DE19533564A1 - Fibrous airgel composite material - Google Patents

Abstract

The invention relates to a composite material containing 5 to 97 % vol. aerogel particles, at least one binder and at least one fibrous material, in which the diameter of the aerogel particle >/= 0.5 mm, a process for its production and its use.

Description

The invention relates to a composite material which contains 5 to 97% by volume of airgel particles, contains at least one binder and at least one fiber material Process for its production and its use.

Aerogels, especially those with porosities over 60% and densities below 0.4 g / cm³, due to their very low density, high porosity and small pore diameter an extremely low thermal conductivity and are therefore used as heat insulation materials such. B. in the EP-B-0 171 722.

The high porosity also leads to low mechanical stability both the gel from which the airgel is dried and the dried airgel itself.

Aerogels in the wider sense, i.e. H. in the sense of "gel with air as Dispersants "are prepared by drying a suitable gel. The term "airgel" in this sense includes aerogels in the narrower sense, Xerogels and cryogels. A dried gel is used as an airgel in the narrower range Designated when the liquid of the gel at temperatures above the critical temperature and starting from pressures above the critical Pressure is removed. However, if the liquid of the gel becomes subcritical, for example, with the formation of a liquid-vapor boundary phase, the resulting gel is called a xerogel.

When using the term aerogels in the present application are aerogels in the broader sense, d. H. in the sense of "gel with air as a dispersant ".  

The molding process of the airgel takes place during the sol-gel transition completed. After formation of the solid gel structure, the outer shape can can only be changed by crushing, for example grinding, for one other form of stress, the material is too fragile.

However, for many applications it is necessary to shape the aerogels to use certain shaped bodies. In principle, the production of Shaped bodies already possible during the gel production. However, the diffusion-determined, typically necessary during production Exchange of solvents (for aerogels: see e.g. US-A-4,610,863, EP-A 0 296 076, regarding airgel composite material: see. e.g. B. WO 93/06044) and also diffusion-determined drying to uneconomically long Lead production times. It is therefore advisable to use the airgel Manufacturing, i.e. after drying, to carry out a shaping step, without a significant change in the internal structure of the airgel in the With regard to the application taking place.

For many applications, an insulating material is used in addition to a good one Thermal insulation also requires good sound insulation. A good Sound absorption typically occurs with porous materials, their porosity lies on a macroscopic scale (<0.1 µm), since then the Speed waves of sound caused by the air rubbing against the Pore walls are dampened. Monolithic materials without Macroscopic porosity therefore has only low sound absorption. Is a material only porous on a microscopic scale, such as B. monolithic Aerogels, so the air cannot flow through the pores, but the Sound waves are transmitted to the framework of the material, which they have no strong Damping passes on.

In DE-A 33 46 180, rigid plates made of pressed bodies are based of silica airgel obtained from flame pyrolysis in connection with a reinforcement described by mineral long fibers. In this out  however, the silica airgel obtained by flame pyrolysis is not an airgel in the above sense, since it is not by drying a gel is produced and thus has a completely different pore structure; thats why it is mechanically more stable and can therefore be destroyed without destroying the microstructure be pressed, but has a higher thermal conductivity than typical Aerogels in the above sense. The surface of such compacts is very large sensitive and must therefore, for example, by using a binder on the Surface hardened or protected by lamination with a film.

In EP-B-0 340 707 an insulating material with a density of 0.1 to 0.4 g / cm³ consisting of at least 50 vol .-% silica airgel particles with a Diameter between 0.5 and 5 mm, using at least one organic and / or inorganic binders are described. Are the airgel particles only on the contact surfaces above the binder connected, the resulting insulation material is not mechanically very stable because in the case of mechanical stress, the part of the Airgel particle tears off, the particle is then no longer bound and the Insulation cracks. Therefore, all gussets between the Airgel particles must be filled with the binder. At very low The resulting material is more stable than pure aerogels, however, cracks can easily occur if not all of Binder's granules are sufficiently enclosed.

With a high volume fraction favorable for a low thermal conductivity of airgel only small volumes of binder remain in the Gusset areas, which is particularly the case with porous binders such as B. foaming with low thermal conductivity results in low mechanical stability. Filling all gusset areas with binder continues through reduced macroscopic porosity (between the particles) to a strong reduced sound absorption in the material.  

