WO1998007780A1

WO1998007780A1 - Transparent composite product containing aerogel and plastic, and method for producing same

Info

Publication number: WO1998007780A1
Application number: PCT/EP1997/004361
Authority: WO
Inventors: Fritz Schwertfeger; Marc Schmidt
Original assignee: Cabot Corporation
Priority date: 1996-08-23
Filing date: 1997-08-11
Publication date: 1998-02-26
Also published as: DE19634109C2; EP0920473A1; DE19634109A1

Abstract

A transparent composite product containing 5 to 97 vol.- % transparent and translucid aerogel particles and at least one transparent or translucid plastic material, the production method and the product application are disclosed.

Description

description

Airgel- and plastic-containing, transparent composite material, process for its production and its use.

The invention relates to novel composite materials of any shape with high thermal insulation capacity, which contain 5 to 97 vol .-% transparent or translucent airgel particles and at least one transparent or translucent plastic.

Conventional insulation materials based on polystyrene, polyolefins and

Polyurethanes are made using blowing agents such as CFCs or CO ₂ . The blowing agent enclosed in the cells of the foam is responsible for the high thermal insulation capacity. However, such blowing agents pollute the environment because they slowly escape into the atmosphere. In addition, such foamed insulation materials are opaque.

Aerogels, especially those with porosities above 60% and densities below 0.6 g / cm ³ , are transparent, translucent or opaque depending on the manufacturing process and have an extremely low thermal conductivity. You will therefore find application as a heat insulation material such. B. is described in EP-A-0 171 722.

Aerogels in the broader sense, ie in the sense of "gel with air as a dispersing agent", are produced by drying a suitable gel. The term "airgel" in this sense includes aerogels in the narrower sense, xerogels and cryogels. A dried gel is referred to as an airgel in the narrower sense if the liquid of the gel is largely removed at temperatures above the critical temperature and starting from pressures above the critical pressure. If, on the other hand, the liquid of the gel is removed subcritically, for example with the formation of a liquid-vapor boundary phase, the resulting gel is often referred to as a xerogel. The use of the term aerogels in the present application is aerogels in the broader sense, ie in the sense of "gel with air as a dispersing agent"

In addition, aerogels can in principle also be divided into inorganic and organic aerogels, inorganic aerogels having been known since 1931 (SS Kistler, Nature 1931, 127, 741), and organic aerogels from a wide variety of starting materials. e.g. from melamine formaldehyde, have only been known for a few years (RW Pekala, J Mater

The various processes for the production of aerogels by supercritical or subcritical drying are described, for example, in EP-A-0 396 076, WO 92/03378, WO 94/25149, WO 92/20623 and EP-A-0658 513

The aerogels obtained by supercritical drying are generally hydrophilic or only hydrophobic for a short time, whereas subcritically dried aerogels are permanently hydrophobic due to their manufacturing process (soiling before drying)

Also known are opaque, airgel-containing composite materials which, because of their low heat conduction, are used as thermal insulation materials. Such composite materials are described, for example, in EP-A-0 340 707, EP-A-0667 370, WO 96/12683, WO 96/15997 and WO 96/15998

The commonality of these composite materials is that, in part, they contain transparent airgel granules, but the additional components themselves are opaque. As a result, the systems as a whole are opaque and cannot be used for transparent or translucent applications

Transparent airgel and xerogel composites are disclosed in DE-A-44 30 642 and DE-A-44 30669 These composites are in the form of a mat containing an airgel or xerogel and fibers distributed therein, the airgel or xerogel fragments being held together by the fibers.

Although good results have been achieved with such materials, there remain disadvantages of a price nature and the need to manufacture these systems in one step.

A transparent component with at least two panes of transparent material arranged in parallel, in the space between which there are fiber-reinforced airgel plates or mats, e.g. disclosed in DE-A-195 07 732. This measure significantly increases the stability of the system, but the complicated structure and the high price remain.

Alternatives to the airgel-containing composites are described in CA-C-1 288 313, EP-A-018 955 and DE-A 41 06 192. According to these writings, airgel monoliths are placed between glass panes in order to

To improve insulation effect by the low thermal conductivity of the airgel and / or to improve the sound attenuation of such panes. The airgel monoliths should partially be firmly connected to the glass plates. In this way, almost crystal-clear, transparent panes can be achieved, but because of the low mechanical stability of the aerogels, the effort for producing correspondingly large monoliths is on the one hand too high, and on the other hand the handling during the manufacture of the panes is too complicated to accommodate such glass panes to be able to use on a larger scale.

