THEREOF

TECHNICAL FIELD OF THE INVENTION

The present invention relates to hydrophobic silica aerogels. More particularly, the present invention relates to hydrophobic silica aerogels that can be produced by a condensation reaction using a siloxane precursor, such as polyalkylalkoxysiloxane or

polyalkylhydroxysiloxane. The invention also provides a process for preparing a hydrophobic silica aerogel.

BACKGROUND OF THE INVENTION

Silica aerogels are porous materials that have unusual properties such as large internal surface area, small refractive index, low thermal conductivity and high visible transparency. Silica aerogels have been used in a variety of applications including Cerenkov radiation detectors, thermal insulations, heat storage systems and catalyst supports. Most silica aerogels described in the prior art are hydrophilic. These hydrophilic aerogels attract atmospheric moisture and as a result deteriorate with time. Therefore, one of the most important requirements for long- term use of the silica aerogels is making them hydrophobic. Furthermore, hydrophobic silica aerogels can be used in a number of applications for which hydrophilic silica aerogels are unsuitable, e.g. for oil spill cleanup.

Hydrophobic aerogels are generally obtained by two methods:

• co-precursor method

• surface derivatization of alcogel

The first method is simple and less time consuming compared to the second method

Zhae et al. {One-pot synthesis of hybrid me soporous xerogels starting with linear

polymethylhydrosiloxane and bridged bis-(trimethoxysilyl)ethane, Microporous and

Mesoporous Materials 163 (2012) 178-185) describe the preparation of hybrid xerogels with organic groups embedded in and anchored onto a framework using l,2-bis(trimethoxysilyl)- ethane (BTME) and polymethylhydrosiloxane (PMHS) as organosilica precursors in the absence of traditional structure-directing agents. It was shown that the representative hybrid materials possessed a significant hydrophobic interface and stable framework structure, and high porosity with tunable high specific surface areas and pore volumes.

Zhou et al. {Hydrophobic silica aerogels derived from polyethoxydisiloxane

and perfluoroalkylsilane, Materials Science and Engineering C 27 (2007) 1291-1294) describe the preparation of hydrophobic aerogels were synthesized from polyethoxydisiloxane (E-40) using perfluoroaklysilane (PFAS) as a coprecursor. With the increase of the volume ratio of PFAS/E-40, the gelation time, shrinkage rate and density

were found to increase. The authors conclude that the fluoroaklyl groups were successfully incorporated into the silica aerogels in hydrolysis-condensation reactions. Yang et al. (Facile Nonsurfactant Route to Silica-based Bimodal Xerogels with

Micro/Mesopores, Chemistry Letters Vol.34, No.8 (2005), 1 138-1139) describe a

nonsurfactant synthesis pathway to prepare silica-based bimodal micro/mesoporous hybrids using mixed polymethylhydrosiloxane (PMHS) and tetraethyl orthosilicate (TEOS) as silica sources. PMHS was dripped into a flask containing ethanol, followed by stirring for 48 h at room temperature to allow PMHS to react with a part of the ethanol and release hydrogen in the presence of NaOH as catalyst. Then TEOS and deionized water were introduced with vigorous stirring for 3 h. The formed sols were statically aged for 4-5 d and finally turned into gels. The obtained gels were heated in a 333K vacuum oven to remove the ethanol. In this way xerogels were made with PMHS/TEOS mass ratio of 1 : 1 and 1 :2.

Huang et al. (Sol-gel composite coatings from methyltriethoxysilane and

polymethylhydrosiloxane, J Sol-Gel Sci Technol (2010) 55 :261-268) describe the preparation of composite coatings by mixing pre-hydrolyzed methyltriethoxysilane (MTES) sol by an acidic catalyst dibutyltin dilaurate (DBTDL) and polymethylhydrosiloxane (PMHS) in gasoline at room temperature. Different basic catalyst [3-aminopropyltriethoxysilane

(APTES) and triethylamine (TEA)] were used and the ratios of pre-hydrolyzed MTES sol and PMHS were varied with various content of active H. The authors found that the composite coating from 2: 1 ratio (w/w) of pre-hydrolyzed MTES sol with equimolar amounts of water and PMHS1.55 under the catalysis of APTES demonstrated high pencil hardness, and excellent resistance against contamination and corrosion.

