WO2010080060A1

WO2010080060A1 - Silica aerogel as transparent panel in energy saving window

Info

Publication number: WO2010080060A1
Application number: PCT/SE2010/050004
Authority: WO
Inventors: Leif Gullberg
Original assignee: AIRGLASS AB
Current assignee: AIRGLASS AB
Priority date: 2009-01-08
Filing date: 2010-01-04
Publication date: 2010-07-15
Anticipated expiration: 2011-07-08
Also published as: SE533051C2; SE0950003A1

Abstract

The present invention describes a body of a silica aerogel material which has excellent properties, such as a very low coefficient of thermal conductivity, i.e. is highly insulating. Furthermore there is described various specific embodiments in which the body according to the present invention has been given various advantageous properties, such as high transparency, increased mechanical stability and improved handling and transportation option, wherein the latter have been made by the fact that the body, e.g. a panel, is encased in an encasing material. Moreover, the present invention also describes processes for the production of a silica aerogel body, such as e.g. a panel.

Description

Silica aerogel as transparent panel in energy saving windows

Field of invention

The present invention relates to a body of a silica aerogel material and to a process for the production of shaped silica aerogel materials, such as e.g. panels. Technical background

It has long been known to produce silica aerogel in powder form. It is also known to produce silica aerogel in the form of blocks. For example, it is described in SE 422 045, a way to produce silica aerogel in the form of a substantially crack-proof, preferably transparent block. The method is carried out by hydrolysis of tetraalkoxysilane, preferably tetramethoxysilane, in an alcohol, preferably methanol, in the presence of a catalyst, preferably ammonia, to the formation of an alcogel, which is aged about 10 days and washed with alcohol to remove water. The alcogel is subsequently treated in an autoclave by temperature increase to over the critical point of the alcohol, isotherm pressure drop by the discharge of alcohol vapour, and temperature drop.

There are several drawbacks to the way or method described in SE 422 045. The method is energy-intensive as it is carried out at comparatively high temperatures, such as e.g. at the after-heat treatment carried out bet- ween 500⁰C and 75O⁰C. Furthermore, the process to perform the method is not quite easy to keep secure because the removal of the solvents from the alcogel and finally out from the autoclave is carried out at relatively high pressures and temperatures, such as for the preferred solvent methanol at about 90 bar and at a final temperature of 275⁰C. Finally, the material produ- ced according to SE 422 045 is further not optimal for various uses. An example of this is that the coefficient of thermal conductivity specified in SE 422 045 is 0.021 W / (m * K) for the obtained aerogel material after evacuation.

WO2007011988 A2 describes aerogel composites comprising organic- inorganic hybrid aerogel particles and binders. Hybrid aerogel materials are materials that include aerogels and other materials. The material described in WO2007011988 A2 is not meant to serve as insulating materials in applications where light must be able to penetrate, which is evident as the material include for example polymers, monomers or oligomers. This implies that the material is not high-transparent and also not particularly heat-resistant or fire- resistant. Furthermore, the material according to WO2007011988 A2 is not super-insulating and cannot be made super-insulating. All of these intrinsic features imply that the material according to WO2007011988 A2 is not appropriate as super-insulating material in applications where light shall penetrate. WO2007146945 A2 describes flexible aerogel foam composites which comprise at least one open cell foam component and at least one aerogel matrix. The material according to WO2007146945 A2 includes at least one component which is not of aerogel type. This allows the material to be made flexible. To make an aerogel material flexible is virtually impossible without the involvement of other material types, which also is apparent from WO2007146945 A2. This also brings about completely different effects. Also in this case, as in the case of WO2007011988 A2, the material can never be high-transparent, heat-resistant or be made super-insulating.

One aim of the present invention is to provide a process for the production of a shaped silica aerogel material body which remedies the above men- tioned problems. Another aim of the present invention is to provide a body, e.g. a panel, of an improved silica aerogel material, such as based on e.g. coefficient of thermal conductivity, with a predetermined desired and decided shape and size. Summary of the invention The latter stated purpose above is achieved by a body of a silica aerogel material which is porous and comprises nanoparticles, wherein the body

- has a density in the range of 60-300 kg/m³;

- has a thermal conductivity λ of less than 0.020 W / (m ^* K) when measured at a density of 150 kg/m³ and temperature of about 2O⁰C, and

- preferably is crack-proof and

- is transparent and has a total transparency which is at least 90%; and - has a refractive index in the range of 1.017-1.060, when measured at a thickness of 14 mm and as non-encased body.