In EP-A-489 319 a composite foam with lower Thermal conductivity disclosed, the 20 to 80 vol .-% silica airgel particles, 20 to 80 vol .-% of a and enveloping the airgel particles connecting styrene polymer foam of density 0.01 to 0.15 g / cm³ and optionally contains conventional additives in effective amounts. The way The composite foam produced is pressure-resistant, but at high Concentrations of airgel particles are not very resistant to bending.

In the as yet unpublished German patent applications P 44 30 669.5 or P 44 18 843.9 are panels or mats made of a fiber-reinforced Airgel described. These plates or mats have a very high airgel content a very low thermal conductivity, but are relative to their manufacture due to the diffusion problems described above long production times necessary.

In the as yet unpublished German patent application P 44 45 771.5 discloses a nonwoven airgel composite material that has at least one Layer of nonwoven fabric and airgel particles, which is characterized in that that the nonwoven contains at least one bicomponent nonwoven, the Fibers with each other and with the airgel particles through the low melting jacket material are connected. This composite material has a relatively low thermal conductivity as well as a high macroscopic Porosity and associated good sound insulation, however through the use of bicomponent fibers the temperature range in which the material can be used, as well as the fire protection class limited. Next are the corresponding composites, in particular complicated moldings, not easy to manufacture.

One of the objects of the present invention was therefore to provide a composite material based on airgel granules that provide a low Has thermal conductivity, is mechanically stable and easy to manufacture leaves.  

Another object of the present invention was to provide a composite material on the basis of airgel granules, which is also a good one Has sound absorption.

The problem is solved by a composite material that contains 5 to 97% by volume Contains airgel particles and at least one binding agent is characterized in that it contains at least one fiber material.

Through the binder, either the fibers and aerogels are mutually and joined together or the binder serves as a matrix material, in that the fibers and airgel particles are embedded. The connection of the Fibers and the airgel particles with each other and with each other through the Binder and possibly embedding in a binder matrix leads to a mechanically stable material with low thermal conductivity.

Compared to a material that only consists of airgel particles that pass through their Surfaces connected or embedded in a binder matrix Surprisingly, even small volume fractions of fibers with the same Volume fraction of binder to an essential mechanical Reinforcement because they take over essential parts of the load. Will be a higher one Volume fraction fibers used and little binder so a porous one Material will be included in which the fibers connected by the binder form a mechanically stable framework in which the airgel particles are embedded. The air pores that occur then lead to a higher porosity and thus improved sound absorption.

The fibers can be natural or artificial, inorganic or act organic fibers such. B. cellulose, cotton or flax fibers, Glass or mineral fibers, polyester, polyamide, or polyaramid fibers. The Fibers can be new or from waste such. B. shredded Fiberglass waste or rags.  

The fibers can be smooth or crimped as individual fibers, as a bulk or as Non-woven or woven fabric is present. Nonwovens and / or fabrics can as a coherent whole and / or in the form of several small pieces in be included in the composite.

The fibers can be round, trilobal, pentalobal, octalobal, ribbon, have fir tree, barbell or other star-shaped profiles. As well hollow fibers can also be used.

The diameter of the fibers used in the composite should preferably be be smaller than the average diameter of the airgel particles to a high To be able to bind the proportion of airgel in the composite. By choosing very thin Fibers make the composite more flexible.

Fibers with a diameter between 1 μm and 1 mm are preferred used. Typically, with a fixed volume fraction of fibers Use of smaller diameters for more break-resistant composite materials.

The length of the fibers is in no way limited. Preferably should however the length of the fibers is greater than the mean diameter of the airgel Particles.

Mixtures of the above types can also be used.

The stability as well as the thermal conductivity of the composite material increases increasing fiber content. Depending on the application, the volume fraction of the Fibers are preferably between 0.1 and 40% by volume, particularly preferably in the range between 0.1 and 15% by volume.

The fibers can still be sized with a better connection to the matrix or contact agents (coupling agents), such as. B. at Glass fibers are common.  

