CA2274911A1 - Method for producing organically modified, permanently hydrophobic aerogels - Google Patents

Abstract

The invention relates to a method for producing organically modified aerogels with permanently hydrophobic surface groups, wherein a) a lyogel is provided;
b) the lyogel provided in step (a) is wahed with an organic solvant; c) the surface of the gel obtained in step (b) is silylated; and d) the silylated surface gel obtained in step (c) is dried. The invention is characterized in that a disiloxane of the formula (I) R3Si-O-SiR3 is used as silylating agent in step (c), wherein the radicals R mean individually, being the same or different, either a hydrogen atom or a non-reactive organic linear, branched, cyclic, saturated or unsaturated, aromatic or heteroaromatic radical.

Description

WO 98l23,6'T PCTIEP97J0659fi Description Specification , Process~for,the preparation of oxganically modified. Pe~entl,y hydrophobic aerogels .
The invention pertains to a process for the preparat~.on of organically modified. Pe~~~Y h°biC aerogels. .
~rerbgels, especially those with porosities above 60% and densities below 0.6 g/~l. ~~e an exlY low fihexmal conductivity and therefore find use as a thermal insulation material as described, for example, in ~A-0 Z?1 722.
Arerogels in the wider sense, i.e. in the sense of a "gel Kith air as the dispersing agent." are prepared by dr».ng a suitable gel. Rerogels is the na~:r~er sense, xerogels and cryogels, are included in the coa,cept of~an "aerogel" is this sense. tn this connection, a dried gel is termed an aerogel in the narrower sense if the li.qui.d of the gel is removed at tempe~catures above the critical temperature and starting out from pressures above the critical pressure. If, by contrast, the liquid of the gel is removed suberitically, e.g. with the formation of a liquid/vapor boundary phase, then one designates the produced gel a xerogel.
when using the terra aerogels iri the present application, one is dealing with aerogels in the wider sense, i.e. in the sense of a "gel with air as the dispezs~g agent."
In addition, one csuz basically subdivide aerogels into inorganic and organic aerogels:.

Inorganic aerogels have been known since 1931 (S.S. Kistler, Nature 1931, 127, 741). Since then, aerogels have been produced from the most varied initial materials. For example, SiOZ, AI203, 'Ti02, Zr4z, Sn02, LizO, Ce02 and V205 aerogels as well as mixtures of these could be prepared (H. D. Lesser) P.
C. Goswami, Chem. Rev.1989, 89, 765 f~.
For several years, organic aerogels have~also been known. One finds in the literature, e.g., organic aerogels based on resor~cinolfformaldehyde, melaminelformaldehyde or resorcinoUfurfurol (R. W. Pekala, J. Mater. Sci.
1989, 24, 3227, US-A 5,508,341, RD 388,047 [388,047?], WO94122,943 and US-A-5,556,892). In addition, organic aerogels of polyisocyanates (V11095/03,358) and poiyurethanes (US-5,484,818) are also known. One proceeds from initial materials such as formaldehyde and resorcinol dissolved in water) as described, for example, in US-A-5,508,341; these are brought to reaction with one another by suitable catalysts, the water in the pores of the gel that forms is exchanged for a suitable organic solvent, and then the gel is dried supercritically.
Inorganic aerogels can be prepared in different ways.
First of all, SiOz aerogels can be produced by acid hydrolysis and condensation of tetraethylorthosilicate in ethanol. A gel is thus formed) which can be dried by supercritical drying while retaining its structure. Production processes based on this drying technique are known, e.g., from EP-A-0 396,076;
WO 92/03,378; and WO 95106,617.
An alternative to the above drying is offered by a process for subcritical drying of SiOz gels) in which these are reacted with a silylation agent containing chlorine prior to drying. The SiOz gel can be obtained, for example, by acid hydrolysis of tetraalkoxysilanes) preferably tetraethoxysilanes (TEOS), in a suitable organic solvent) preferably ethanol, by means of water. After exchange of the solvent for a suitable organic solvent, the obtained gel is reacted with a chlorine-containing silylation agent in an additional step.
Methylchlorosilanes {Me,~."SiCI° with n = 1 to 3) are thus preferably used as silylation agents, due to their reactivity. The Si02 gel that is thus formed and is modified with methylsilyl groups on its surface can then be dried in air from an organic solvent. Thus aerogels with densities below 0.4 glcm3 and poros'rties greater than 60% can be obtained. The production process based on this drying technique is described in detail in WO 94125,149.
The above-described gels can be reacted with tetraalkoxysilanes in aqueous alcohol solution prior to drying and can be aged in order to increase the gel network strength, as disclosed) e.g., in WO 92120,623.
The tetraalkoxysilanes used in the above-described process as initial materials, however, represent an extraordinarily high cost factor.
A considerable reduction in cost can be achieved by the use of water glass as the initial material for the production of Si02 gels. For this purpose, for example, a silicic acid can be praduced from an aqueous water~lass solution by means of an ion-exchange resin, and this aad can be polycondensed by add~ion of a base to fom~ a SiOz gel. After exchange of the aqueous medium for a suitable organic solvent, the obtained gel is then converted with a chlorine-containing silylation agent in an additional step. Methylchlorosilanes (Me,4."SiCI"