The body or the silica aerogel material which it is composed of according to the present invention has a very low thermal conductivity. At a density of 150 kg/m³ and temperature of about 2O⁰C and in non-evacuated state, i.e. a state where there is still air left in the pores of the material, the thermal conductivity (the coefficient of thermal conductivity) is below 0.020 W / (m ^* K), which shall be compared with 0.021 W / (m ^* K) for the aerogel material described in SE 422 045, the latter, however, measured at a higher density of 240 kg/m³ but yet in evacuated state (air-free state for the pores), which in itself implies a much lower thermal conductivity than the non-evacuated state. By comparison, for vacuum the thermal conductivity is 0.022 W / (m ^* K) at about room temperature. Specific embodiments of the invention There are many different possible uses of a body, such as e.g. a panel, according to the present invention. Depending on the intended application, the silica aerogel material according to the present invention may be given different properties which are appropriate for the specific application. An intrinsic property of a silica aerogel material according to the present inven- tion is the possibility of a high transparency of the material. This implies that the material according to one specific embodiment may provide a high clarity, which means that it is appropriate to use in applications where both light is to penetrate and where one should to be able to see through the material almost as good as for ordinary window glass. This in turn implies that a panel in accordance with the present invention may be used in energy-window applications, which also are transparent, but also save energy because of the fact that the coefficient of thermal conductivity is so low for the material. As mentioned above, the following apply to the body according to the present invention: - is transparent and has a total transparency which is at least 90%, and

- has a refractive index in the range of 1.017-1.060, when measured at a thickness of 14 mm and as non-encased body.

Transparency in the field of optics is synonymous with transparent and translucent, i.e. clear. The transparency of a material can be divided into the transparency that can be derived from the light or radiation that directly penetrates straight through the material, so-called normal illumination, and light or radiation which is distributed in the material, but subsequently penetrates, so- called scattered light or diffused light. The total transparency can be defined as transparency which is derived from both direct light penetration and the penetration of scattered or diffused light.

The bodies may, however, be produced so that they have other characteristics than just high transparency. One such example is a body in accor- dance with the present invention which is translucent by means of glass fibers being admixed in the silica aerogel material or by means of the surface of the body being heat treated or roughened. Such treatment may for example be made by surface treatment by spraying with water or by burning on the surface. A translucent material implies a material that passes light through but is not transparent. There are applications where translucent bodies according to the present invention could be very useful. One such example is for panels in energy-windows in e.g. a bathroom where you want to let light through, but do not wish to have any transparency. Through the mixing of e.g. glass fibers in the panel material, the mechanical stability of the material also increases, which of course may be desirable for certain applications. The glass fibers which can be admixed may have different forms, such as for example nets, sticks or chips. To heat treat the surface to make the material translucent may be accomplished by overheating the surface, causing the pores to "crash" and thus the surface to rise.

In addition to glass fibers, there are many other substances or materials which for certain application purposes may be advantageous to admix in the material according to the present invention. Examples of these are wavelength shifters and light filters. This may e.g. be performed in order to alter the colour of the material. There is naturally a bluish in a standard panel according to the present invention. With the help of e.g. a wavelength shifter, the blue light as the standard panel material normally emits may be shifted to another colour, e.g. green light. Moreover, a wavelength shifter may be ad- mixed to reduce the proportion of diffused light, by reduced scattering of light, and thus give the effect of an increased proportion of direct penetrated light. According to one specific embodiment of the present invention, a wavelength shifter is therefore admixed in the silica aerogel material. As mentioned above, certain types of components may be admixed in the material according to the present invention. Such examples are carbon fibers, glass wool, mineral wool and/or rock wool. This, however, implies that the material to some extent becomes opaque, which is not advantageous for applications where light must penetrate. With an opaque material is meant in this respect, a material that is untransparent, which means that light does not go through the material and obviously that one neither is able to see through the material. By admixing carbon fibers, glass wool, mineral wool and/or rock wool in the silica aerogel material according to the present invention, the mechanical stability of the material is increased, but transparency is, how- ever, completely lost. Opaque bodies, such as panels, can preferably be used as insulating materials when a low thermal conductivity and mechanical stability are important while transparency is not interesting.

