DE102004026260A1 - New spinnable mass, obtained by reacting aluminum triacylate with carbonic acid and mixing with silicon dioxide containing solution, useful for preparing green fibers and/or ceramic fibers based on aluminum oxide - Google Patents

Abstract

Spinnable mass (I) (obtained by reacting aluminum triacylate with carbonic acid, which can displace the acylate group of the aluminum triacylate, to form solution or suspension and mixing the solution or suspension with silicon dioxide containing sol or gel as far as the fibers should contain silicon dioxide), for preparing green fibers and/or ceramic fibers based on aluminum oxide or aluminum oxide-silicon dioxide, is new. An independent claim is also included for the preparation of (I).

Description

The invention relates to the synthesis of a chloride and nitrate-free spinning composition for the production of staple and other fibers in the system Al ₂ O ₃ -SiO ₂ , in particular with an aluminum oxide content higher than 60%. The new dope can be spun with an industrial centrifugal spinning plant to green fibers and processed to the ceramic end product (fiber mats or the like).

From the German patent DE 34 47 670 C2 It is known to produce fibers in the system Al ₂ O ₃ -SiO ₂ from a chloride-containing precursor. This method is also used commercially. In this case, hydrogen chloride precipitates as a waste product. This leads to corrosion in the system and despite exhaust gas scrubbing in the air pollution control to problems that are exacerbated by the statutory lowering of emission limits.

In the DE 21 63 678 C2 For the production of fibers in the system Al ₂ O ₃ -SiO ₂ the use of commercially available aluminum formate is proposed, but mandatory that the SiO ₂ content in the form of freshly hydrolyzed tetraalkoxysilane is present. If it is attempted, as hot as it is there, to add the SiO ₂ content in the form of customary, ie stable, silica or silica sols, unstable spinning solutions are obtained or spontaneous precipitations occur.

The inventors have made an attempt to reproduce the method proposed in said patent. For example, aluminum formate was first dissolved in hot water. The freshly hydrolyzed silica sol was prepared by combining 200 g of silicic acid ester in the form of tetramethoxysilane, 38 g of methanol, 0.4 ml of 1N HCl, and 48 g of water with stirring. After stirring for another half hour, the SiO ₂ sol was ready for use. The then combined aluminum formate-silicic acid solution was concentrated at 50 ° C and a pressure of 20 mbar to a clear concentrate with a total oxide content of 12.5% Al ₂ O ₃ and 0.25 SiO ₂ . By stirring 0.9% polyethylene oxide (based on the total mass of the solution) with a mean molecular weight of 100,000 as a spinning aid, which was dispersed in a little cold methanol, a solution was obtained after 15 hours of slow stirring and a final filtration, although the required viscosity of 270 mPas, but did not form any threads in Glasstabziehversuchen. Further concentration to improve the spinnability did not lead to success, as it forms a white precipitate.

In The description of this patent will also make use of others Aluminum carboxylates proposed. Accordingly, by dissolving aluminum or aluminum hydroxide in a monobasic low carboxylic acid, such as e.g. Formic acid, acetic acid, propionic acid, preferably but formic acid, at temperatures between 40 ° C and 100 ° C an aluminum salt solution in general, 1.7 to 3.2 moles of organic Acid on 1 mol applied so that in solution either basic or neutral salts are present. This gives homocarboxylates, the contain either formate, acetate or propionate. This will become then the conclusion is drawn that it is also possible without further ado be, commercially available Use aluminum formates or aluminum acetates as starting material. However, corresponding attempts by the inventors did not lead to success.

aluminous Spun masses of Cl-free precursors are also disclosed in US Patent 4,954,462 received from Thomas Wood. The aluminum source is aluminum nitrate formate acetate or basic aluminum formate acetate. When concentrating the solutions comes it due to the heterogeneous composition with two different carboxylic acids not to the precipitation of a solid, but to the emergence of a clear viscous solution. By adding very small amounts of lactic acid (0.04 mol, based on Al) becomes the thread draw greatly increased.

Aluminum nitrate formate acetate can not be considered as a substitute for chlorine-containing aluminum compounds due to the nitrate content. A solution of basic aluminum formate acetate is prepared by reacting aluminum with a mixture of aqueous formic and acetic acid. The reaction time is enormously long; even with reflux cooking, 12 hours are required to react 27 grams of aluminum. Although this period can be according to DE 21 63 678 C2 shorten by amalgamation, but for this step HgCl _{2 is} needed, a substance that is itself very toxic and whose reaction products are also difficult to dispose of.