CN 101 774 592 describes a method of preparing a SiCh aerogel for insulation film material having high transparency and high porosity, using a tetraethylorthosilicate [Si(OCH2CH3)4] .

CN 103 861 555 describes a method of a preparing a multi-porous silica gel liquid chromatographic monolithic column. The monolithic column is obtained by performing a crosslinking reaction on PHMS and 2,4,6,8-tetravinyl-2,4,6,8-tetramethylcyclotetrasiloxane (D4Vi) which serve as precursors and polystyrene (PS) microspheres which are prepared by a dispersion and polymerization method and serve as a template.

SUMMARY OF THE INVENTION

The inventors have developed a novel hydrophobic silica aerogel that can be prepared by a simple (co-precursor) process. Furthermore, the properties of this silicagel can easily be tuned by manipulating process parameters (solvent, precursor ratios, solvent to water ratios, aging time etc.) and by varying the chain length of the siloxane precursor that is employed in the preparation of the silica aerogel.

The silica aerogel of the present invention has a porosity of at least 80% and comprises a hydrophobic polysiloxane that can be obtained by a condensation reaction that employs one or more siloxanes represented by formula (I)

(R ¹ )3SiO-[(R ² )R ³ SiO] _i - [(R ² )R ⁴ SiO]j-[(R ² )HSiO]k-Si(R ¹ ) ₃ (I)

wherein:

• each R ¹ independently represents H, OH, Ci-4 alkyl, Ci-4 alkoxyl;

• each R ² independently represents a Ci-4 alkyl;

• each R ³ independently represents a Ci-4 alkyl;

· each R ⁴ independently represents OH or C 1-4 alkoxyl;

• 1 < i+j+k < 50;

• 0 < i < 40;

• j≥i ; • 0 < k < 49;

and optionally one or more silanes represented by formula (Ha) or (lib)

Si(R ⁵ ) ₄ (Ha)

R ⁶ -[(R ⁵ ) ₂ Si-0-Si(R ⁶ )2]u-R ⁶ (lib)

wherein

• each R ⁵ independently represents H, OH, Ci-4 alkyl or Ci-4 alkoxyl; and wherein at least one R ⁵ residue represents OH or Ci-4 alkoxyl;

• each R ⁶ independently represents H, OH, Ci-4 alkyl or Ci-4 alkoxyl; and wherein at least one R ⁶ residue represents OH or Ci-4 alkoxyl;

• u > 1 ; and

wherein at least 70 wt.% of the hydrophobic polysiloxane is derived from the siloxanes represented by formula (I).

An example of a siloxane of formula (I) is alkoxylated polymethylhydrosiloxane (PMHS) which can be prepared by combining PMHS with alkanol and a base catalyst. PMHS is an abundantly available non-toxic byproduct of the silicon industry. Alkoxylated PMHS is a hydrophobic siloxane that can suitably be used in the production of a hydrophobic aerogel according to the present invention by reacting the alkoxyl residues of the alkoxylated PMHS with the hydroxyl residues of one or more silanes represented by above mentioned formula (Ha) or (lib). It is also feasible to hydrolyse one or more of the alkoxyl groups in the alkoxylated PMHS and to react the resulting hydroxyl residues with alkoxyl and/or hydroxyl residues of one or more silanes represented by formula (Ila) or (lib).