It is also known to fill the intermediate space with airgel granules that are easier to produce and also easier to handle. However, the panes are no longer crystal clear, but rather translucent, comparable to frosted glass panes or structured glass panes. This is not a significant limitation, since there are a number of applications, for example as skylights, in which drop shadows should be avoided by direct light, such as windows in factory, warehouse, trade fair or museum halls. However, such windows also have some disadvantages. So the granules can compress over time, which leads to a change in the fill level in the window, which is undesirable.The compression can take place, for example, as a result of persistent small vibrations or through the destruction of airgel spheres, particularly in the case of evacuation between glass pane and

Granule balls are only transferred over very small contact surfaces. This results in relatively high compressive and / or shear stresses, which can lead to the destruction of airgel particles

It was therefore proposed in DE-A-35 33 805 to bring the airgel granulate between two foils between the glass panes and to evacuate the space between the foils. The flexibility of the foil allows the surface between the foil and the surface of the spheres to be enlarged, What led to an improvement in the long-term stability is disadvantageous, however, that the structure of the panes is very complicated. In addition, the additional film layers reduce the transparency

Vacuum panel systems filled with airgel are also known, as disclosed, for example, in EP-A-0468 124, EP-A-0 114 687 and DE-A-33 47 619

The disadvantage of vacuum panel systems, however, is that they can no longer be changed in shape or size at the point of use

It was therefore an object of the present invention to develop a transparent, heat-insulating composite material which can be produced simply and in any shape and size and whose size can still be changed at the point of use

This task is solved by a plastic-containing, transparent composite material which contains 5 to 97% by volume of transparent or translucent airgel particles and at least one transparent or translucent plastic The plastic forms a matrix that connects or encloses the airgel particles and runs as a continuous phase through the entire composite material.

If the content of airgel particles was significantly below 5% by volume in the composition, the positive properties of the airgel particles in the composition would be largely lost due to the low proportion. Such a composition would no longer have low densities and thermal conductivities.

A content of airgel particles that is significantly above 97% by volume would lead to a content of plastic of less than 3% by volume. In this case, its proportion would be too low to ensure sufficient connection of the airgel particles to one another and mechanical pressure and bending strength.

The proportion of airgel particles is preferably in the range from 10 to 97% by volume and particularly preferably in the range from 40 to 95% by volume.

A particularly high proportion of airgel particles in the composite material can be achieved by using a bimodal distribution of the grain sizes.

Another possibility of achieving a particularly high proportion of airgel particles in the composite material is the use of airgel particles which have a logarithmic distribution of the grain size.

In order to achieve the highest possible degree of filling, it is also advantageous if the airgel particles are small in relation to the total thickness of the molded part.

However, the airgel particles should be large enough so that the transparency of the composite material is as high as possible. The number of scattering centers through interfaces of the airgel particles can thereby be kept as low as possible.

The size of the airgel particles is therefore preferably in the range from 250 μm to 10 mm. In addition, it is important that the airgel granulate itself has the highest possible transparency. On the one hand, this is due to the structure of the airgel being as homogeneous as possible, ie above all the narrowest possible pore and particle radius distribution, and on the other hand by the surface of the airgel being as smooth as possible -Particles (no cracks, depressions, microfuses or dents) reached

Generally used aerogels for the compositions according to the invention are those based on metal oxides which are suitable for sol-gel technology (CJ Bnnker, GW Scherer, Sol-Gel-Science, 1990, chapters 2 and 3), such as Si or Al compounds, or those based on organic substances which are suitable for the sol-gel technique, such as melamine formaldehyde condensates (US Pat. No. 5,086,085) or resorformaldehyde condensates (US Pat. No. 4,873,218). The compositions can also based on mixtures of the above materials. Aerogels containing Si compounds and in particular SiO ₂ aerogels are preferably used

To reduce the radiation level of the thermal conductivity, the airgel can contain IR opacifiers, such as titanium dioxide or zirconium dioxide, as well as mixtures thereof. Care must be taken that these IR opacifiers do not adversely affect the transparency in the visible area of the airgel particles

