DE102004046496A1 - dispersants - Google Patents

Abstract

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

Description

object The invention is a dispersant especially for hydrophobic Aerogels in aqueous solution and a method for its production.

dispersants on an organic basis 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 are still able to 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 dry, whereby forcibly a granulate is formed ( US 6,131,305 A ). The subcritical drying of silica aerogels can, with the exception of the method US 6,131,305 A not realized until today. It is also state of the art silica gels as granules to gel quickly in a drop tower and supercritical drying by known methods in an autoclave. The above US 6,131,305 A describes a manufacturing process for aerogels wherein 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 4,610,863 A describes, for example, a production 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 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 also describes the US 6,287,987 B1 a manufacturing method 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.

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

In a first embodiment, this object is achieved by a hydrophilic, powdery, fluorine-doped, inorganic dispersant containing 70-100 wt.% SiO ₂ at a density in the range from 0.1 g / cm ³ to 1 g / cm ³ .

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

Prefers contains the dispersant of the invention 5 to 30% by weight and more preferably about 10% by weight ammonium fluoride. This is not only in the preparation of the dispersant added catalytically but in sufficient quantity so that it no later than at the subsequent preparation of the drying of the dispersant fails. The advantage of using ammonium fluoride was surprising to a person skilled in the art, since ammonium fluoride in the production of silica gels so far rather with disadvantages, such as the low stability and strength and the lack of transparency of the products. How much ammonium fluoride indeed present in the dispersant depends for one, of course on how much ammonium fluoride is used as starting material, and of course how much of it reacts with the silica gel. It gets away assumed that the ammonium fluoride in water in the chemical Equilibrium with hydrofluoric acid located. These in turn can have hydroxyl groups on the surface of the Replacing silica particles with fluorine and forming water with them. On the other hand, it can not be ruled out that ammonium fluoride too directly with the hydroxyl groups on the surface of the silica particles below Exchange of the hydroxyl groups for fluorine and concurrent Formation of ammonia and water can react. A detailed Treatment of these mechanisms and their importance for morphology The resulting gels and solids, for example, in an article by Poddenezhny et al. (Materials Science, Vol. 20, No. 2, 53-61) treated.

The Dispersing agent according to the invention can over it also contain other excipients and fillers.

The dried dispersing agent according to the invention contains advantageously no organic additives or auxiliaries, the one fire resistance of the resulting material or composite impair could because then the resulting materials or composites resistant to organic solvents and are also fireproof and thus used in fire safety can.

The Dispersant is preferably not transparent or opaque, as it is then easier to visual in production processes can be perceived and thus dosed.

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 , so that it is easier to use in contrast to the much lighter airgel Can mix water.

The Dispergierung designed with the dispersant of the invention especially good because the airgel powder is hydrophilic and a preferred particle size of 0.1 up to 1 μm has, so that liquid in the capillaries and between the particles can penetrate. A classic Problem of all ultrafine powders also made of metals is that water is not able to dissolve agglomerates, but agglomeration strengthened. This is usually because of the interfacial tension of the water and the wetting angle to the powder is high and thus water is the particles can not separate. In the dispersant according to the invention by the hydrophilic character, the wetting angle is small. The particles the dispersant of the invention preferably have a platelike shape and hang not just punctually together. Taken together, this causes the dispersant of the invention in aqueous solutions is easily recorded.

When hydrophilic is the dispersant of the invention viewed when there is a static contact angle with respect to water of less than 10 °. Due to the hydrophilicity, the dispersant is light with water wetted. The dispersant of the invention encloses it is a hydrophobic airgel to be dispersed in water and thus in direct contact with this hydrophobic airgel. This can be done the hydrophobic and thus otherwise floating airgel easily disperse in water. Hydrophobic within the meaning of the invention means that the surface hydrophobic airgel has a static contact angle to water in air of more than 120 °.

advantageously, is used in the production of the aqueous Dispersion using the dispersant of the invention the water a small amount of alcohol, preferably 0.1 to 5 wt.%, Based to the water, admitted. This increases the viscosity of the water considerably and the dispersion succeeds much better.

The Dispersing agent according to the invention allows ie wetting of the hydrophobic airgel particles with water, but is not reactive in itself. In contrast, binders need such as cement, which are inherently reactive are solidified and solid materials. Binders are used not the production of dispersions but of solid materials.

Preferably is the mean particle size of the dispersant of the invention in a range of 100 to 1000 nm, more preferably in one Range of 200 to 500 nm. The pore size of the dispersant of the invention is advantageously in a range of 0.5 to 3 microns, preferably in a range of 1 to 2 μm.

By Addition of a strong and fast catalyst and extremely fast Gelation of the gel, whose network consists of particles in the Contrary to conventional silica aerogels particle sizes in the range from 200 to 500 nm, and accordingly pores which are approximately 1 to 2 μm are in diameter. In particular, these large pores cause the vaporizing liquid from the gel network instead of conventional airgel voltages in the range of about 1 MPa to exercise the network, only Stresses in the range of 1 kPa, which the network obviously can resist.

