MXPA97002936A - Composition containing an aerogel, method to paraproad such composition and use of the - Google Patents

Abstract

The present invention relates to a composition comprising between 30 and 95% by volume of airgel particles and at least one aqueous binder, wherein the particle diameter of the airgel particles is less than 0.5 mm and the airgel particles are preferably they have hydrophobic surface groups, a procedure for the preparation thereof and the use of the same

Description

COMPOSITION CONTAINING A QEROGEL. METHOD PORO PRODUCE DICHFT CQ? FQSICION AND USE, PE LA? ISGIR DESCRIPTIVE MEMORY The invention relates to a composition comprising from 30 to 95% by volume of airgel particles and at least one aqueous binder, a process for the preparation thereof and the use thereof. Due to their very low density and high porosity, aerogels, especially those with porosities greater than 60% and densities less than 0.6 g / cm3, have extremely low thermal conductivity and therefore find application as heat insulating materials, as described in EP-fi-0 171 722. However, the high porosity also results in low mechanical stability, both of the gel from which the airgel dries and of the dry airgel itself. It is also known that aerogels have fairly low dielectric constants that have values that have values between 1 and 2, depending on the density of the airgel. Aerogels are therefore also predestined for electronic applications, for example for high frequency applications (5.CU.Hrubesh et al. 3. Mater, Res. Vol. 8, No. 7, 1736-1741). In addition to the aforementioned mechanical disadvantages of aerogels, it is very advantageous for electronic applications if the dissipation factor is high. It is known that the dissipation factor on the inner surface is increased by hydrophilic and polar surface groups or absorbed molecules. Aerogels are prepared by drying a suitable gel. A dry gel is referred to as an airgel if the gel liquid is removed at temperatures above the critical temperature and from pressures above the critical pressure. Conversely, if the liquid in the gel is removed with the formation of an adjoining vapor-liquid phase, the resulting gel is also referred to as a xerogel. Unless stated otherwise, the term "airgel" in the present application also includes xerogels as well as mixtures thereof. The procedure of configuring the airgel is completed during the sol / gel transition. Once the solid structure of the gel has been formed, the outer shape can be changed only by grinding, for example by grinding. However, for many applications it is necessary to use aerogels in particular ways. For this it is necessary, after preparation of the airgel, that is, after drying, to carry out a configuration step without a significant change in the internal structure of the airgel with respect to its application. EP-ft-0 377 301 discloses a stable, boable, aqueous suspension whose amorphous silica content, however, is restricted to a value of 5 to 15% by weight. The low silica content makes possible the formation of a stable, pumpable aqueous suspension, but excludes any application as a heat insulating material. EP-fl-0 340 707 discloses an insulating material having a density of 0.1 to 0.4 g / cma comprising at least 50 volume% of silica airgel particles having a diameter between 0.5 and 5 rnm, joined together by less by an organic and / or inorganic binder. The comparatively thick particle size results in prepared insulating materials that have a non-uniform appearance, which is disadvantageous for many applications. Particularly the thin layers («0.5 m) are necessarily special for electronic applications and these can be prepared using the aforementioned airgel particles. fidemás, the coarse particle size of the airgel makes it possible to produce heat insulation articles or film form having a thickness of less than 0.5 mm. The film layers of greater thickness are also adversely affected by airgel particles that are comparatively large in relation to the thickness of the film, since especially at the perimeters, an increased binder portion is necessary and this has an adverse effect on the thermal conductivity and the electronic properties of the dry film or the configured article.

EP-ñ-0 340 707 describes a process in which the airgel particles are coated and / or mixed with a binder, and the paste thus obtained is then cured in a mold. Due to the large difference in density between the airgel and the inorganic and / or organic binder and the size of the airgel particles, the mixed paste tends to separate as it is unstable for a relatively long period which is necessary for the application and storage. As a consequence, the configuration can be carried out only by comparatively rapid hardening of the mixed paste in a cover mold or by reducing the density difference between the airgel particles and the binder, for example by further producing binder foaming. Since the binder has a crucial effect on the final thermal conductivity of the dry shaped article, foaming of the binder assumes additional importance in that if the binder is cured without foaming to give a compact solid, the result is a thermal conductivity typically. high. The foaming allows the proportion of the binder to be reduced, as described in the German patent application not yet published DE-fl-44 04 701, giving a lower thermal conductivity. However, foaming is an additional and expensive process step that must be done in situ and includes the use of a blowing agent. Foaming can not be used for electronic applications.

