IMPROVED PROCESS FOR PRODUCING SILICA AEROGEL THERMAL INSULATION PRODUCT WITH INCREASED EFFICIENCY

Abstract

The invention relates to an improved method for producing silica aerogel in pure and flexible sheet form having effective suppression of radiative heat transport at high temperatures and increased thermal insulation property. The suppression of radiative heat transport was achieved by in-situ production of titanium dioxide nanoparticles in very minor concentrations during gelation of silica precursor, with nanoporous surface area more than 300 m2/g and acts as an infra red reflecting agent. When aerogel is subjected to heat during hot object insulation, it automatically turn into infra red reflecting material. Said silica aerogel can be incorporated into the inorganic fibre mat matrix individually or into two or more layers with organic sponge sheet placed in between and stitched together to form a sandwich sheet to form highly insulating flexible sheet.

Claims

1.-18. (canceled)

19. A method for preparation of a silica aerogel thermal insulation product, the method comprising: (a) preparing a hydro-alcoholic solution containing at least one alkali; (b) adding a solution of a metal oxide precursor to the hydro-alcoholic solution to form a dispersion effecting in-situ formation and precipitation of nanoparticles of at least one metal oxide, wherein the metal oxide precursor comprises one or more metals selected from the group consisting of iron, manganese, magnesium, zirconium, zinc, chromium, cobalt, titanium, tin, and indium; (c) mixing at least one silica precursor with the dispersion to form a first mixture; (d) stirring the first mixture to obtain a viscous mixture with the nanoparticles entrapped therewithin; (e) aging the viscous mixture to form an aged viscous mixture; and (f) effecting supercritical drying of the aged viscous mixture, to obtain the silica aerogel thermal insulation product.

20. The method of claim 19, wherein the at least one silica precursor is selected from the group consisting of tetraethylorthosilicate, tetramethylorthosilicate, hexamethyldisiloxane, methyl trimethoxysilane, and sodium silicate, individually or in combination.

21. The method of claim 19, wherein aging comprises aging of the viscous mixture for a time period ranging from about one hour to about twenty-four hours at a room temperature.

22. The method of claim 19, further comprising effecting a solvent exchange before effecting supercritical drying of the aged viscous mixture, wherein the solvent exchange comprises immersing the aged viscous mixture in an alcoholic solvent for about three days while replacing the alcoholic solvent with a fresh batch of alcoholic solvent approximately daily.

23. The method of claim 22, wherein the alcoholic solvent further comprises the metal oxide precursor.

24. The method of claim 19, wherein effecting supercritical drying of the aged viscous mixture comprises: (a) maintaining the aged viscous mixture in a pressure vessel filled with the alcoholic solvent, wherein the pressure vessel is maintained at a temperature ranging from about 260 C. to about 350 C. and at a pressure ranging from about 80 bars to about 150 bars for a time period ranging from about 0.2 hours to about 3 hours; and (b) allowing the pressure vessel to cool down while venting out solvent vapours at a rate of about 0.5 bar/minute.

25. The method of claim 19, wherein the metal oxide precursor is titanium isopropoxide, titanium butoxide, or a combination thereof.

26. The method of claim 19, wherein the method further comprises impregnating an inorganic fibre mat with the viscous mixture before aging and supercritical drying of the viscous mixture, wherein the inorganic fibre mat comprises an individual inorganic fibre mat, a pair of individual inorganic fibre mats with an organic sponge sandwiched therebetween, or a combination thereof.

27. The method of claim 26, wherein the individual inorganic fibre mat comprises a woven fabric made of one or more materials selected from the group consisting of ceramic fibres, refractory fibres, glass fibres, e-glass fibres, and oxide fibres.

28. The method of claim 26, wherein the individual inorganic fibre mat comprises a non-woven fabric made of one or more materials selected from the group consisting of ceramic fibres, refractory fibres, glass fibres, e-glass fibres, and oxide fibres.

29. The method of claim 26, wherein the organic sponge comprises one or more polymers selected from the group consisting of polyethylene, polypropylene, polyolefin, polyurethane, and polyvinyl chloride.

30. The method of claim 26, wherein the inorganic fibre mat comprises the pair of individual inorganic fibre mats with the organic sponge sandwiched therebetween, and wherein the supercritical drying of the viscous mixture impregnated in the pair of individual inorganic fibre mats degrades the organic sponge.

31. The method of claim 19, wherein the method further comprises casting the viscous mixture in a mold before aging and supercritical drying of the viscous mixture.

32. The method of claim 19, wherein the solution of the metal oxide precursor is an alcoholic solution containing a precursor of titanium dioxide in an amount of less than or equal to 5% by weight of the alcoholic solution.

