation of inorganic silica, dissertation, KIT, 2010)

[0025] - Supercritical drying times are highly dependent on the preceding solvent exchange and the sample size.

[0026] In contrast, the following process times are observed in the production of silica aerogels based on sodium silicate solution using subcritical drying:

[0027] - Gel formation times

[0028] • Addition of sulfuric acid and water to sodium silicate solution over 90 minutes, 30 minutes gel formation time at 8.6% SiO2 content after addition of the sulfuric acid (see Kerstin Quarch, Product design on colloidal agglomerates and gels, gelation and fragmentation of inorganic silicon dioxide, dissertation, KIT, 2010),

[0029] • Gel formation times when using colloidal silica solutions are 15 minutes at pH values ​​between 5 and 6 (see Friederike Kleinert et al., Microstructure and Transmittance of Silica Gels for Application as trans parent Heat Insulation Materials, Journal Sol-Gel Science Technol. 75, pp. 602-616, 2015).

[0030] • Gel formation times when using sodium silicate solution (8% S1O2 content) via ion exchanger are approximately 10 minutes.

[0031] - Aging times of the gels

[0032] • Aging times of sodium silicate-based gels are approximately 50 hours at 50 °C (see Schwertfeger, F., Hydrophobie Waterglass based Aerogels without solvent Exchange or supercritical Drying) • Aging times of silica gels are approximately 1.5 hours (see Schwertfeger, F., Hydrophobie Waterglass based Aerogels without solvent Exchange or supercritical Drying) Typical process times for the production of sodium silicate aerogels with subcritical drying, in combination with solvent exchange and hydrophobing, are typically

[0033] - Gel formation and aging times: 1 second to 2 hours

[0034] - The washing times required to remove the sodium from the gel are unknown.

[0035] - A solvent exchange using acetone takes approximately 2 hours.

[0036] - Duration of silanization, i.e., hydrophobization, using hexamethyldisiloxane at room temperature: 5 hours

[0037] - Subcritical drying

[0038] * 17 hours at 150 °C or

[0039] • 1 hour in a 200 °C hot nitrogen / hexamethyldisilazane stream.

[0040] Even in the best-case scenario, the process takes at least 8 hours, and this does not include washing times to obtain a sodium-free gel.

[0041] A significant improvement is brought about by the one-pot method developed by the Swiss Federal Laboratories for Materials Science and Technology (EMPA), the individual steps of which require the following time:

[0042] - The gel formation and aging process using hexamethyldisilazane (HDMSO), ammonia, water, ethanol, and TEOS takes approximately 2 hours. - The hydrophobization of the moist gel is carried out using a mixture of HCl and HDMSO over a period of 1 hour.

[0043] - The supercritical drying time is approximately 1 hour. The total process time is therefore between 4 and 6 hours.

[0044] However, these process times still pose major challenges for large-scale industrial production, particularly in the case of hydrophobization, where large excesses of hydrophobizing agents must be used to obtain the necessary hydrophobization for solvent exchange.

[0045] In the context of aerogel production, it is known that the gelation of silica aerogels can be induced by carbon dioxide. The influence of carbon dioxide on the gelation of silica-based aerogels is described by Smirnova (Journal of Sol-Gel Science 28, pp. 175-184, 2003) and Xiaodong Wu (Journal Ceramics International 44, pp. 821-829, 2018). Smirnova's investigation is based on tetramethyl orthosilicate-based (TMOS-based) systems with a molar composition of TMOS:MeOH:H₂O of 1:2, 4:4. These are examined under pressures ranging from 5 to 50 bar. Under these conditions, the gelation time can be reduced from 230 minutes to just under 50 minutes. In Xiaodong Wu's publication, gaseous CO2 is passed through a water glass solution with a pH of 13, so that gelation started in about 35 minutes by lowering the pH to 9.

[0046] A problem common to all of the aforementioned aerogel production methods is that they typically yield undefined particles without a regular external shape, making them difficult to use in loose fill or for incorporation into insulating plaster systems. These irregular particles are significantly less mechanically robust and do not form dense sphere packings as well as would be expected for regular, especially spherical, particles. For this reason, the effectiveness of aerogel in practice often falls short of the calculated values.

[0047] The prior art thus still lacks a system for reproducibly producing aerogels with significantly reduced process times, thereby enabling continuous or quasi-continuous production at reduced costs. Furthermore, it is also not possible to produce aerogels with a defined geometric structure on an industrial scale and reproducibly. For many applications, spherical aerogel particles are particularly preferred, as these are likely to exhibit significantly higher mechanical strength. Likewise, it is currently not possible to selectively produce aerogel particles with preselected particle sizes. The object of the present invention is therefore to eliminate, or at least mitigate, the disadvantages associated with the prior art described above.In particular, one object of the present invention is to provide a process for the production of aerogel particles that can be carried out with significantly shorter process times and preferably continuously or quasi-continuously. A further object of the present invention is to enable the targeted production of aerogels with defined properties, in particular also with defined external shape and defined particle size.

[0048] Furthermore, another object of the present invention is to provide an aerogel which is mechanically resilient and is particularly suitable for use in insulating materials.

[0049] The problem set out above is solved according to the invention by a method according to claim 1; further advantageous developments and embodiments of the method according to the invention are the subject of the relevant dependent claims.

[0050] A further subject matter of the present invention, according to a second aspect of the present invention, is an aerogel according to claim 55. Further advantageous embodiments of this aspect of the invention are the subject of the relevant documents.

[0051] Subclaims.

[0052] A further subject matter of the present invention according to a third aspect of the present invention is the use of an aerogel according to claim 68.

[0053] A further subject matter of the present invention according to a fourth aspect of the present invention is the use of an aerogel according to claim 69. Further advantageous embodiments of this aspect of the invention are the subject matter of the corresponding dependent claim.

[0054] Another further subject matter of the present invention - according to a fifth

[0055] Aspect of the present invention - a device for producing aerogels according to claim 71. Further advantageous developments and embodiments of this aspect of the invention are the subject of the corresponding dependent claims.

[0056] Finally, the subject matter of the present invention – according to a sixth aspect of the present invention – is a method for producing a lyogel according to claim 74.

[0057] It goes without saying that special features, characteristics, designs and embodiments as well as advantages or the like, which are subsequently listed – for the purpose of avoiding unnecessary repetition – only with regard to one aspect of the invention, naturally apply accordingly to the other aspects of the invention without the need for express mention.

[0058] Furthermore, it is assumed that all values ​​or parameters mentioned below, or similar, can in principle be determined or ascertained using standardized or explicitly specified determination procedures or methods familiar to the person skilled in the art in this field.

[0059] Furthermore, it goes without saying that all percentages relating to weight or quantity are selected by a specialist in such a way that the total results in 100%; this, however, is self-evident.

[0060] Having said that, the present invention will now be described in more detail.

[0061] The subject matter of the present invention – according to a first aspect of the present invention – is thus a method for producing a silica aerogel by means of a sol-gel process, wherein a lyogel is first produced from a sol and the lyogel is subsequently converted into an aerogel, wherein the formation of the lyogel is carried out at least partially under a pressure of more than 30 bar.

[0062] For, as the applicant surprisingly discovered, by applying pressures exceeding 30 bar during the production of the lyogel from a sol, in particular a precursor sol (i.e., a solution or dispersion of a precursor), a dimensionally stable gel can be produced almost instantaneously. In this way, for example, by injecting or dripping a sol into an autoclave, lyogel particles and ultimately also aerogel particles can be obtained, the external shape of which corresponds to the droplets introduced into the autoclave. This means that the process according to the invention makes it possible to obtain nearly spherical or cylindrical aerogels, in particular silica aerogels, which are not previously known in the prior art.

[0063] Furthermore, within the scope of the present invention, the process time for the production of silica aerogels, from gel formation to completion of drying—when all process steps are carried out under increased pressure—can be reduced to 1 to 2 hours, and in particular to less than 1.5 hours. This represents a considerable time saving compared to the prior art and thus enables the continuous or quasi-continuous production of aerogels. In addition to the faster production times, the process according to the invention also significantly reduces costs, thereby opening up further application areas for aerogels in industrial environments.

[0064] Since spherical or cylindrical aerogel particles are accessible with the method according to the invention, these are ideally suited as thermal insulation materials due to their excellent mechanical properties and resistances as well as the possibility of forming dense sphere packings, especially in loose fill, but also for incorporation into insulating plaster systems.