In addition, the particles used as in EP-FL-0 340 707 do not have the long-term stability that would be necessary for the storage of aqueous composition since the OH groups of the aerogeo particles have been only methanol-containing. Ester groups of that type are, however, susceptible to hydrolysis as described in R. Iler, The Chernistry of tilica, Wiley a Sons, 1'379, PP. 6 < 34ff. The object of the present invention, therefore, is to provide a composition comprising an airgel containing a sufficiently homogeneous structure for each of the applications, it is stable for a relatively long period and therefore has storage life. good and has airgel particles that are stable to hydrolysis, so that water can be used as a solvent and because it is + or the composition is easy to dry. An additional object of the present invention is to provide a composition that satisfies the aforementioned criteria, but which also possess, after drying, good adhesion to the surface of the substrate. A further object of the present invention is to provide a composition which, in a dry form, has a dielectric constant ba and a dissipation factor which is as low as possible. These objects are achieved according to the invention by means of a composition comprising from 30 to 95% on volume of airgel particles and at least one aqueous binder, wherein the diameter of the airgel particles is less than 0.5 mm. The particle diameter refers to the average diameter of the individual airgel particle since the method of preparing the airgel particles, for example by grinding, means that they do not necessarily need to have a spherical shape. The particle diameter distribution depends on the application. Therefore, for requirements in electronics, a structure that is as homogeneous as possible can be desired, that is, a narrow distribution. For applications that require a high volume ratio (> 66%) of airgel particles in the composition, the distribution may be broader or even bomodal. In each case, the upper limit of the particle diameter depends on the homogeneity and storage life that are required by the composition. Therefore, the only airgel particles that can be used are those that have a diameter smaller than 0.5 mm. A content of? airgel artifacts that is significantly less than 30% by volume in the composition, this low proportion of airgel particles means that the advantageous properties in the composition would be lost to a considerable degree. A composition of this type would not have the high porosities, low densities and low thermal conductivities that are typical of airgel particles. A content of airgel particles significantly greater than 95% by volume would lead to a binder content of less than 5% by volume. In this case, the proportion of binder would be too low in the dry state, ie, after removing the water in the binder to ensure adequate agglutination of the airgel particles with each other. The proportion of the airgel particles is preferably in the range of 50 to 90% by volume, particularly preferably in the range of 50 to 80% by volume. According to the invention, the particle diameter of the airgel particles is less than 0.5 nm, preferably less than 0.2 mm. For applications in electronics, the diameter can yield even significantly less. Airgel particles whose average particle diameter is larger would give rise to an inhomogeneous, ie non-uniform, distribution in the composition according to the invention which is generally a suspension due to the difference in density compared to the aqueous binder and the size of airgel particles. In addition, problems would arise from the particle size, for example in the course of the suspension to produce thin films and molded parts. The dielectric constant of the dry composition would preferably be especially for applications in electronics <; 2. In addition, the dissipation factor would be as small as possible. Aerogels suitable for the compositions according to the invention are those based on metal oxides which are suitable for the sun and / or gel technique (CJ Brin er, GU Scherer, Sol-Gel Science, 1990, Chapters 2 and 3), such as, for example, silica or aluminum compounds, or those based on organic substances that are suitable for the sol and gel technique, such as melamine-aldehyde condensates (US-A-5 086 085) or resorcinol condensates. -formaldehyde IUS-A-4 873 218). They can also be based on mixtures of the aforementioned materials. Preference is given to aerogels comprising silicon compounds, in particular, "aerogels" and very particularly preferably SiOa-xerogels. To reduce the radioactive contribution to thermal conductivity, the airgel can include IR opacifiers such as, for example, carbon black, titanium dioxide, iron oxides or zirconium dioxide, or mixtures thereof. It is also the case that the thermal conductivity of aerogels decreases with the increase in porosity and with the decrease in density. For this reason, aerogels having porosities greater than 60% and densities less than 0.6 g / cm 3 are preferred. In a preferred embodiment, the airgel particles have hydrophobic surface groups. It is advantageous to stabilize the aqueous suspension if hydrophobic groups that are not removed by the action of water are covalently bound to the inner surface of the aerogels. Suitable groups for permanent hydrophobization are mono-, di- or trisubstituted groups of the formulasR \ / Si-R Si Si- 'V und wherein R is hydrogen or a non-reactive linear, branched cyclic, aromatic or heteroarytic radical, preferably a linear, branched or cyclic Cx-Cis alkyl radical or an aryl radical of C, aC, and Rz and R3, independently one of the other, are identical or different and each is hydrogen or a linear, branched, cyclic, aromatic or unreactive heteroaromatic organic radical, preferably a linear, branched or cyclic C: 1_-CIB radical, an aryl radical of C ^ -Cx, an OH or OR 'group, wherein R' is a linear or branched Cx-C * alkyl radical; preferably trialkyl and / or trialkylsilyl groups. It is particularly preferable if R, Ra and R3, independently of each other, are identical or different and are C? -C alkyl? < s, cyclohexyl or phenyl. The use of trirnethyl and