Suitable aerogels for the composite materials according to the invention are those based on metal oxides that are suitable for the sol-gel technique (see e.g. C.J. Brinker, G.W. Scherer, Sol-Gel-Science, 1990, chap. 2 and 3) how for example Si or Al compounds or those based on organic Substances that are suitable for the sol-gel technique, such as Melamine formaldehyde condensates (US-A-5,086,085) or Resorcinol formaldehyde condensates (US-A-4,873,218). You can also click on Mixtures of the above materials are based. Preferably used become aerogels containing Si compounds, particularly preferably aerogels containing SiO₂, in particular SiO₂ aerogels, optionally organic are modified.

The airgel can be used to reduce the radiation contribution to thermal conductivity IR opacifiers, such as. B. carbon black, titanium dioxide, iron oxide, zirconium dioxide or Mixtures of the same contain.

In addition, the thermal conductivity of the aerogels with increasing porosity and decreasing density decreases up to one Density in the range of 0.1 g / cm³. For this reason, aerogels are included Porosities above 60% and densities between 0.1 and 0.4 g / cm³ are preferred. The Thermal conductivity of the airgel granules should preferably be less than 40 mW / mK, particularly preferably less than 25 mW / mK.

In a preferred embodiment, hydrophobic airgel particles used by introducing hydrophobic surface groups on the Pore surfaces of the aerogels during or after the production of the aerogels are available.

The term “airgel particles” is intended in the present application Particles that are either monolithic, i.e. H. from a Consist of pieces, or the essentially airgel particles with a Diameter smaller than that of the particle contained by a suitable one  Binders are connected and / or by pressing into a larger particle are put together.

The size of the grains depends on the application of the material. Around However, to be able to bind a high proportion of airgel granules Particles larger than the fiber diameter, preferably larger than 30 microns. To achieve high stability, the granules should not be too coarse preferably the diameter of the grains should be less than 1 cm. For special applications it may also be particularly preferred that the diameter of the airgel particles is less than 0.5 mm.

On the one hand, a low thermal conductivity, on the other hand, an adequate one to achieve mechanical stability of the composite material, the Volume fraction of the airgel preferably between 20 and 95 vol .-%, are particularly preferably between 40 and 95 vol .-%, high Volume fractions lead to lower thermal conductivity and strength. For Achieving a high porosity of the overall material and thus increased Sound absorption should still contain air pores in the material, which is why Volume fraction of the airgel should be below 85 vol .-%.

Granules can preferably be used to achieve high airgel volume fractions a favorable bimodal grain size distribution can be used. Depending on Application, e.g. B. in the field of sound insulation, other distributions Find use.

Fibers and airgel particles as well as fibers and airgel particles with each other are connected by at least one binder. The binder can either just connecting the fibers and airgel particles to each other and effect with each other or serve as matrix material. All are known for this Suitable binder. So inorganic binders such as. B. Water glass glue, or organic binders such as hot melt adhesives (e.g. Ethylene-vinyl acetate copolymers, polyamides or polyolefins),  Reaction adhesives (e.g. epoxy resin adhesives, reactive polyurethane adhesives, Phenol, resorcinol, urea and melamine formaldehyde resins, Silicone resin adhesives, polyimide and polybenzimidazole resins) or aqueous Dispersion adhesives (e.g. styrene-butadiene and styrene-acrylic ester copolymers) be used. Another is the use of mixtures of these Substances possible.

If the binder forms a matrix in which aerogels and fibers are embedded, because of their low thermal conductivity, they are advantageously porous Materials with densities less than 0.75 g / cm³ such as foams, preferably Polymer foams (e.g. polystyrene or polyurethane foams) are used.

In order to distribute the binder in the gusset cavities at high To achieve airgel content and the best possible adhesion, should that one starts from binders in solid form, the grains of the binder preferably be smaller than that of the airgel granules. Likewise, a Processing at elevated pressure may be necessary.

Must the binder at elevated temperatures such. B. in the case of Hot melt adhesives or reaction adhesives such as B. melamine formaldehyde resins, processed, the binder must be chosen so that its Melting temperature does not exceed the melting temperature of the fibers.

The binder is generally in an amount of 1 to 50 vol .-% of Composite material used, preferably in an amount of 1 to 30 vol .-%. The choice of binder depends on the mechanical and thermal requirements for the composite as well as the requirements in With regard to fire protection.  