with n = 1 to 3) are also preferably utilized as sifylation agents, due to their reactivity. The modified 5102 gel that is formed and is modified with methylsilyl groups on the surface can then also be dried in air from an organic solvent.
The production process based on this technique is described in detail) e.g., in DE
A-4,342)548.
Hydrogen chloride (HCI) as well as a multiple number of byproducts combined therewith are necessarily formed in very large quantities in the silylation by means of chlorine-containing silylation agents, and these require sometimes a very expensive and cost intensive cleaning of the silylated Si02 gels by multiple washings with a suitable organic sohrent.
The use of a silylation agent free of chlorine is described in DE-C 195 02,453. For this purpose, for example, a silicate-type Iyogel produced according to the above-described method is proposed and is reacted with a chlorine-free silylation agent. Preferably methylisopropene oxysilanes (Me,."Si(OC(CH3)CHz)n with n = 1 to 3) are used as silyfation agents. The thus-formed SiOz gel that is modified with methylsilyl groups on the surface can then also be dried in air from an organic solvent.
By the use of chlorine-free silylation agents, in fact, the problem of formation of HCl is solved) but the chlorine free silylation agents that are used represent a very high cost factor.
In WO 95!08,817 and in the German Patent Application 195-41 279.fi, methods are disclosed for the production of silicic acid aerogels with hydrophobic surface groups.