33. The method of claim 19, wherein the at least one silica precursor and the metal oxide precursor are utilized in a molar ratio ranging from about 1:0.0014 to about 1:0.7.

34. The method of claim 19, wherein molar ratio of the at least one silica precursor to an alcohol present in the hydro-alcoholic solution ranges from about 1:4 to about 1:50, and wherein molar ratio of the at least one silica precursor to water present in the hydro-alcoholic solution ranges from about 1:0.5 to about 1:4, and wherein the at least one silica precursor comprises a combination of tetraethylorthosilicate and methyl trimethoxysilane in a molar ratio ranging from about 5:1 to about 5:5.

35. The method of claim 19, wherein the at least one alkali and the at least one silica precursor are utilized in a molar ratio ranging from about 1:0.05 to about 1:0.1.

36. The method of claim 19, wherein the silica aerogel thermal insulation product comprises silica aerogel with a specific surface area of about 300 m.sup.2/g.

37. The method of claim 19, wherein the silica aerogel thermal insulation product is substantially devoid of SiOTi.

38. The method of claim 19, wherein the hydro-alcoholic solution comprises methanol, ethanol, isopropanol, or a combination thereof.

Description

BRIEF DESCRIPTION OF DRAWING

[0059] These and other features, aspects and advantages of the present invention will become better understood when the detailed description is read with reference to the accompanying figures and drawing.

[0060] FIG. 1: Flow chart showing the formation of silica aerogel granules infiltrated flexible sheet sandwiched with organic sponge sheet in between according to one preferred embodiment under the invention.

[0061] FIG. 2: Flow chart showing the formation of silica aerogel granules infiltrated flexible sheet according to another embodiment under the invention.

[0062] FIG. 3: Flow chart showing the formation of pure silica aerogel produced according to the invention

[0063] FIG. 4: Schematic of the stitching pattern of inorganic fibre matorganic sponge sheetinorganic fibre mat sandwich

[0064] FIG. 5: Graph showing chemical analysis done by energy dispersive x-ray analysis (EDAX) of the sample as prepared in Example 1.

[0065] FIG. 6: Infra red reflectivity for the sample prepared by similar process described in Example 2 with variation in titanium dioxide content 0%, 0.1% and 1%

[0066] FIG. 7: Comparative graph on isotherm which plots the quantity of nitrogen adsorbed with respect to the relative partial pressure in nitrogen adsorption studies with the respective surface area for the silica aerogel in the flexible sheet prepared by a method as described in Example 2 and Example 4 and their comparison with the commercially available samples prepared by the process described in the patents mentioned in the prior art.

[0067] FIG. 8. Comparative graph on cumulative pore volume for the silica aerogel in the flexible sheet prepared by a method as described in Example 2 and Example 4 and their comparison with the commercially available samples prepared by the process described in the patents mentioned in the prior art.

DETAILED DESCRIPTION OF THE INVENTION

[0068] The most popular and promising application area of silica aerogels is thermal insulation. If compared with all the conventional high and cryo temperature insulation materials, silica aerogel tops the list of thermal insulation material in its class. Further being an inorganic material, it is structurally and chemically stable at wide temperature range in cryo and above ambient temperatures, which makes it a unique choice. Additionally its ultra low density is an additional advantage for insulation weight management. These advantages to silica aerogel are due to the nanoporous open network present in it which is being formed during its preparation by sol-gel method. The extent of this nanoporosity determines the density and thermal insulation property. Higher is the nanoporosity, better the thermal insulation property. The porosity in the silica aerogel is measured in terms of surface area, pore volume and pore area using the standard technique of nitrogen adsorption which is known as BET analysis. Typically pure silica aerogels possess specific surface area of about 500 to 1000 m.sup.2/g. When the composite of the silica aerogel is made by using fibre reinforcement to form the flexible or non flexible sheets, the specific surface area is reduced compared to the pure silica aerogel. The perfection in the manufacturing process can give rise to higher specific surface area even in the composite form, similar to the pure silica aerogel.

[0069] According to the process disclosed in the present invention, we are able to achieve high specific surface area due to the nanopores in the range of 1-100 nm.