As a method for the production of spun fibers is in the US 4,954,462 centrifugal spinning is listed. The stated viscosity range of the spin masses is given there as 15,000 to 1,000,000 cP (corresponding to 15 to 1,000 Pas). In the DE 34 47 670 C2 Viscosities of 70 to 200 Pas are recommended. Commercially applied dry spinning or centrifugal spinning processes, however, often require much less fluid solutions, for example in the range from about 1.5 to 10 Pas. However, spinnable solutions with such viscosities have never before been described.

The present invention is based on the object, a storable over a long time, chloride-free, preferably nitrate-free spinning mass for the production of staple and other fibers in the system Al ₂ O ₃ or Al ₂ O ₃ -SiO ₂ with an alumina content higher than 60% to provide the total oxide content, which can be adjusted to a viscosity of about 1.5 to 10, preferably 2.0 to 8.0 Pas and is spinnable with this viscosity in a dry spinning / centrifugal spinning process.

This object is achieved by providing a spinnable composition which is obtainable by reacting an aluminum triacylate, selected in particular from formates, acetates, propionates and butyrates and mixtures thereof, with an acid which can displace acylate groups from the triacylate. If the system is also to contain silicon, the solution or suspension obtained in this reaction is mixed with an SiO ₂ -containing gel or sol.

According to the invention, it is avoided as far as possible to use a basic or nitrate-containing aluminum precursor. In addition, of course, the absence of chloride of SiO ₂ source and all other additives and auxiliaries must be guaranteed.

Of the Aluminum precursor can be either in the form of a heteroacylate (carboxylate) or a homoacylate (carboxylate). Examples are aluminum formate acetate, aluminum formate propionate or aluminum triformate.

The Acylate groups from the aluminum triacylate displacing acid is preferably selected from carboxylic acids with a pKs value of less than 3.75. Very particularly preferred is it is a chelating carboxylic acid. Particularly preferred the use of carboxylic acids with a pKs less than 2.5, even more preferably less than 1.5. carboxylic acids show with such acid strengths in addition to the carboxy group usually one or more electron-withdrawing Groups such as other carboxylate groups, hydroxy groups or keto groups. Particularly preferred are di- or polycarboxylic acids.

Becomes the spinnable mass added a spin aid as from the State of the art, in some cases, their spinnability be increased. As a spinning aid, for example, lactic acid, polyethylene oxide or polyvinyl alcohol (PVA). Quantities of e.g. 1 to 5, in particular of about 2.5% by weight of spinning aids such as PVA, based on the Dope, are well suited; sometimes, however, can be much lower Quantities are enough, e.g. then, if lactic acid is used.

The spinning composition according to the invention is spinnable with the process and the plant to green fibers, as they are in principle in the DE 3447670 C2 are described, and can then be processed into a ceramic end product. Their stability and their spinning behavior (viscosity, fiber-forming capacity, take-off speed and adjustable diameter of the green fiber) can readily be adapted to the required conditions or requirements with common means within a wide range. In particular, it is desirable to adjust a viscosity in the range of 1.5 to 10 Pas, preferably in the range of 2 to 8 Pas. It has been found that the quality of the final product compared to that which can be produced with the chloride-containing dope according to this document, can be achieved unchanged.

The inventors have found that it is not possible to obtain a storage-stable, spinnable mass with the desired properties, if one of the doctrine of US 4,954,462 follows, but to avoid the disadvantages described above, the basic aluminum acylate used there exchanged for an aluminum triacylate. In the example of the US patent described as successful, an Al (OH) ₁ , ₈ (O ₂ CCH ₃ ) _{o, 6} (O ₂ CH) _0.6 prepared from metallic aluminum is used. Instead, the inventors of the present application added acetic acid or propionic acid in various concentrations to dissolved aluminum triformate to form aluminum formate acetate or aluminum formate propionate. It is also possible to use a triacetate with butyrate as an ingredient.