[0065] Due to the almost instantaneous, i.e. immediate, gel formation, it is also possible to specifically adjust both the particle size and the particle size distribution of the resulting lyogel particles and thus also of the aerogel particles.

[0066] 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, the so-called sols. In a sol-gel process, particles in the nanometer range are typically obtained from a colloidal dispersion, the sol, by aggregation. These particles then form a gel, i.e., a three-dimensional network, through further condensation and aggregation, the pores of which are filled with a fluid, wherein the fluid is either a liquid or a gas.

[0067] Within the scope of the present invention, a gel is a dimensionally stable, dispersed system rich in liquids and / or gases, consisting of at least two components. These components comprise at least a solid, colloidally divided substance with long or widely branched particles, such as gelatin, silica, montmorillonite, bentonite, polysaccharides, pectins, etc., and a fluid, in particular a gas or liquid, as the dispersion medium. The solid substance is coherent, meaning it forms a three-dimensional network within the dispersion medium, with the particles adhering to one another at various points, the so-called flap points, by secondary or primary valences. If the spaces between the particles are filled with a liquid, the system is called a lyogel. If the dispersion medium is air, the gel is called an aerogel.For further details on the term gel, reference is made to the entry for the keyword "Gels" in ROEMPP Chemistry Lexicon, 9th expanded and revised edition, Volume 2, 1999, p. 151 1.

[0068] A lyogel is a gel, i.e., a three-dimensional network whose pores are filled with a liquid. Special cases of lyogels include the flydrogel, in which the liquid is water, and the alcogel, in which the liquid is an alcohol, usually ethanol. Lyogels containing organic solvents are also called organogels. Within the scope of the present invention, a sol is understood to be a solution or a finely divided dispersion, i.e., a colloidal dispersion.

[0069] Within the scope of the present invention, a solution is understood to be a single-phase mixture in which a substance – the solute – is homogeneously dispersed in a second substance – the solvent. Within the scope of the present invention, a dispersion is understood to be a two-phase mixture in which a first phase containing the dispersed substance, the so-called discontinuous phase, is finely dispersed, and in particular homogeneously dispersed, in a second phase, the dispersion medium or continuous phase. The transition from solutions to dispersions is fluid and not strictly distinguishable; for example, colloidal solutions cannot be unambiguously classified as either solutions or dispersions. Even with "solutions" of highly polymeric macromolecules, it is not possible to definitively determine whether a solution or a dispersion is present. Therefore, within the scope of the present invention, a sol is preferably understood to be a solution or a finely dispersed, i.e.,h. colloidal dispersion. According to a preferred embodiment of the present invention, the formation of the lyogel is carried out entirely under increased pressure. In this context, it has proven particularly advantageous if the formation of the lyogel takes place in an autoclave, for example by introducing the sol into an autoclave.

[0070] Preferably, the process according to the invention, in particular the lyogel formation, is carried out in a pressurized process medium, especially a compressed gas or a supercritical substance or mixture of substances. Carbon dioxide and / or inert gases, in particular nitrogen and / or argon, optionally in combination with other gases or substances, are used as the process medium. It has proven particularly effective to use carbon dioxide and / or nitrogen, optionally in combination with other gases or substances. Carbon dioxide, mixtures of carbon dioxide and nitrogen, or mixtures of nitrogen and ammonia are commonly used as the process medium. For the purposes of this invention, a substance is understood to be, in particular, a chemical substance, i.e., a chemical compound or an element with specific physical or chemical properties.

[0071] Within the scope of the present invention, particularly good results are obtained when the lyogel formation is carried out in compressed carbon dioxide, especially supercritical carbon dioxide. The use of supercritical carbon dioxide has the particular advantage that acidification of the sol to initiate gel formation during the production of silica aerogels can be omitted, and no other electrolytes need to be added to the sol, which would then have to be removed. Initiation of gel formation by a shift in pH can also be achieved by using mixtures of nitrogen or argon with ammonia, in which case the sol preferably has a pH in the acidic range.

[0072] As regards the pressure at which the method according to the invention is carried out, this can naturally vary over a wide range. However, it has proven advantageous if the pressure is more than 40 bar, in particular more than 50 bar, preferably more than 60 bar, more preferably more than 70 bar, and most preferably more than 74 bar. Likewise, within the scope of the present invention, it can be provided that the pressure is set between 30 and 300 bar, in particular in the range of 40 to 250 bar, preferably in the range of 50 to 200 bar, more preferably in the range of 60 to 180 bar, more preferably in the range of 70 to 160 bar, and most preferably in the range of 74 to 150 bar.

[0073] Particularly good results are obtained when the lyogel is produced at elevated temperatures. In this context, it has proven effective to produce the lyogel at temperatures above 50 °C, preferably 70 °C, preferably 80 °C.

[0074] It can also be provided that the production of the lyogel is carried out at temperatures in the range of 50 to 200 °C, in particular 60 to 180 °C, preferably 70 to 160 °C, preferably 80 to 140 °C.

[0075] At the aforementioned pressures and temperatures, particularly rapid gel formation can be achieved, enabling, for example, the production of nearly spherical lyogels that are dimensionally stable and retain their shape even in subsequent processes. Furthermore, it has proven advantageous to carry out the conversion of the lyogel to an aerogel at a pressure exceeding 50 bar. Within the scope of the present invention, the conversion of the lyogel to an aerogel preferably encompasses all measures and process steps necessary to remove the liquid solvent or dispersion agent from the lyogel.

[0076] According to a preferred embodiment of the present invention, the formation of the lyogel and its conversion into an aerogel are provided for continuously or quasi-continuously. With the process according to the invention, the process times, in particular the times of the individual process steps, can be shortened to such an extent that continuous or at least quasi-continuous production of aerogels, especially silica aerogels, is possible. The production can be carried out either as a one-pot process, i.e., in a single reaction vessel, in particular an autoclave, or in successive apparatuses, in particular several autoclaves. Within the scope of the present invention, it is typically provided that the sol is a solution or dispersion of a precursor.

[0077] In the context of the present invention, a precursor is understood to be a precursor substance from which the desired target compound, in particular an S1O2 network, is formed by chemical reaction, in particular by hydrolysis or solvolysis and subsequent condensation.

[0078] Within the scope of the present invention, in principle all compounds capable of forming a gel from precursors can be used. Gel formation can occur at acidic, neutral, or basic pH values. In this context, it is particularly advantageous for gel formation to take place in an acidic pH range, as this significantly reduces gel formation times, especially when using supercritical CO2 as the process medium, and eliminates the need for electrolytes. Alternatively, gel formation can also occur at basic pH values, for example, by using mixtures of nitrogen and ammonia as the process medium. Particularly good results are obtained when the precursor is selected from silica, especially colloidal silica, silica sols, silanes (preferably tetraalkoxysilanes), siloxanes, and mixtures thereof.The aforementioned compounds form, upon hydrolysis, a possibly organically modified silicon dioxide network, which is ideally suited for the production of silica aerogels.

[0079] Particularly good results are obtained in this context when the precursor is selected from silicas, especially collodionic silica, silica sols, and tetraalkoxysilanes, preferably tetraethoxysilanes and / or tetramethoxysilanes. It is especially preferred if the precursor is a silica.

[0080] Within the scope of the present invention, it is usually provided that the sol comprises at least one solvent or dispersant.

[0081] In this context, it has proven advantageous if the solvent or dispersion agent 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.

[0082] Particularly good results are obtained in this context when the solvent or dispersant consists of alcohols, especially methanol, ethanol, isopropanol, water, and mixtures thereof. Mixtures of organic solvents and water, especially ethanol and water, are particularly preferred within the scope of the present invention, since, on the one hand, the water causes rapid hydrolysis and condensation of the precursor compound, and on the other hand, a proportion of organic solvents promotes the removal of the solvent or dispersant from the pores of the lyogel.

[0083] The use of organic solvents such as ethanol, acetone, dimethyl sulfoxide for gel synthesis offers the possibility of also using hydrophobizing agents such as trimethylsilanol, methyltriethoxysilane, diphenylsilanediol, hexamethyldisilazane etc. directly during the gelation process.

