WO2006032651A1

WO2006032651A1 - Dispersant

Info

Publication number: WO2006032651A1
Application number: PCT/EP2005/054674
Authority: WO
Inventors: Lorenz Ratke; Sabine BRÜCK; Michael Reuss
Original assignee: Deutsches Zentrum für Luft- und Raumfahrt e. V.
Priority date: 2004-09-23
Filing date: 2005-09-20
Publication date: 2006-03-30
Also published as: DE102004046496A1

Abstract

The invention relates to a dispersant for hydrophobic aerogels in aqueous solution and to a method for producing the same.

Description

dispersants

The invention relates to a dispersant, in particular for hydrophobic aerogels in aqueous solution and a process for its preparation.

Organic-based dispersants for the preparation of dispersions of hydrophobic aerogels in aqueous solution are known.

Dispersions with organic additives or auxiliaries have the disadvantage that the resulting materials or composites are neither resistant to organic solvents nor can they be used in fire protection.

It is state of the art to gel and supercritically dry silica airgel from a sol-gel process using TEOS (tetraethyloxysilane), ethanol and water in the acidic or alkaline range. It has also been proposed to prepare silica aerogels as granules while avoiding supercritical drying by silylating the silica particles in the state of the wet gel, exchanging the liquid in the gel network with heptane, and adding the wet gel by rapid evaporation of the heptane drying, whereby forcibly a granulate is formed (US 6,131,305 A). The subcritical drying of silica aerogels, with the exception of the process from US Pat. No. 6,131,305 A, can not be realized until today. It is also state of the art silica gels as granules to gel quickly in a drop tower and supercritical by known methods in an autoclave dry. The above-mentioned US Pat. No. 6,131,305 A describes a production process for aerogels in which the resulting airgel particles are necessarily hydrophobic. Due to the hydrophobicity of the airgel particles, they can not be mixed with water and thus are not suitable as dispersants for aqueous dispersions.

US Pat. No. 4,610,863 A describes, for example, a preparation process for the production of airgel particles in which airgel particles are dried overcritically. Furthermore, the aerogels are transparent and have a particle size of 1 to 5 nm and a pore size of 3 to 5 nm.

US 4,840,653 A describes the doping of silica gels with fluorine during the preparation of the gel itself using initially fluorine-free starting materials to form a high-density vitreous material.

US 5,753,305 A describes a process for the preparation of aerogels in which the aging step of the gels is accelerated. This acceleration is achieved by the use of steam or gaseous ammonia, hydrofluoric acid or ammonium fluoride in catalytic amounts. The fact that these highly corrosive catalysts are in the vapor phase, this process is complicated and expensive. Furthermore, the solvent must be replaced before the drying step in the described process. The airgel is then either supercritically dried or surface modified by silanes and dried slowly in a controlled atmosphere. Similar to the patent just described, US Pat. No. 6,287,987 B1 also describes a production process for porous silica airgel. Again, hydrofluoric acid or ammonium fluoride is used in catalytic amounts and in the vapor phase. This in turn has the above-mentioned disadvantages.

It is therefore an object of the present invention to provide a dispersant which makes hydrophobic aerogels dispersible in aqueous solution without organic additives or auxiliaries.

In a first embodiment, this object is achieved by a hydrophilic, powdery, fluorine-doped inorganic dispersant at least 70 wt.% Si02 at a density in a range of 0.1 g / cm ³ to 1 g / cm ³ .

The inorganic dispersant according to the invention can be used in particular without further additives and auxiliaries being necessary to produce dispersions of hydrophobic and / or light particles, preferably hydrophobic airgel particles, in polar solvents, preferably water. The airgel particles thus dispersed are preferably hydrophobic silica airgel particles. However, hydrophobic organic airgel particles, hydrophobic carbon airgel particles or hydrophobic other inorganic airgel particles can also be dispersed in water with the dispersant according to the invention.

Unlike the prior art, in the fluorochemical gels and resulting high density glassy materials usually by the addition of HF during gel preparation are obtained, it is the object of the present inventive method to obtain a very loose and light powder as a dispersant. The present invention dispels the previous prejudice that accelerating the gelling process by adding fluoride can only result in solid and high density glassy materials. It was previously unknown that accelerating the gelation process by adding fluoride can lead to very light materials that can even be dried subcritically.