Interestingly, could be made from the mixture of such an aluminum mixed acylate solution with a silica sol after concentration, that is also without addition of spinning aid, threads are pulled with the help of a glass rod, and that Thread-forming ability improved as expected by the addition of spin aid. Without the addition of an acylatverdrängenden Acid, e.g. a chelating acid or an acid with low pKs value, however, these approaches failed industrial centrifugal spinning machine are spun into fibers. The obtained in this way formate acetate spinning masses in the presence of aged silica also lost its precipitates Thread-forming ability (Glass rod) already after a few days. The solutions could not by spinning aids either be further stabilized.

According to the invention, however, it turned out that the spinning masses can then be stored for a long period of time without losing their thread-forming ability, and moreover have such good spinning properties that they can be spun even after prolonged storage on the dry spinning plants in question here, if the aluminum triacylate used (Homo - or Mischacylat) is reacted with at least one carboxylic acid which can displace at least a portion of the Acylatgruppen from the aluminum triacylate. For this purpose, chelating acids or acids with a relatively low pKs value are suitable, for example citric acid or oxalic acid. The use of both acids has been studied in detail; Stable sols were obtained from which, after addition of SiO ₂ with subsequent vigorous concentration, threads could also be drawn without spinning aid with the aid of the glass rod drawing test. They are useful in contrast to their predecessors over several weeks, with the thread-forming capacity in approaches using oxalic acid to approaches using citric acid is significantly better. However, the filament-forming property of oxalic acid-containing spinning compositions only starts at higher viscosities than that of citric acid-containing spinning compositions.

Spinning compositions with particularly favorable properties are obtained by reacting the aluminum triacylate in solution with the acylate-displacing acid and then concentrating the solution to dryness. The resulting brittle residue (eg Aluminiumformiatoxalat) usually has an oxide content of nearly 30% by mass. It can be taken up with water or a water-containing, eg alcoholic, solvent (under certain circumstances also in alcohols such as ethanol or methanol, if the acylate groups of aluminum are less polar than formates), whereby an aqueous solution with approx. % Al ₂ O ₃ , more preferably 10-15 mass% Al ₂ O ₃ . Optionally, an appropriate amount of silica sol is added to this solution. The amount is chosen so that the dope obtains an elemental ratio of aluminum to silicon in the range of 1: 0 to preferably 3: 2. If mullite fibers are to be produced from the mass, this ratio is about 3: 1 or 3: 2. After addition of the silica sol and optionally a spinning aid, the mixture can be concentrated by concentration to the desired viscosity of 1.5 to 10 Pas, for example of about 3 Pas. Such an approach can be easily spun on dry spinning or centrifugal spinning equipment.

Next the aluminum compound and optionally silica may be the Spun mass according to the invention if required or desired, contain further metal compounds, for example Oxides of yttrium, zirconium, boron and / or magnesium. The concentration these oxides should total 10 mass% in the baked fiber do not exceed. The said elements are e.g. by water-soluble compounds (acetates) introduced into the dope.

The Dope can also if necessary, contain suitable additives such as defoamers or others.

below the invention will be explained in more detail by way of examples.

example 1

2,226 kg of aluminum formate are added to 4,400 kg of deionized water, which was then added 1.718 kg of glacial acetic acid. After a short simmer at reflux creates an almost clear solution that then 0.976 kg of an aged silica sol having a solids content offset by 20.5%. This solution is determined by mixing to 39.8 kg with an oxide concentration of 19 mass% produced. The aluminum formate oxalate concentration is thus 45.6% by mass. From the solution can be directly After making, use a glass rod to pull threads. After a few days However, the formation of a precipitate is observed, and the thread pull goes lost.

Example 2

121.4 g of aluminum formate (23.0% by mass of aluminum oxide) are mixed with 69.76 g of oxalic acid and 240 g of deion. Water boiled for 10 minutes at reflux. The resulting clear solution is concentrated at reduced pressure until a viscous residue forms. This is dried in a convection oven at 200 ° C until a brittle, slightly yellowish residue. This one comes with 185 g deion. Water is brought into solution by boiling at reflux. After cooling to room temperature, 53.0 g of silica sol (20.5% by mass of silica) and 33.7 g of PVA solution are added. At reduced pressure and a maximum temperature of 60 ° C, the mixture is concentrated to about 19 mass oxide content. The molar ratio of Al: Si in this spinning mass is 3: 1, the mass ratio of Al ₂ O ₃ : SiO ₂ at 72:28.