In V11O 95I06,61y) silicic acid aerogels are obtained by reacfion of a water-glass solution with an acid at a pH value of 7.5 to 11, extensive release of the silicic acid hydrogel that forms from the ionic components by washing with water or dilute aqueous solutions of inorganic bases, whereby the pH of the hydrogel is maintained in the range of 7.5 to 11) expelling the aqueous phase obtained in the hydrogel by an alcohol and subsequent supercritical drying of the obtained alkogel.
In German Patent Application 195-~41 279.6) in a way similar to that described in WO 95106,617, silicic-acid aerogels are produced and are then dried subcritically.
In both methods) however, the omission of chlorine-containing silylation agents leads only to an aerogel with hydrophobic surface groups bound via oxygen. These can easily be cleaved again in a water containing atmosphere.
Thus the described aerogel is only hydrophobic for a short time.
It is also possible to utilize organically modified gels without final drying to the aerogel in the most varied fields of technology, such as, e.g., in chromatography) in cosmetics, and in the pharmaceutical field.
The task of the present invention was thus to prepare a method for the production of permanently hydrophobic aerogels, in which a commercially available, inexpensive silylation agent can be used) without incurring the other disadvantages described above, which are known from the prior art This task is resolved by a process for the production of organically modfied aerogels with permanently hydrophobic surface groups, in which one a) provides a lyogel, b) washes the lyogel provided in step a) with an organic solvent, c) surtace.silylates the gel obtained in step b), and d) dries the surface-silylated gel obtained in step c), characterized in that in sfiep c), as the silylation agent, a disiloxane of formula I is used R3Si-O-SIRS (I) whereby the residues R, independently of one another, the same or different, indicate in each case a hydrogen atom or a nonreactive organic, linear) branched, cyclic, saturated or unsaturated, aromatic or heberoaromatic residue, preferably C~-C~8 alkyl or C~C~4 aryl, and particularly preferred C~-Ce alkyl, cyclohexyl or phenyl, particularly methyl or ethyl:
In the present invention, a lyogel is understood to mean a gel dispersed in at least one solvent. The solvent may also be water. tf the water component in the solvent amounts to at least 50%, then one also speaks of a hydrogel.
The network of the lyogel may be present in any organic andlor inorganic base composition. All of the systems of the prior art known to the person skilled in the art can be used as the organic base composition. An inorganic base composition is preferably based on oxidic silicon, tin) aluminum, gallium, indium, titanium andlor zirconium compounds, and particularly preferred are those based on oxidic silicon, aluminum) titanium andlor zirconium compounds. Most preferred is a silicate-type hydrogel, which may contain fractions of zirconium) aluminum, titanium, vanadium andlor iron compounds) particularly a pure silicate-6 .

type hydrogel. In the case of organic andlor inorganic base compositions, the different components must not necessarily be distributed homogeneously andlor form a continuous network. It is also possible that individual components are present partially or completely in the form of inclusions, individual nuclei and/or agglomerations in the network.
The disiloxanes used according to the invention) when compared with the chlorine-containing silylation agents known from the prior art) have the advantage that no chlorine-containing byproducts are formed. In addition) they can easily be separated from aqueous phases based on their insolubility, which makes possible the recovery of excess reagents. In this way, it is possible to minimize silylation times by the use of excess concentrations.
The preparation of the lyogels provided in step a) can be produced according to all methods known to the person skilled in the art.
Three preferred forms of embodiment for the preparation of silicate-type lyogels will be described in more detail in the following, but without, however limitation to these.
In a first preferred form of embodiment, in step a) a silicate-type lyogel is provided, which is obtained by hydrolysis and condensation of Si alkoxides in an organic solvent with water. A tetraalkoxysilane) preferably tetraethoxy- or tetramethoxysilane is used as the Si alkoxide. The organic solvent is thus preferably an alcohol) and particularly preferred ethanol or methanol, to which up to 20 vol.°/a water can be added. In the hydroiysis and concentration of Si alkoxides in an organic solvent with water, acids andlor bases may be added in a one- or two-step process as catalysts.
The lyogel provided in step a) may also contain zirconium, aluminum) tin andlor titanium compounds suitable for condensation.
In addition, before andlor during the gel preparation, opacif~ers can be added as additives, particularly IR opacifiers far reduction of the radiation contribution to the heat conductivity, such as, e.g., carbon black, titanium oxides, iron oxides, andlor zirconium oxides.
In addition, fibers can be added to the sol in order to increase the mechanical stability. Inorganic fibers, such as) e.g., glass fibers or mineral fibers, organic fibers, such as, e.g., polyester fibers, aramide fibers, nylon fibers or fibers of plant origin) as well as mixtures of these can be used as the fiber materials. The fibers may also be coated, such as, e.g., polyester frbers, which are metallized with a metal, such as, e.g., aluminum.
The production of the lyogel is generally conducted at a temperature between the freezing point of the solution and 70°C. In this way, if necessary, a shaping step can be conducted simultaneously, such as, e.g., spray forming, extrusion or drop formation.
The obtained lyogel can also be subjected to an aging. This is generally done between 20°C and the boiling point of the organic solvent. If necessary, aging can also be conducted under pressure at higher temperatures. The time generally amounts to up to 48 hours) preferably up to 24 hours.