[0070] The low density of aerogels leads to minimise the heat conduction through solid. The density has direct relation to the porosity and surface area in the silica aerogels. Hence higher the surface area and lower the density, lower the thermal conductivity in silica aerogels. The nano size pores having diameter less than the mean free path of air molecules at ambient pressure, minimizes the convectional heat flow. The average mean free path of the air molecules in ambient atmospheric pressure is about 70 nm. If majority of the pores in silica aerogel are equivalent or less than 70 nm, the heat transfer through the air is minimized to large extent. Hence to improve the thermal insulation property of silica aerogels, the pore size is to be controlled to achieve average pore size less than 70 nm. Another part of heat conduction is through radiation, mainly via infra red radiation. If the thermal insulation material can restrict the infra red radiation emitted by the heated object on which the insulation is applied, the heat losses will be minimized to greater extent. In the strategy to improve the thermal insulation performance, There are various ways by which thermal insulation performance, can be improved such as reducing the density further to low values by controlling the reaction parameter, controlling the pore size distribution, reducing the mean free path of air molecules by lowering the air pressure in the pores and finally combining the infra red reflecting material with silica aerogel by dispersing, enclosing, layering etc.

[0071] It is clearly seen from the prior art that, many types of infra red opacifiers are added to the silica aerogel or its composites, where it either absorb or reflect the said radiation. This invention deals with the infra red reflection property of aerogel. The concentration of such additives claimed in various patents varies in number. When infra red pacifiers are externally added to silica sol, the dispersion of such materials in the form of particles or fibres doses not guaranty the uniformity in distribution as these particles or fibres vary in density, surface chemistry and surface charge. If the same material is added in ultra small size, not only there is an improvement in the dispersion uniformity, but also results in the reduction of the quantity required to have same functionality. However, the production of such nanoparticles is a specialized process and lot of expertise is involved in doing so. Such nanomaterials are also available commercially in powder and dispersion form with higher cost. The dispersion of nanopowders in liquids is a challenge and it is a subject of R&D itself. Hence we have addressed this issue and have come out with easiest and cheapest way to produce such infra red opacifying dispersants in-situ in the silica porous network.

[0072] Among all the inorganic infra reflecting materials, metal oxides and among them titanium dioxide is the best known materials due to its temperature stability, abundance of occurrence in nature, cheaper price, compatibility of reaction conditions with silica forming reactions and aesthetics of bright white light reflecting colour and more importantly its ability to reflect the infra red radiation. The titania occurs in major three crystalline phases, anatase, rutile and brookite. Generally, as prepared titania by chemical sol-gel route in ambient condition is amorphous in structure. Thus formed amorphous titania when subjected to heating, starts becoming crystalline and may transform from anatase to rutile or directly in rutile structure depending on the reaction conditions of its preparation. Although both, anatase and rutile structured titania has infra red reflecting property, the rutile structure performs the best. It is known that due to the quantum size effects of nanoparticles, all the physical properties change as the size of the particles is reduced. The smaller particle size leads to the crystallization at lower temperature.

[0073] The silica-titania composite aerogels are well studied. Hitherto the mixed oxide aerogel such as silica-titania aerogel mainly focus on incorporating titania in silica to form SiOTi bonding. This bonding is achieved by adding the mixture of silica and titania precursors to solvent-catalyst or making sols of silica and titania separately and then mixing together. Formation of ultra-fine particles dispersed in the solvents defined as sol. In fact when two sols are mixed, the two types of ultra fine particles, such as silica and titania, bond with each other to form SiOTi bond. We do not envisage the prior art practice of forming SiOTi bonding during the process of preparation of mixed oxide aerogel such as silica-titania aerogel, but prefer first precipitating titania in solvent-catalyst mixture and then trapping them in silica matrix formed later. The pure titania particles without any bond to silica are found to be more effective in infra red reflection than SiOTi bonding.

[0074] The metal oxides such as iron, manganese, magnesium, zirconium, zinc, chromium, cobalt, titanium, tin, indium etc or mixtures thereof can be prepared in-situ using their salts or organometallic precursors. Titanium isopropoxide, butoxide, tetrachloride, trichloride, and sulphonate are various precursors used in the synthesis of titanium compounds. Out of which titanium isopropoxide and butoxide are the organometallic precursors which can take part in sol-gel reaction to form nano titania in certain reaction conditions. There will not be any unwanted by-products in the form of compounds and ions. Hence these two precursors are the most preferred ones. Both of these chemicals are most hygroscopic and react with water or moisture very vigorously. For any nanoparticle preparation, control over the rate of reaction is extremely important. So the precursor is initially diluted in alcohol and then used in the preparation. The synthesis of silica aerogel is well known, heavily documented and is available in published literature where tetraethyl orthosilicate (TEOS) or tetramethylorthosilicate (TMOS) is used as silica precursor. The typical procedure includes mixing of precursor in ethyl or methyl alcohol adding water as hydrolysing process and acid or base as a catalyst to complete the sol-gel reaction. The present innovation involves the steps where alcohol, water and catalyst are mixed, to which the diluted titanium precursor is added so that it reacts with water to form first hydroxide and then oxide i.e. titanium dioxide nanoparticles. The transparent milky colour with excellent dispersion in the liquid mixture confirms the formation nano titanium dioxide. Then silica precursor is added which undergoes hydrolysis and polycondensation to form silica network arresting nano titanium dioxide into the pores. Due to the nano size the volume of the titania nanoparticles increases and even 0.1 percent of titania particles almost doubles the infra red reflection properties when subjected to heat compared to the sample without presence of titanium dioxide. Rest all the process of aerogel formation including solvent exchange and supercritical drying remain same.

