MXPA97005686A - Procedure for the preparation of modified aerogels, and its - Google Patents

Abstract

The present invention relates to a process for the production of modified SiO 2 aerogels in which, a) a silicate lyogel is prepared, b) if desired, the lyogel prepared in step a) is subjected to a solvent exchange with another organic solvent, c) the gel obtained in weight a) or b) is reacted with at least one chlorine-free silylating agent, and d) the gel obtained in weight c) is dried subcritically and to the use of the same

Description

PROCEDURE FOR THE PREPARATION OF MODIFIED AEROGELS, AND THEIR USE FIELD OF THE INVENTION The invention relates to a process for the preparation of modified aerogeyes and their use.

BACKGROUND OF THE INVENTION Aerogels, especially those with porosities above 50% and densities below 0.5 g / cm3, have an extremely low thermal conductivity and are therefore used as a thermal insulating material, as described for example in EP- fl-0 171. 722. Aerogels, in the broadest sense, ie in the sense of "gel with air as a dispersion medium", are prepared by drying a suitable gel. In this sense, the term "airgel" includes aerogels in the narrowest sense, xerogsles and cryogels. A dry gel is described as an airgel in the narrowest sense if the liquid is removed from the gel at temperatures above the critical temperature and starting from pressures above the critical pressure. If, in contrast, the gel liquid is removed subcritically, for example with formation of a liquid-vapor boundary phase, then the resulting gel is described as a xerogel. It should be noted that novel gels are aerogels in the sense of gels with air as dispersion medium. However, since these gels are prepared by subcritical drying, they can also be described as xerogels. SIOO2 aerogels can be prepared, for example, by the acid hydrolysis of tetraethyl orthosilicate in ethanol. Hydrolysis gives a gel whose structure is determined by the temperature, pH and duration of the gelation process, among other things. However, generally the structure of the gel is crushed when the wet gels are dried, since the capillary forces that arise during drying are extremely large. The crushing of the gel can be inhibited by carrying out the drying above the critical temperature and the critical pressure of the solvent. Since the limit of the liquid and gaseous phases disappears in this region, capillary forces are no longer applied and the gel does not undergo any change during drying, that is, there is no shrinkage of the gel during drying. Preparation procedures based on this drying technique are known, for example, from EP-O-0 395 075 and UO 92/03378. However, this technique requires, if ethanol is used for example, a temperature of about 240 ° C and pressures above 50 bar. If CO2 is exchanged for ethanol before drying, the drying temperature decreases to about 30 ° C, but then the pressure required is above 70 bar. An alternative to the drying mentioned above is given by a method for subcritical drying of S1.O2 gels, if these are reacted with a silylating agent containing chlorine before drying. The SIO2 gel of the present can be obtained, for example, by means of acid hydrolysis of tetraalkoxysilane, preferably tetraethoxysilane (TEOS), using water in a suitable organic solvent, preferably ethanol. After exchanging the solvent for a suitable organic solvent, the gel obtained is reacted, in a subsequent step, with a silylating agent containing chlorine. As the silylating agent, rnethylchlorosilanes (Ile / i-n SiCln, with n = 1 to 3) are preferably used, due to their reactivity. The resulting S.I.O2 gel, which is modified on the surface with rnetilsilyl groups, can then be dried from the organic solvent in the air. In this way, aerogels with densities below 0.4 g / cm3 and porosities above 60% can be obtained. The preparation process based on this drying technique is described in detail in 94/25149. To increase the strength of the gel structure, the gels described above can also be mixed and left to stand with tetraalkoxysilanes in the aqueous alcoholic solution, before drying, as described in UO 92/20623. Nevertheless, the tetraalkoxysilanes used as starting materials in the processes described above are exceptionally expensive. Furthermore, during silylation with chlorine-containing silylating agents, large quantities of hydrogen chloride (HC1) and a large variety of by-products associated therewith inevitably occur and a very complicated and expensive purification of silylated SiO2 gels may be required. by means of repeated washing with a suitable organic solvent. Likewise, production plants especially resistant to corrosion associated with this are very expensive. In addition, the safety risk associated with the production of very large quantities of HC.1 gas. , it requires very complicated engineering and therefore it is also very expensive. A first and significant cost reduction can be achieved by using water glass as starting material for the preparation of the SiO2 gels. For example, by using an ion exchange resin and starting with an aqueous solution of water glass, a silica can be prepared which is polycondensed by the addition of a base to give a SIOO2 gel. After exchanging the aqueous medium with a suitable organic solvent, the gel obtained is reacted in a subsequent step with a silylating agent containing chlorine. Likewise, ethylchlorosilanes (Me? -nSiCln with n = 1 to 3) are preferably used as a silylating agent, due to their reactivity. Thereafter, the resulting gel of SIO2, which is modified on the surface with rnetilsilyl groups, can also be dried from an organic solvent in the air. The preparation process based on this technique is known from DE-fl-43 42 548.