In a second preferred form of embodiment, a silicate-type hydrogel is introduced in step a), which is prepared by bringing an aqueous water-glass solution to a pH of s 3 by means of an acidic ion-exchanger resin, a mineral acid, or a hydrochloric acid solution, then polycondensing the silicic acid that forms thereby by addition of a base to a SiOz gel, and) if a mineral acid or a hydrochloric acid solution has been used, the gel is washed with water essentially free of electrolyte. The polycondensation to the Si02 gel can be undertaken both in one step as well as in multiple steps.
Preferably sodium andlor potassium water glass are used as the water glass. Preferably an acidic resin is used as the ion-exchanger resin) whereby in particular, those resins are suitable, which contain sutfonic acid groups. If one uses mineral acids, hydrochloric acid andlor sulfuric acid are particularly suitable.
if one uses hydrochloric acid solutions, particularly aluminum salts are suitable, especially aluminum sulfate andlor chloride. As the base, generally NH40H, NaOH, KOH, AI(OH)3 andlor colloidal silicic acid are used.
The hydrogel preferably prepared from the above-described silicate-type initial compounds may also contain zirconium, aluminum, tin andlor trtanium compounds capable of condensation.
In addition, prior to and/or during the gel production, opacifiers may be added as additives, particularly IR opacifiers, for the reduction of the radiation contribution to the heat conductivity, such as, e.g., carbon black, titanium oxides, iron oxides andlor zirconium oxides.

In addition, fibers can be added to the sol for increasing the mechanical stability. Inorganic fibers such as) e.g., glass fibers or mineral fibers) organic fibers such as e.g., polyester fibers, aramide fibers, nylon fibers or fibers of plant origin) as well as mixtures of the same can be used as fiber materials. The fibers rnay also be coated, such as, e.g., polyester fibers, which are metallized with a metal, such as, e.g., aluminum.
The production of the hydrogel is generally conducted at a temperature between the freezing point and the boiling point of the solution. Thus) if necessary, a shaping step can be conducted simultaneously, such as) e.g., spray forming, extrusion or drop formation.
The obtained hydrogel can also be subjected to an aging. This aging may be produced prior to andlor after an above-described possible washing with water, with which the gel is essentially washed free of electrolyte.
The aging is conducted generally at a temperature in the range of 20 to 100°C, preferably at 40 to 100°C and particularly at 80 to 100°C) and at a pH
value of 4 to 11, preferably 5 to 9, and particularly 5 to $. The time for this generally amounts to up to 48 hours, preferably up to 24 hours, and particularly preferably up to 3 hours.
In a third preferred form of embodiment, in step a) a silicate-type hydroge) is provided, which is prepared by obtaining a SiOz gel from an aqueous water glass solution by means of at least one organic andlor inorganic acid through the intermediate step of a silicic acid sol.

In general, a 6 to 25 wt.9~6 (with respect to the Si02 content) sodium andlor potassium water-glass solution is used as the water-glass solution. Prefierred is a 10 to 25 wt°% water-glass solution, and particularly preferred is a 10 to 18 wt.°%
water--glass solution.
In addition, the water-glass solution may also contain up to 90 wt.%, with respect to the SiOz) of zirconium, aluminum, tin andlor titanium compounds capable of condensation.
As acids, generally 1 to 50 wt.% acids are used, preferably 1 to 10 wt.%
acids. Preferred acids are sulfuric acid, phosphoric acid, hydrofluoric acid, oxalic acid, andlor hydrochloric acid. Particularly preferred is hydrochloric acid.
However, mixtures of the corresponding cads may also be utilized.
In addition to the mixing, properly speaking, of the water-glass solution and the acid, it is also possible to introduce a part of the acid into the water-glass solution andlor a part of the water-glass solution into the acid prior to the mixing itself. It is possible in this way to vary the ratio of the material flows of water glass solutionlacid over a very broad range.
After mixing the two solutions, preferably a 5 to 12 wt.% SiOz gel is obtained. A 6 to 9 wt% Si02 gel is particularly preferred.
In order to assure an optimal intermixing of the water glass solution and the acid, before a Si02 gel is formed) both solutions should preferably have, independent of one another) a temperature between 0 and 30°C, particularly preferred between 5 and 25°C, and particularly between 10 and 20°C.