[0075] Once the gel is formed, the drying of the gel is performed by most popular super critical drying process to be carried out in an autoclave which can be performed using alcohol or liquid carbon dioxide as a solvent. Before drying, the solvent and water mixture in the gel is completely replaced by a pure alcohol or liquid carbon dioxide. There are advantages and limitations of either of the process of supercritical drying. If alcohol is used as a solvent, the process needs to be carried out at elevated temperatures above 250 C. and after venting it, the same can be easily water condensed and reused. However, this has higher power requirement and has the risk of handling highly flammable solvent. In other case of using liquid carbon dioxide which has lower critical temperature i.e. 31 C., the supercritical process can be performed at much lower temperature i.e. at 40 C. This process takes longer autoclave operation where total process may take 3 to 4 days. This process needs extra facility to scrub or re-condense the vented carbon dioxide during drying process. Being a green house gas, if released in atmosphere, the carbon footprint is very high for the process. In case of leakage due to any reason, the increased concentration of carbon dioxide in air may become lethal for life. In both the cases, requirement of high pressure is a common parameter. The ethanol is a preferred alcohol as a solvent in the drying process with the advantage of its higher critical temperature at 243 C. which helps to initiate the crystallization process of the nano titania loaded silica gels which can not happen if liquid carbon dioxide is used as a drying solvent.

[0076] Controlling the surface chemistry of silica aerogel is extremely important. The hydrophobic nature of silica aerogels is most preferred as it avoids the atmosphere moisture and rain water absorption and protects the insulation property. The hydrophobic silica aerogels are formed mainly by two methods. The first method is the silica gel surface modification by alkilation process. As prepared silica gel surface is covered with hydroxyl groups which makes silica aerogel hydrophilic. The hydroxyl groups are reacted with some alkoxy compounds such as hexamethyldisilazane, methyl trimethoxysilane, trimethylchlorosilane to convert them to a group ending with alkyl group. This process is called as alkilation. If the silane containing chemicals are used for this purpose, the process is called as silation. These gels after alkilation or silation treatment are dried either by super or sub critical drying to produce hydrophobic silica aerogels. In second method, silica precursor or a combination of precursors is selected such that it contains at least one alkyl group in the precursor molecule. Hexamethyldisilazane, methyl trimethoxysilane are the most preferred precursors for producing hydrophobic silica aerogels. Other way is that the alkyl group containing precursors can be added in a proportion to other silica precursors as a hydrophobising agent during the sol preparation stage of the synthesis. The ethanol drying process carried out at higher temperature above 250 C., enhances the reaction of surface hydroxyl groups with hydrophobising agent and ethanol molecule itself to increase the hydrophobic nature of silica aerogel.

[0077] As described above, the infra red reflecting pure silica aerogel with smaller fraction of opacifier generated in-situ and with hydrophobic nature are produced by simple way following preferably ethanol based supercritical drying method. The flexible sheet form of silica aerogel sheet with fibre reinforcement is the most successful product which is available commercially. The prior art describes all the claimed process for making the same. The general procedure for making such flexible sheets is preparation of silica sol which is in-filtered in the mat of non-woven fibres followed by gelation of the in-filtered sol to form the fibre and gel wet composite., After drying this composite sheet supercritically, flexible aerogel sheet is obtained. Basically, more the content of aerogel, higher is the thermal insulation property of the sheet. The content of aerogel in the sheet is determined by the porosity available or in turn density of the fibre mat used as reinforcement material. There is limitation, on the density based on their commercial availability and if used too low dense mat, the mechanical strength of the sheet is compromised. So the increase in the aerogel content in the sheet beyond certain value near to 50% is impossible. The present invention relates to increase the aerogel content by applying novel strategy where it can reach upto 90%.