Possible admixing in the material according to the present invention, described above, should not be associated with such admixing made in the materials of WO2007011988 A2 or WO2007146945 A2. To admix polymers, such as according to WO2007011988 A2, implies that the material loses heat-resistance and the possibility of high transparency or translucency. In addition, such a material can never be made super-insulating. In the same way for the material according to WO2007146945 A2, in which components are admixed to make the material flexible, this renders the fact that the material cannot be transparent or translucent and that it cannot be made super- insulating.

There are also other purely procedural possible measures to implement to enhance certain properties that might be important for different types of applications. As mentioned above, evacuation of the material, which means that essentially all the air left in the pores of the material are driven out, is a possible method which implies that the coefficient of thermal conductivity of the present material may be further improved. According to one specific embodiment of the present invention, the body is evacuated and is super- insulating and has a thermal conductivity λ which is 0.007 W / (m ^* K) or less, when measured at a density of 150 kg/m³ and temperature of about 2O⁰C. This is a value of the coefficient of thermal conductivity which is significantly lower than for both vacuum and the material described in SE 422 045. Since the coefficient of thermal conductivity varies with the density, it may be of interest to regulate the obtained density of the material for some applications. As noted, the density of the silica aerogel material according to the present invention may be between 60 and 300 kg/m³, such as between 80 and 250 kg/m³, but in a number of applications the desired density is between 100 and 200 kg/m³, such as in the range of 125-175 kg/m³, e.g. at about 150 kg/m³.

To evacuate the material and make it super-insulating is not possible to do for the materials according to WO2007011988 A2 or WO2007146945 A2. To evacuate materials, such as according to the present invention, implies that the material is encased between e.g. a pair of discs, such as glass panels, and that the air in the material is exhausted by suction. In view of the fact that the material according to WO2007011988 A2 is not solid, but more as a kind of felt carpet, it is not possible to perform an evacuation of the material in this way. Would such an evacuation be performed, this would flatten the material enormously. The same applies to the material according to

WO2007146945 A2. The material according to the present invention is however a solid and therefore withstand very high outspread pressures.

Furthermore, there are other properties which may be desirable to donate to the material according to the present invention. According to one specific embodiment of the present invention, the silica aerogel material is further water-resistant in view of the fact that a silane compound is bound into the silica aerogel material. This may be an advantage because the material thus becomes more easily handled. An example of silane compounds which can be bound is hexamethyldisilazane (HMDS). The body according to the present invention may also be encased in a surrounding material. This is also something that could lead to the fact that panels according to the present invention will be easier to handle and transport. According to one specific embodiment of the present invention, an en- casing material therefore surrounds the body, such as when the material is evacuated as according to above. According to another embodiment of the invention, the encasing material is chosen from the group consisting of transparent materials, plastic foils, glass panels and plastic sheets. What type of encasing material used of course depends on the desired application. For example, when an encased body with high transparency is the desired one, the encasing material must of course also be transparent, e.g. a transparent plastic foil. Another possibility is to use a glass material to encase the body. In the case when a panel of the silica aerogel material according to the present invention is encased between two insulating glasses, these insulating glasses may e.g. be coloured. This to reduce the impression of the blue mist sight that otherwise might occur from the silica aerogel material. For an encased opaque body, however, non-transparent materials may of course be used, e.g. plastic foils in this case. With body according to the present invention is meant a geometric body. As indicated above panels are very conceivable geometric bodies according to the present invention. According to one specific embodiment of the present invention, the body is therefore a panel, e.g. a panel with a thickness of up to 5 cm. According to another specific embodiment of the present invention, the body is a halfpipe-shaped body.