The composite can in effective amounts of other additives such. B. Dyes, pigments, fillers, flame retardants, synergists for Flame retardants, antistatic agents, stabilizers, plasticizers and Contain IR opacifiers.

The composite can also contain additives that contribute to its Manufacturing used or arise in the manufacture, such. B. Pressing lubricants, such as zinc stearate, or the reaction products of acidic or acid-releasing hardening accelerators when used of resins.

The fire class of the composite material is determined by the fire class of the airgel, the fibers and the binder and any further contained Substances determined. To make the composite material fire class as favorable as possible to obtain, preferably non-flammable fiber types, such as. B. glass or mineral fibers, or flame-retardant fiber types such. B. TREVIRA C® or melamine resin fibers, aerogels on an inorganic basis, especially preferably based on SiO₂, and flame retardant binders such as e.g. B. inorganic binders or urea and melamine formaldehyde resins, Silicone resin adhesives, polyimide and polybenzimidazole resins are used.

If the material in the form of flat structures, such as. B. slabs or mats, used, it can be on at least one side with at least one Top layer is laminated to improve the properties of the surface, so z. B. to increase the robustness, train them as a vapor barrier or against to protect easy pollution. The cover layers can also Improve mechanical stability of the composite molding. Be on cover layers are used, they can be the same or to be different.  

All materials known to the person skilled in the art are suitable as cover layers. she can be non-porous and thus act as a vapor barrier, such as. B. Plastic films, preferably metal foils or metallized plastic films, the Reflect thermal radiation. However, porous cover layers can also be used are used that allow air to penetrate into the material and thus lead to better sound absorption, such as. B. porous foils, papers, Woven or non-woven.

The cover layers themselves can also consist of several layers. The cover layers can be attached with the binder through which the Fibers and the airgel particles are interconnected and interconnected, however, another adhesive can also be used.

The surface of the composite material can also be at least by introduction a suitable material in a surface layer and be solidified. As materials are e.g. B. thermoplastic polymers such. B. Polyethylene and polypropylene, or resins such. B. Melamine formaldehyde resins suitable.

The composite materials according to the invention have thermal conductivities between 10 and 100 mW / mK, preferably in the range from 10 to 50 mW / mK, particularly preferably in the range from 15 to 40 mW / mK.

Another object of the present invention was to provide a method for To provide production of the composite material according to the invention.

The binder is initially in powder form, at elevated temperature and optionally increased pressure in the case of hot melt adhesives and melts in In the case of reactive adhesives reacting, the composite material can, for example can be obtained as follows: Using conventional mixing devices Airgel particles, fiber material and binder mixed. This mixture will then subjected to a shaping. The hardening of the mixture in  depending on the type of binder, the shape is optionally carried out under pressure Heating, e.g. B. in reaction adhesives, or in hot melt adhesives by heating above the melting point of the binder and subsequent cooling below the Melting point of the binder. A material that is porous on a macro scale can in particular by the following procedure: If the fibers are not already in bulk (e.g. small balls of cut fibers or small ones Pieces of a nonwoven) are present, they become known to the person skilled in the art Methods processed into small balls. Already in this step if necessary, the airgel granules are placed between the fibers. After that, these balls together with the binder and optionally the airgel particles e.g. B. mixed in a mixer until Binder and optionally airgel particles as evenly as possible between distributed the fibers. The mass is then placed in a mold and optionally heated under pressure to a temperature which in the case of Hot melt adhesives above the melting temperature of the adhesive and in the case of Reaction adhesives are above the temperature required for the reaction. After the binder has melted or reacted, the material becomes cooled down. Melamine formaldehyde resins are preferably used here. The density of the composite material can be increased by using higher pressures increase.