The rapid intermixing of the two solutions is conduced in devices known to the person skilled in the art) such as, e.g., boilers with stirring apparatus) mixing nozzles and static mixers. Preferred are semicontinuous or continuous processes, such as, e.g., moving nozzles.
If necessary, a forming step can be conducted simultaneously with production, e.g.) by spray forming, extrusion or drop formation.
The obtained hydrogel may also be subjected to an aging. This is generally done at 20 to 100°C, preferably at a0 to 100°C) particularly at $0 to 100°C and a pN value of 2.5 to 11, prefierably 5 to 8. The time for this generally amounts to up to 12 hours,.preferably up to 2 hours, and particularly preferred, up to 30 minutes.
The prepared gel is preferably washed with water, particularly preferably until the wash water used is free of electrolyte. If an aging of the gel is to be conducted, the washing can be conducted prior to, during and/or after the aging, whereby the gel in this case is preferably washed during or after the aging.
For the washing, a part of the water can be replaced by organic solvent. The water content, however, should preferably be high enough that the salts in the pores of.
the hydrogel do not crystallize out.
In order to remove sodium andlor potassium ions as extensively as possible, the hydrogel can also be washed with a mineral salt prior to, during andlor after the washing with water. Preferred mineral salts arse thus also the mineral salts named as preferred for the production of the hydrogel.

Further, opacifiers can be added to the water glass, the acid and/or the sol as additives, particularly IR opacifiers, for the reduction of the radiation contribution to the heat conductivity such as, e.g., carbon black, titanium oxides, iron oxides and/or zirconium oxides.

In addition, fibers can be added to the water glass, to the acid and/or to the sol in order to increase the mechanical stability. Inorganic fibers, such as, e.g., glass fibers or mineral fibers, organic fibers, such as, e.g., polyester fibers, aramide fibers, nylon fibers or fibers of plant origins as well as mixtures of the same can be used as fiber materials, The fibers may also be coated such as, e.g., polyester fibers, which are metallized with a metal, such as, e.g., aluminum.

In step b), one washes the gel obtained from step a) with an organic solvent, preferably until the water content of the gel is ~ 5 wt. %, particularly preferred ~ 2 wt. % and in particular ~ 1 wt. %. Generally aliphatic alcohols, ethers, esters or ketones as well as aliphatic or aromatic hydrocarbons are used as solvents. Preferred solvents are methanol, ethanol, acetone, tetrahydrofuran, acetic acid ethyl ester, dioxane, pentane, n-hexane, n-heptane and toluene.
Particularly preferred as the solvent is acetone, tetrahydrofuran, pentane and n-heptane. Mixtures of the named solvents may also be used. Further, the water can also be washed out first with a water-miscible solvent, e.g., an alcohol, acetone or THF, and then the latter is washed out with a hydrocarbon.
Preferably pentane or n-heptane is used as the hydrocarbon.