[0084] For the production of silica aerogels and lyogels in particular, precursor solutions, preferably based on silica sols, colloidal silicic acids, and tetraethyl silica esters, are first prepared and presented. In the case of the silica sols and the silicic acid, these are pre-silicified water glass (polysilicic acids) with varying degrees of silicification and reduced alkali content. The monosilicic acids, generally produced by ion exchange, exist predominantly as di- and trisilicic acids due to condensation processes.

[0085] In contrast, silica sols exhibit a significantly higher degree of silicification and typically have a primary particle size between 5 and 40 mm. Compared to tetraethyl silica esters (TMOS, TEOS) and potassium silicates often used in aerogel production, the use of silica sols and silicas offers the possibility of targeted control of the gelation and subsequent maturation process of the hydrogels. In silica sols and silicas, the silica nanoparticles are generally stabilized in solution via ionic charges.

[0086] One way to obtain polysilicic acids with low water content and a higher proportion of organic solvents is to use alcoholic tetraethyl silicates, which, however, must first be pre-hydrolyzed to ensure sufficiently rapid polycondensation of the resulting monosilicic acid. To increase the proportion of monosilicic acids in the precursor solution, aqueous silica solutions can be added after hydrolysis of the tetraethyl silicates, and gel formation can then be initiated to produce an organogel with low water content.

[0087] According to a particular and equally preferred embodiment of the present invention, the solvent or dispersing agent is water. As for the times in which the lyogel is formed, these are, as previously described, extremely short. Within the scope of the present invention, it is typically provided that the formation of the lyogel under pressure occurs within 0.1 to 60 seconds, in particular 0.2 to 30 seconds, preferably 0.2 to 10 seconds, more preferably 0.3 to 5 seconds, and most preferably 0.3 to 3 seconds. Thus, within the scope of the present invention, as already explained, almost instantaneous, i.e., immediate, gel formation is possible.

[0088] In the context of the present invention, it has also proven advantageous if the sol has a pH value greater than or equal to 7, in particular greater than 7, preferably greater than 8, preferably greater than 8.5.

[0089] The use of a sol with a pH in the basic range generally prevents premature gel formation and, especially when using supercritical CO2 as the process medium, particularly to generate high pressure, allows for rapid gel formation. This is because carbon dioxide reacts as an acid in the presence of water, and the sol is thus acidified, preferably under elevated temperature and pressure—that is, under strongly reactive, accelerating conditions. This leads to immediate gel formation, enabling the synthesis of regular spherical or cylindrical lyogel particles.

[0090] Within the scope of the present invention, it can further be provided that the sol has a pH value in the range of 7 to 14, in particular 8 to 1.1, preferably 8.5 to 1.1. Specifically for the production of silica aerogels or lyogels, in addition to gelation using carbon dioxide as the process medium, other methods of formation can be used alternatively or additionally. In the case of silicic acids, gelation can be most easily initiated by shifting the pH value into the neutral pH range. The gelation times can be set in the range of seconds. Preferably, within the scope of the present invention, at least partially aqueous dilute solutions or dispersions based on silica sols, silicic acids, or tetraalkoxysilane are added dropwise to an autoclave containing compressed carbon dioxide for the production of silica aerogels or lyogels.Here, compressed carbon dioxide can be used for targeted gelation and continuous gel production of the precursor solution by shifting the pH value. Surprisingly, gelation occurs immediately upon entry of the sol into the autoclave when the pH value is set above 7, preferably between 8.5 and 1.1. Gel formation proceeds so rapidly that dimensionally stable spherical or cylindrical particles are obtained. The aging times of the hydrogels or organogels produced in this way are, at room temperature, in the range of approximately 30 minutes. By increasing the temperature to 100 °C during the droplet phase, the aging time can be reduced to just a few minutes.

[0091] Alternatively, precursor solutions or brines, especially silica solutions, with an acidic pH can be used and brought into contact with a basic process medium, for example, a mixture of nitrogen and ammonia. This also induces gel formation by shifting the pH. Furthermore, the sol typically requires a certain solids content to form a stable gel. The solids content of the sol refers to the proportion of the sol remaining after the removal of all liquid components. Within the scope of the present invention, it has proven advantageous for the sol to have a solids content of at least 2% by weight, in particular 2.5% by weight, preferably 3% by weight, more preferably 4% by weight, and particularly 5% by weight, based on the sol.According to a preferred embodiment of the present invention, the sol has a solids content in the range of 2 to 30 wt.%, in particular 2.5 to 20 wt.%, preferably 3 to 15 wt.%, more preferably 4 to 10 wt.%, and most preferably 5 to 9 wt.%, based on the sol. With solids contents in the aforementioned range, shape-stable lyogels can be obtained particularly quickly, which also exhibit the desired high porosity.

[0092] Within the scope of the present invention, the sol may contain a hydrophobizing agent, in particular a silanizing agent. The use of a hydrophobizing agent, in particular a silanizing agent, in the sol leads in particular to the incorporation of hydrophobic groups into the lyogel framework. This, in turn, results in a more elastic gel structure, which is significantly more resilient during any solvent exchange or drying than, for example, a pure SiO2 structure.

[0093] Within the scope of the present invention, it is preferred that the hydrophobizing agent is selected from organosilanes, in particular monoorganosilanes, diorganosilanes, triorganosilanes, silazanes, silanoes, in particular monoorganosilanols, diorganosilanols, and mixtures thereof. For the purposes of the present invention, organosilanes or organosilanols are understood to be silanes or silanoes with organic groups, in particular hydrophobic organic groups, such as alkyl, alkenyl, or aryl.

[0094] When a silane is used as a water-repellent agent within the scope of the present invention, its chemical nature can also vary widely. Particularly good results are obtained, however, when a silane of the general formula I is used.

[0095] R 1 nSiR 2 4-n (I) with

[0096] n = 1 to 3, in particular 1 or 2, preferably 1 ;

[0097] R 1= Cr to C3o-alkyl and / or Ce to C3o-aryl,

[0098] especially C2- to C2o-alkyl and / or Ce- to C2o-aryl,

[0099] preferably C3- to C2o-alkyl and / or Ce- to C2o-aryl,

[0100] preferably C4 to C- 15 -Alkyl and / or Ce- to C- 15 -Aryl,

[0101] especially preferred C5 to C- 12 -Alkyl and / or Ce- to C- 12 -Aryl,

[0102] especially preferred C5 to C- 12 -Alkyl;

[0103] R 2 = Halide, especially chloride, bromide and / or iodide,

[0104] OX with X = hydrogen, alkyl, aryl, polyether and / or carboxylic acid derivative,

[0105] especially alkyl, preferably Cr to Cs-alkyl,

[0106] preferably C2- to C4-alkyl;

[0107] is used.

[0108] Particularly good results are obtained within the scope of the present invention when the hydrophobizing agent is selected from organochlorosilanes, in particular monoorganochlorosilanes, diorganochlorosilanes, triorganochlorosilanes, methoxyorganosilanes, especially trimethoxyorganosilanes, dimethoxydiorganosilanes, methoxytriorganosilanes, ethoxyorganosilanes, especially triethoxyorganosilanes, diethoxydiorganosilanes, ethoxytriorganosilanes, hexamethylenedisilazane, trimethylsilanol, diphenylsilanediol, phenyltriethoxysilane, trimethylisopropenoxysilane, and mixtures thereof. By using hydrophobizing agents, especially silanizing agents, early in the process before gel formation, the developing network structure can be influenced and the resulting pore sizes controlled. Furthermore, elasticization of the gel network can be achieved by incorporating mono- and difunctional silanizing agents.Both can be used to accelerate a subsequent solvent exchange of the produced hydrogel.

[0109] Within the scope of the present invention, it is preferred if the sol is introduced in the form of drops into a pressurized device, in particular by dripping and / or injecting.

[0110] By introducing the sol in droplet form, for example by dripping or injecting it into a pressurized device, particularly an autoclave, it is possible to synthesize aerogels with a nearly circular cross-section. Depending on the drip rate and injection conditions, nearly spherical and / or cylindrical particles can be obtained. The nozzle can be designed, for example, as a slot nozzle or a capillary, and the sol can be introduced into the device by a pump, especially a high-pressure pump. Introducing the sol in droplet form into a pressurized device thus makes it possible to obtain nearly spherical aerogel particles that retain their shape throughout the process.This makes spherical aerogels accessible, which exhibit improved mechanical properties compared to the prior art and can form denser sphere packings, thus making them better suited as thermal insulation materials, both in loose fill and, for example, for incorporation into insulating plaster systems. According to a particular embodiment of the present invention, the sol is pre-gelled before applying a pressure exceeding 30 bar, especially before being introduced into the pressurized device. Pre-gelling refers to the formation of larger network structures and aggregates, although a continuous spatial network is not yet obtained. The pre-gelled sol remains fluid and can, for example, be dripped or injected into a device.