The invention also provides a process of preparing the aforementioned hydrophobic silica aerogel, the process comprising:

• providing a liquid reaction mixture containing solvent, one or more siloxanes as defined above and optionally one or more silanes as defined above;

• subj ecting the one or more siloxanes and the optional one or more silanes to a

condensation reaction to produce a hydrophobic polysiloxane;

• continuing the condensation reaction until a gel has been formed ;

• washing the gel with solvent;

• removing solvent from the gel. DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the invention relates to a silica aerogel having a porosity of at least 80%, said aerogel comprising at least 50% by weight of dry matter of a hydrophobic polysiloxane that can be obtained by a condensation reaction that employs one or more siloxanes represented by formula (I)

[(R ² )R ⁴ SiO]j-[(R ² )HSiO]k-Si(R ¹ ) ₃ (I)

wherein:

• each R ¹ independently represents H, OH, CM alkyl, CM alkoxyl;

• each R ² independently represents a CM alkyl;

• each R ³ independently represents a CM alkyl;

• each R ⁴ independently represents OH or C 1-4 alkoxyl;

• 1≤ i+j+k < 50; preferably 1 < i+j+k < 30; most preferably I+j+k =1 ;

• 0 < i < 40; preferably 0 < i < 30 ; most preferably i=0;

• j > 1 ; preferably 1 < j < 40; more preferably 1 j≤ 30; most preferably j=l;

• 0 < k < 49; preferably 0< k < 30; more preferably 0 < k < 10; most -preferably k=0;

and optionally one or more silanes represented by formula (Ila) or (lib)

Si(R ⁵ ) ₄ (Ha)

R ⁶ -[(R ⁵ ) ₂ Si-0-Si(R ⁶ )2]u-R ⁶ (lib)

wherein

• each R ⁵ independently represents H, OH, CM alkyl or CM alkoxyl; and wherein at least one R ⁵ residue represents OH or CM alkoxyl;

• each R ⁶ independently represents H, OH, CM alkyl or CM alkoxyl; and wherein at least one R ⁶ residue represents OH or CM alkoxyl;

• u > 1; preferably 1 < u < 50; most preferably 5 < u < 20;

wherein at least 70 wt.% of the hydrophobic polysiloxane is derived from the siloxanes represented by formula (I).

The term "aerogel" as used herein refers to is a synthetic porous material derived from a gel by replacing the liquid component of the gel with air or another gas. The term "aerogel" as used herein, unless indicated otherwise, also encompasses "xerogel". The siloxane represented by formula (I) can be an oligomer or a polymer. Examples of siloxane polymers that may be employed include homopolymers, statistical copolymers, block copolymers and combinations. Most preferably, the siloxane polymer is a homopolymer.

The aerogel of the present invention can be obtained by a condensation reaction in which hydroxyl residues in one precursor molecule react with alkoxyl residues in another precursor molecule (loosing alkanol) or in which hydroxyl residues one precursor molecule react with hydroxyl residues in another precursor molecule (loosing water).

The present invention encompasses an aerogel that is obtained by polycondensation of a siloxane represented by formula (I), i.e. in the absence of silanes represented by formula (Ila) or (lib). Examples of siloxanes that can be used to produce a hydrophobic polysiloxane by intermolecular self-condensation include:

· (CH ₃ )3SiO-(CH ₃ )(CH ₃ 0)SiO-Si(CH3)3

• (CH ₃ )3 SiO-(CH ₃ )(HO)SiO-Si(CH3)3

The present invention enables the preparation of a hydrophobic silica aerogel by employing a hydrophobic siloxane and reacting said siloxane using not more than a limited amount of the one or more silanes represented by formula (Ila) or (lib). Accordingly in a particularly preferred embodiment, at least 75 wt.%, most preferably at least 80 wt.% of the hydrophobic polysiloxane is derived from the siloxanes represented by formula (I).

The aerogel preferably comprises at least 80% by weight of dry matter, more preferably at least 90% by weight of dry matter and most preferably at least 95% by weight of dry matter of the hydrophobic polysiloxane.

The siloxane represented by formula (I) typically has a molecular weight in the range of 220 to 10,000 g/mol. More preferably, said molecular weight is in the range of 230 to 5,000 g/mol, most preferably of 235 to 2,500 g/mol.