dimethylsilyl groups for permanent hydrophobization of the airgel is particularly advantageous. These groups can be introduced as described in UO 94/25 149 or by gas phase reaction between the airgel and for example an activated tyalkylaminosilane derivative, such as, for example, a chlorotpalkylsilane or a hexa-alkyldi silazane (cf. R. Tler , The Chemistry of Silica, Uliley &Sons, 1979). The hydrophobic surface groups prepared in this manner also reduce the dissipation factor compared to aerogels having only OH groups on the inner surface. Aqueous binder are preferably aqueous dispersions which are also used as binders for dyes, surface coatings and adhesives. The aqueous binder comprises at least one organic polymer and / or inorganic binder dispersed in water and has a water content less than 90% (v / v). Said dispersions can also be obtained in a wide variety with commercial products. Examples are: vinyl acetate homo- and copolymer dispersions, ethylene-vinyl acetate dispersions, styrene-acrylic copolymer and styrene-butadiene dispersions and acrylate dispersions. Inorganic binders, such as, for example, aqueous glass, aqueous solutions of phylocilicates, or colloidal iOK are also suitable. Also suitable are mixtures of inorganic and / or organic binders. The dispersions can be stabilized either by means of surfactants - or by means of protective colloids; Stabilization by a combination of one or more protective colloids with one or more ionic and / or nonionic surfactants is also possible. The binder must have good compatibility with the airgel. If necessary, binders can be used that give, after drying, a water-resistant agglomeration of the airgel particles. Water resistance of this type can be achieved by known methods such as, for example, in relase. It is also advantageous if the solids content of the binder is as high as possible and its volume ratio in the mixture with the airgel is as small as possible. In order to obtain shaped articles that are hard and sufficiently stable, a dispersion must be chosen in which the glass transition temperature of the polymer is above ambient temperature. On the contrary, for the production of soft insulation materials, it is necessary to choose a dispersion in which the glass transition temperature of the polymer is lower than the service temperature. For thermal applications, it may also be advantageous if the composition includes fibers. The fibrous material may be either inorganic fibers, such as for example glass fibers, mineral fibers, silicon carbide fibers or carbon fibers, or organic fibers, such as for example polyester fibers, aramid fibers or nylon fibers. . The fibers can also be coated, such as polyester fibers metallized with a metal such as aluminum. The flammability classification of the article obtained after drying is determined by the flammability classification of the airgel and the binder and, if used, of the fiber material. In order to obtain the best possible flammability classification for the mixed material (low flammability or non-flammable), the binder must consist of inorganic binders and non-combustible material fibers such as mineral or glass fibers, or low-fiber fibers. flammability, such as for example melamine resins, specific polyester fibers ("CS") or polybenzyl idazoles (PBI). To avoid an increase in thermal conductivity produced by the added fibers, a. the volume ratio of the fibers should be from 0.1 to 30%, preferably from 1 to 10%, and b. The thermal conductivity of the fiber material should preferably be <; 1 U / rnk. By means of a suitable choice of fiber diameter and / or fiber material, the contribution of radiation to the thermal conductivity can be reduced and an increased mechanical strength can be achieved. For these purposes, the fiber diameter must be a. for non-metallized fibers preferably from 0.1 to 30 um and / or. b. for metallized fibers preferably from 0.1 to 20 μin. The contribution of radiation to the thermal conductivity can be further reduced by using blackened fibers, such as, for example, polyester fibers enriched with carbon black or simply carbon fibers. It is also possible to add carbon black as an additive to the composition. For this, varieties of carbon black having small particle diameters and as low a density as possible are particularly suitable. The mechanical strength of the article obtained after drying is also influenced by the length and distribution of the fibers in the composition. The compositions according to the invention can be prepared by mixing the airgel particles, and if desired the fibers and / or the carbon black, with the aqueous binder. This mixing can be carried out in any desired manner. Therefore, any of the components, of which at least two, can be introduced simultaneously into the mixing apparatus, or one of the components can be introduced first and the others can be added later. There is also no restriction on the mixing apparatus necessary for mixing. Any * mixing apparatus known for this purpose can be used by one skilled in the art. The mixing operation is continued until an almost uniform distribution of the airgel particles is present in the composition. It can be controlled both by means of sweating and, for example, by means of the speed of the agitation device. After drying, the compositions according to the invention are suitable, due to their low thermal conductivity, as heat insulation materials, or due to their small dielectric constant and their small dissipation factor, for electronic applications. The compliance composition of the invention is preferably dried on a temperature scale of 0 to 100 ° C. The drying process to be used can be selected from a variety of procedures known to one skilled in the art. The compositions according to the invention are also suitable as binders for other materials, in particular insulating materials, such as, for example, airgel particles (having a large particle diameter) in the form of granules. The suspensions according to the invention, due to their flowability can be emptied or pumped in any type of mold so that after drying corresponding shaped articles of insulating material are formed.