[0078] We had applied for a patent for the process of making aerogel granules by template method vide Indian patent application No. 2406/DEU2010 dated Oct. 8, 2010 where silica sol is in-filtered into the pores of organic sponge to make wet composite of silica gel and sponge. When dried supercritically in ethanol solvent, the ethanol supercritical temperature degrades the organic sponge releasing the aerogel granules in their pores. This invention is further taken ahead in this patent application to form flexible sheets as per the following process. Initially, the organic sponge sheet is placed in between two inorganic fibre mats as a sandwich structure and stitched using high temperature stable thread in a grid structure making pockets in the stitched sheet as shown in FIG. 2 but not limited to the shown stitching pattern. The thread used for stitching can be of any suitable thickness and composition depending upon the thickness of sheet and the usage temperature in the application. For usage at high temperature, the stitching thread is preferably made of the fibres or yarn of silica, silica-alumina, zirconia with or without re-inforced metal threads/metal threads. The silica sol is then soaked in these sheets where it gets absorbed by inorganic fibre mat as well as organic sponge sheet. The wet gel composite thus formed is subjected to solvent exchange and ethanol supercritical drying process where the middle layer of organic sponge degrades at supercritical temperature and releases the aerogel granules within the pockets of the stitched sheet, holding them in the sheet. There is no limit to number of such alternate layers of organic sponge and inorganic fibre mat and their thickness as long as they are able to stitch. With this we can increase the aerogel content in the sheet in a controlled way and as large as possible. The silica sol described earlier is used to make these sheets to gain all the infra red opacification properties. The organic sponge is selected which has degradation temperature more than or equal to 250 C. so that the organic part of the sponge is completely degraded during the supercritical drying process to releases the aerogel granules trapped in its pores. For this purpose the range of polymeric sponges made up of polymers like but not limited to polyethylene, polypropylene, polyolefin, polyurethane, polyvinyl chloride more preferably polyurethane. The pore sizes and the total porosity in the organic sponge determine the final aerogel granule size and the quantity of the aerogel granules respectively. Hence the selection of the organic sponge should be done depending on the desired size of the aerogel granules and it should be highly porous so as to produce larger quantity of aerogels granules per volume of the organic sponge.

[0079] Hence we have come out with an improved process for producing silica aerogel thermal insulation product having titanium dioxide formed in situ capable of suppressing radiative heat transport as shown in the flow diagram in FIG. 1. It comprise of the following steps: [0080] a) preparing an aqueous solution of alcohol selected from methanol, ethanol, isopropanol preferably ethanol, in which an aqueous solution of ammonium fluoride and ammonia solution is added as alkaline catalysts; [0081] b) addition of metal oxide precursor preferably titanium isopropoxide in alcohol as titania precursor and dissolve in into the solution of step (a) during which titania nano particles are precipitated in the solution; [0082] c) mixing silica precursor comprising alkoxides of silica selected from tetramethylorthosilicate (TEOS), tetraethyl orthosilicate, hexamethyldisiloxisilane, methyl trimethoxisilane (MTMS), sodium silicate, more preferably TEOS and MTMS, individually or in combination, in the dispersion formed in the step (b); [0083] d) stirring the resulting mixture continuously till the total mixture starts becoming viscous; [0084] e) soaking an inorganic fiber mat in the liquid formed in step (c), wherein the inorganic fibre mat is having two or more layers with organic sponge sheet placed in between the layers and stitched together to form a sandwich sheet of desired size, shape and thickness [0085] f) ageing the resultant product of step (e) for 1-24 hr at room temperature; [0086] g) immersing the resultant product (f) in pure solvent preferably ethanol to replace all the original solvent and water mixture used in step (a) for at least 3 days; [0087] h) replacing the solvent and water mixture used in step (a) every day with the fresh batch of said pure solvent till the complete exchange of liquid present in gel is replaced by the solvent; [0088] i) subjecting the resultant product to supercritical temperature by keeping the gel in a pressure vessel filled with said solvent used in step (g) and maintain a temperature of 260 C. to 350 C., and pressure of 80 bars to 150 bars for 0.2 to 3 hours; [0089] j) venting out the vapours of the solvent completely at the rate of about 0.5 bar/min from the pressure vessel by opening the release valve and putting off the heater to cool down the pressure vessel and recovering the silica aerogel products from the pressure vessel.

[0090] In another embodiment under the invention said inorganic fibre mat of desired size, shape and thickness is soaked in an inorganic fibre mat in the liquid formed in step (d) instead of said sandwich sheet as shown in the flow diagram in FIG. 2.

[0091] In yet another embodiment under the invention the liquid formed in step (d) is poured into the mould to form the silica gel in pure form having the desired size, shape and thickness as shown in the flow diagram in FIG. 3.

[0092] Now various steps involved in the process of making such silica aerogel in pure form and flexible sheet form are explained in details in the following paragraphs along with examples.