The hydrophobic polysiloxane typically has a molecular weight in the range of 400-40,000 g/mol, more preferably of 410-10,000 g/mol and most preferably of 420-5,000 g/mol. In the aforementioned formula (I) preferably each R ¹ independently represents H, OH, methyl, ethyl, methoxy or ethoxy. Most preferably, R ¹ independently represents methyl.

Preferably, each R ² in formula (I) independently represents methyl or ethyl. Most preferably R ² represents methyl.

R ³ in formula (I) preferably represents methyl or ethyl, most preferably it represents methyl.

According to another preferred embodiment, in formula (I) each R ⁴ independently represents OH, methoxyl or ethoxyl. Even more preferably, each R ⁴ represents OH or methoxyl.

The ratio j/(j+k) in formula (I) preferably exceeds 0.6, more preferably it exceeds 0.8 and most preferably it equals 1.

The ratio j/(i+j+k) preferably exceeds 0.5, more preferably it is in the range of 0.6 to 1 and most preferably it it is in the range of 0.8 to 1.

In formula (Ila) preferably each R ⁵ independently represents H, OH, methyl, ethyl, methoxyl or ethoxyl. More preferably, each R ⁵ independently represents OH, methyl or methoxyl.

Preferably, at least two R ⁵ residues represent OH or Ci-4 alkoxyl, more preferably OH, methoxyl or ethoxyl, and most preferably OH or methoxyl.

In formula (lib) preferably each R ⁶ independently represents H, OH, methyl, ethyl, methoxyl or ethoxyl. More preferably, each R ⁶ independently represents OH, methoxyl or ethoxyl. Most preferably, each R ⁶ independently represents OH or ethoxyl.

Preferably, at least two R ⁶ residues represent OH or Ci-4 alkoxyl, more preferably OH, methoxyl or ethoxyl, and most preferably OH or methoxyl.

The hydrophobic silica aerogel preferably contains not more than a limited number of hydrophilic moieties. Accordingly, it is preferred that the aerogel has a contact angle of at least 80°, more preferably of at least 85° and most preferably of 90-170°. The contact angle of the aerogel can suitably be determined by the static sessile drop method. The hydrophobic aerogel of the present invention typically has a porosity in the range of 70% to 99%, more preferably of 80% to 98% and most preferably of 85% to 95%. The porosity of the hydrophobic aerogel can be suitably be determined using a helium pycnometer (e.g. AccuPyc II 1340 ex Micromeritics).

The density of the hydrophobic aerogel typically does not exceed 0.4 g/cm ³ . More preferably, the density of the aerogel does not exceed 0.25 g/cm ³ , most preferably it does not exceed 0.15 g/cm ³ .

The hydrophobic silica aerogel of the present invention typically has a volume weighted average pore diameter of 2-10,000 nm, more preferably of 4-2,000 nm even more preferably of 5-500 nm and most preferably of 6-200 nm The present aerogel typically has a surface area of at least 100 m ² /g, more preferably of at least 200 m ² /g and most preferably of 400 to 1000 m ² /g.

The thermal conductivity of the hydrophobic aerogel is preferably less than 50 mW(mK) ^_1 , more preferably of 5-30 mW(mK) ^"1 measured according to ASTM C518.

The aerogel of the present invention preferably is a monolith, a granulate or coating. In one preferred embodiment the aerogel is a monolith.

In another preferred embodiment, the aerogel of the present invention is a coating. An aerogel coating according to the present invention may suitably be applied onto a porous substrate. Consequently, the present invention also encompasses a coated porous product that comprises a porous substrate that has been coated with the aerogel of the present invention.

The porous substrate can take the shape of, for instance, a fabric, a membrane, a foam or a sponge. Most preferably, the porous substrate is a woven or non-woven fabric. Preferably, the porous substrate comprises at least 70 wt.%, more preferably at least 80 wt.% of synthetic polymer, cotton, wool, silk, glass (glass wool), mineral (rockwool), metal, carbon (carbon fibres) or a combinations thereof. More preferably, the porous substrate contains at least 70 wt.%, most preferably at least 80 wt.% of polyester, nylon, polypropylene, glass wool, cellulose based fibres and combinations thereof.