In a preferred embodiment, the airgel particles have permanently hydrophobic surface groups. Suitable groups for permanent hydrophobization are, for example, silyl groups of the general formula

-Sι (R) _n , where n = 1, 2 or 3, preferably tπsubstituted silyl groups, the radicals R in general, independently of one another, the same or different, each being a hydrogen atom or a non-reactive, organic linear, branched, cyclic, aromatic or heteroaromatic The rest, preferably C ₁ -C ₁₈ alkyl or C ₆ -C ₁ aryl, particularly preferably CC ₆ alkyl, cyclohexyl or phenyl, especially methyl or ethyl, are particularly advantageous for the permanent hydrophobization of the airgel is the use of trimethylsilyl groups These groups can be introduced as described in WO 94/25149, or by gas phase reaction between the airgel and, for example, an activated trialkylsilane derivative, such as, for example, a chlorotrialkylsilane or a hexaalkyldisilazane (compare R. Her, The Chemistry of Silica, Wiley & Sons, 1979). Compared with OH groups, the hydrophobic surface groups produced in this way further reduce the dielectric loss factor and the dielectric constant. Airgel particles with hydrophilic surface groups can adsorb water depending on the air humidity, which means that the dielectric constant and the dielectric loss factor can vary with the air humidity. This is often not desirable for electronic applications. The

The use of airgel particles with hydrophobic surface groups prevents this variation since no water is adsorbed. The selection of the residues also depends on the typical application temperature.

If airgel particles with hydrophobic surface groups are used in conjunction with hydrophobic plastics, a hydrophobic composite material is obtained.

In addition, the thermal conductivity of the aerogels decreases with increasing porosity and decreasing density. Aerogels with porosities above 60% and densities below 0.6 g / cm ³ are therefore preferred. Aerogels with densities below 0.2 g / cm ³ are particularly preferred.

Basically, all known plastics are suitable for producing the composite materials according to the invention if they are transparent or translucent after production in the visible light wavelength range.

It is not critical whether the plastic is amorphous, semi-crystalline and / or crystalline.

The plastic is used either in liquid form, ie as a liquid, melt, solution, dispersion or suspension, or else as a solid powder. Suitable transparent or translucent plastics are, for example, polymethyl methacrylates (PMMA, for example Degalan®), cycloolefin copolymers (COC, for example Topas®), polyvinyl butyrals (for example Mowital®), polycarbonates and polyethylene terephthalates (PET, for example Hostaglas®), with polyvinyl butyrals, polycarbonates and polymethyl methacrylate are preferred.

The plastic is generally used in an amount of 3 to 95% by volume of the composite material, preferably in an amount of 3 to 90% by volume and particularly preferably in an amount of 5 to 60% by volume. The choice of plastic is made in each case according to the desired mechanical and thermal properties of the composite material. Mixtures of different plastics can also be used

When selecting plastics, preference is also given to selecting those products which essentially do not penetrate into the interior of the porous airgel particles. The penetration of the plastic into the interior of the airgel particles can, in addition to the selection of the plastic material, also be influenced by various parameters, such as pressure, temperature or processing time.

In addition, one preferably selects those plastics which have a low thermal conductivity.

In order to avoid strong light scattering and the associated reduced transmission, the gusset between the airgel particles should preferably be filled as completely as possible with transparent plastic

To reduce the radiation contribution of the thermal conductivity, the composite material can contain IR opacifiers, such as titanium dioxide or zirconium dioxide, as well as mixtures thereof, as long as the transparency is not significantly impaired, which is particularly advantageous for applications at higher temperatures and / or in evacuated systems. In addition, the composite material can also contain fillers, for example for coloring or for achieving special decorative effects.

Furthermore, the composite material can also up to 85 vol .-% of transparent fillers such. B. film snippets and / or fibers, for example to improve the mechanical properties. The proportion of the fillers, based on the composite material, is preferably below 70% and particularly preferably in the range from 0 to 50% by volume.

If the composite material is hydrophilic due to the plastic used and / or due to hydrophilic airgel particles, a subsequent treatment can optionally be carried out which gives the composite material hydrophobic properties. All substances known to the person skilled in the art for this purpose are suitable for this purpose, which give the composite material a hydrophobic surface, such as, for. B. paints, films, silylating agents, silicone resins, inorganic and / or organic binders.