However, the problems described above of the exceptionally high costs of the processes, aiated with the use of silylating agents containing chlorine, are not solved by using water glass as the starting material.

DESCRIPTION OF THE INVENTION Therefore, the object of the present invention is to provide a process for the preparation of modified Si-2 aerogels which do not have the disadvantages of the prior art, for example the formation of elimination products or even reaction products having problems of safety or treatment, and that also cause high costs of the procedure. This object is achieved by means of a process for the preparation of aerogels, in which, a) A silicate lyogel is prepared, b) If desired, the lyogel prepared in step a) is subjected to a solvent exchange with another organic solvent, c) The gel obtained in step a) or b) is reacted with at least one silylating agent, and the silylated gel obtained in step c) is dried suberically, The process comprises, in step c), reacting the gel obtained in step a) or b) with at least one chlorine-free silylating agent having the formula I, R2 R3 1 S i (0- C) (I) R * wherein each R1, identical or different, is a linear or branched alkyl radical of C -Ce, a cyclohexyl radical or a phenyl radical, R2, R3 R1 are identical or different and are hydrogen atoms, linear or branched alkyl radicals of Ci-Ce, cyclohexyl radicals or phenyl radicals, and n = 1, 2 or 3. In a preferred embodiment, in step a) a silicate alkali is prepared which is obtained by hydrolysis and condensation of Si alkoxides in a organic solvent, with water (i). The Si alkoxide used is a tetraalkoxysilane, preferably tetraethoxy- or tetraethoxysilane. The organic solvent is preferably an alcohol, particularly preferably ethanol or methanol, to which up to 20% by volume of water can be added. In the hydrolysis and condensation of the Si alkoxides in an organic solvent with water, acid and / or base catalysts can be added in a one or two stage step. In a particularly preferred embodiment, in step a), a silicate lyogel is prepared using an ion exchange acidic resin and a mineral acid to carry an aqueous solution of water glass to a pH < 3, adding a base to polycondensen the resulting silica to give a SiO2 gel and, if a mineral acid was used, washing the gel with water until substantially free of electrolytes (ii). Sodium water glass and / or potassium water glass are generally used. The ion exchange resin is preferably an acid resin, with one containing sulfonic acid groups being especially suitable. If mineral acids are used, hydrochloric acid and sulfuric acid are especially suitable. The base used is generally NH 4 OH, KOH, fil (0H) 3 and / or colloidal silica. If a mineral acid is used, the SiO2 gel produced using the base is washed with water until substantially free of electrolytes. It is preferable to continue washing until the discharged washing water has the same electrical conductivity as the demineralized water. It is preferable to let the gel sit before washing, to be precise, generally at a temperature of 20 to 100 ° C, preferably 40 to 100 ° C and particularly preferred of 80 to 100 ° C, and at a pH of 4 to 11. , preferably from 5 to 9. The time for this is generally from 1 second to 48 hours, preferably from 1 second to 3 hours. The lyogel obtained in (i) or (ii) is washed with an organic solvent until the water content of the gel is less than 5% by weight, preferably less than 2% by weight, and particularly preferred less than 1% by weight . The solvents used are generally aliphatic alcohols, ethers, esters or ketones, or aliphatic or aromatic hydrocarbons. The preferred solvents are methanol, ethane.1. , acetone, tetrahydrofuran, ethyl acetate, dioxane, n-hexane and toluene. The ketone is particularly preferred as a solvent. However, it is also possible to use mixtures of the aforementioned solvents. In addition, water can be washed first with an alcohol which is iscible with water and then can be washed with a hydrocarbon. The step of a) is generally carried out at a temperature between the freezing point of the solution and 70 ° C. The lyogel prepared in step a) can also be prepared starting from silicate compounds, as described above, which additionally contain zirconium, aluminum and / or titanium compounds capable of condensation. The organic solvents used in step b) are in general, aliphatic alcohols, ethers, esters or ketones or aliphatic or aromatic hydrocarbons. It is also possible to use mixtures of the aforementioned solvents. Preferred solvents are methanol, ethanol, propanol, acetone, tetrahydrofuran, ethyl acetate, dioxane, n-hexane, n-heptane and toluene. Acetone is used particularly preferably as a solvent in step b).