In a preferred embodiment, the mixture is pressed. It is possible for the specialist, the suitable one for the respective application Press and select the appropriate press tool. If necessary, you can for pressing lubricants known to the person skilled in the art, such as, for. B. zinc stearate Melamine formaldehyde resins can be added. Because of the high proportion of air of the airgel-containing molding compounds, the use of vacuum presses is advantageous. In a preferred embodiment, the airgel-containing molding compounds pressed into plates. To prevent the molding compound from baking on the ram can avoid, the airgel-containing mixture to be compressed with release paper be separated against the ram. The mechanical strength of the Airgel-containing plates can be made by laminating screen fabrics, fleeces  or papers on the plate surface can be improved. The sieve cloth, Nonwovens or papers can both be added to the airgel-containing plates are applied, with the screen cloth, nonwovens or papers beforehand for example, can be impregnated with melamine resins and then in one heatable press can be connected to the plate surfaces under pressure, as well, in a preferred embodiment, in one working step Insert the sieve cloth, nonwovens or papers, if necessary with Melamine resin can be impregnated into the mold and hanging on the Airgel-containing molding compound to be compressed and then under pressure and Temperature are pressed to a composite sheet containing airgel.

The pressing takes place in the depending on the binder used generally at pressures from 1 to 1000 bar and temperatures from 0 to 300 ° C in any shape.

In the case of phenol, resorcinol, urea and melamine formaldehyde resins pressing preferably at pressures of 5 to 50 bar, especially preferably 10 to 20 bar and temperatures preferably from 100 to 200 ° C, particularly preferably 130 to 190 ° C and in particular between 150 and 175 ° C in any shape.

If the binder is initially in liquid form, the composite can can be obtained for example as follows: With usual Mixing devices are used to mix the airgel particles and the fiber material. The mixture thus obtained is then coated with the binder, e.g. B. by spraying, brought into a mold and hardened in the mold. The Depending on the type of binder, the mixture may be cured under pressure by heating and / or evaporating the solution used or dispersant. The airgel particles with the fibers in are preferred swirled in a gas stream. A mold is filled with the mixture, whereby the binder is sprayed on during the filling process.

A material that is porous on a macroscale can be in particular as follows  Processes are obtained: If the fibers are not already in bulk (e.g. small balls of cut fibers or small pieces of a fleece) are present, they become too small using methods known to those skilled in the art Small balls processed. This may already be the case in this step Airgel granules are placed between the fibers. Otherwise these balls then together with the airgel granules z. B. in one Mixer mixes until the airgel particles between them as evenly as possible distributed the fibers. In this step or after the binder sprayed as finely as possible onto the mixture, which is then in a mold if necessary, under pressure to the temperature necessary for binding brought. After that, the composite is made using conventional methods dried.

If a foam is used as the binder, the composite material can according to the type of foam can also be produced as follows.

The foam is expanded into expandable granules in a mold as in the case of expanded polystyrene, so are all Components intimately mixed and then typically heated, advantageous using hot air or steam. Due to the resulting expansion of the particles the pressure in the mold is increased, causing the gusset volume of the Foam filled and the airgel particles are fixed in the composite. After cooling, the composite molding is removed from the mold and optionally dried.

Is the foam by extrusion or expansion with a liquid mixture subsequent solidification, the fibers of the liquid be added. The airgel particles are created with the Mixed liquid, which then foams.

If the material is to be provided with a cover layer, this can be done for example, inserted into a mold before or after filling it  be so that the lamination and the shape in one step can take place, preferably as the binder for the lamination Composite binder is used. But it is also possible that Only add a top layer to the composite afterwards.

The shape of the molded part, which consists of the composite material according to the invention, is in no way limited; in particular, the composite in Plate shape.

Due to the high proportion of airgel and its low thermal conductivity the composites are very well suited for thermal insulation.

The composite can e.g. B. in the form of plates, as sound absorption material directly or in the form of resonance absorbers for sound insulation be used. In addition to cushioning the airgel material occurs namely, depending on the porosity due to macroscopic pores Damping by air friction on these macroscopic pores in the Composite material. The macroscopic porosity can be changed of fiber content and diameter, grain size and proportion of airgel particles and type of binder can be influenced. The sound absorption in their Frequency dependence and its size can be determined by the choice of the top layer Plate thickness and macroscopic porosity known to those skilled in the art Way to be changed.

The composite materials according to the invention are also suitable on the basis of macroscopic porosity and especially the large porosity and specific Surface of the airgel also as adsorption materials for liquids, Vapors and gases.