The lyogel obtained in step b) may be subjected to an aging. This is generally done between 20°C and the boiling point of the organic solvent. If necessary, aging may be conducted also under pressure at higher temperatures.
The time generally amounts to up to 48 hours, preferably up to 24 hours. After such an aging, if necessary, another solvent exchange for the same or a different solvent can be conducted. This additional aging step may also be repeated several times.
In step c), the solvent-containing gel is reacted with a disiloxane of formula I as the silylation agent.
FZ3Si-0-SiR~ (I) whereby the residues R, independently of one another, either the same or different) each time represent a hydrogen atom or a nonreactive organic, linear, bunched, cyclic, saturated or unsaturated, aromatic or heberoaromatic residue, preferably C~-C~e alkyl or C~C,4 aryl, particularly preferred C~-C6 alkyl, cyclohexyl or phenyl, particularly methyl or ethyl.
The solvent-containing gel in step c) is preferably reacted with a symmetric disiloxane, whereby a symmetric disiloxane is understood to be a disiloxane in which both Si atoms have the same residue R.
Particularly preferred, disiloxanes are used in which all residues R are the same. In particularly, one uses hexamethyldisiloxane.
The reaction is generally conducted at 20°C up to the boiling point of the silylation agent, if necessary, in a solvent. Preferred solvents here are the solvents described as preferred in step b). Particularly preferred is acetone, tetrahydrofuran, pentane and n-hep#ane. It the silylation is produced in a solvent, then the silylation is generally conducted between 20°C and the boiling point of the solvent.
In a preferred form of embodiment, silylation is conducted in the presence of a catalyst, for example an acid or base. Preferably, acids are utilized as the catalyst. Particularly preferred acids are hydrochloric acid, sulfuric acid) acetic acid and/or phosphoric acid.
In another form of embodiment, the silylation is conducted in the presence of catalytic quantities of a silylation agent, which forms acids in the presence of water. Preferably, chlorosilanes are [used], and particularly preferred is trimethylchlorosilane ('T'MCS). In addition, a combination of acids or bases and TNlCS is also possible.
Prior to step d)) the silylated gel is preferably washed with a erotic or aromatic solvent, until unreacted silylation agent is essentially removed (residual content 51 wt.%). Suitable solvents are those named in step b). Analogously) the solvents named there as preferred are also preferred here.
In step d), the silylated and) if necessary, washed gel is preferably dried subcritically, preferably at temperatures from -30°C to 200°C) and particularly preferred, 0 to 100°C, as well as pressures prefierably from 0.001 to 20 bars, and particularly prefierably 0.01 to 5 bars, particularly 0.1 to 2 bars) for example by radiation, convection andlor contact drying. Drying is preferably conducted until the gel has a residual solvent content of less than 1 wt.%. The aerogels obtained in the drying are permanently hydrophobic.

The gel obtained in step c) may also be dried supercritically. This requires temperatures higher than 200°C andlor pressures higher than 20 bars, depending on the respective solvent. This is possible without anything further, but it is associated with increased expenditure and does not offer essential advantages.
In another form of embodiment) the gel can be subjected to another network reinforcement, each time depending on application) prior to the silylation in step c). This is done by reacting the obtained gel with a solution of an orthosilicate of formula R'4.f,Si(OR2)~ capable of condensation, preferably an alkyl and/or aryl orthosilicate, whereby n = 2 to 4 and R' and R2) independently of one another, are hydrogen atoms, linear or branched C~-Cg alkyl, cyclohexyl or phenyl residues, or with an aqueous silicic-acid solution.
In another form of embodiment, after the shaping polycvndensation andlor each subsequent process step, the gel can be comminuted according to techniques known to the person skilled in the art) such as) e.g., milling.
The aerogels produced according to the process of the invention fend particular use as heat insulation materials.
The process according to the invention is described in more detail in the following, based on examples of embodiment, without thereby being limited to these.
Example 1 2 liters of a sodium water-glass solution (Si02 content of fi wt.% and Na20:Si02 ratio of 1:3.3) are passed through a sheathed glass column (length =