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

• a) Vermengen von wenigstens 0,1 bis 2 Stoffmengenäquivalenten TEOS, 4 bis 7 Stoffmengenäquivalenten Ethanol und 3 bis 7 Stoffmengenäquivalenten Wasser,
• b) Zugabe von 0,1 bis 1 Stoffmengenäquivalenten NH₄F in fester oder flüssiger Form zu der Mischung aus a),
• c) Gelieren der Lösung, und
• d) Unterkritisches Trocknen des Gels.

In a further embodiment, the aforementioned 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) addition of 0.1 to 1 molar equivalent NH ₄ F in solid or liquid form to the mixture of a),
• c) gelling the solution, and
• d) Subcritical drying of the gel.

By Subcritical drying especially of silica gels can be the method not only greatly simplified but also much more cost-effective be designed.

advantageously, the gelation time is below 60 s, more preferably below set from 20 s. This will not only make the production time shortened. By adding a big one Amount of catalyst, the gelation happens in very many places within the sol at the same time. This creates many small ones gel body, the more by clashing because by true bonds grow together to a loose gel body. Thereby arise gel-grain boundaries, which together with the precipitated ammonium fluoride to the white Color the dispersant. Too slow gelation, that is too low catalyst concentration, arise wet gel body with much smaller particles at a few or only one point in sol, so then a supercritical Drying is necessary. Small particles means small pores and thus high capillary forces (Liquid-gas).

advantageously, becomes the molar ratio from TEOS to ethanol in a range of 0.1 to 0.3, especially preferably at about 0.2. This can cause a uniform gelation be achieved at the same time sufficiently short gel time.

Preferably be in the process of the invention 0.8 to 1.2 molar equivalents, preferably 1 molar equivalent, TEOS, 4.5 to 5.5 molar equivalents Ethanol, preferably 5 molar equivalents Ethanol, and 4 to 5 molar equivalents Water, preferably 4.5 molar equivalents of water, mixed. This allowed the gelling time to be set below 10 s.

The Gel is advantageously in the process according to the invention directly in air dried. In addition to short gelling time and the use of ammonium fluoride in solid or more fluid Shape, can the process costs compared the conventional one Procedures are thereby significantly reduced.

apart of hydrophobic aerogels, the dispersant of the invention may of course be used for others light and / or hydrophobic substances are used in polar solvents. With the present dispersant, the viscosity of the polar solvent for example, water varies from thin to honey-tough become. The viscosity of the solvent For example, it may be more than 5000 cps more than 10000 cps, can be set. The viscosity is determined by means of a Brookfield viscometer at 25 ° C according to DIN 1342 measured.

Farther the dispersant according to the invention is suitable also as a filler for plastics.

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.

After that The gel was simply dried in air or in a drying oven at up to 60 ° C. Depending on Volume developed within a few hours a dry, white dispersant (typical cylindrical volume in the laboratory of about 20 mm in diameter and 50 mm height need at 20 ° C about 20 h, at 60 ° C about 2 hours). Although the resulting dispersant was monolithic, especially with slow drying, but without real cohesion. It crumbled On contact. Scanning electron micrographs showed particle sizes of about 300 nm.

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

wobei die Anteile von NH₄F und H₂O wie folgt variiert wurden. Den Einfluss auf die Gelierzeit kann man an den Tabellen 2 bis 4 und den korrespondierenden Diagrammen in den 1 bis 3 ablesen: Tabelle 2:

Tabelle 3:

Tabelle 4:

In order to determine the optimum 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 gel time can be found in tables 2 to 4 and the corresponding diagrams in the 1 to 3 read off: Table 2:

Table 3:

Table 4:

The 4 to 6 show SEM images of the dispersant of 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 (9)

Hydrophilic powdered fluorine-doped inorganic dispersant containing 70-100 wt% SiO ₂ at a density in a range of 0.1 g / cm ³ to 1 g / cm ³ . Dispersant according to claim 1, characterized that there is a mean particle size in a range from 100 to 1000 nm, especially in a range of 200 to 500 nm. Dispersant according to Claim 1 or 2, characterized that there is a pore size in one area from 0.5 to 3 μm, in particular in a range of 1 to 2 microns. 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. 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 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) drying the gel. Method according to claim 5, characterized that the gel time is below 60 seconds, in particular below 20 seconds. Method according to one of claims 5 or 6, characterized that one the molar ratio of TEOS to ethanol to a value in the range of 0.1 to 0.3, in particular at 0.2, sets. Method according to one of claims 5 to 7, characterized that you have 1 molar equivalent TEOS, 5 substance equivalents Ethanol and 4.5 molar equivalents Water mixed. Method according to one of claims 5 to 8, characterized that you dry the gel directly in air.