The invention is explained in more detail below on the basis of exemplary embodiments described:

example 1 Molded body made of airgel, microcellulose and melamine-formaldehyde resin

There are 140 g of an organically modified SiO₂ airgel (77 vol .-%), 75 g Melamine formaldehyde powder resin Madurit® MW 909 (10 vol.%) And 17.5 g Microcellulose type 402-2B from Mikrotechnik, Miltenberg am Main (13 vol.%) Intimately mixed. The airgel granulate, which is analogous to the example of DE-A-43 42 548 has a grain size in the range of 50 to 250 µm, a bulk density of 0.117 g / cm³, a BET surface area of 540 m² / g and a thermal conductivity of 18.4 mW / m · K. The bottom of the Press mold with a base area of 30 cm × 30 cm is made with release paper laid out, a sieve mesh with a mesh size of 5 mm. Thereon the airgel-containing molding compound is evenly distributed with a sieve covered by 5 mm mesh size and the whole with a release paper covered. It is at a temperature of 160 ° C and a pressure of 20 bar pressed for 9 minutes with subsequent recooling. The one as a stable plate obtained molded body has a density of 0.38 kg / m³ and a Thermal conductivity of 37 mW / m · K.

Example 2 Moldings made of airgel, melamine-formaldehyde resin and various Fillers

140 g of an organically modified SiO₂ airgel from Example 1 are 75 g melamine formaldehyde powder resin Madurit® MW 909 and those in Table 1 specified fillers intimately mixed and in a press with a Base area of 30 cm x 30 cm at a temperature of 160 ° C and one Pressed from 10 to 20 bar for 10 minutes. The densities of the plates obtained are listed in Table 1.  

Table 1

Composition of the molding compounds in vol .-% and densities of the moldings obtained

Comparative example Molded body made of melamine-formaldehyde resin and silica

90 g melamine formaldehyde powder resin Madurit® MW 396 with 100 g Perkasil® KS 404 silica, 30 g microcellulose and 2.5 g zinc stearate thoroughly mixed and in a press with a base of 12 cm × 12 cm at a temperature of 155 ° C and a pressure of 270 bar 4 Minutes pressed. The molded body obtained has a density of 1.37 kg / m³ and a thermal conductivity of 150 mW / m · K.  

The thermal conductivities of the airgel granules were measured using a Heating wire method (see e.g. O. Nielsson, 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.

Claims (17)

1. Composite material containing 5 to 97 vol .-% airgel particles and at least one binder, characterized in that it has at least one fiber material. 2. Composite material according to claim 1, characterized in that the Volume fraction of the fiber material is 0.1 to 40 vol .-%. 3. Composite material according to claim 1 or 2, characterized in that the fiber material contains glass fibers as the main component. 4. Composite material according to claim 1 or 2, characterized in that the fiber material contains organic fibers as the main component. 5. Composite material according to one of claims 1 to 4, characterized characterized in that the proportion of airgel particles in the range of 20 to 95 vol .-% is. 6. Composite material according to at least one of claims 1 to 5, characterized in that the airgel particles have porosities over 60%, Densities below 0.4 g / cm³ and thermal conductivities of less than Have 40 mW / mK. 7. Composite material according to at least one of claims 1 to 6, characterized in that the airgel is a SiO₂ airgel, the is optionally organically modified. 8. Composite material according to at least one of claims 1 to 7, characterized in that at least part of the airgel particles Have hydrophobic surface groups.   9. Composite material according to at least one of claims 1 to 8, characterized in that the binder has a density that is less than 0.75 g / cm³. 10. Composite material according to at least one of claims 1 to 9, characterized in that the binder as a main ingredient contains inorganic binder. 11. Composite material according to claim 10, characterized in that the inorganic binder is water glass. 12. Composite material according to at least one of claims 1 to 9, characterized in that the binder as a main ingredient contains organic binder. 13. Composite material according to claim 12, characterized in that the organic binder is melamine formaldehyde resin. 14. Composite material according to at least one of claims 1 to 13, characterized in that at least part of the airgel particles and / or the binder contain at least one IR opacifier. 15. Composite material according to at least one of claims 1 to 14, characterized in that it has a flat shape and on at least one side is laminated with at least one cover layer. 16. A method for producing a composite material according to at least one of claims 1 to 14, characterized in that the Airgel particles and the fiber material mixes with the binder The mixture undergoes shaping and hardening.   17. Use of a composite material according to at least one of the Claims 1 to 14 for thermal and / or acoustic insulation.