In step c), the solvent-containing gel is reacted with at least one chlorine-free silylating agent of the formula (I), R2 P3 R1 s. (0- c (I) R * wherein each R1, identical or differently, is a linear or branched alkyl radical of Ci -C & , a cyclohexyl radical or a phenyl radical, 2, R3 and 4 are identical or different and are hydrogen atoms, linear or branched alkyl radicals of Ci-C, cyclohexyl radicals or phenyl radicals, and n = 1, 2 or 3. Preferably, R1 is a methyl or ethyl group, each of R2, R3 and R41 identical or differently, is preferably a hydrogen atom or a methyl group and n is preferably 1 or 2. Tri-ethyl isopropenoxy-silane is particularly preferred (Ri = CH3, R2 = 3 = H, R * = CH3 and n = 1). Trimethylisopropenoxysilane, like most other silylating agents according to the invention, is commercially available or can be prepared by methods known from the literature. A relevant description is for example found in DE-A-12 48 049. The reaction is preferably carried out at a temperature of -20 to 100 ° C in an organic solvent; in particular, those solvents used for washing in step a) and solvent exchange in step b) are used. Acetone is particularly preferred. In a preferred embodiment, the system is mixed with small amounts of at least one acid, before or during silylation, to accelerate the silylation reaction. For this purpose, inorganic acids or acetic acid are preferred. In addition, it is also possible to add other silylating agents, to accelerate the silylation reaction. Suitable compounds for this are the silanes of the formula R ^ -nSiCln, with n = from 1 to 3, wherein R 1 and R 2, independently of one another, is Ci-C 3 alkyl, cyclohexyl or phenyl. Trimethylchlorosilane is preferably used. The amounts are generally from 0 to 1% by weight (based on the lyogel), with concentrations of 0 to 0.5% by weight being preferred, and those of 0 to 0.2% by weight being particularly preferred. In the reaction of the gel with the chlorine-free silylating agents according to the invention, of formula I, α-unsaturated alcohols are formed which are immediately converted into the corresponding ketones or aldehydes (keto-enolic tautornerisrno). Thus, when trirnethyl isopropenoxy silane is used, acetone is produced as the sole elimination product, which is identical with the preferred solvent. When the elimination product is identical to the solvent employed, all the purification steps associated with silylation can be dispensed with for the purposes of solvent and product treatment. In spite of this, under certain circumstances it may be advantageous to wash the silylated gel with a protic or aprotic solvent before drying it, using in particular those solvents which are also used for washing in step a) and for the exchange of solvent in the Step b). A further advantage of the novel process is that in the silylation with the chlorine-free silylating agents according to the invention, no HC.1 gas is produced. , and therefore also byproducts containing chlorine that are corrosive under certain circumstances. In step d), the silylated gel is dried subcritically, preferably at temperatures of -30 to 200 ° C, particularly preferably 0 to 100 ° C, and pressures of 0.001. at 20 bar, particularly preferred from 0.01 to 5 bar, in particular from 0.1 to 2 bar, for example by means of drying by radiation, convection and / or contact. Drying is generally continued until the gel has a residual solvent content of less than 0.1% by weight. The aerogels obtained during drying are permanently hydrophobic. In a further embodiment, the gel can be ground by means of techniques known to the person skilled in the art., for example grinding after having been shaped in the polycondensation in step a) and / or after each subsequent treatment step. In addition, IR opacifiers, such as carbon black, titanium dioxide, iron oxides and / or zirconium dioxide, can be added to the sun prior to gel preparation to reduce the radiant contribution to thermal conductivity. In a further embodiment, depending on the application, the gel can also be subjected to structure reinforcement prior to silylation. This can be carried out, for example, by reacting the obtained gel with a solution of an alkyl and / or aryl orthosilicate which is capable of condensation and having the formula Ri / in Si (0R2), where n ~ 2 at 4 and R and R2, independently of one another, are straight or branched alkyl radicals of Ci-Cß, cyclohexyl radicals or phenyl radicals, or with an aqueous solution of For many applications of aerogels in which other components are added before the surface modification, it is particularly advantageous to be able to do without a very acid silylation, as in the case of chlorosilanes, without losing the effectiveness of the surface modification.