1) Step of Preparing Silica Sol

[0093] Initially a stirred reactor is charged with a solvent such as either one or the mixture of methanol, ethanol, isopropyl alcohol. To this, water is added as a hydrolyzing agent in a certain proportion. The catalyst, preferably alkali such as ammonia solution, ammonium fluoride, ammonium hydroxide and sodium hydroxide, more preferably ammonia solution and ammonium fluoride in aqueous solution form are added to the mixture of solvent and water. Optionally, solution of metal oxide precursor of metals such as but not limited to iron, manganese, magnesium, zirconium, zinc, chromium, cobalt, titanium, tin, indium etc or mixtures of them is prepared in a separate container. Most preferably the titanium precursor such as titanium isopropoxide, butoxide, tetrachloride, trichloride, sulphonate more preferably titanium isopropoxide is diluted using the same solvent which was used in earlier step. This diluted titanium precursor is then added to the mixture of solvent, water and catalyst. The solution becomes milky white in few seconds. Then predetermined quantity of silica precursor such as tetramethylorthosilicate, tetraethyl orthosilicate, hexamethyldisiloxisilane, methyl trimethoxisilane sodium silicate or combination of them, more preferably mixture of tetraethyl orthosilicate (TEOS) which is also commercially known as ethyl silicate and methyl trimethoxysilane (MTMS) are added to the milky white solution. The total mixture is then mildly stirred and observed it for the beginning of the increase in viscosity. The concentration ratio of precursor:solvent is used preferably between 1:4 to 1:50 moles and the ratio of TEOS and MTMS precursors used is between 5:1 to 5:5. The catalyst concentration used is preferably between 1:0.05 to 1:0.1 moles. The precursor-water molar ratio used is preferably in the range of 1:0.5 to 1:4 moles.

II) Step of Casting the Gel

[0094] The sol prepared in step I is then poured in any desired shape and size container preferably plastic or glass container. The sol solidifies to form a gel in some time. This gelation time can be within 2 minutes to 24 hours depending upon the reactant concentrations.

[0095] In another embodiment, the sol prepared in step I is soaked in the pores of inorganic flexible fibre mat of any desired thickness and length. The sol in the pores of the inorganic fibre mat is converted into gel to composite of inorganic fibre mat and wet gel. The inorganic fibre mat used can be made up of woven or non woven ceramic fibres, refractory fibres, glass fibres, e glass fibres, any other oxide or mixture of oxide fibres of any desired thickness, size and density.

[0096] In one more embodiment, the sol prepared in the step I is soaked in the layered structured flexible sheet made up of inorganic fibre mat and organic sponge. This composite mat of inorganic fibre and organic sponge is prepared by stitching two layers of inorganic fibre mat with organic sponge sheet sandwiched between them in a grid structure as shown in the FIG. 2 which is a representative grid structure but not limited to this pattern. The thread used for stitching can be of any suitable thickness and composition depending upon the thickness of the sheet and the usage temperature in the application. For usage at high temperature, the stitching thread is preferably made up of the fibers or yarn of silica, silica-alumina, zirconia with or without metal thread re-inforcing and metal threads. There is no limit to number of such alternate layers of organic sponge and inorganic fibre mat and their respective thickness as long as they are able to stitch. Organic sponge selected is made up of polymers like but not limited to polyethylene, polypropylene, polyolefin, polyurethane, polyvinyl chloride more preferably polyurethane of desired pore size. The inorganic fibre mat used can be made up of woven or non woven ceramic fibres, refractory fibres, glass fibres, e glass fibres, any other oxide or mixture of oxide fibres of any desired thickness, size and density.

[0097] All the types of casted gels in pure or composite forms as described above are then are then further kept undisturbed in air tight container for completing the cross linking reaction and aging for about one day and subjected to solvent exchange process to make them ready for supercritical drying. Optionally, these gels are immersed in the titanium isopropoxide or its solution before going to the next step in the preparation process.

III) Step of Supercritical Drying

[0098] The solvent exchanged gel prepared in step II is then placed in the high pressure reactor and then the solvent preferably ethanol is pored to cover the gel completely. The reactor is closed and slowly heated to the temperature upto 260 C. The pressure developed during heating is maintained at 80-150 bar at 260-350 C. Once these temperature and pressure conditions are achieved in the high pressure reactor, it is maintained for 0.2-3 hours as a soaking period. Then the pressure is released slowly at the rate of 0.5-0.1 bar/min by venting the ethanol vapours in the reactor. The vented ethanol vapours as collected by liquefying them in cool water condenser connected to the vent valve. Once the pressure reaches the atmospheric pressure, the heater is made off and the reactor is allowed to cool naturally. The silica aerogel products are collected from the cooled reactor.