In a particularly preferred embodiment, the coated porous product is selected from the group consisting of textiles, filters, panels, granules and sponges. More preferably, the coated porous product is selected from textiles and filter Most preferably, the coated porous product is textile.

The aerogel coating preferably represents at least 10 wt.%, more preferably at least 12 wt.%, even more preferably at least 15 wt.% and most preferably 20-95 wt.% of the coated porous product.

The coated porous product typically has a porosity of at least 60%, more preferably of at least 80%.

Yet another aspect of the present invention relates to the use of the aforementioned coated product as clothing, insulation material, filter, adsorbent, absorbent, catalyst support. Most preferably, the coated product is used as insulation material or filter. Another aspect of the present invention relates to the use of the present hydrophobic silica aerogel as a thermal insulator, as an acoustic insulator, as an absorbent, as catalyst support, as a clothing material. In these applications the aerogel may be applied as such in the form of e.g. monoliths. Alternatively, the aerogel can be combined with other materials, e.g. by blending the aerogel in powder-form with paint, mortar, plaster etc.

Yet another aspect of the present invention relates to a process of manufacturing the aerogel described herein, said process comprising:

• providing a liquid reaction mixture containing solvent, one or more siloxanes as defined herein before and optionally one or more silanes as defined herein before;

· subjecting the one or more siloxanes and the optional one or more silanes to a

condensation reaction to produce a hydrophobic polysiloxane;

• continuing the condensation reaction until a gel has been formed;

• washing the gel with solvent; removing solvent from the gel.

The present process encompasses embodiments in which additional components are introduced into the reaction mixture, including insoluble components, such as particles. The process also encompasses embodiments wherein the gel formation occurs on the surface of a substrate so as to provide such substrate with an aerogel coating. In a preferred embodiment, the process comprises the step of impregnating a porous substrate (e.g. a fabric) with the liquid reaction mixture and carrying out the subsequent process step to produce an aerogel coated porous substrate.

According to a particularly preferred embodiment of the present process, the one or more siloxanes employed in the present process are prepared by combining a

polyalkylhydrosiloxane with Ci-4 alcohol and a base catalyst. According to a particularly preferred embodiment, the polyalkylhydrosiloxane is polymethylhydrosiloxane. The Ci-4 alcohol preferably is selected from methanol, ethanol and mixtures thereof. Most preferably, the alcohol is methanol. Sodium hydroxide is an example of a base catalyst that can suitably be used.

The aforementioned polyalkylhydrosiloxane typically has a molecular weight in the range of 200 to 9,000 g/mol. More preferably, said molecular weight is in the range of 210 to 4,500 g/mol, most preferably of 220 to 2,200 g/mol.

According to another preferred embodiment, the one or more silanes are prepared by hydrolysing a silicon alkoxide, preferably a silicon alkoxide represented by formula (Ilia) or (Illb):

Si(R ⁷ ) ₄ (Ilia)

R ⁸ -[(R ⁸ ) ₂ Si-0-Si(R ⁸ ) ₂ ] _u -R ⁸ (Illb)

wherein:

• each R ⁷ independently represents Ci-4 alkyl or Ci-4 alkoxyl, and wherein at least two, more preferably at least three of R ⁷ represent Ci-4 alkoxyl;

• each R ⁸ independently represents Ci-4 alkoxyl; more preferably methoxyl or ethoxyl; most preferably ethoxyl;

• u > 1 ; preferably 1 < u < 50; most preferably 5 < u < 20. According to a particularly preferred embodiment, each of R ⁷ independently represents mehyl, ethyl, methoxyl or ethyoxyl. Most preferably, the silicon alkoxide of formula (Ilia) is tetraethylorthosilicate or methyltrimethoxysilane.