The novel process is described in greater detail below, using working examples.

EXAMPLE 1 1 1 of a sodium water glass solution (with a content of 7% by weight of Si 2 and a Na 2:: Si 2 ratio of 1: 3.3) was stirred, together with 0.5 1 of an acid exchange resin. Ionic (styrene-divinylbenzene copolymer with sulfonic acid groups, commercially available under the name "Duolite C20), until the pH of the aqueous solution was 2.7.The ion exchange resin was then separated by filtration and the aqueous solution was adjusted to a pH of 4.7 using 1 molar NaOH solution, then the resulting gel was allowed to stand for 3 hours at 85 ° C and then the water was extracted with 3 1 of acetone.Afterwards, the acetone-containing gel was silylated with trirnethylisopropenoxysilane (50 g of trirnethylisopropenoxysilane per gram of wet gel) The gel was dried in the air (3 hours at 40 ° C, then 2 hours at 50 ° C and 12 hours at 150 ° C) The transparent airgel obtained from this way had a density of 0.15 g / cm3. BET specific was 780 rn2 / g. The lambda value was 0.018 Ul / rnk. The thermal conductivity was measured by means of a hot wire method (see for example 0. Nielsson, G. Rüschenpóhler, 3. Groß, 3. Fricke, High Te peratures - High Pressures, Vol. 21, 267-274 (1989) ).

EXAMPLE 2 1 1 of a sodium water glass solution (containing 7% by weight of SiO 2 and a Na 2:: Si 2 ratio of 1: 3.3) was stirred together with 0.5 1 of an acid ion exchange resin ( styrene-divinylbenzene copolymer with sulphonic acid groups, commercially available under the name RDuolite C20), until the pH of the aqueous solution was 2.7. Then, the ion exchange resin was separated by filtration and the aqueous solution was adjusted to a pH of 4.7 using a 1 molar NaOH solution. Subsequently, the resulting gel was allowed to stand for 3 hours at 85 ° C and then an acetone solvent exchange was carried out using 3 liters of acetone. The acetone-containing gel was then silylated with trirnethylisopropenoxysilane (50 mg of trirnethylisopropenoxysilane per gram of wet gel). The gel was dried in air (3 hours at 40 ° C, then 2 hours at 50 ° C and 12 hours at 150 ° C). The transparent airgel obtained in this way had a density of 0.145 g / cm3. The specific surface area of BET was 690 rn2 / g. The long value was 0.017 W / rnk. The thermal conductivity was measured by means of a hot wire method (see for example 0. Nielsson, G. Rüschenpóhler, 3. Groß, 3. Fricke, High Ternperatures - Hi gh Pressures, Vol. 21, 267-274 (1989)).

EXAMPLE 3 1 1 of a sodium water glass solution (containing 7% by weight of SiO 2 and a Na 2:: Si 2 ratio of 1: 3.3) was stirred together with 0.5 1 of an acid ion exchange resin ( styrene-divinylbenzene copolymer with sulphonic acid groups, commercially available under the name RDuo.lite C20), until the pH of the aqueous solution was 2.7. Then, the ion exchange resin was separated by filtration and the aqueous solution was adjusted to a pH of 4.7 using a 1 molar NaOH solution. The resulting gel was then allowed to stand for 3 hours at 85 ° C and then a solvent exchange of isopropanol was carried out using 3 liters of isopropanol. The gel containing isopropanol was then silylated with 5% by weight of trinetylisopropenoxysilane and 0.1% by weight of trimethylchlorosilane (data based on wet gel). The gel was dried in air (3 hours at 40 ° C, then 2 hours at 50 ° C and 12 hours at 150 ° C). The transparent airgel obtained in this way had a density of 0.13 g / cm3. The specific surface area of BET was 553 rn2 / g. The lambda value was 0.0.15 W / rnk. The thermal conductivity was measured by means of a hot wire method (see for example O. Niel son, G. Rüschenpóhler, 3. Groß, J. Fricke, High Temperatures - High Pressures, Vol. 21, 267-274 (1989) ).

