TECHNICAL FIELD OF THE INVENTION

The present invention relates to a process of applying a polymethylsilsesquioxane (PMS) aerogel coating onto a porous substrate, such as a woven or non-woven fabric, a membrane, a foam or a sponge. The invention also relates to an aerogel coated product that can be obtained by such a process.

BACKGROUND OF THE INVENTION 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. Aerogels are essentially the solid framework of a gel isolated from the gel's liquid medium. Almost all aerogels are derived from gels made through sol-gel chemistry. The term sol-gel refers to a process in which solid nanoparticles dispersed in a liquid (a sol) agglomerate together to form a continuous three-dimensional network extending throughout the liquid (a gel).

The sol-gel process that is used in the production of silica aerogels can ordinarily be divided into the following steps:

• forming a solution,

• gelation,

· aging, and

• drying. The majority of silica aerogels are prepared using silicon alkoxide precursors. The most common of these are tetramethyl orthosilicate (TMOS, Si(OCH3)4) and tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4). The balanced chemical equation for the formation of a silica gel from TEOS is:

Si(OCH ₂ CH ₃ ) ₄ (liq.) + 2 H ₂ 0 (liq.)→ Si0 ₂ (solid) + 4 HOCH ₂ CH ₃ (liq.)

The above reaction is typically performed in ethanol, with the final density of the aerogel being dependent on the concentration of the silicon alkoxide monomers in the solution.

After gelation, the gel is aged for a sufficient period of time to strengthen the gel network(the silica backbone of the freshly formed gel still contains a significant number of unreacted alkoxide groups). The aged gel is then submitted to drying. Silica aerogels are usually quite fragile. This fragility hampers the use of silica aerogels in applications in which the aerogel is exposed to conditions of stress and/or shear.

Furthermore, most silica aerogels are rigid, i.e. non- flexible. Such rigid aerogels are unsuitable for applications in which these aerogels are subject to bending, stretching etc.

Hayase et al. {New flexible aerogels and xerogels derived from methyltrimethoxysilane/ dimethyldimethoxysilane co-precursors, J. Mater. Chem. (2011) 21 : 17077-17079) describe the preparation of highly flexible aerogels from the methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) co-precursor systems, using a 2-step acid/base sol-gel process and surfactant (n-hexadecyltrimethylammonium chloride) to control the phase separation of the hydrophobic networks that yields porous monolithic gels.

US 2013/0022769 describes a method for preparing an insulating material comprising a polymer material and an aerogel base material comprising:

a) synthesizing an aerogel to form discrete aerogel geometric bodies; and

b) impregnating the polymer material prior to curing the polymer material with the aerogel geometric bodies; and

c) curing the polymer material to form the insulating material US 2015/0082590 describes a method for providing a mat containing aerogel, comprising the steps of:

• immersing a ribbon of fabric or non-woven fabric, unwound from a reel, in a solution containing aerogel in suspension,

· drying said ribbon impregnated with aerogel solution,

• winding said dried ribbon containing aerogel onto a rewinding reel.

Huang et al. {Sol-gel composite coatings from methyltriethoxysilane and polymethylhydrosiloxane, J Sol-Gel Sci Technol (2010) 55:261-268) describe composite coatings that were prepared by mixing pre-hydrolyzed methyltriethoxysilane (MTES) sol with an acidic catalyst and polymethylhydrosilane (PMHS) in gasoline at room temperature.

Brzezihski et al. (Nanocoat Finishing of Polyester/Cotton Fabrics by the Sol-Gel Method to Improve their Wear Resistance, FIBRES & TEXTILES in Eastern Europe (2011) Vol. 19, No. 6 (89): 83-88.) describe the formation of xerogel coats on fibre surfaces using a hybrid S1O2/AI2O3 sol, synthesised with the use of two precursors: (3-glycidoxypropyl) trimethoxysilane (GPTMS) and aluminium isopropoxide (ALIPO). The fabric susceptibility to form pilling was practically completely eliminated and the fabric abrasion resistance was increased.

US 2008/200432 describes a method of treating fibers with a fiber-treating agent, which contains an alkoxysilane (a), an organic acid (b) and water (c).The treatment method includes the step (i) of bringing the fiber-treating agent into contact with fibers to penetrate, into the fibers, the silanol compound formed by hydrolysis of the alkoxysilane (a) and step (ii) of polymerizing the silanol compound.