According to another particularly preferred embodiment, the silicon alkoxide of formula (Illb) is polymethoxydisoloxane or polyethoxydisiloxane, most preferably polyethoxydisiloxane.

Preferably, the condensation reaction is carried out in a solvent containing at least 50 wt.%, more preferably at least 70 wt.% and most preferably at least 80 wt.% of C1 alcohol.

Preferably, the alcohol is selected is selected from methanol, ethanol, n-propanol, iso- propanol and combinations thereof.

The condensation reaction is preferably carried out at a temperature in the range of 10-60 °C, more preferably of 15-40 °C and most preferably of 18-30 °C.

In one embodiment of the present process the reaction mixture contains an organosilica precursor in the form of a siloxane represent by formula (I) and no silane precursors represented by formula (Ila) or (lib).

In another embodiment of the present process the reaction mixture provided contains a siloxane represented by formula (I) and a silane represented by formula (Ila) or represented by formula (lib). Preferably, said siloxane and said silane are present in the reaction mixture in a molar ratio that lies in the range of 1 :4 to 50: 1, more preferably in the range of 1 :3 to 20: 1 and most preferably in the range of 1 :2 to 15 : 1.

The gel that is formed by the condensation reaction is washed with solvent to remove excess reactants and condensation products (e.g. water). Examples of solvents that may be used to wash the gel include alcohols, ketones, hydrocarbons. Preferably, the solvent used to wash the gel is selected from methanol, ethanol, iso-propanol, acetone and combinations thereof.

In the present process the solvent may be suitably be removed from the gel by means of evaporation or by supercritical drying. More preferably, the solvent is removed by supercritical drying, preferably by replacing the solvent with supercritical carbon dioxide, followed by controlled depressurization.

The invention is further illustrated by the following non-limiting examples.

EXAMPLES

Example 1

The following reactions were carried out at room temperature (20°C). 0.577 ml of polyethoxydisiloxane (PEDS: P75E20, 75% hydrolysed, containing 20% silica, obtained from PCAS, France) was hydrolysed with 0.24 g of distilled water in 5.58 g isopropanol. 3.776 g polymethylhydrosiloxane (PMHS) with a molecular weight of 222 g was diluted with 15 g methanol and treated with 0.12 g sodium hydroxide in another beaker.

5-10 minutes after addition of the sodium hydroxide, the hydrolysed PEDS was added drop- wise to the base-treated and cooled PHMS solution. The solution was transferred into a vial and kept for gelation at room temperature. A gel was formed after 10-15 mins. The gel was aged at room temperature for 1 hr, and then transferred into an aging solution where the gel was aged for another 24 hrs. The aging solution had been prepared by mixing 14.5 L of 2- propanol, 14.5 L of ethanol, 500 ml water and 620 ml of 20% NH ₄ OH.

After the aging, the gel was removed from the vial and washed with ethanol, followed by drying with supercritical dioxide in a supercritical extractor.

The properties of the aerogel so obtained are summarized in Table 1.

Table 1

Example 2 The following reactions were carried out at room temperature (20°C). 7.29 ml of tetraethoxyorthosilicate (TEOS) was hydrolysed with 29 ml of O.OOIM oxalic acid in the presence of 166.5 ml methanol for hydrolysis. The solution was stirred for 3h to accomplish the hydrolysis of TEOS.

At the end of the completion of 3h of hydrolysis of TEOS, 45.9ml of PHMS (MW 222) was mixed with 166.5 ml of methanol in a separate beaker and stirred. To this solution, 1.08g of NaOH (powder form) was slowly added. The reaction was exothermic with strong evolution of ¾. The solution was stirred for 5 minutes more until all the NaOH was dissolved. The solution was cooled with ice to prevent phase separation. Next, the hydrolysed TEOS was added to the cooled solution.