With panel according to the present invention is further meant geometric shapes where length and width are larger than the thickness. According to the present invention, completely different thicknesses, lengths and widths are possible. According to one specific embodiment, the panel has a size of up to 1.5 m ^* 2 m (width * length), such as e.g. 600 mm ^* 1200 mm (width ^* length).

The present invention also describes a process for the production of a silica aerogel body which is porous and comprises nanoparticles. According to one specific embodiment of the present invention, the process comprises the steps of:

- supply of recipe start components to a mixer, wherein the recipe start components at least include a silane compound, an alcohol chosen from the group consisting of methanol, ethanol and propanol, and a catalyst which is either ammonia or titanium lactate;

- mixing in the mixer under temperature control for the formation of a solgel which through polymerization transforms into a wetgel;

- filling of the wetgel into at least one mould; - duration of the wetgel in the mould;

- form removal of the mould and transfer of the wetgel to a transport device;

- supply of the transport device and the above lying wetgel to an autoclave to which autoclave either supercritical carbon dioxide or liquid carbon dioxide is added gradually, to achieve extraction where removal of alcohol from the wetgel in the autoclave is carried out, for the commencement of the formation of a silica aerogel;

- emptying of the autoclave and further processing of the silica aerogel in a heating process at 200⁰C or above for further removal of alcohol and formation of a substantially alcohol-free silica aerogel material; - discharge of the formed substantially alcohol-free silica aerogel material from the heating process and optionally further processing of the formed substantially alcohol-free silica aerogel material for shaping into desired shape and size, where supply of further recipe components optionally is made at either the mixing in the mixer, in the extraction or in the heating process. In connection with the description of the process above, the following explanations of certain terms and elements may be important to understand.

A solgel is, by definition, a suspension consisting of a solvent and a sol which then may polymerize and aggregate to form a gel. Polymerization above implies binding together into longer chains by chemical reaction, and not that any polymer is admixed into the material. The solvent, in this case methanol, ethanol or propanol, may then be evaporated away and the result is a porous material.

An example of a material for a mould, in this case, is e.g. glass, but there are other materials that are equally possible. The step of duration of the wetgel in the mould may for instance be performed by the mould being immersed in a water bath and kept there for about 1-3 days. It would, according to the present invention, also be possible to speed up the polymerization during the duration step. This could for example be possible by treating the wetgel with radiation, such as ultraviolet light, or ultra sound or by heat treatment.

An example of the form removal step is that said mould is immersed in a form removal bath containing solvent, then the top side of the mould is removed so that the wetgel releases from the mould and floats up into the solvent, when the wetgel then becomes heavier by interference from the solvent the wetgel sinks down on a transport device placed in the form removal bath between the wetgel and mould after the wetgel has floated up into the solvent. The solvent in this case is preferably the same solvent used in the remaining part of the process.

The transport device described above may be of different type. There are various examples that are possible. A grill has been tested and this may act under this procedure adequate for certain application types. On the other hand, there may occur small marks in the gel where the grid holds this. For other applications, such as during the manufacture of transparent panels for window applications, that is not fully acceptable and then other types of transport devices should be used, such as a supporting plane, e.g. a membrane, which is gas permeable. There are also other techniques that could be possible to use according to the present invention. This is e.g. transportation of the gel on an air bed which keeps the gel floating. Also in the autoclave, technology may be used to ensure that the gel does not need to rest against a transport device, such as a grill or a supporting plane. This could for example be to direct a gas inflow of suitable gas, e.g. an inert gas, with sufficient force to keep the gel floating. Furthermore, devices having elements which in relation to each other varying hold up the gel are also possible.

These technology solutions brings about the risk of deformation of the gel to be reduced or eliminated, and also gives assurance that carbon dioxide in the autoclave is in contact with all parts and sides of the gel.

Thus, at the form removal step it is often important that the wetgel is not changed in terms of structure and shape, and therefore the transport device or transport technology should be chosen carefully. In this regard, also the remaining part of the process may have a significant importance. By having a vertical layout for subsequent steps, i.e. when panels are produced, such as during transport and delivery to the autoclave, instead of a horizontal layout, the problem of deformation of panels may further be decrease.