[0099] The invention is described in details with reference to the Examples given below which are provided solely to illustrate the invention and hence should not be construed to limit the scope of the invention.

Example 1

[0100] In the first step, 412 ml ethanol, 385 ml distilled water, 16.5 ml NH.sub.4F (0.5M) and 1.65 ml NH.sub.3 solution taken in flat round bottom flask under stirring. The titanium 2.75 ml isopropoxide was diluted in 165 ml of ethanol and added to above mixture slowly. Then 275 ml tetraethoxyorthosilicate and 110 ml of methyl trimethoxysilane was added to this mixture while stirring. The resulting sol was transferred into a plastic container where it was converted into gel in 5-7 minutes. Thus formed gel was kept for aging to strengthen the gel network at room temperature for 1 day. Finally, the gel was removed form the plastic container and immersed into ethanol for 3 days to exchange the liquid and bi-products inside the gel. The ethanol was replaced with a fresh lot everyday. The gel was then submitted to high temperature supercritical drying in the pressure reactor. The reactor temperature and pressure was raised to 260 C. and 80 bars pressure. This temperature and pressure condition was maintained for 180 minutes. Then the vapours in the reactor were vented completely at 0.5 bar/min rate and then the heater was made off to cool down the reactor. The highly porous silica aerogels with hydrophobic property and having loading of titania nanoparticles were obtained after opening the cooled reactor. The graph in FIG. 3 is a chemical analysis of the aerogel formed by energy dispersive x-ray analysis (EDAX) where presence of titanium element is seen.

Example 2

[0101] In the first step, 375 ml ethanol, 350 ml distilled water, 25 ml NH.sub.4F (0.5M) and 1.5 ml NH.sub.3 solution taken in a beaker under stirring. The titanium 5 ml isopropoxide was diluted in 150 ml of ethanol and added to above mixture slowly. Then 250 ml tetraethoxyorthosilicate and 100 ml of methyl trimethoxysilane was added to this mixture while stirring. This sol was soaked in 10 mm thick ceramic fibre non-woven blanket of 30 cm30 cm size. Within 5-10 minutes the sol soaked in the fibre blanket was solidified. Thus formed composite gel was kept for aging in an air tight plastic container to strengthen the gel network at room temperature for 1 day.

[0102] Finally, the composite gel was removed form the plastic container and immersed into ethanol for 3 days to exchange the liquid and bi-products inside the gel. The ethanol was replaced with a fresh lot everyday. The gel was then submitted to high temperature supercritical drying in the pressure reactor. The reactor temperature and pressure was raised to 260 C. and 80 bars pressure. This temperature and pressure condition was maintained for 180 minutes. Then the vapours in the reactor were vented completely at 0.5 bar/min rate and then the heater was made off to cool down the reactor. The highly porous silica aerogels flexible sheet re-inforced with ceramic fibres with hydrophobic property and having loading of titania nanoparticles were obtained after opening the cooled reactor.

Example 3

[0103] The silica aerogel prepared as per the procedure described in Example 2 except where in place of 5 ml titanium isopropoxide, 0.5 ml is added which leads to in-situ formation of about 0.1% titanium dioxide in the final product. In another experiment no titanium isopropoxide is added. to get pure silica aerogel flexible sheet sample without any titanium dioxide The infra red radiation reflection property was tested for these two samples with 0.1% titanium dioxide and no titanium dioxide after heating it in air at 400 C. FIG. 4 depicts the increase in the infra red reflectivity in the wavelength range of 3-7 m due to the presence of titanium dioxide which is 1% in concentration compared to the sample with no titanium dioxide.

Example 4

[0104] Two pieces of ceramic fibre non-woven blanket of about 5 mm thickness with 30 cm30 cm size were taken. Then two number of polyurethane foam sheet of 2 mm thickness and 30 cm30 cm size were cut. These two cut sheets of polyurethane foam were placed between the two ceramic fibre non-woven blankets. The total sheet layers thus formed were stitched using six layers of silica thread to form a stitched blanket as shown in FIG. 1.