Furthermore, so-called "coupling agents" can also be used for bonding. They bring about better contact of the plastics with the surface of the airgel particles and can also form a firm bond with both the airgel particles and the plastic.

The molded articles produced according to the invention from airgel granules preferably have a density of less than 0.6 g / cm ³ and preferably a thermal conductivity of less than 100 mW / mK. The thermal conductivity is particularly preferably below 50 mW / mK.

The fire class of the composite material obtained after drying is determined by the fire class of the airgel and the plastic. In order to obtain the most favorable fire class of the composite material (flame-retardant or non-flammable), the composite materials can also be laminated with suitable materials, such as. B. window panes or silicone resin adhesives. Furthermore, the use of fire protection agents known to the person skilled in the art is possible. In addition, all coatings known to the person skilled in the art are also possible which, for. B. dirt-repellent, IR semi-transparent and / or hydrophobic.

Another object of the present invention was to develop a manufacturing method for the composite materials described above.

To produce the composite materials according to the invention, the airgel particles are connected to one another by means of at least one plastic. The individual particles can be connected to one another in a quasi-punctiform manner. Such a surface coating can be achieved, for example, by spraying the airgel particles with the plastic. The coated particles are then filled into a mold, for example, and cured in the mold.

Another object of the invention is therefore a method for producing a composite material containing 5 to 97 vol .-% transparent or translucent airgel particles and at least one transparent or translucent plastic, which is characterized in that airgel and plastic are mixed into the brings the desired shape and hardens.

In a preferred embodiment, the gusset volume between the individual particles is also completely or partially filled by the plastic. If a particularly transparent version is required, make sure that there are as few gas bubbles as possible in the gusset volume. On the other hand, the degree of transparency can be determined by specifically including gas bubbles in the

Control gusset volume. Such a composition can be produced, for example, by mixing the airgel particles with the plastic granulate.

The mixing can be carried out in any conceivable way. So on the one hand it is possible to simultaneously insert the at least two components into the

Introduce mixing device, but on the other hand, one of the components can also be submitted and the other (s) can then be added. The mixing device necessary for the mixing is in no way limited. Any mixing device known to those skilled in the art for this purpose can be used.

The mixing process is carried out until there is an approximately uniform distribution of the airgel particles in the composition. The mixing process can be regulated both over the duration and, for example, over the speed of the mixing process.

This is followed by the shaping and curing of the mixture in the mold, which, depending on the type of plastic, is carried out by heating and / or evaporating the solvent and / or dispersing agent used or, if melts are used, by cooling below the melting temperature of the plastic.

In a preferred embodiment, the mixture is pressed. It is possible for the person skilled in the art to select the suitable press and the suitable pressing tool for the respective application. The use of vacuum presses is advantageous due to the high air content of the airgel-containing molding compounds. In a preferred embodiment, the airgel-containing molding compounds are pressed into sheets. In order to avoid caking of the molding compound on the pressing tool, for example a pressing die, the airgel-containing mixture to be compressed can be separated off against the pressing tool using release paper or release film. The mechanical strength of the airgel-containing plates can be improved by laminating screen fabrics, foils or glass panes onto the plate surface. The screen fabrics, foils or glass panes can be applied to the airgel-containing plates both subsequently and during the production of the composite material. The latter is preferred and can preferably be carried out in one working step by inserting the screen cloth, foils or glass panes into the mold and placing them on the airgel-containing molding compound to be compressed and then pressing them under pressure and temperature to form an airgel-containing composite panel. Depending on the plastic used, the pressing generally takes place at pressures of 1 to 1000 bar and temperatures of 0 to 300 ° C. in any shape.

In the case of composite materials which contain a particularly high volume fraction of airgel particles and whose thermal conductivity is correspondingly low, heat can additionally be brought into the plates with the aid of suitable radiation sources. If, as in the case of polyvinyl butyrals, the plastic used couples with microwaves, this radiation source is preferred.

After curing, the composite materials according to the invention are suitable, for example, because of their low thermal conductivity

Thermal insulation materials.

Due to their transparency or translucency, they are also suitable e.g. as a window in factory, warehouse, exhibition or museum halls. Another one

Area of application are skylights and so-called daylighting systems.