It is also possible to apply the compositions according to the invention, for example in the form of a suspension, on surfaces on a coating; their adhesion makes them very suitable for this purpose. It may be advantageous if the surface is activated in advance by common procedures, such as, for example, electric discharges, thus improving adhesion. It is also possible to additionally apply specialized adhesion promoters and / or intermediate layers, for example silicate, to the actual substrate before the composition is applied. Suitable application methods are, for example, spraying, knife coating, brushing or immersion. The drying of the applied layers gives surface coatings that have good adhesion, are heat insulators and sound absorbers, have good dielectric constant and small factor of dissipation. The compositions according to the invention can also be used as sound absorbing materials, either as such or in the form of resonance absorbers, whether they have a low sound transmission rate and a sound isolation effect greater than the aerogels not reinforced. It is also possible to increase the performance of ultrasound transmitters by applying such a composition in the form of a layer. The invention is described in further detail hereinafter by the working examples.

EXAMPLE 1 125 ml of hydrophobic airgel granules having a particle size on the scale of 50 to 250 u (volumetric density of 0.08 g / cm) are mixed with an initial charge of 60 nrl of an aqueous dispersion of styrene-acrylate copolymer solids content: 19% by weight (34 ml of Mo ilith DM 760, 26 rnl of water) in a 200 ml beaker, using a propeller stirrer at 1200 rpm for 20 minutes until a flowable suspension is formed. The suspension is then carried between two barriers corresponding to the final thickness of the casting and dried for four hours at 80 ° C. After drying, a white casting having a density of 0.1.75 g / cm is formed. The thermal conductivity is 0.035 Ul / rnk.