The evaporation of alcohol carried out in the autoclave, which autoclave is filled with the alcohol in question, can either be done by using supercritical carbon dioxide or liquid carbon dioxide. An example of how this may be accomplished is with supercritical carbon dioxide at for example from 90 bar to 150 bar and at from 45⁰C to 8O⁰C or with liquid carbon dioxide at for example 50-70 bar and temperature of 10-20⁰C. This step is much easier and much safer than the alcohol evaporation step as described in SE 422 045. According to another specific embodiment of the present invention, the process for the preparation of a silica aerogel body, which is porous and comprises nanoparticles, comprises the following steps:

- direct supply of the mould to an autoclave to which autoclave either supercritical carbon dioxide or liquid carbon dioxide is added successively, to achieve extraction where removal of alcohol from the wetgel in the autoclave is carried out, for the commencement of the formation of a silica aerogel; - emptying of the autoclave and further processing of the silica aerogel in a heating process at 200⁰C or above for further removal of alcohol and formation of a substantially alcohol-free silica aerogel material;

- discharge of the formed substantially alcohol-free silica aerogel material from the heating process and optionally further processing of the formed substantially alcohol-free silica aerogel material for shaping into desired shape and size, where supply of further recipe components optionally is made at either the mixing in the mixer, in the extraction or in the heating process. The above specific embodiment renders the fact that the step with form removal of the wetgel does not have to be carried out before the extraction step in the autoclave. This of course does present new demands on the mould when this will be sustained in the autoclave. Also in this case it is possible that a transport device is inserted into the bottom of the mould, such as e.g. a transport device in the-form of a supporting plane, e.g. a membrane, which is gas permeable, to find use in a later step during and possibly after extraction in the autoclave. Form removal of the mould is therefore made in this case after extraction or after the heating process. As described above, for the various specific embodiments, the subsequent heating process is performed at a temperature above 200⁰C. The temperature chosen depends largely on the desired final material for the silica aerogel body. According to one specific embodiment of the present invention, the heating process is performed at 250°C-350°C, which is adequate for those solvents intended according to the present invention. This implies that the heating process according to the present invention is significantly less energy consuming than that described in SE 422 045, where a temperature range of between 500⁰C and 75O⁰C is current, i.e. to achieve a similar effect. As is apparent from the description above, the produced material according to the present invention is heat-resistant and sustains at least 35O⁰C, e.g. at least 400⁰C. The material according to the present invention has according to one specific embodiment e.g. a heat-resistance of 600⁰C. Also this feature is something that distinguishes the material according to the present invention substantially from the materials according to WO2007011988 A2 and WO2007146945 A2.

There are also other possible solvents which can be used generally to produce a silica aerogel material. This is e.g. ethylacetoacetate. This is, however, not something that is desirable because the heating process then must be effected at above 400⁰C for the silica aerogel material to become clear again, after at a couple of hundred degrees being brownish due to use of the solvent. In addition, this known process implies HF to be used as a catalyst in the formation of the solgel, which is also not desirable. However, there are applications according to the present processes when an elevated temperature is desirable in the heating process. This may for example be to achieve shrinkage and glass transition of the material, resulting in a form changing of the material and which may provide an in- creased mechanical stability. According to one specific embodiment of the present invention, the heating process is therefore performed at at least 500⁰C in order to achieve shrinkage of the essentially alcohol-free silica aerogel material. Shrinkage increases with increasing temperature, and if a more powerful shrinkage of the material is desired, the heating process may thus be performed at e.g. at least 700⁰C.

Sealing the pores on the surface can also be conducted on the material according to the present invention. This could with another word be called surface glazing or shrinkage of only the surface. This can be performed by different types of technologies, such as by "sputtering", which is also known as sputtering or cathode flocking in Swedish, of the surface or by overheating of the surface.

Furthermore, what is listed above for definitive removal of alcohol in connection with the first specific process embodiment is also valid for the other specific embodiments according to the present invention. In the case of catalysts which may be used in accordance with the present invention, this process may be run basic as well as acidic. In the acidic case, possible alternatives are HF, HCI and H₂SO₄, but where HF is the least desirable. However, it is desirable to use basic catalysts, such as ammonia or titanium lactate. An example of a commercially available titanium lactate catalyst which can be used in the present processes is furthermore Tyzor^®. Different types of mixers can be used in the present processes, e.g. a Sulzer^® mixer.