US 6,472,067 discloses a process of preparing non-flammable high-tensile strength, cured fibrous-siloxane composites having a density of 1-3 g/cc, comprising:

• polymerizing in an aqueous medium about 50 to 95 parts by weight of at least one trialkoxysilane, 5.0 to 50 parts by weight of at least one dialkoxysilane, and 0 to 10 parts by weight of at least one tetraalkoxysilane to obtain liquid polyalkylsiloxane resins,

• impregnating fibrous materials with an effective amount of said siloxane resins to obtain fibroussiloxane prepregs, • drying said fibrous-siloxane prepregs, and

• subsequently subjecting at least two plies of said fibrous-siloxane prepregs to pressures ranging from about 25 psi to 700 psi at temperatures ranging from about 50 to 300 °C.

SUMMARY OF THE INVENTION

The inventors have discovered a process that enables coating of porous substrates with a flexible silica aerogel. Thus, reinforced pliable silica aerogels with improved robustness can be produced. Furthermore, the novel process makes it possible to apply a resilient pliable silica aerogel onto e.g. fabrics, thereby imparting special properties to these fabrics, such as excellent thermal insulation.

The present process applies a polymethylsilsesquioxane (PMS) aerogel coating onto a porous substrate by:

• providing a liquid reaction mixture comprising:

o water;

o trialkoxysilane selected from alkyltrialkoxysilane, alkenyltrialkoxysilane and combinations thereof;

o dialkoxysilane selected from dialkyldialkoxysilane, dialkenyldialkoxysilane, alkylalkenyldialkoxysilane and combinations thereof;

o surfactant;

o acid catalyst;

• soaking a porous substrate with the liquid reaction mixture to produce a porous substrate that is covered with liquid reaction mixture coating;

• gelling and aging the liquid reaction mixture coating to transform it into an aqueous PMS gel coating, thereby producing an aqueous PMS gel-coated porous substrate;

• replacing the water in the aqueous PMS gel coating with organic solvent, thereby producing a water-free PMS gel-coated porous substrate;

• drying the water-free PMS gel-coated porous substrate to produce a porous substrate coated with a PMS aerogel. In the present process, the aerogel coating is formed in situ, thereby enabling maximum coverage of the available surface area.

The invention also relates to a coated porous substrate that can be obtained by the aforementioned process and to the use of such a coated product in clothing, insulation materials, filters, adsorbents, absorbents or catalyst supports.

DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the invention relates to a process of applying a polymethylsilsesquioxane (PMS) aerogel coating onto a porous substrate, said process comprising:

• providing a liquid reaction mixture comprising:

o water;

o trialkoxysilane selected from alkyltrialkoxysilane, alkenyltrialkoxysilane and combinations thereof;

o dialkoxysilane selected from dialkyldialkoxysilane, dialkenyldialkoxysilane, alkylalkenyldialkoxysilane and combinations thereof;

o surfactant;

o acid catalyst;

• soaking a porous substrate with the liquid reaction mixture to produce a porous substrate that is covered with liquid reaction mixture coating;

• gelling and aging the liquid reaction mixture coating to transform it into an aqueous PMS gel coating, thereby producing an aqueous PMS gel-coated porous substrate;

• replacing the water in the aqueous PMS gel coating with organic solvent, thereby producing a water-free PMS gel-coated porous substrate;

• drying the water-free PMS gel-coated porous substrate to produce a porous product coated with a PMS aerogel.

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 term "fabric" as used herein, refers to any material made from fibers or yarn through weaving, knitting, crocheting, or bonding (such as non-woven material).

The porous substrate that is employed in the present process can be made of various materials. Examples of such materials include synthetic polymers, cellulose based fibres (e.g. cotton), wool, silk, glass wool, mineral wool, metals, carbon (fibres) and combinations thereof. In order to ensure that the PMS aerogel coating adheres to the substrate it may be necessary to apply a suitable pre-coating.

The benefits of the present invention are particularly appreciated if the porous substrate that is coated in the present process has a high porosity. Preferably, the (non-coated) substrate has a porosity of at least 30%, more preferably of at least 70% and most preferably of at least 90%. Porosity of the substrate can suitably be determined using a helium pycnometer (e.g. AccuPyc II 1340 Pycnometer ex Micromeritics).

Preferably, the porous substrate employed in the present process 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 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.