[0111] As previously described, the lyogel is preferably in the form of particles with a circular cross-section, in particular in the form of spherical or cylindrical particles.

[0112] A method for producing lyogel and subsequently also aerogel particles with a circular cross-section is not known, especially for silica aerogels. Within the scope of the present invention, it is possible, in particular by adjusting the conditions under which the sol is introduced into the pressurized device, to selectively influence and adjust both the particle size and the particle size distribution. As previously described, several reactor configurations are possible to achieve gelation of the precursor sols.

[0113] Preferably, the precursor solution is metered into a carbon dioxide-filled pressure vessel during gelation. Ideally, the metering is carried out in such a way that a droplet chain or a gel strand forms. Upon entering the pressure vessel, the precursor solution is liquid or partially gelled and preferably gels completely immediately upon contact with the carbon dioxide, forming carbonic acid and a corresponding change in the pFI value. The solvent contained in the resulting gel, such as water or ethanol, is then dissolved in the compressed carbon dioxide, particularly during gel storage, so that a dry particle, the aerogel, can be obtained. Generally, it is possible to induce gel formation in the precursor solutions by selectively destabilizing the silica sols and silicic acids, especially in a two-step process, e.g., by pH shifting or electrolyte addition.To initiate hydrogel formation, electrolyte additions, pH shifts using acids or bases, and denaturing solvents such as ethanol and acetone can be employed, thereby accelerating the hydrolysis and condensation rates of silica sols or polysilicic acid. The polycondensation capacity of the silicas represents the rate-determining step in the formation of a dimensionally stable, three-dimensional network. It has been shown that the targeted gelation of silica or silica sols is possible through the use of ethanol and electrolytes. Organogels with a 66 vol% ethanol content can be synthesized. These are characterized by a high hydrolysis and condensation rate as well as the formation of a dimensionally stable organogel network.

[0114] Investigations into the droplet properties of precursor solutions for the production of silica aerogels show that gelation can be initiated by contact of a precursor solution with compressed carbon dioxide at the inlet of a high-pressure vessel filled with carbon dioxide. The precursor sol, consisting of a silica sol, silica solution, and / or tetraethyl silicate, is liquid or partially gelled at the inlet of the high-pressure vessel and gels completely immediately thereafter. The droplet size can be controlled, in particular, by selecting the nozzle orifice and the gelation rate, and is typically in the range of 0.5 to 5 mm when using a 2 mm nozzle. By selecting a smaller nozzle, the gel particle size can be further reduced. The resulting particles preferably have a spherical shape and retain this shape even in subsequent process steps.

[0115] For pre- or partial gelation of the precursor sols, acids or bases can be added to the basic or acid-stabilized silica, particularly in the binary feed, whereby the gelation times can be adjusted via the resulting pH value. However, the preferred method is to shift the pH value using the drying gas or process medium employed. With compressed CO2, a decrease in pH can be achieved through the formation of carbonic acids, as previously described, while a decrease in pH can be achieved, for example, by using inert gases, especially nitrogen and / or argon, in combination with ammonia. The gelation of the silica can be carried out analogously to that described previously for the silicas. Additionally, the silica sols can be induced to gel by the use of electrolyte additives, such as polyvalent metal salts and denaturing solvents like ethanol or acetone.

[0116] Tetraethyl silicates, such as tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS), offer—as previously described—the possibility of producing organol gels with low water content, which significantly accelerates the subsequent solvent exchange. To accelerate the gelation rates of these precursorsols, pre-hydrolysis of the metal alcoholates can be performed, which can be carried out in both acidic and basic pH ranges, with the formation of three-dimensional networks being favored in acidic conditions. Mineral acids such as hydrochloric acid can be used as catalysts for the pre-condensation. In particular, the pre-condensation can be accelerated by the use of catalysts such as organic acids, especially acetic acid, inorganic acids such as hydrochloric acid, or Lewis acids, such as titanium tetrabutanoate.Pre-condensation with acetic acid at pH values ​​of 3.5 to 4.5 and a stoichiometric water content of 2.5 to 3.5 produces precursorsols within a few hours, which can be gelled by pH adjustments and the addition of water. Additionally, it is possible to shift the pH of these pre-condensed tetraethyl orthosilicate solutions or sols into the alkaline range, thus triggering CO2-induced gelation, analogous to the gelation of silica. Furthermore, the solutions are compatible with silica sols and, in particular, with silicas, so that they can be used in combination, which significantly reduces gelation times. According to a preferred embodiment of the present invention, the present invention relates to a previously described process, wherein...

[0117] (a) in a first process step a sol, in particular a solution or dispersion of a precursor, is provided, and

[0118] (b) In a second process step following the first process step (a), the sol is introduced, in particular by dripping or injecting, into a device pressurized to more than 30 bar, thereby obtaining a particulate lyogel. All the advantages, special features, and characteristics mentioned above can be applied equally to this particular embodiment of the process according to the invention.

[0119] Within the scope of the present invention, it may further be provided that the lyogel is allowed to age after its formation. If the lyogel is allowed to age, it is preferred that it be allowed to age for a period of 1 minute to 1 hour, in particular 2 to 50 minutes, preferably 3 to 40 minutes, more preferably 5 to 35 minutes, and most preferably 10 to 20 minutes. Aging the lyogel particularly strengthens the gel structures, making them significantly more stable and resistant in the subsequent drying process. Preferably, the aging of the lyogel is carried out at the temperature at which the lyogel is formed. In this context, it is preferred that the aging of the lyogel is carried out in the temperature range of 50 to 130 °C, in particular 60 to 120 °C, and more preferably 80 to 110 °C.The pressures at which the aging process is carried out can vary widely. However, within the scope of the present invention, it is particularly preferred if the aging of the lyogel is carried out at pressures similar to those used during its formation. The high pressure results in significantly faster gel formation and aging of the lyogel, especially in a CO2 atmosphere.

[0120] Within the scope of the present invention, it is therefore possible to reduce the aging time of a lyogel, in particular a hydrogel, which is usually at least 2 hours, to less than 15 minutes.

[0121] Within the scope of the present invention, it may be provided that, after the formation of the lyogel, particularly following process step (b), a solvent exchange is carried out, especially in a third process step (c). A solvent exchange may be particularly necessary to facilitate subsequent drying of the lyogel to form an aerogel. Water is difficult to remove from the typically hydrophilic network, particularly the SiO2 network, of the lyogel during the drying process by supplying thermal energy. This also applies if the lyogel has been made hydrophobic. In particular, to reduce the water content of the previously produced lyogels, especially hydro- or organogels, before the actual drying step, it may be necessary to subject the gels to a solvent exchange by, for example, coating the particles with an organic solvent.

[0122] The produced particles, preferably with a circular cross-section, especially hydrogel particles, have a water content that usually makes drying difficult.

[0123] It has been shown that reducing the water content of the initial silica solution significantly accelerates the drying rates, depending on the added organic solvent. In this context, it is preferred to bring the lyogel into contact with a liquid or gaseous organic solvent to carry out the solvent exchange.

[0124] The organic solvent can be introduced into the reaction chamber in gaseous form and then displaces water or other organic solvents trapped in the pores of the lyogel. Alternatively, the lyogel can be brought into contact with the liquid solvent, in particular dispersed in it or layered with it, thus achieving extensive solvent exchange, for example, by repeatedly layering with solvents and removing the mixture of water and / or organic solvents. Preferably, the solvent used for solvent exchange is soluble in a drying gas, especially carbon dioxide. This makes it possible to carry out supercritical drying with carbon dioxide significantly faster and more gently.