Example 3

Example 2 is repeated except that 23.67 kg of aluminum formate (oxide content 23.0) is reacted with 13.57 kg of oxalic acid. After drying, the resulting aluminum formate oxalate gives an oxide content of 29.5% by mass. By 27.68 kg of water, an aqueous solution with 11.8% by weight of Al ₂ O _{3 is} prepared. For this purpose, 10.378 kg of an approximately 20% SiO ₂ sol are added. After adding 6.4 kg of PVA solution and a few grams of defoamer, the following concentrations are present:
Total Oxide Content: 12.0 mass% (8.65 mass% Al ₂ O ₃ and 3.30 mass% SiO ₂ ). This corresponds to a mass ratio Al ₂ O ₃ to SiO ₂ of 72 to 28 and an element ratio of 3: 1. The Aluminiumformiatoxalat concentration in this solution is 28.8% by mass.

Example 4

Out the approaches Examples 2 and 3 are green fibers in a conventional manner pulled, which are dried at room temperature. The dried ones Gel fibers are placed in a crucible and within 13 hours at 1,300 ° C heated. After one hour at 1300 ° C, the fibers are left in the oven cool to room temperature. You get colorless, mechanically stable fibers.

Claims (16)

A spinnable composition for producing Al ₂ O ₃ or Al ₂ O ₃ -SiO _{2 based} green fibers or ceramic fibers obtainable by reacting an aluminum triacylate with a carboxylic acid capable of displacing acylate groups from the aluminum triacylate to form a solution or suspension , and mixing this solution or suspension with a SiO ₂ -containing sol or gel, if the fibers are to contain SiO ₂ . Spinnable composition according to claim 1, characterized the aluminum triacylate is chosen from aluminum homo heteroacylates of formic acid, acetic acid and propionic acid. Spinnable composition according to claim 1 or 2, characterized that the carboxylic acid, can displace the acylate groups from the aluminum triacylate, selected is under carboxylic acids with a pKs value of less than 3.75. A spinnable composition according to claim 3, wherein said carboxylic acid selected is under carboxylic acids with a pKs value of less than 2.5, preferably less than 2.0, and especially less than 1,5 and under carboxylic acids with at least two carboxyl functions or at least one carboxyl function and at least one hydroxy and / or keto group. A spinnable composition according to claim 4, wherein said carboxylic acid citric acid, Oxalic acid, pyruvic acid or a mixture of these acids is. Spinnable composition according to one of the preceding claims, which, optionally after mixing the solution or suspension with the SiO ₂ -containing sol or gel, was concentrated and has a viscosity in the range from 1.5 to 10 Pas, preferably from 2 to 8 Pas , A spinnable composition according to any one of the preceding claims, further containing a spinning aid, preferably polyvinyl alcohol. Spinnable composition according to one of the preceding claims, characterized in that the SiO ₂ -containing sol or gel is an aged silica sol. Spinnable composition according to one of the preceding claims, characterized characterized in that it has a content of aluminum compound plus Having silica, the 15 to 25% by mass of the sum of the oxides equivalent. Spinnable composition according to one of the preceding claims, characterized characterized in that it has an elemental ratio of aluminum to silicon of 1: 0, preferably 1: 0.01 to 3: 2. A process for preparing a spinnable composition according to any one of claims 1 to 10, characterized in that the aluminum triacylate is reacted in a suitable solvent with the carboxylic acid which can displace acylate from the aluminum triacylate, and the solution or suspension formed with a SiO ₂ -containing Sol or gel is mixed. A method according to claim 11 or 12, characterized in that the solution formed is concentrated to dryness and then resumed in the same or a different solvent before mixing with the SiO ₂ -containing sol or gel is carried out. Method according to claim 11 or 12, characterized that the solvent (s) selected are under water and water-based solvents. Method according to one of claims 11 to 13, characterized in that the solution is added simultaneously with the SiO ₂ -containing sol or gel or subsequently to the addition of a spinning aid. Method according to one of claims 11 to 14, characterized that the resulting mixture, preferably at reduced pressure and / or a temperature of up to 60 ° C, is concentrated until a viscosity of 1.0 to 10 Pas, preferably from 1.5 to 2.0 Pas, is reached. Use of a spinnable mass according to one of claims 1 to 10 for the production of green fibers or of ceramic fibers.