Due to the simple production, it is also possible to produce slightly curved surfaces or other more complicated geometries. They are therefore particularly suitable for the production of skylights.

The invention is described in more detail below on the basis of exemplary embodiments, without being restricted thereby.

The hydrophobic aerogels were produced analogously to the process disclosed in DE-A-4342 548.

The thermal conductivities of the airgel granules were measured using a heating wire method (see, for example, 0.Nielsen, G. Rüschenpöhler, J. Groß, J. Fricke, High Temperatures - High Pressures, Vol. 21, 267-274 (1989)).

The thermal conductivities of the moldings were measured in accordance with DIN 52612.

The transparency of the moldings was determined using an apparatus consisting of a Voltage measuring device (Keithley multimeter), a pyranometer (Lambrecht company, type CM3, sensitivity +/- 0.5%, 16.30 ^* 10 ^"6 V / Wm ² ), an inside matt white painted aluminum cube and a halogen lamp (500 W) The shaped bodies are placed directly in front of the round opening (0 80 mm) of the aluminum cube painted matt white on the inside. The halogen lamp set up in 400 mm serves as the light source. The pyranometer placed in the aluminum cube behind a panel converts the detected light into a voltage The transmission in percent of l _{0 was} determined by measuring the voltage without (l ₀ ) and with the respective insulation board (l _D ).

Example 1a

Shaped body made of 50 vol .-% airgel and 50 vol .-% polyvinyl butyral

50% by volume of hydrophobic airgel granules (solid density 130 kg / m ³ ) and 50% by volume of a polyvinyl butyral powder (solid density 1100 kg / m ³ ) are mixed intimately. The percentage volume relates to the target volume of the shaped body. The hydrophobic airgel granulate has a grain size greater than 650 μm, a BET surface area of 640 m ² / g and a thermal conductivity of 11 mW / mK. ®Mowital (Polymer F) (Hoechst AG, Frankfurt, Germany) with a grain size of around 50 μm is used as the polyvinyl butyral powder.

The bottom of the mold with a base area of 30 cm x 30 cm is lined with release paper. The airgel-containing molding compound is then evenly distributed and the whole thing is covered with a release paper. It is pressed at 220 ° C for 30 minutes to a thickness of 18 mm.

The molded body obtained has a density of 481 kg / m ³ and a thermal conductivity of 57 mW / mK. The transmission is 28%.

Example 1b

Shaped body made of 50 vol .-% airgel and 50 vol .-% polyvinyl butyral 50% by volume of hydrophobic airgel granules (solid density 130 kg / m ³ ) and 50% by volume of a polyvinyl butyral powder (solid density 1100 kg / m ³ ) are mixed intimately. The percentage volume relates to the target volume of the shaped body. The hydrophobic airgel granulate has a grain size of less than 650 μm, a BET surface area of 640 m ² / g and a thermal conductivity of 11 mW / mK. ®Mowital (Polymer F) (Hoechst AG, Frankfurt, Germany) with a grain size of around 50 μm is used as the polyvinyl butyral powder.

The molded body obtained has a density of 510 kg / m ³ and a thermal conductivity of 59 mW / mK. The transmission is 20%.

Example 2

Shaped body made of 65 vol .-% airgel and 35 vol .-% polyvinyl butyral

65% by volume of hydrophobic airgel granules (solid density 130 kg / m ³ ) and 35% by volume of a polyvinyl butyral powder (solid density 1100 kg / m ³ ) are mixed intimately. The percentage volume relates to the target volume of the shaped body. The hydrophobic airgel granulate has a grain size greater than 650 μm, a BET surface area of 640 m ² / g and a thermal conductivity of 11 mW / mK. ®Mowital (Polymer F) (Hoechst AG, Frankfurt, Germany) with a grain size of around 50 μm is used as the polyvinyl butyral powder.

The bottom of the mold with a base area of 30 cm x 30 cm is lined with release paper. The airgel-containing molding compound is then evenly distributed and the whole thing is covered with a release paper. It is pressed at 220 ° C for 30 minutes to a thickness of 18 mm. The molded body obtained has a density of 539 kg / m ³ and a thermal conductivity of 32 mW / mK. The transmission is 31%.