The dispersant according to the invention preferably contains from 5 to 30% by weight and more preferably about 10% by weight of ammonium fluoride. This is so not only added catalytically in the preparation of the dispersant, but in sufficient quantity so that it precipitates at the latest at the subsequent drying of the dispersant to the production. The advantage of using ammonium fluoride was surprising to the person skilled in the art, since ammonium fluoride in the preparation of silica gels has hitherto been associated with rather disadvantages, such as the low stability and strength and the lack of transparency of the products. How much ammonium fluoride actually exists in the dispersant depends, of course, on how much ammonium fluoride is used as the starting material, and of course how much of it reacts with the silica gel. It is assumed that the ammonium fluoride in water is in chemical equilibrium with hydrofluoric acid. This, in turn, can replace hydroxyl groups on the surface of the silica particles with fluorine and form water therewith. On the other hand, it is not excluded that ammonium fluoride also directly with the hydroxyl groups on the Surface of the silica particles can be reacted by exchanging the hydroxyl groups for fluorine and with simultaneous formation of ammonia and water. A detailed discussion of these mechanisms and their importance for the morphology of the resulting gels and solids is described, for example, in an article by Poddenezhny et al. (Materials Science, Vol. 20, No. 2, 53-61).

In addition, the dispersant of the invention may also contain other auxiliaries and fillers.

The dried dispersant according to the invention advantageously contains no organic additives or auxiliaries that could impair the refractory strength of the resulting material or composite, since then the resulting materials or composites are resistant to organic solvents and also fireproof and thus can be used in fire protection.

The dispersant is preferably not transparent or opaque, since it can then be more easily visually perceived and thus metered in production processes.

The density of the dispersant is preferably 0.1 g / cm ³ to 1 g / cm ³ , preferably 0.4 g / cm ³ to 0.7 g / cm ³ , in order that it is easier to use in contrast to the much lighter airgel Can mix water.

The dispersion with the dispersant according to the invention is particularly good, since the airgel powder is hydrophilic and has a preferred particle size of 0.1 to 1 .mu.m, so that Liquid can penetrate into the capillaries and between the particles. A classic problem of all fine powders also made of metals is that water is not able to dissolve agglomerates, but rather agglomerate formation. This is usually because the interfacial tension of the water and the wetting angle to the powder are high and thus water can not separate the particles. In the dispersant according to the invention by the hydrophilic character of the wetting angle is small. The particles of the dispersant according to the invention preferably have a platelet-like shape and not only point-to-point. Taken together, this causes the dispersant of the invention to be easily absorbed in aqueous solutions.

The dispersant according to the invention is considered to be hydrophilic if it has a static contact angle with respect to water of less than 10 °. Due to the hydrophilicity, the dispersant is slightly wetted with water. The dispersant according to the invention encloses a hydrophobic airgel to be dispersed in water and is thus in direct contact with this hydrophobic airgel. This makes it easy to disperse the hydrophobic and thus otherwise floating airgel in water. Hydrophobic in the sense of the invention means that the surface of the hydrophobic airgel has a static contact angle with respect to water in air of more than 120 °.

Advantageously, in the preparation of the aqueous dispersion using the dispersant according to the invention, a small amount of alcohol, preferably 0.1 to 5% by weight, is added to the water, based on the water added. As a result, the viscosity of the water increases significantly and the dispersion succeeds much better.

Thus, the dispersant according to the invention makes it possible to wetting the hydrophobic airgel particles with water, but is not reactive per se. In contrast, binders such as cement, which are inherently reactive and solidify into solid materials, must be seen. Binders are therefore not used for the production of dispersions but of solid materials.

Preferably, the average particle size of the dispersant according to the invention is in a range from 100 to 1000 nm, particularly preferably in a range from 200 to 500 nm. The pore size of the dispersant according to the invention is advantageously in a range from 0.5 to 3 μm, preferably in one range from 1 to 2 μm.

By adding a strong and fast catalyst and extremely rapid gelation of the gel, its network consists of particles which, in contrast to conventional silica aerogels have particle sizes in the range of 200 to 500 nm, and accordingly pores, which are about 1 to 2 microns in Diameter are. In particular, these large pores cause the evaporating liquid from the gel network to exert only stresses in the range of 1 kPa, as evidenced by conventional network airgel voltages in the range of about 1 MPa, on the network, which the network can obviously resist.

The dispersant of the invention preferably has a specific surface area in the range of 300 to 400 m ² / g, especially preferably in a range of 300 to 330 m ² / g. This results in a particularly good wetting of the otherwise hydrophobic airgel particles with water.

In a further embodiment, the abovementioned object is achieved by a process for the preparation of the dispersant according to one of claims 1 to 4, characterized in that it comprises the following steps: a) blending of at least 0.1 to 2 molar equivalents of TEOS, 4 to 7 molar equivalents of ethanol and 3 to 7 molar equivalents of water, b) adding 0.1 to 1 molar equivalents of NH ₄ F in solid or liquid form to the mixture of a), c) gelling the solution, and d) subcritically drying the gel.

By subcritical drying, especially the silica gels, the process can not only be considerably simplified but also made much more cost-efficient.