[0105] The sol is prepared by first mixing 375 ml ethanol, 350 ml distilled water, 25 ml NH.sub.4F (0.5M) and 1.5 ml NH.sub.3 solution taken in a beaker under stirring. The titanium 5 ml isopropoxide was diluted in 150 ml of ethanol and added to above mixture slowly. Then 250 ml tetraethoxyorthosilicate and 100 ml of methyl trimethoxysilane was added to this mixture while stirring. This sol was soaked in the stitched blanket of ceramic fibre and polyurethane sponge as described earlier. Within 5-10 minutes the sol soaked in the stitched blanket was solidified. Thus formed composite gel was kept for aging in an air tight plastic container to strengthen the gel network at room temperature for 1 day. Finally, the composite gel was removed form the plastic container and immersed into ethanol for 3 days to exchange the liquid and bi-products inside the gel. The ethanol was replaced with a fresh lot everyday. The gel was then submitted to high temperature supercritical drying in the pressure reactor. The reactor temperature and pressure was raised to 260 C. and 80 bars pressure. This temperature and pressure condition was maintained for 180 minutes. Then the vapours in the reactor were vented completely at 0.5 bar/min rate and then the heater was made off to cool down the reactor. The highly porous silica aerogels flexible sheet having silica granules sandwiched and placed in the pockets of the stitched blanket with hydrophobic property and having loading of titania nanoparticles were obtained after opening the cooled reactor.

Example 5

[0106] The nitrogen adsorption studies were carried out on the samples prepared as per the procedure described in Example 2 and 4 and the two commercially available silica aerogel flexible sheets re-inforced with inorganic fiber mat which are prepared as per the process described in the patents from prior art. The nitrogen adsorption studies were carried out as per the standard procedure which includes important steps as follows. The sample was accurately weighed and heated in vacuum at 300 C. for 3 hours prior to the analysis. Then these samples were kept in liquid nitrogen bath to attain the liquid nitrogen temperature. Then the extra pure quality nitrogen gas was dosed to the sample to allow it to adsorb on the available surface area in the sample. The dosage was continued till the pressure ratio of P/P.sub.0 is 0.99 to obtain the isotherm graph of quantity of nitrogen adsorbed in cm.sup.3/g vs P/P.sub.0. Using this data and applying BJH standard theory, the data of cumulative pore volume vs pore size was generated.

[0107] FIG. 5 gives the comparative graph on isotherm which plots the quantity of nitrogen adsorbed with respect to the relative partial pressure in nitrogen adsorption studies with the respective surface area for the silica aerogel in the flexible sheet prepared by a method as described in Example 2 and Example 4 and their comparison with the two samples which are commercially available and prepared by the process described in the patents mentioned in the prior art. It is evident that the samples prepared by the process mentioned in this invention, show much higher porosity.

[0108] FIG. 6 shows the comparative graph on cumulative pore volume for the silica aerogel in the flexible sheet prepared by a method as described in Example 2 and Example 4 and their comparison with the two commercially available samples prepared by the process described in the patents mentioned in the prior art. It is very clear that the flexible sheet having silica aerogel granules sandwiched between two ceramic fibre re-inforced silica aerogel sheets prepared as per example 4 possess extra content of aerogel, hence possess more pore volume and hence will possess the greater thermal insulation property.

Advantages of the Invention

[0109] 1. The process produces [0110] Slica aerogel having dispersion of nano titanium dioxide nanoparticles which are prepared in-situ while formation of silica gel network. [0111] Silica aerogel having dispersion of metal oxide nanoparticles, preferably titanium dioxide nanoparticles which are prepared in-situ while formation of silica gel network in-filtered into the inorganic fibre non-woven blanket to form infra red reflecting flexible insulation sheets when applied on hot surface. Silica aerogel having dispersion of preferred nano titanium dioxide nanoparticles which are prepared in-situ while formation of silica gel network in-filtered into the inorganic fibre non-woven blanket to form infra red reflecting flexible insulation sheets when applied on hot surface. [0112] Silica aerogel flexible sheet where the infra red reflecting silica aerogel granules are sandwiched and placed in the pockets between two the infra red reflecting silica aerogel in-filtered flexible sheets of ceramic fibre blanket or their multilayer structures. [0113] Silica aerogels in all forms described in this invention having nanoporous surface area greater than 300 m.sup.2/g which is important criteria for better thermal insulation property. [0114] 2. The process is cost effective as the metal oxide nanoparticles, preferably titanium dioxide nanoparticles are required in smaller quantity even 2% than conventionally used micron size particles and is a single step easy process to incorporate them into silica aerogel network with uniform distribution. [0115] 3. The process facilitates increasing the silica aerogel content in the flexible sheet and increasing the ability to have better thermal insulation property.

[0116] We have brought out the novel features of the invention by explaining some of the preferred embodiments under the invention, enabling those skilled in the art to understand and visualize our invention. It is also to be understood that the invention is not limited in its application to the details set forth in the above description. Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, various modifications can be made without departing from the spirit and scope of the invention as described herein above and as defined in the following claims.