Example 3a

Shaped body made of 50 vol.% Airgel and 50 vol.% Polymethyl methacrylate (PMMA)

50% by volume of hydrophobic airgel granules (solid density 130 kg / m ³ ) and 50% by volume of a polymethyl methacrylate (solid density 1200 kg / m ³ ) are mixed intimately. The percentage volume relates to the target volume of the shaped body. The hydrophobic airgel granulate has a grain size greater than 650 μm, a BET surface area of 640 m ² / g and a thermal conductivity of 11 mW / mK. ®Degalan (PMMA) (DEGUSSA, Frankfurt, Germany) with a grain size of around 50 μm is used as the polymethyl methacrylate.

The molded body obtained has a density of 615 kg / m ³ and a thermal conductivity of 59 mW / mK. The transmission is 25%.

Example 3b

Shaped body made of 50 vol.% Airgel and 50 vol.% Polymethyl methacrylate (PMMA)

50% by volume of hydrophobic airgel granules (solid density 130 kg / m ³ ) and 50% by volume of a polymethyl methacrylate (solid density 1200 kg / m ³ ) are mixed intimately. The percentage volume relates to the target volume of the shaped body. The hydrophobic airgel granulate has a grain size of less than 500 μm, a BET surface area of 640 m ² / g and a thermal conductivity of 11 mW / mK. As polymethyl methacrylate ®Degalan (PMMA) (DEGUSSA, Frankfurt, Germany) with a grain size of 50 μm.

The bottom of the mold with a base area of 30 cm x 30 cm is lined with release paper. The airgel-containing molding compound is then distributed evenly and the whole thing is covered with a release paper. It is pressed at 220 ° C for 30 minutes to a thickness of 18 mm.

The molded body obtained has a density of 628 kg / m ³ and a thermal conductivity of 64 mW / mK. The transmission is 17%.

Example 4

Shaped body made of 50 vol .-% airgel and 50 vol .-% cycloolefin copolymer (COC)

50% by volume of hydrophobic airgel granules (solid density 130 kg / m ³ ) and 50% by volume of a cycloolefin copolymer (solid density 1200 kg / m ³ ) are mixed intimately. The percentage volume relates to the target volume of the shaped body. The hydrophobic airgel granulate has a grain size greater than 650 μm, a BET surface area of 640 m ² / g and a thermal conductivity of 11 mW / mK. ^® Topas (COC) (Hoechst AG, Frankfurt, Germany) with a grain size of around 250 μm is used as the cycloolefin copolymer.

The molded body obtained has a density of 585 kg / m ³ and a thermal conductivity of 62 mW / mK. The transmission is 25.6%.

Claims

Claims:

1. Composite material which contains 5 to 97% by volume of transparent or translucent airgel particles and at least one transparent or translucent plastic.

2. Composite material according to claim 1, characterized in that SiO ₂ airgel particles are used as airgel particles.

3. Composite material according to claim 1 or 2, characterized in that the airgel particles have hydrophobic surface groups.

4. Composite material according to at least one of claims 1 to 3, characterized in that the airgel particles have porosities above 60% and densities below 0.6 g / cm ³ .

5. Composite material according to at least one of claims 1 to 4, characterized in that a plastic, a polymethyl methacrylate, a cycloolefin copolymer, a polyvinyl butyral, a polycarbonate and / or a polyethylene terephthalate is used.

6. Composite material according to claim 5, characterized in that a polyvinyl butyral, a polycarbonate or a polymethyl methacrylate is used as the transparent plastic.

7. Composite material according to at least one of claims 1 to 6, characterized in that the airgel particles and / or the composite material contain IR opacifiers.

8. Composite material according to at least one of claims 1 to 7, characterized in that the composite material contains fillers and / or fibers.

9. Composite material according to at least one of claims 1 to 8, characterized in that the thermal conductivity is less than 100 mW / mK.

10. Composite material according to at least one of claims 1 to 9, characterized in that the composite material is coated on at least one side.

11. Use of a composite material according to at least one of claims 1 to 10 for thermal insulation.

12. A method for producing a composite material according to at least one of claims 1 to 10, characterized in that airgel and plastic are mixed, brought into the desired shape and cured.

13. The method according to claim 12, characterized in that shaping and curing are carried out by pressing at a pressure of 1 to 1000 bar and a temperature of 0 to 300 ° C.

14. The method according to claim 13, characterized in that the pressing is carried out by means of vacuum pressing.