EXAMPLE 2 125 ml of hydrophobic airgel granules having a particle size in the range of 50 to 250 μm (volumetric density of 0.08 g / cm) are mixed with an initial charge of 60 ml of an aqueous dispersion of esterino-acrylate copolymer, solids content: 19% by weight (34 rnl of Mowilith DM 760, 26 ml of water) in a 200 ml beaker, using a propellant stirrer at 1200 rpm for 20 minutes until a flowable suspension is formed. The suspension is then carried between two barriers corresponding to the final thickness of the casting and dried for 12 hours at room temperature. After drying, a casting of thickness of 1 ml having a density of 0.177 g / cm is formed. The thermal conductivity is? .035 w / mK.

EXAMPLE 3 125 rnl of hydrophobic airgel granules having a particle size on the scale of 50 to 250 μm (volumetric density of 0.08 g / cm) are mixed with an initial charge of 60 ml of an aqueous dispersion of esterino-acrylate co-polymer. , solids content: 28% by weight (34 nm of Mowilth DM 611, 26 ml of water) and 3.5% by weight (based on airgel mass) of glass fibers having a length of 4.5% rnl in a beaker of precipitate of 200 rnl, using a propeller stirrer at 1200 rpm for 15 min. until a flowable suspension is formed. The suspension is then carried between two barriers corresponding to the final thickness of the cast and dried for 7 hours at 80 ° C. After drying, a 6 mm thick cast is formed having a density of 0.17 g / cm. The thermal conductivity is 0. 046 l / rnK.

EXAMPLE 4 125 rnl of hydrophobic airgel granules having a particle size on the scale of 50 to 250 μm (volumetric density of 0.08 g / crn) are mixed with an initial charge of 60 rnl of an aqueous dispersion of acrylate-acetate copolymer. vinyl-ethylene, solids content: 28% by weight (25 ml of Mowilith VDM 1340 34 ml of water) in a beaker of 200 rpm, using a propeller stirrer at 1200 rpm for 20 min. until a slightly pasty suspension is formed. The suspension is then carried between two barriers corresponding to the final thickness of the casting that is dried for 6 hours at 80 ° C. After drying, a casting with a thickness of 5 mm is formed which has a density of 0.14 g / cm. The thermal conductivity is 0.032 U / mK. The thermal conductivity of the castings in Examples 1 to 4 was measured using a hot wire method (see, for example, 0. Nielsson, G., R? schenpohler, 3. Groß 3. Fricke, High Temperaturee-Hi gh Pr sures, Vol. 21, 267-274 (1989)).

Claims (11)

NOVEPRP PE THE INVENTION REIVIN ICATIONS

1. - A composition is comprised of 30 to 95% by volume of particles in airgel and at least one aqueous binder, wherein the particle diameter of the airgel particles is less than 0.5 mm, and the airgel particles have surface groups permanently hydrophobic on its internal surface.

2. A composition according to claim 1, further characterized in that the proportion of airgel particles are in the range of 50 to 90% volume.

3. A composition according to claim 1 or 2, further characterized in that the particle diameter of the airgel particles is less than 0.2 nm.

4. A composition according to at least one of claims 1 to 3, characterized in that the airgel is an airgel of SiO_ > .

5. A composition according to at least one of claims 1 to 4, further characterized in that the hydrophobic surface groups are trialkylsilyl groups and / or triarylsilyl groups.

6. A composition according to at least one of claims 1 to 5, further characterized by the airgel particles have porosities greater than 60% and densities less than 0.5 g / cm.

7. A composition according to at least one of claims 1 to 6, further characterized in that the aqueous binder is a dispersion comprising at least one organic polymer and / or inorganic binder and has a water content of less than 90. % (v / v).

8. A composition according to at least one of claims 1 to 7, further characterized in that the composition includes 0.1 to 30% by volume of fibers.

9. A composition according to at least one of claims 1 to 8, further characterized in that the composition includes an IR opacifier.

10. A process for preparing a composition according to at least one of claims 1 to 9, further characterized in that the airgel particles are mixed with the aqueous binder.

11. A method for using a composition according to at least one of claims 1 to 9 to produce an insulating material, an adhesive and / or a film, or as an auxiliary material in electrical engineering.