According to the present invention it is possible to produce silica aerogel bodies, such as for example panels or halfpipe-shaped bodies. According to one specific embodiment of the present processes, the formed silica aerogel body therefore is panel or halfpipe-shaped. According to another embodiment of the present processes, the formed silica aerogel body is encased finally in an encasing material. This can e.g. be done for panels, but also other shapes.

According to one embodiment of the present invention, the recipe start components include at least one silane compound which is tetramethyl ortho- silicate (TMOS), also known as tetramethoxysilane, or a mixture of tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS), the latter also known as tetraethoxysilane. As mentioned above, supply of further recipe components are optionally performed at either the mixing in the mixer, in the extraction or in the heating process. Examples of such possible further recipe components are carbon fibers, rock wool, glass wool, mineral wool, glass fibers, wavelength shifters, iron, titanium or tin compounds, boron compounds and silane compounds, or a mixture thereof. Several of those are described above, but with respect to various possible metal compounds, such as various metal salts, of which only a number has been specified above, these are admixed in order to achieve colouring of the silica aerogel material according to the present invention. In addition to these, there are also other possible admixing components or recipe components. Examples of such are for example dendrimers. Other examples include ethylene glycol or boron oxide, which could be admixed to break some bonds in the silica aerogel material. This could e.g. be done to increase the elasticity of the material according to the present invention and as an example such components could be admixed already during the mixing stage of the procedure.

Furthermore, one specific embodiment is a panel or a halfpipe-shaped body which is produced by a process according to the present invention. Two other possible specific embodiments is an energy window comprising a transparent or translucent panel according to the present invention and an insulating material comprising an opaque panel according to the present invention. Conclusions

The present invention describes a body of a silica aerogel material which has excellent properties, such as a very low coefficient of thermal conductivity, i.e. is highly insulating. Furthermore, various specific embodiments are described where the body, such as e.g. a panel, according to the present invention has been given various advantageous properties, such as high transparency, increased mechanical stability and improved handling and transportation option, the latter being made by the fact that the panel has been encased in an encasing material.

Furthermore, the present invention describes processes for the produc- tion of a shaped silica aerogel material, such as e.g. a panel, which are both relatively energy efficient and safe in comparison with known technologies.

Claims

1. Body of a silica aerogel material which is porous and comprises nanoparticles, wherein the body

- has a density in the range of 60-300 kg/m³;

- has a thermal conductivity λ of less than 0.020 W / (m ^* K) when measured at a density of 150 kg/m³ and temperature of about 2O⁰C;

- preferably is crack-proof; and

- is transparent and has a total transparency which is at least 90%; and

- has a refractive index in the range of 1.017-1.060, when measured at a thickness of 14 mm and non-encased body.

2. Body according to claim 1 , wherein a wavelength shifter is admixed in the silica aerogel material.

3. Body according to claim 1 or 2, wherein the body is evacuated and is super-insulating and has a thermal conductivity λ which is 0.007 W / (m ^* K) or less, when measured at a density of 150 kg/m³ and temperature of about 2O⁰C.

4. Body according to anyone of claims 1-3, wherein the silica aerogel material is water-resistant by a silane compound being bound into the silica aerogel material.

5. Body according to anyone of claims 1-4, wherein an encasing material surrounds the body.

6. Body according to claim 5, wherein the encasing material is chosen from the group consisting of transparent materials, plastic foils, glass panels and plastic sheets.

7. Body according to anyone of the preceding claims, wherein the body is a panel.

8. Body according to anyone of the preceding claims, wherein the body is a panel with a thickness of up to 5 cm.

9. Body according to anyone of the preceding claims, wherein the body is a panel which has a size of up to 1.5 m * 2 m (width ^* length).

10. Body according to anyone of claims 1-6, wherein the body is a halfpipe- shaped body.

11. Energy window comprising a transparent panel according to anyone of claims 1-9.