[0125] Within the scope of the present invention, it is also preferred if, through solvent exchange, the water content of the lyogel is reduced to a value of less than 30 wt.%, particularly less than 20 wt.%, preferably less than 15 wt.%, and preferably less than 10 wt.%, based on the lyogel. Reducing the proportion of water, in particular, in the lyogel enables effective and gentle drying with carbon dioxide in the critical range. Within the scope of the present invention, it is preferably provided that the solvent exchange, in particular the contacting of the lyogel with the solvent, is carried out under increased pressure.Solvent exchange under increased pressure significantly accelerates the solvent exchange process. In particular, only small amounts of gaseous organic solvents need to be mixed into a compressed and pressurized gas phase, yet these amounts are still sufficient to displace water or other solvents from the pores of the lyogel. Preferably, according to the present invention, during solvent exchange either liquid solvent or a mixture of water and organic solvent is removed from the device, or the water-contaminated gas phase is at least partially removed from the reactor and new solvent in a gaseous state is introduced into the reactor to achieve the most complete solvent exchange possible.

[0126] Within the scope of the present invention, particularly good results are obtained when the solvent exchange, in particular the contacting of the lyogel with the solvent, is carried out at pressures of more than 30 bar, in particular more than 50 bar, preferably more than 70 bar, preferably more than 100 bar, and especially preferably more than 120 bar.

[0127] It is also possible that the solvent exchange, in particular the contacting of the lyogel with the solvent, is carried out at pressures in the range of 30 to 300 bar, in particular 50 to 250 bar, preferably 70 to 200 bar, preferably 100 to 180 bar, particularly preferably 120 to 170 bar.

[0128] Regarding the temperature range at which the solvent exchange is carried out, it has proven advantageous to perform the solvent exchange at elevated temperatures. Particularly good results are obtained in this context when the solvent exchange, especially the contacting of the lyogel with the solvent, is carried out at temperatures above 50 °C, particularly above 70 °C, preferably above 90 °C, more preferably above 100 °C, and most preferably above 110 °C. A high temperature, especially in combination with high pressure, achieves the fastest and most complete solvent exchange possible.

[0129] In this context, it can also be provided that the solvent exchange, in particular the contacting of the lyogel with the solvent, is carried out at temperatures in the range of 50 to 180 °C, particularly 70 to 160 °C, preferably 90 to 150 °C, preferably 100 to 140 °C, and especially preferably 110 to 130 °C. As regards the organic solvent, it has proven advantageous if the solvent is selected from the group consisting of hydrophilic organic solvents, hydrophobic organic solvents, and mixtures thereof. Within the scope of the present invention, it is particularly preferred if the organic solvent is soluble in carbon dioxide.

[0130] In the context of this invention, an organic solvent is understood to be a solvent or dispersant containing an organic group. Regarding the organic solvent, it has proven effective to select one from the group consisting of alcohols, ethers, dimethyl sulfoxide, N,N-dimethylformamide, Cs- to Cs-alkanes, and mixtures thereof. Particularly good results are obtained in the context of this invention when the organic solvent is selected from methanol, ethanol, isopropanol, dimethyl sulfoxide, n-pentane, n-hexane, n-heptane, cyclohexane, and mixtures thereof. The aforementioned solvents not only facilitate solvent exchange and subsequent drying but are also ideally suited for contacting the lyogel with modification reagents.

[0131] Within the scope of the present invention, it is particularly possible for the organic solvent to be brought into contact with the lyogel together with a hydrophobizing agent, in particular a silanizing agent. Within the scope of the present invention, it is thus possible to carry out hydrophobization, in particular silanization, of the lyogel even during the solvent exchange, in order to subsequently enable simple drying and conversion of the hydrogel into an aerogel. To achieve particularly effective hydrophobization, in particular silanization, it is advantageous if, at the beginning of the contact between the organic solvent and the hydrophobizing agent, the water content of the lyogel is at least 50% by weight, in particular at least 60% by weight, preferably at least 70% by weight.In this way, rapid hydrolysis and reaction of the reactive groups of the hydrophobizing agent, especially the silanizing agent, is achieved.

[0132] Regarding the chemical nature of the water-repellent agent, it has proven effective to select it from organosilanes, in particular monoorganosilanes, diorganosilanes, triorganosilanes, silazanes, silanoes, especially monoorganosilanols, diorganosilanols, and mixtures thereof. When a silane is used as the water-repellent agent within the scope of the present invention, its chemical nature can vary considerably. However, particularly good results are obtained when a silane of the general formula I is used.

[0133] R 1 nSiR 2 4-n (I) with

[0134] n = 1 to 3, in particular 1 or 2, preferably 1 ;

[0135] R 1 = Cr to C3o-alkyl and / or C Ö - up to C3o-acrylic,

[0136] especially C2- to C2o-alkyl and / or Ce- to C2o-aryl,

[0137] preferably C3- to C2o-alkyl and / or Ce- to C2o-aryl,

[0138] preferably C4 to C- 15 -Alkyl and / or Ce- to C- 15 -Aryl,

[0139] especially preferred C5 to C- 12 -Alkyl and / or Ce- to C- 12 -Aryl,

[0140] especially preferred C5 to C- 12 -Alkyl;

[0141] Halide, in particular chloride, bromide and / or iodide,

[0142] OX with X = hydrogen, alkyl, aryl, polyether

[0143] and / or carboxylic acid derivative,

[0144] especially alkyl, preferably C1- to Cs-alkyl,

[0145] preferably C2- to C4-alkyl;

[0146] is used.

[0147] Particularly good results are obtained in this context when the hydrophobizing agent is selected from organochlorosilanes, in particular monoorganochlorosilanes, diorganochlorosilanes, triorganochlorosilanes, methoxyorganosilanes, in particular trimethoxyorganosilanes, dimethoxydiorganosilanes, ethoxytriorganosilanes, ethoxyorganosilanes, in particular triethoxyorganosilanes, diethoxydiorganosilanes, ethoxytriorganosilanes, hexamethyldenisilazane, trimethylsilanol, diphenylsilanediol, phenyltriethoxysilane, trimethylisopropenoxysilane and mixtures thereof.

[0148] The hydrophobizing agents preferably used during the solvent exchange thus correspond to the hydrophobizing agents that are also used in the hydrophobization or silanization of the sol. Within the scope of the present invention, it is particularly preferred if both the precursor sol is treated with a hydrophobizing agent, in particular a silanizing agent, and further hydrophobization is carried out after lyogel formation.

[0149] Hydrophobization of the lyogel pores is achieved by hydrophobization after its formation, particularly as part of a solvent exchange or as a separate process step. During solvent exchange, pore hydrophobization, especially pore silanization, can be carried out using additional hydrophobizing agents, particularly silanizing agents. It has been found that the use of additional hydrophobizing agents, such as hexamethyldisilazane, can significantly accelerate the required solvent exchange step. For successful silanization, the residual water content of the lyogel should be sufficiently high, preferably above 50% by weight of the lyogel.

[0150] The pH values ​​of the solutions or dispersions of the hydrophobizing agent, especially the silanizing solutions, can vary depending on the hydrophobizing agents used, particularly the silanizing agents. When using trimethylsilanol, diphenylsilanediol, hexamethyldisilazane, and hexamethyldisiloxane, as well as other silanoic or silanol-forming substances, pH values ​​greater than 8 have proven advantageous. Organic solutions such as nonpolar substances (hexane), aprotic solvents, or alcoholic solvents, such as methanol, ethanol, isopropanol, or the like, can be used as silanizing solutions, to which the aforementioned hydrophobizing agents, especially silanizing agents, are added. The lyogels can be immersed in or overlaid with the solution or dispersion containing the hydrophobizing agent, with contact times of up to 30 minutes.

[0151] Alternatively, the hydrophobizing agents, in particular silanizing agents, can also be used in a dense phase saturated or partially saturated with organic solvents, especially the process medium, preferably a CO2 phase, wherein the phase can be either a subcritical gas phase or a supercritical phase. Suitable organic solvents include nonpolar solvents such as hexane, aprotic solvents such as dimethyl sulfoxide, or alcoholic solvents such as ethanol. The solvents used can improve the solubility of the hydrophobizing agents, in particular the silanizing agents, in the dense CC>2 phase. If the hydrophobizing agents, in particular the silanizing agents, have sufficient solubility in the process medium, especially in the dense CO2, the use of organic solvents can be omitted.