Advantageously, the gelling time is set below 60 s, more preferably below 20 s. This not only shortens the production time. By adding a large amount of catalyst, gelation occurs simultaneously at very many locations within the sol. This results in many small gel bodies that grow together more by clashing than by real bonds to a loose gel body. This results in gel-grain boundaries, which together with the precipitated ammonium fluoride lead to the white color of the dispersant. Too slow gelation, that is too low catalyst concentration, wet gel body with much smaller particles arise at a few or only one point in the sol, so that then a supercritical drying is necessary. Small particles means small pores and thus high capillary forces (liquid-gaseous).

Advantageously, the molar ratio of TEOS to ethanol is set in a range of 0.1 to 0.3, more preferably about 0.2. As a result, a uniform gel formation can be achieved at the same time sufficiently short gel time.

In the process according to the invention, preferably 0.8 to 1.2 molar equivalents, preferably 1 molar equivalent, TEOS, 4.5 to 5.5 molar equivalents of ethanol, preferably 5 molar equivalents of ethanol, and 4 to 5 molar equivalents of water, preferably 4.5 molar equivalents of water, are mixed , This allowed the gelling time to be set below 10 s.

The gel is advantageously dried directly in air in the process according to the invention. In addition to the short gelling time and the use of ammonium fluoride in solid or liquid form, the process costs compared to conventional methods can be significantly reduced.

Apart from hydrophobic aerogels, the dispersant of the invention may of course be used for other light and / or hydrophobic substances in polar solvents. With the present dispersant, the viscosity of the polar solvent such as water can be varied from fluid to honey tough. The viscosity of the solvent can for example be adjusted to a value of more than 5000 cps, preferably more than 10000 cps. The viscosity is measured by means of a Brookfield viscometer at 25 ^° C. according to DIN 1342.

Furthermore, the dispersant of the invention is also suitable as a filler for plastics.

Exemplary embodiment:

A solution with the following composition was prepared:

1 mol of TEOS

5 moles of ethanol

4.5 mol of water and as a catalyst

0.2 mol of NH ₄ F

The solution was stirred rapidly and gelled within 8 seconds after addition of the NH ₄ F.

Thereafter, the gel was simply dried in air or in a drying oven at up to 60 ⁰ C. Depending on the volume, a dry, white dispersant was formed within a few hours (typical cylindrical volumes in the laboratory of approx. 20 mm diameter and 50 mm height require approx. 20 h at 20 ^° C., approx. 2 h at 60 ^° C.). Although the resulting dispersant was monolithic, especially at slow drying, but no real cohesion. It crumbled when touched. Scanning electron micrographs showed particle sizes of about 300 nm.

With this dispersant according to the invention, for example, the hydrophobic airgel known from US 6,131,305 A can easily be dispersed in water.

In order to determine the optimal mole fraction of NH ₄ F and H ₂ O, the following mixtures were prepared (Table 1): Table 1 :

wherein the proportions of NH ₄ F and H ₂ O were varied as follows. The influence on the gelling time can be seen in Tables 2 to 4 and the corresponding diagrams in FIGS. 1 to 3:

Table 2:

Table 3:

FIGS. 4 to 6 show SEM images of the dispersant according to the invention. In the illustrated case, it can be seen that the average particle size of the dispersant is about 250 nm and in this case the dispersant has a pore size of about 1 to 2 μm.

Claims

claims:

1. Hydrophilic, pulverulent, fluorine-doped, inorganic dispersant containing at least 70 wt.% Si02 at a density in a range of 0.1 g / cm ³ to 1 g / cm ³ .

2. Dispersing agent according to claim 1, characterized in that it has an average particle size in a range of 100 to 1000 nm, in particular in a range of 200 to 500 nm.

3. Dispersant according to claim 1 or 2, characterized in that it has a pore size in a range of 0.5 to 3 m, in particular in a range of 1 to 2 m.

4. Dispersant according to one of claims 1 to 3, characterized in that it has a specific surface area in the range of 300 to 400 m ² / g, in particular in a range of 300 to 330 m ² / g.

5. A process for the preparation of the dispersant according to one of claims 1 to 4, characterized in that it comprises the following steps: a) blending of at least 0.1 to 2 molar equivalents of TEOS, 4 to 7 molar equivalents of ethanol and 3 to 7 molar equivalents of water, b) Adding from 0.1 to 1 molar equivalents of NH ₄ F in solid or liquid form to the mixture of a), c) gelling the solution, and d) drying the gel.

6. The method according to claim 5, characterized in that one sets the gelling time below 60 seconds, in particular below 20 seconds.

7. The method according to any one of claims 5 or 6, characterized in that one sets the molar ratio of TEOS to ethanol to a value in the range of 0.1 to 0.3, in particular at 0.2.

8. The method according to any one of claims 5 to 7, characterized in that one mixes 1 molar equivalent TEOS, 5 molar equivalents of ethanol and 4.5 molar equivalents of water.

9. The method according to any one of claims 5 to 8, characterized in that the gel is dried directly in air.