Claims (12)

1. 6 NOVELTY OF THE INVENTION CLAIMS

1. - A process for the preparation of aerogels in which a) a silicate lyogel is prepared, b) if desired, the lyogel prepared in step a) is subjected to an exchange of solvent with another organic solvent, c) the gel obtained in step a) or b) is reacted with at least one silylating agent, and d) the silylated gel obtained in step c) is dried subcritically; the process comprises, in step c), reacting the gel obtained in step a) or b) with at least one chlorine-free silylating agent of formula I, • s ¡(o - c)) 4 4 - nn \ \ 'n wherein each R, identical or different, is a linear or branched alkyl radical of C? ~ Ce, a cyclohexyl radical or a phenyl radical, R2, R3 and A They are identical or different and are hydrogen atoms, linear or branched alkyl radicals Ci-Cß, cyclohexyl radicals or phenyl radicals, and n - • 1, 2 or 3.

2. The process according to claim 1, further characterized in that a silicate lyogel is prepared which is obtained by means of hydrolysis and condensation. of Si alkoxides in an organic solvent, with water, and then the gel obtained is washed with an organic solvent until the water content of the gel is < 5% by weight.

3. The process according to claim 1, further characterized in that a silicate lyogel is prepared using an acid ion exchange resin or a mineral acid to carry an aqueous solution of water glass to a pH <1. 3, adding a base to polycondensen the resulting silica to give an IO2 gel, and if a mineral acid was used, washing the gel with water until free of electrolytes, and then washing the gel obtained with an organic solvent until the content of gei water is < 5% by weight.

4. The process according to claim 3, further characterized in that NH4OH, NaOH, KOH, A1 (0H) 3 and / or colloidal silica are used as the base.

5. The process according to at least one of claims 1 to 4, further characterized in that the SIOO2 gel obtained in step a) is allowed to stand at a temperature of 20 to 1 D0 ° C and a pH of 4 to 11 for a period of one second to 48 hours before being washed.

6. - The method according to at least one of claims 5, further characterized in that in step b), the organic solvent used is methanol, ethanol, propanol, acetone, tetrahydrofuran, ethyl acetate, dioxane, n-hexane and / or toluene.

7. The method according to at least one of claims 1 to 6, further characterized in that the gei obtained in step a) or b) is reacted before the reaction with an alkyl orthosilicate solution and / or aplo that is capable of condensation and has the formula Ri-in -S? (0R) n, where n • - from 2 to 4 and R and R2, independently of each other, are linear or branched alkyl radicals of Ci- Cß, cyclohexyl radicals or phenyl radicals, or with an aqueous solution of silica.

8. The process according to at least one of claims 1 to 7, further characterized in that in step c), the solvent-containing gel is reacted with at least one chlorine-free silicone agent of formula I , wherein R1 is a methyl or ethyl group; R2, R3 and R < / identical or different, respectively, are a hydrogen atom or a methyl group and n = 1 or 2.

9. The process according to claim 8, further characterized in that in step c), the solvent-containing gel is made react with tri-ethyl isopropenoxy silane.

10. The method according to at least one of claims 1 to 9, further characterized in that the reaction in step c) is carried out in an organic solvent at a temperature in the range of -20 to 100 ° C. .

11. The process according to at least one of claims 1 to 10, further characterized in that the silylated gel obtained in step c) is washed with a protic or aprotic solvent before drying in step d).

12. The process according to any of claims 1 to 11, further characterized in that in step d), the silylated gel is dried at a temperature of -30 to 200 ° C and at a pressure of 0.001 to 20 bar.