[0152] In the solvent exchange process, a CO₂-soluble solvent is preferably introduced. The solvent exchange then takes place, for example, under a pressure of 80 bar and 120 °C. In this process variant, dissolving the solvent into the CC phase is sufficient to displace the water from the pores. Alternatively, the gel, which is stored particularly in an autoclave, can be layered with liquid solvent. This is preferably done at a pressure of 160 bar and a temperature of 120 °C. According to the present invention, the solvent exchange can be carried out in several process stages, in particular in 2 to 15, preferably 3 to 10, and more preferably 3 to 4, process stages. In this context, it can be provided that the lyogel is repeatedly brought into contact with the organic solvent.Preferably, it is specified that in each process stage at least a part of a mixture of solvent and water or solvent to be replaced is removed from the reactor and new organic solvent is introduced.

[0153] Particularly preferred within the scope of the present invention is if the water content of the lyogel is reduced to below 20 vol.%, preferably below 15 vol.%, preferably below 10 vol.%, based on the total volume of solvents or dispersion agents, by means of solvent exchange.

[0154] According to a preferred embodiment, solvent exchange can be carried out using water-miscible solvents such as ethanol, methanol, isopropanol, and dimethyl sulfoxide. It is found that the residual water content in the spherical lyogel particles should preferably be reduced to less than 10 vol% before subsequent drying. Alternatively, and equally preferably, hydrophobic organic solvents such as hexane, pentane, or cyclohexane can be used for this process step. With sufficient pre-silanization, these solvents can displace the water trapped in the pores of the lyogel. The solvent exchange preferably takes place in compressed carbon dioxide. In this process, the solvent is metered under pressure into a reactor, particularly an autoclave.Surprisingly, it turns out that solvent exchange can be successfully carried out even if the solvent does not come into contact with the gel particles in liquid form. Rather, it is sufficient if the solvent dissolves in the compressed CO2 and thus penetrates the gel and displaces the water from the pores.

[0155] According to a preferred embodiment of the present invention, the present invention relates to a method for producing aerogel as previously described, wherein

[0156] (a) in a first procedural step a sol, in particular a solution or dispersion of a precursor, is provided,

[0157] (b) in a second process step following the first process step (a), the sol is introduced, in particular by dripping or injecting, into a device pressurized to more than 30 bar, whereby a particulate lyogel is obtained, and

[0158] (c) in a third process step following the second process step (b) a solvent exchange and / or a hydrophobization of the lyogel is carried out.

[0159] The solvent exchange in process step (c) can take place over a period of up to 50 minutes, in particular up to 40 minutes, and in particular up to 30 minutes. Particularly preferred within the scope of the present invention is a period of 10 to 50 minutes, in particular 20 to 40 minutes, and preferably 20 to 30 minutes.

[0160] All previously mentioned further embodiments, features, and special characteristics apply to the embodiment of the process according to the invention described above. Within the scope of the present invention, it is typically provided that the lyogel is converted into an aerogel by removing the solvent or dispersant, particularly in a subsequent process step (d). In this context, it may be provided that, following solvent removal and / or hydrolysis of the lyogel, particularly following process step (c), the lyogel is converted into an aerogel. Within the scope of the present invention, it is preferred if the solvent removal is carried out under increased pressure.

[0161] In general, to convert the lyogel into an aerogel, the lyogel is brought into contact with a drying medium, in particular a drying gas or a supercritical medium. Preferably, the drying medium is carbon dioxide. In this context, the lyogel can be brought into contact with the drying medium, in particular the drying gas or the supercritical medium, continuously or discontinuously. In discontinuous contact, the lyogel is brought into contact with a predetermined quantity of the drying medium in a device for a preselected period of time. Subsequently, the solvent-contaminated drying medium is removed and, if necessary, replaced with fresh drying medium until the desired degree of dryness is achieved.In continuous drying, where the lyogel is continuously brought into contact with the drying medium, the lyogel is brushed or flowed through by the drying medium in a device until the desired degree of dryness is reached.

[0162] Particularly good results are obtained in this context when the solvent removal is carried out at pressures above 50 bar, especially above 60 bar, preferably above 70 bar, and more preferably above 74 bar. Likewise, it can be provided that the solvent removal is carried out in the range of 50 to 180 bar, especially 55 to 160 bar, preferably 60 to 140 bar, preferably 70 to 130 bar, and most preferably 74 to 130 bar. Regarding the temperatures at which the solvent removal is carried out, it has proven advantageous to perform it at elevated temperatures. Typically, the solvent removal is carried out at temperatures above 50 °C, especially above 55 °C, and preferably above 60 °C.In this context, it may also be provided that the removal of the solvent is carried out at temperatures in the range of 50 to 160 °C, in particular 70 to 160 °C, preferably 90 to 150 °C, preferably 100 to 140 °C, and most preferably 10 to 130 °C.

[0163] By removing the solvent at the aforementioned pressures and temperatures, an aeolgel can be obtained particularly quickly, especially by supercritical drying using CO2. Typically, within the scope of the present invention, the solvent is removed from the lyogel within 10 to 50 minutes, preferably 20 to 30 minutes.

[0164] The present invention preferably relates to a method for producing an aerogel as described above, wherein

[0165] (a) in a first procedural step a sol, in particular a solution or dispersion of a precursor, is provided,

[0166] (b) in a second process step following the first process step (a), the sol is introduced, in particular by dripping or injecting, into a device pressurized to more than 30 bar, whereby a particulate lyogel is obtained,

[0167] (c) optionally, in a third process step following the second process step (b), a solvent exchange and / or a hydrophobization of the lyogel is carried out, and

[0168] (d) in a subsequent process step (d) the lyogel is converted into an aerogel by removing the solvent or dispersant.

[0169] All the aforementioned process features and embodiments, in particular advantages and special features, can be applied without restriction to this particular and preferred embodiment of the present invention.

[0170] As regards the total duration of the previously described process, the process according to the invention is usually carried out over the process steps (a) to (d) with the implementation of process step (c) in a period of 1 to 2 hours, preferably 1 to 1.5 hours.

[0171] The process according to the invention can be carried out either as a one-pot synthesis or process, i.e., in an autoclave. However, it is equally possible for the individual steps to be carried out in several devices connected in series, in particular autoclaves. However, it is particularly preferred within the scope of the present invention if all process steps are carried out under increased pressure, in particular in a CO2 atmosphere.

[0172] The particles are preferably dried in supercritical CO2. The drying time of the resulting spherical gel particles with a size of 0.5 to 5 mm can be reduced to 10 to 30 minutes using the inventive method by hydrophobizing the lyogels.

[0173] In particular, by feeding in compressed carbon dioxide as a drying medium, the gas stream can be used for targeted continuous drying of the organogels, and a single-stage aerogel particle production, i.e. in a reactor vessel or reactor, can be reliably achieved.

[0174] Due to the spherical particle shape and the typical particle diameters between 0.5 and 5 mm, supercritical drying can be carried out in a time window of up to 30 minutes at a pressure of 120 bar and a temperature of 60 to 120 °C.

[0175] The figure depictions show, according to

[0176] Fig. 1 shows a cross-section of a device according to the invention for carrying out the method according to the invention.

[0177] Fig. 2 Sorption isotherms of the commercially available aerogel P300 and the hydrophobized aerogel H-8 according to the invention. A further object of the present invention, according to a second aspect of the present invention, is an aerogel, obtainable in particular by the methods described above, wherein the aerogel is in the form of particles with a substantially circular cross-section. As already explained above, the aerogels according to the invention are characterized by a particularly circular cross-section, which significantly increases both the mechanical strength and the ability to form dense spherical clusters.

[0178] Within the scope of the present invention, it is usually provided that the aerogel particles are spherical or cylindrical.

[0179] The aerogels according to the invention offer advantages in processing due to their shape. For example, the spherical aerogels can be mixed much more easily into powder mixtures. Due to their improved flowability, higher strength under uniaxial pressure, and higher packing density compared to conventional aerogel powders based on irregular or cubic particles, the aerogels according to the invention, preferably spherical, are particularly suitable for use in powder beds or powder mixtures, such as thermal insulation plasters.

[0180] As for the particle size of the aerogel particles, these can naturally vary widely. However, it has proven advantageous if the aerogel has particle sizes in the range of 0.1 to 10 mm, particularly 0.2 to 8 mm, preferably 0.3 to 7 mm, and more preferably 0.5 to 5 mm.

[0181] Similarly, within the scope of the present invention, it may be provided that the aerogel particles have a monodisperse particle size distribution.