The resulting mixture was poured into a mould. The gelation time was 40-50 mins at room temperature. The gel was aged at room temperature for 1 hr and then transferred into an aging solution where the gel was aged for 24h. The aging solution had been prepared by mixing 14.5 L of 2-propanol, 14.5 L of ethanol, 500 ml water and 620 ml of 20% NH ₄ OH. After aging the gel in the aging solution, the gel was washed with ethanol and then transferred into an autoclave for drying.

The properties of the aerogel so obtained are summarized in Table 2. Table 2

Example 3

0.76 g of tetraethoxyorthosilicate (TEOS) was hydrolysed with 3.22 g of O.OOIM oxalic acid in the presence of 15 g methanol. The solution was stirred for 3h to accomplish the hydrolysis of TEOS.

At the end of the 3h of hydrolysis of TEOS, 3.22 g of PHMS (MW 1900) was mixed with 15g of methanol and stirred. To this solution, 0.12g of NaOH (powder form) was slowly added. The ensuing reaction was exothermic with strong evolution of H ₂ . The solution was stirred for 5 more minutes until all the NaOH was dissolved. The solution was then cooled with ice. To the cooled solution the hydrolysed TEOS was added.

The mixture was poured into a mould. The gelation time was 10-15 minutes at 50°C. The gel was aged for 24h at 50°C. After aging, the gel was washed with ethanol (at ambient conditions) and then transferred into an autoclave for drying. The properties of the aerogel so obtained are summarized in Table 3.

Table 3

Example 4

A blanket made of glass wool was coated with a silica aerogel according to the present invention using the procedure described below. 2.29 g of methyltrimethoxysilane (MTMS) was mixed with 240 g of methanol. 3 g of cetyltrimethylamonium bromide (CTAB) was added to the mixture, followed by 19.32 g of 0.001M oxalic acid for hydrolysis. The mixture was stirred for 3h to complete the hydrolysis.

After the MTMS had been hydrolysed, 22.65g of polymethylhydrosiloxane (PHMS, MW 1900) was mixed with 120 g of methanol and stirred. Next, 0.72 g of NaOH (powder form) was added slowly. The reaction that followed was exothermic with strong evolution of ¾. The solution was stirred for 5 minutes more until all the NaOH was dissolved. The resulting solution was cooled with ice to prevent phase separation. Next, the hydrolysed MTMS was mixed with the cooled solution.

The resulting mixture was poured into a dish containing a blanket made of glasswool. The dimension of the blanket was 9.5x14.5x1 cm. The gelation time was 40-50 mins at room temperature. The gel infiltrated blanket was aged overnight (at room temperature) and subsequently washed with ethanol. The washed blanket was transferred into an autoclave for supercritical carbon dioxide drying. The drying conditions were 45 °C and 1 10 bar. After drying a glaswool blanket coated with silica aerogel was obtained. The density of the aerogel coated blanket was found to be 0.095 g/cm ³ . The coated blanket had a porosity of 95%.

Comparative Example A

The experiments described by Yang et al. {Facile Nonsurfactant Route to Silica-based Bimodal Xerogels with Micro/Mesopores, Chemistry Letters Vol.34, No.8 (2005), 1138-1139) were reproduced.

PMHS (2.338 g or 4.676 g) was dripped into a flask containing 70 ml ethanol, followed by stirring for 48 h at room temperature to allow PMHS to react with a part of the ethanol and release hydrogen in the presence of 0.15 g NaOH as catalyst. Then TEOS (4.676 g) and deionized water (20 g) were introduced with vigorous stirring for 3 h The formed sols were statically aged for 4-5 d and finally turned into gels. The obtained gels were heated in a 333K vacuum oven to remove the ethanol. In this way xerogels were made with PMHS/TEOS mass ratio of 1 : 1 and 1 :2.

The two xerogels so obtained showed serious cracking. The densities of the xerogels were measured as 0.37 g/cm ³ (2.338 g PMHS) and 0.57 g/cm ³ (4.676 g PMHS), respectively. The actual densities of the xerogels were significantly higher as the interstitial spaces resulting from cracking inflated the volume determination.