The porous substrate can take the shape of a fabric, a membrane, a foam, or a sponge. More preferably, the porous substrate is in the form of a fabric, even more preferably a non-woven or knitted fabric, most preferably a non-woven fabric.

As explained herein before, the present invention enables the preparation of a PMS aerogel coating that is pliable and resilient. Hence, the benefits of the present invention are particularly appreciated in case the PMS aerogel coating is applied onto a porous substrate that is also pliable.

The liquid reaction mixture that is employed in the present process preferably comprises:

30-80 wt.% water;

8-20 wt.%) of the trialkoxysilane;

3-15 wt.%) of the dialkoxysilane;

0.5-8 wt.%) surfactant 0.005-0.1 wt.% acid catalyst;

The trialkoxysilane that is employed in the present process is preferably selected from methyltrimethoxysilane,vinyltrimethoxysilane and combinations thereof. Most preferably, the trialkoxysilane is trimethoxysilane.

The dialkyldialkoxysilane is preferably selected from dimethyldimethoxysilane, dimethyldiethoxysilane and combinations thereof. Most preferably, the dialkyldi alkoxysilane is dimethyldimethoxysilane.

The use of a surfactant in the present process is essential in order to maintain homogeneity of the reaction media. The surfactant employed preferably has an HLB of at least 8, more preferably of 9-15. The acid catalyst is employed in the present process to catalyze hydrolysis of the alkoxysilane components. The acid catalyst is preferably selected from acetic acid, formic acid, citric acid, sulfuric acid, hydrochloric acid and combinations thereof. At the beginning of the gelling step, the reaction mixture typically has a pH of not more than 5.5, more preferably of not more than 3 and most preferably of not more than 2.

In the present process the aqueous PMS gel coating is preferably formed by an acid/base two- step sol-gel reaction, wherein the base is used to increase pH in the second condensation step, thereby accelerating gelation relative to phase separation. Preferably, the base is added to increase the pH of the reaction mixture with at least 0.5, more preferably to increase the pH of the reaction mixture with at least 1, even more preferably to increase the pH of the reaction mixture with at least 2.

The base may be added after the initial stage of gelation or the base may be produced in situ. Accordingly, in one embodiment of the present process, base is added to the reaction mixture after at least a fraction of the alkoxysilane components has been hydrolysed. Preferably, base is added to increase the pH of the reaction mixture to at least pH 3, more preferably to at least pH 5.5. In another embodiment of the process, the liquid reaction mixture contains a base precursor that forms base in situ at a later stage of the gel forming reaction. Typically, sufficient base is formed during this later stage of the gel forming reaction to increase the pH of the mixture to at least pH 3, more preferably to at least pH 5.5.

Preferably, the base is produced in situ. This may be achieved, for instance, by including urea in the reaction mixture. In the present process, urea is hydrolyzed to release ammonia which raises the solution pH. Typically, urea is employed in the reaction mixture in a concentration of 3-40 wt.%, more preferably of 5-25 wt.%.

In order to adequately soak the porous substrate with the liquid reaction mixture, typically the total amount of liquid reaction mixture soaked into the porous substrate should exceed 500% by weight of the substrate, more preferably 1,000% by weight of the substrate and most preferably 2500%) by weight of the substrate. Generally, the amount of liquid reaction mixture that is soaked into the porous substrate does not exceed 8,000%> by weight of the porous substrate.

The liquid reaction mixture coating is typically gelled and aged at a temperature of at least 40°C, more preferably of 50-100°C, most preferably of 60-80°C. Typically, gelation and aging of the liquid reaction mixture coating is achieved by keeping said coating at the aforementioned temperature for at least 20 minutes, preferably for 0.5 to 20 hours and most preferably for 1-10 hours.

In the present process the water in the aqueous PMS gel coating is replaced by an organic solvent in order to facilitate the drying of the PMS gel. The organic solvent employed is preferably selected from iso-propanol, ethanol, methanol, acetone and combinations thereof. Most preferably, the organic solvent is selected from iso-propanol, ethanol and combinations thereof. The water in the aqueous PMS gel coating may suitably be replaced with organic solvent by washing the gel coated substrate with the organic solvent. Preferably, the gel coated substrate is washed at least twice with the organic solvent. Typically the water content of the gel coated substrate is reduced to less than 1.0%, more preferably less than 0.5 % and most preferably less than 0.1%> by weight of the water-free PMS gel coated porous substrate. The drying of the water-free PMS gel-coated porous substrate may suitably be done by means of evaporation or by means of supercritical drying. Most preferably, the drying is done by means of supercritical drying.