[0182] However, within the scope of the present invention, it is also possible for the aerogel particles to have a polydisperse particle size distribution. The particle size distribution can be specifically controlled, in particular, by varying the injection or dripping conditions into the reactor.

[0183] The aerogel particles according to the invention are highly porous solids. Typically, the aerogel has a porosity of over 94%, in particular over 95%, preferably over 96%. Likewise, it can be provided that the aerogel has a porosity of 94 to 99.5%, in particular 95 to 99%, preferably 96 to 98%. Furthermore, the aerogels according to the invention have high internal surface areas. Thus, it can be provided that the aerogel has a BET surface area of ​​at least 500 m². 2 / g, especially 600 m 2 / g, preferably 650 m 2 / g, preferably 700 m 2 / g, especially preferred 800 m 2 / g, exhibits.

[0184] Similarly, it may be intended that the aerogel has a BET surface area in the range of 500 to 1,000 m². 2 / g, especially 600 to 1,050 m 2 / g, preferably 650 to 1,000 m 2 / g, preferably 700 to 950 m 2 / g, especially preferably 800 to 900 m 2 / g, exhibits.

[0185] Regarding the thermal conductivity of the aerogel, this can vary widely. However, in the context of the present invention, the aerogel typically exhibits very low thermal conductivities. Particularly good results are obtained when the aerogel has a thermal conductivity of at most 0.025 W / mK, more specifically at most 0.022 W / mK, preferably 0.020 W / mK, and more preferably 0.019 W / mK.

[0186] Typically, the aerogel has 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, preferably 0.015 to 0.019 W / mK.

[0187] Furthermore, within the scope of the present invention, it may be provided that the aerogel has a density in the range of 0.01 to 0.60 g / cm³. 3 , especially 0.11 to 0.55 g / cm³ 3 , preferably 0.12 to 0.50 g / cm³3 preferably 0.13 to 0.50 g / cm³ 3 , exhibits.

[0188] For further details on the aerogel according to the invention, reference can be made to the above descriptions of the method according to the invention, which apply accordingly to the aerogel according to the invention.

[0189] A further object of the present invention according to a third aspect of the present invention is the use of the previously described aerogel for insulation purposes, in particular for sound insulation, electrical insulation or thermal insulation, especially for thermal insulation.

[0190] For further details regarding the use according to the invention, reference may be made to the descriptions of the other aspects of the invention, which apply accordingly to the use according to the invention. A further object of the present invention, according to a fourth aspect of the present invention, is the use of an aerogel, as described above, for insulating purposes, in particular as or in thermal insulation.

[0191] In this context, it may be intended that the aerogel is used in loose form, in a powder mixture or in an insulating material, for example an insulating plaster.

[0192] For further details regarding the use according to the invention, reference may be made to the above statements concerning the further aspects of the invention, which apply accordingly to the use according to the invention. A further object of the present invention – according to a fifth aspect of the present invention – is a device for producing aerogel under pressure, wherein the device

[0193] a) at least one pressurizable reactor,

[0194] (b) at least one inlet opening, in particular a nozzle, arranged on the reactor for introducing fluids, in particular liquids, into the reactor, and

[0195] (c) has at least one outlet opening, in particular a sluice gate, arranged on the reactor for the removal of liquids or solids from the reactor.

[0196] Within the scope of the present invention, it can be provided in particular that a sol for the production of a lyogel is dripped or injected into the reactor via at least one inlet opening. Preferably, the reactor has several inlet openings for the introduction of fluids, in particular liquids, namely at least one nozzle for introducing the sol into the reactor and at least one nozzle for introducing further solvents.

[0197] The outlet opening of the reactor is preferably designed in the form of a sluice gate to allow for the rapid removal of the lyogel or aerogel from the reactor, or to ensure multiple solvent exchanges by layering and subsequent draining of the contaminated solvent from the reactor.

[0198] According to a preferred embodiment of the present invention, the device has at least one inlet and / or outlet opening arranged on the reactor for introducing and / or removing gases into and / or from the reactor.

[0199] Preferably, the pressure in the reactor is regulated by the amount of substance, particularly in the gas phase and / or a supercritical phase, and / or the temperature. Pressure regulation can be achieved, for example, by introducing gas into or removing it from the reactor.

[0200] Furthermore, it is typically provided within the scope of the present invention that the device includes a temperature control unit. Temperature control also allows for the targeted influence and control of the processes within the reactor and thus within the device as a whole. In particular, it is possible to heat or cool the reactor. The device also typically includes a control unit, especially for controlling the pressure and / or temperature within the reactor.

[0201] The device according to the invention can either have one reactor or several reactors, in particular reactors arranged sequentially and / or interconnected, so that the individual process steps of the inventive process are each carried out in separate reactors. In this way, continuous aerogel production can be achieved.

[0202] For further details on the device according to the invention, reference can be made to the above statements on the further aspects of the invention, which apply accordingly to the device according to the invention.

[0203] Finally, a further aspect of the present invention—a sixth aspect thereof—is a method for producing a lyogel using a sol-gel process, wherein the formation of the lyogel is carried out at least partially under a pressure exceeding 30 bar. With regard to the production of the lyogel, all the advantages, features, and embodiments previously mentioned in connection with the method for producing an aerogel apply accordingly to the lyogel. For further details concerning the method for producing a lyogel according to the invention, reference may be made to the above descriptions of the further aspects of the invention, which apply accordingly to the method for producing a lyogel according to the invention.

[0204] The subject matter of the present invention is below illustrated in a non-limiting manner and by way of example with reference to the figures and the embodiments.

[0205] Fig. 1 schematically shows a device 1 according to the invention with a reactor 2. The reactor 2 has several inlet openings, in particular nozzles 3, 4, 5 for the inlet of liquids and / or gases and has an outlet opening 6 for the removal of substances from the reactor 2, such as aerogels or lyogels or liquid solvents.

[0206] To carry out the process according to the invention, a precursor solution 7 is provided, which is placed in a container 8 and introduced or injected into the reactor 2 by means of the inlet opening 7, in particular a nozzle. The precursor solution 7 is in particular an aqueous solution of silica, a silica sol or a silane hydrolysate, which has a pH value in the basic range, preferably between 8.5 and 10.

[0207] Reactor 2 preferably has an atmosphere 9 of supercritical CO2, in particular at a pressure of 80 to 120 bar and a temperature of 120 °C. This causes the sol to form directly into a nearly spherical and dimensionally stable lyogel 10. The lyogel particles 10 collect at the bottom of reactor 2 and can either be removed from reactor 2 or further processed within the reactor. Preferably, after the formation of the lyogel 10, a solvent exchange is carried out, simultaneously making the lyogel 10 hydrophobic by means of a suitable organic solvent and a hydrophobizing agent, in particular a silanizing agent. The solvent and hydrophobizing agent are introduced into reactor 2 via the inlet opening 5. It is preferred that the organic solvent is soluble in CO2 to enable supercritical drying with CO2.Gases, such as CO2, can be introduced into the reactor via the inlet opening 5 and, if necessary, removed again. After solvent exchange, the lyogel 10 is dried, specifically by first draining the solvent through the outlet opening 6 and then carrying out supercritical drying of the lyogel using CO2, resulting in an aerogel.

[0208] The subject matter of the present invention is explained below in a non-limiting manner by means of exemplary embodiments:

[0209] Examples of implementation

[0210] Silica aerogels are made from silicic acids and are examined for their properties:

[0211] 1. Production of the aerogels Production of the starting material:

[0212] The silica is produced from sodium silicate using an ion exchanger. The solids content is adjusted to 5 to 10 wt%, preferably 7 to 8 wt%. For storage, the silica can be stabilized at a pH of 1–2 using HCl. A few minutes before use, the pH of the silica is then adjusted to 8.5–10.5 by adding NH3.

[0213] Process description: The produced silica is introduced dropwise into a container pressurized with CO2 using a high-pressure pump. Depending on the capillary used, droplets with a diameter of 2 to 6 mm are formed. The pressure inside the container can be varied between 30 bar and 300 bar for gelation, at a minimum temperature of 60 °C. Gelation occurs immediately upon entry of the silica into the pressure vessel due to the change in pH caused by the CO2 diffusing into the water.