100 crn, diameter = 8 cm), which is packed with 4 liters of an acidic ion-exchanger resin (styrene-divinylbenzene copolymer with sulfonic acid groups, commercially available under the name ~Duolite C20j (at approximately 70 mllmin). The column is operated at a temperature of approximately 7°C.
The silicic-acid solution exiting at the lower end of the column has a pH value of 2.3.
This solution is brought to a pH of ~4.7 for the polycondensation with a 1.0 molar NaOH solution. After this, the gel that forms is aged for another 3 hours at 85°C
and then the water is exchanged for acetone with 3 liters of acetone. Then the acetone-containing gel is silylated with hexamethyldisiloxane at room temperature for 5 hours (2.5 wt.% hexamethyldisilvxane per gram of wet gel.
After washing the gel with 3 liters of acetone, drying of the gel is conducted in air (3 hours at 40°C) then 2 hours at 50°C and 12 hours at 150°C). The thus-obtained transparent aerogel has a densihr of 0.15 glan3, a heat conductivity of 15 [16?] mWImK, a specific surface according to BET of 600 mz/g and is permanently hydrophobic.
Example 2 424 g of a 7.5% HCI solution cooled to 10°C is reacted dropwise with g of a sodium water-glass solution cooled to 10°C (with a content of 13 wt.°Xo Si02 and a Na20:SiOz ratio of 1:3.3j. A pH value of 4.7 is thereby adjusted.
The hydrogel formed after several seconds is aged for one hour at 85°C. It is then washed with 3 liters of hot water and the water is exchanged for acetone with liters of acetone. Then the acetone-containing gel is silylated with hexamethyldisiloxane (2.5 wt.96 hexamethyldisiloxane per gram of wet gel) for hours at room temperature. After washing the gel with 3 liters of acetone, it is dried in air (3 hours at 40°C, then 2 hours at 50°C and 12 hours at 150°C).
The thus-obtained aerogel has a density of 0.15 gloms, a heat conductivity of 17 mWImK, a specific surface according to BET of 580 m2lg and is permanently hydrophobic.
Example 3 The hydrogel is produced as described in Example 2. The hydrogel aged for one hour at 85°C is then washed with 3 liters of warm water and the water is exchanged for acetone with 3 liters of acetone. Then the acetone-containing gel is silylated with hexamethyldisiloxane (2.5 wt.% hexamethyldisiloxane per gram of wet gel) in the presence of 0.1 wt.% trirnethylchlorosilane (0.? wt.%
trimethylchlorositane per gram of wet gel) for 5 hours at room temperature.
After washing the gel with 3 liters of acetone, it is dried in air (3 hours at 40°C) then 2 hours at 50°C and 12 hours at 150°C).
The thus-obtained aerogel has~a density~of 0.14 glcm3, a heat conductivity of 16 mWImK, a specific surface according to BET of 590 m2lg and is permanently hydrophobic.
Example 4 The hydrogel is produced as described in Example 2. The hydrogel aged far 1 hour at 85°C is then washed with 3 liters of warm water and the water is exchanged for acetone with 3 liters of acetone. Then the acetone-containing gel is silylated with hexarnethyldisiloxane (2.5 wt.% hexamethyldisiloxane per gram is of wet get) in the presence of 0.1 wt.% '! N aqueous hydrochloric acid (0.1 wt.°Xo 1 N aqueous hydrochloric acid per gram of wet gel) for 5 hours at room temperature. After washing the gel with 3 liters of acetone, it is dried in air (3 hours at 40°C) then 2 hours at 50°C and 12 hours at 750°C).
The thus-obtained aerogel has a density of 0.'14 g/cm3, a heat conductivity of 16 rr~W/mK, a specific BET surface of 570 m=lg and is permanently hydrophobic.
The heat conductivities were measured with a resistance-wire method (see) e.g., O. Nielssen, G. Ruschenpohler, J. Gross, J. Fridce, High Temperatures - High Pressures, Vol. 21) 267-274 (1989)).