An important objective of supercritical drying is to eliminate the organic solvent from the water-free PMS gel-coating without generating a two-phase system and the related capillary forces. This is suitably achieved by:

• introducing the water-free PMS gel-coated substrate into an autoclave,

· pressurizing the autoclave containing the coated substrate with a gas to a pressure above the critical pressure and at a temperature that is above the critical temperature;

• maintaining the supercritical conditions until the organic solvent in the PMS gel coated porous substrate has been replaced by supercritical gas; and

• decompressing the autoclave to atmospheric pressure whilst ensuring that the gas (e.g. carbon dioxide) changes its state from supercritical to gas phase without any condensation.

According to a particularly preferred embodiment, supercritical drying of the water-free PMS gel-coated substrate is done using supercritical carbon dioxide.

Another aspect of the present invention concerns a coated porous product that can be obtained by the aforementioned process. More preferably, said coated porous product is actually obtained by said process.

According to a particularly preferred embodiment, the coated porous product is a pliable product.

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 filters. Most preferably, the coated porous product is a textile product. According to a particularly preferred embodiment, the textile product is non- woven product. The PMS 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%). Porosity of the coated product can suitably be determined using a helium pycnometer (e.g. AccuPyc II 1340 ex Micromeritics).

The coated porous product of the present invention typically has a density of less than 0.3 g/ml, more preferably a density of less than 0.25 g/ml and most preferably of 0.05-0.2 g/ml.

Yet another aspect of the present invention relates to the use of the aforementioned coated product in clothing, insulation materials, filters, adsorbents, absorbents or catalyst supports. Most preferably, the coated product is used in clothing, insulation materials or filters.

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

EXAMPLES

Example 1

5 g of Urea and 0.8 g of hexadecyltrimethylammonium chloride (CTAC) was vigorously stirred in 25 ml of acetic acid (5 mM). Next, 4 g of methyltrimethoxysilane (MTMS) and 2.5 g of dimethyldimethoxysilane (DMDMS) were added to the mixture and stirred well for 45 minutes.

Non- woven polyester blankets (thickness 1.5 cm) were impregnated with the reaction mixture in a dish. About 100 ml of solution was used to soak a 140 cm ³ blanket (~4 g) in order to cover the entire blanket with liquid. The dishes containing the impregnated blankets were subsequently transferred into an oven that was maintained at 80 °C to age the coated blankets for 3 hrs. Subsequently, the blankets were taken out of the oven, washed with 2-propanol 3 times with a time span of 3-4 hours between the washings. After the washing, the gel coated blankets were dried in an oven at 100 °C. The whole process took 2-2.5 days (including drying time). The weight of the blankets after aerogel coating was found to be approximately 14 g.

The PMS aerogel coated blanket so obtained had a density of 0.14 g/cm ³ . PES blankets have a thermal conductivity of 0,040W/m/K while impregnated blankets had a conductivity of 0,030W/m/K. Furthermore, it was found that the aerogel coated blanket was capable of absorbing up to 400% by weight of kerosene.

Example 2

Example 1 was repeated, except that this time the blankets (~4.1 g) were washed with ethanol (3 times with a time-span of 3-4 hrs between the washings), followed by supercritical drying using C0 ₂ . Drying was done at 120 bars and 45 °C for 2 days, followed by slow depressurization. The weight of the PMS aerogel coated blanket was found to be 15.8 g. The coated blanket had a density of 0.09-0.12 g/cm ³ .

Example 3

Example 1 was repeated, except that this time glass wool blankets (thickness 1 cm) were impregnated with the reaction mixture. About 105 ml of solution was used to soak a 120 cm ³ blanket (-7.2 g) in order to cover the entire blanket with liquid.

The impregnated blankets were subsequently transferred into an oven that was maintained at 80 °C to age the coated blankets for 3 hrs. Subsequently, the blankets were taken out of the oven, washed with 2-propanol (3 times with a time span of 3-4 hours between the washings). After the washing, the gel coated blankets were dried in an oven at 100 °C. The whole process took 2-2.5 days (including drying time).

The weight of the PMS aerogel coated blanket was found to be 17.0 g. The coated blanket had a density of 0.14 g/ cm ³ .