[0214] The spherical hydrogel particles collect at the bottom of the container. The water contained in the hydrogels interferes with the drying process and must therefore be replaced with a suitable CO₂-soluble solvent. For this purpose, the pressure is preferably set in the supercritical range, for example to 140 bar, and ethanol with 5% hexamethyldisilizazane (HDMZ) is metered into the container. This initially leads to the formation of a liquid ethanol phase at the bottom of the container and a CO₂ phase saturated with ethanol. It has been shown that both covering the gels with the liquid ethanol-HDMZ mixture and exclusive contact of the gels with the ethanol-saturated gas phase result in sufficient solvent exchange. The simultaneous addition of HDMZ leads to hydrophobization of the gels. After a residence time of 30 minutes, the liquid ethanol is drained from the container.Two further cycles of adding ethanol to the container follow, with the aim of saturating the CO2 phase with ethanol. After each 20-minute cycle, the saturated gas phase and the liquid ethanol phase are exchanged. It has proven particularly advantageous to carry out the first solvent exchange in such a way that the gels are layered with the liquid ethanol phase.

[0215] After solvent exchange under pressure, the gels contain less than 5% water and can be supercritically dried. For this purpose, the pressure in the column is preferably varied between 100 and 160 bar, preferably between 120 and 160 bar, for 45 minutes, at a vessel temperature of 80 to 120 °C. The supercritical drying can be carried out either continuously or batchwise. In batch drying, the gel is brought into contact with a defined quantity of the drying medium, in particular carbon dioxide, in the column. After an adjustable residence time, the solvent-enriched drying medium is partially or completely removed from the column and replaced with fresh drying medium. This process is repeated until the desired degree of dryness is achieved.Alternatively, during continuous drying, the column can be continuously purged with the drying medium, especially carbon dioxide. In continuous drying, the pressure can either be kept constant or varied, particularly periodically. After completion of the drying step, dry spherical aerogels can be removed. 2. Properties of the aerogels.

[0216] By investigating solvent exchange in hydrogels using compressed carbon dioxide, the influence of silanizing agents and the timing of their addition during the manufacturing process is examined.

[0217] This shows that the addition of silanizing agents prior to gel formation has positive effects on the developing gel matrix. The silanizing agent is incorporated into the forming Si-O network. This leads to partial elasticization of the network, which is reflected in smaller pore radii, accelerated solvent exchange, and reduced shrinkage.

[0218] To assess the degree of hydrophobization, the produced aerogel samples are stored in liquid water at 98% relative humidity. The results show that while pre-silanization is advantageous for structural formation, it is often insufficient for complete silanization. Post-silanization during the drying process results in only small amounts of moisture being absorbed into the pores of the aerogels over a storage period of four weeks in water.

[0219] Table 1: Overview of the results from nitrogen adsorption BET

[0220] Samples A5 to A8 underwent pre-silanization using hexadimethyldisilazane at pH 7.0, while samples H-5 to H-8 were both pre- and post-silanized. As a result, the mean pore radius could be varied between 30 and 15 nm. Shrinkage due to drying was reduced in the samples with pre- and post-silanization. Furthermore, sample H-8 exhibited the lowest shrinkage and the highest porosity.

[0221] Thermal conductivity: To determine the thermal conductivity, a Hot Disk device from C3 Prozess und Analysetechnik GmbH with a sensitivity of up to 0.005 W / m*K is used. The Hot Disk sensor consists of a nickel double spiral that serves both as a heating source and for measuring the temperature rise during the measurement.

[0222] Table 2: Overview of results from thermal conductivity measurements

[0223]

[0224] Pore ​​volume and density

[0225] Mercury porosimetry was used to determine the density and pore volume. In this process, the sample is subjected to pressures of up to 400 MPa, which destroys the sample but also allows for complete detection of the internal pore volume. Table 3: Overview of the mercury porosimetry results

[0226]

[0227] The commercially available, subcritically dried and hydrophobized aerogel Enova P300 (Cabot Corporation), which has a mean density of 150 kg / m³ according to its data sheet, is used as a reference. 3 exhibits, and uses the aerogel Enova 31 10 (Cabot Corporation).

[0228] The measured values ​​of the analogous sorption measurements are shown in Figure 2F, together with the hydrophobized aerogel H-8. Both aerogels exhibit a similar isothermal profile. A constant value for the adsorbed volume is not achieved. This behavior indicates that P300 also contains pores that are not detected by the sorption measurement and the evaluation according to BET or BJH. Additionally, P300 exhibits flysteresis that is extended at lower pressures, caused by increased flame desorption of nitrogen.

[0229] Table 4: Derived properties from the sorption isotherms of commercial aerogel P300 and sample H-8

[0230] sample

[0231]

[0232] P300 754.6 3.61 37 19 25 8

[0233] H-8 656.3 3.38 97 20.6 40 10 Reference symbol list:

[0234] 1 Device

[0235] 2 reactors

[0236] 3 Inlet opening 15

[0237] 4. Entrance opening

[0238] 5. Entrance opening

[0239] 6. Outlet opening

[0240] 7. Precursor solution

[0241] 8. Container 20

[0242] 9. Carbon dioxide atmosphere

[0243] 10. Lyogel particles

Claims

Patent claims:

1. A method for producing a silica aerogel using a sol-gel process, wherein a lyogel is first produced from a sol and the lyogel is subsequently converted into an aerogel, characterized by that the formation of the lyogel is carried out at least partially under a pressure of more than 30 bar.

2. Method according to claim 1, characterized in that the formation of the lyogel is carried out in a compressed gas or a supercritical substance or mixture of substances.

3. Method according to claim 1 or 2, characterized in that, that the pressure is more than 40 bar, in particular more than 50 bar, preferably more than 60 bar, preferably more than 70 bar, particularly preferably more than 74 bar, and / or that the production of the lyogel is carried out at temperatures above 50 °C, in particular 60 °C, preferably 70 °C, preferably 80 °C.

4. Method according to one of claims 1 to 3, characterized in that the conversion of the lyogel into an aerogel is carried out at a pressure of more than 50 bar.

5. A method according to any of the preceding claims, characterized in that the sol is a solution or dispersion of a precursor.

6. Method according to claim 5, characterized in that the precursor is selected from silicas, in particular colloidal silica, silica sols, silanes, silica sols, preferably tetraalkoxysilanes, siloxanes and mixtures thereof.

7. Method according to one of the preceding claims, characterized in that the sol contains a hydrophobizing agent, in particular a silanizing agent.

8. A method according to any one of the preceding claims, characterized in that the sol is introduced in the form of droplets into a pressurized device, in particular by dripping and / or injecting.

9. A method according to any one of the preceding claims, characterized in that a solvent exchange is carried out after the formation of the lyogel.

10. Method according to claim 9, characterized in that the solvent exchange, in particular the contacting of the lyogel with an organic solvent, is carried out under increased pressure. 1 1. Method according to claim 10, characterized in that the organic solvent is brought into contact with the lyogel together with a hydrophobizing agent, in particular a silanizing agent.

12. Method according to claim 11, characterized in that following a solvent exchange and / or a hydrophobization of the lyogel, in particular following process step (c), the lyogel is converted into an aerogel.

13. Aerogel, in particular available according to one of the preceding claims, characterized in that the aerogel is in the form of particles with, in particular, at least substantially circular cross-sections.

14. Aerogel according to claim 13, characterized in that the aerogel particles are spherical or cylindrical.

15. Aerogel according to claim 13 or 14, characterized in that the aerogel has particle sizes in the range of 0.1 to 10 mm, in particular 0.2 to 8 mm, preferably 0.3 to 7 mm, preferably 0.5 to 5 mm.

16. Use of an aerogel according to any one of claims 13 to 15 for insulation purposes, in particular for sound insulation, electrical insulation or thermal insulation, in particular for thermal insulation, or as a carrier material, as an absorber or as an adsorbent.

17. Use of an aerogel according to any one of claims 13 to 15 for insulating purposes, in particular as or in thermal insulation.

18. Device (1) for producing aerogel under pressure, characterized by that the device (1) (a) at least one pressurizable reactor (2), (b) at least one inlet opening (3) arranged on the reactor (2), in particular a nozzle, for introducing fluids, in particular liquids, into the reactor, and (c) has at least one outlet opening (4) arranged on the reactor (2), in particular a sluice gate, for removing liquids or solids from the reactor.

19. Method for the production of a silica lyogel using a sol-gel process, characterized by that the formation of the lyogel is carried out at least partially under a pressure of more than 30 bar.