Claims (19)

Claims

1. Process for the preparation of organically modified aerogels with permanently hydrophobic surface groups in which one a) introduces a lyogel into the reactor;
b) washes the lyogel introduced into the reactor in step a) with an organic solvent;
c) surface-silylates the gel obtained in step b) and d) dries the surface-silylated gel obtained in step c), characterized by the feature that. as the silylating agent in step c), one uses a disiloxane of formula I

R~Si-O-SiR~ (I) whereby the residues R, independently of one another, identically or, differently, signify in each case a hydrogen atom or a nonreactive, organic, linear. branched, cyclic, saturated or unsaturated. aromatic or heteroaromatic residue.

2. Process in accordance with Claim 1, characterised by the feature that, in step a), one introduces a silicate-type lyogel into the reactor.

3. Process is accordance with Claim 2, characterized by the feature that, in step a), one introduces into the reactor a silicate-type lyogel which is obtainable by hydrolysis and condensation of Si alxoxides in as organic solvent with water.

4. Process in accordance with Claim 2, characterised by the feature that, in step a), one introduces into the reactor a silicate-type hydrogel that is prepared by bringing an aqueous water glass solution to a pH value ~3 with the aid of an acidic ion-exchanged resin or an inorganic acid and, via the addition of a base, polycondensing the silicic acid, which is produced in this way, to give a SiO2 gel and, if an inorganic acid has been used, washing the gel essentially free from electrolytes with water.

5. Process in accordance with Claim 2, characterized by the feature that one introduces into the reactor in step a) a silicate-type gel (which is prepared by obtaining it from an aqueous water glass solution with the aid of at least one organic and/or inorganic acid via the intermediate stage of a silicic acid sol.

6. Process in accordance with one of Claims 1 through 5.
characterized by the feature that, prior to and/or during the preparation of the gel, one adds IR turbidity-promoting agents.

7. Process in accordance with one of Claims 1 through 6, characterized by the feature that fibers are added prior to, and/or during, the preparation of the gel.

8. Process in accordance with at least one of the preceding claims, characterized by the feature that one allows the lyogel obtained in step a) to age before it is washed in step b).

9. Process in accordance with at least one of the preceding claims, characterized by the feature that one washes the gel in step b) for a sufficiently long time until the water content of the gel is ~5 wt%.

10. Process in accordance with at least one of the preceding claims, characterized by the feature that use is made of aliphatic alcohols, ethers, esters, or ketones and aliphatic or aromatic hydrocarbons as the organic solvents in step b).

11. Process in accordance with at least one of the preceding claims, characterized by the feature that use is made of a symmetrical disiloxane as the silylating agent in step c).

12. Process in accordance with at least one of the preceding claims, characterized by the feature that, as the silylating agent in step c), a disiloxane is used in which all the residues R in the disiloxane are identical.

13. Process in accordance with at least one of the preceding claims, characterized by the feature that hexamethyldisiloxane is used as the silylating agent is step c).

14. Process in accordance with at least one of the preceding claims, characterized by the feature that the silylation process is carried out in a solvent.

15. Process in accordance with at least one of the preceding claims, characterized by the feature that the silylation process is carried out in the presence of a catalyst, preferably an acid.

16. Process in accordance with at least one of the preceding claims, characterized by the feature that the silylation process is carried out in the presence of catalytic quantities of trimethylchlorosilane.

17. Process in accordance with at least one of the preceding claims, characterized by the feature that, prior to step d), one washes the surface-silylated gel with a protic or aprotic solvent.

18. Process in accordance with at least one of the preceding claims, characterized by the feature that one subcritically dries the surface-silylated gel.

19. Process in accordance with at least one of the preceding claims, characterized by the feature that, prior to silylation, one reacts the gel obtained in step b) with a solution of an orthosilicate, which is capable of bringing about condensation, of formula R~~~~Si (OR2)~, preferably an alkyl orthosilicate and/or an aryl orthosilicate. whereby n = 2 through 4 and R1 and R2, independently of one another, are hydrogen atoms, linear or branched C1-C6 alkyl residues, cyclohexyl residues or phenyl residues, or with as aqueous silicic acid solution.