GB2184430A - Preparation of ceramic fibres/filaments from a liquid precursor - Google Patents

Abstract

Filaments and fired ceramic fibres are formed from a precursor solution containing a magnesium or a calcium salt dissolved in an alkyl silicate hydrolysate, an aluminium hydroxyhalide solution, or a mixture of the two. The aluminium hydroxyhalide is preferably in the form of an organic complex e.g. aluminium chlorohydrate propylene glycol complex.

Description

SPECIFICATION Preparation of fibres or other articles from a liquid precursor This invention relates to the preparation of a ceramic fibre or a monolithic material directly from a liquid phase or precursor which is caused to gel. The resultant gel is subsequently fired to form the ceramic fibre or monolithic material. In this specification the product obtained from the prepared gelling mixture by processes such as extruding, spinning or drawing will, in general, be referred to as a "filament" and the final fired filamentary product will be referred to as "fibre".

In general, ceramic oxide fibre precursors may be made by dissolving the appropriate metal compound or compounds in a suitable solvent together with an organic polymer which will enable a fibre to be formed from the resulting solution. This procedure is described in British Patent No. 1,360,197. According to this specification, the metal compound may be a salt or a sol and the solvent may be water. The organic polymer content should be less than 10% by weight of the metal compound, preferably 0. 1-2.0% by weight and the molecular weight of the polymer should be in the range 10-10. There should be little or no reaction between the metal compound and the organic polymer.British Patent No. 1,360,199 describes the use of a polyethylene oxide in this process and British Patent No. 1,360,200 describes the use of a polyvinyl alcohol or partially hydrolysed polyvinyl alcohol in this process. Alumina ceramic fibre precursors may be obtained by these procedures if the metal compound is an aluminium chlorhydrate or an aluminium chlorophosphate. The precursor is converted to an alumina ceramic fibre on firing.

An alumina ceramic fibre precursor may also be obtained by dissolving an aluminium chlorhydrate in water, concentrating the solution until the viscosity is in the range 1505-3000 centipoise and extruding this concentrated solution into a coagulating bath, which is ethanol, isopropanol or tertiary butanol, or a mixture of acetone with either ethanol or isopropanol. The preferred aluminium chlorhydrate is Al2(OH)5Cl. This process is described in United States Patent No.

3,865,917.

An object of the present invention is to provide a precursor system that does not require the addition of an organic polymer. Therefore there is less organic material to be removed during firing. Another advantage of the procedures of the present invention is that the precursor liquids do not require a coagulating bath to form a filament.

Gels for binding refractory grains may be obtained from solutions of aluminium halohydrates, preferably chlorhydrates, optionally with certain magnesium salts, in water/alcohol mixtures. The preparation of these gels is described in United States Patent No. 3,975,202. The preparation of solutions containing silicon and aluminium for binding refractory grains is described in British Patent No. 1,356,248 and in United States Patent No. 3,979,215. The preparation of a solution containing silicon and aluminium in the oxide stoichiometry required for mullite and the use of this solution to bind refractory grains is described in British Patent No. 2,004,263. All of these solutions containing aluminium and silicon are prepared by the hydrolysis of a lower alkyl silicate in the presence of an aluminium compound soluble in water and in the solvent used in the hydrolysis.United States Patent No. 3,979,215 and British Patent No. 1,356,248 also describe the preparation of solutions for binding refractory grains by the hydrolysis of a lower alkyl silicate in the presence of a magnesium or calcium salt which is soluble in water and in the solvent required for the hydrolysis reaction. All these solutions require a gel-inducing accelerator for useful binding of refractory grains. Normally, this gelation-inducing accelerator is basic. A dilute solution of ammonia, optionally containing ammonium acetate to control the pH, is usually used.

The preparation of a solution for binding refractory grains by dissolving magnesium oxide in a solution containing aluminium and silicon obtained by hydrolysing ethyl silicate in the presence of aluminium chloride is described in West German Offenlegungschrift 2,154,764. This solution may be gelled by treatment with a dispersion of light magnesia in a mixture of water and glycerol (1, 2, 3-trihydroxypropane).

According to one aspect of the present invention a settable precursor liquid for the preparation of a ceramic fibre or a monolithic material comprises a solution of a calcium or a magnesium salt dissolved in a lower alkyl silicate hydrolysate or in a solution of an aluminium hydroxyhalide in water or a mixture of water and an alcohol, the said precorsor liquid, with solvent evaporation if necessary, setting to a clear coherent gel which on firing forms a ceramic fibre or a monolithic material.

Thus the settable precursor solution may comprise a lower alkyl silicate in the presence of a calcium salt which is soluble in water and in the solvent required for the hydrolysis reaction, or an aluminium hydroxyhalide and a calcium or magnesium salt dissolved in water or in a mixture of water and an alcohol.

The preferred calcium salt is a salt of an inorganic acid, for instance the nitrate or a halide. Of these salts, calcium chloride is preferred, the hydrates being well defined crystalline material.

When the precursor solution is prepared by the hydrolysis of a lower alkyl silicate the use of a hydrate of calcium chloride enables the total amount of water used in the hydrolysis reaction to be determined exactly. The preferred solvent for the hydrolysis reaction is a monohydric alcohol miscible with or soluble in water. To obtain a precursor liquid suitable for forming a filament the hydrolysis reaction should be carried out at the reflux temperature of the solvent system.

The preferred magnesium salt is a salt of an inorganic acid, for instance the nitrate or a halide.

Of these salts magnesium chloride is preferred, the hydrates being well defined crystalline material. It is important that the magnesium salt and the aluminium hydroxyhalide do not react to form an insoluble precipitate. The alcohol used must be miscible with or soluble in water. The preferred alcohols are the C1-C4 monohydric alcohols for instance ethanol or isopropanol.

The lower alkyl silicate is an organic silicate containing at least one C,-C6 alkyl group. The preferred lower alkyl silicate is ethyl silicate. The aluminium hydroxyhalide has the general formula Al2(0H)nX6n or a polymer or hydrate thereof, where X is Cl, Br or I and n is less than 6.

Preferably X is Cl.

According to a second aspect of the invention a process for the preparation of a ceramic fibre and/or a monolithic material comprises forming a homogeneous precursor solution by dissolving a calcium or magnesium salt in a hydrolysate of an organic silicate containing at least one C1-C6 (lower) alkyl group or in a solution of an aluminium hydroxyhalide of the formula Al2(0H)nX6n or a polymer or hydrate thereof where X is Cl, Br or I and n is less than 6 in water or a mixture of water and an alcohol selected from the group comprising C1-C4 monohydric alcohols forming from the solution a filament or clear coherent gel, evaporating solvent if necessary then firing to form a ceramic fibre or monolithic material.

According to a third aspect of the invention a settable precursor liquid for the preparation of a ceramic fibre and/or a monolithic material comprises a hydrolysate of a lower alkyl silicate formed by hydrolysing a lower alkyl silicate in the presence of a magnesium salt and an aluminium hydroxyhalide organic complex formed from an aluminium hydroxyhalide and a polyol containing 2 to 6 carbon atoms and at least 2 hydroxy groups both being soluble in water and the solvent required for the hydrolysis reaction, the said precursor liquid, with solvent evaporation if necessary setting to a clear coherent gel which on firing forms a ceramic fibre or a monolithic material.

The lower alkyl silicate is an organic silicate containing at least one C1-C6 alkyl group. The preferred lower alkyl silicate is ethyl silicate. The preferred solvent for the hydrolysis reaction is a monohydric alcohol miscible with or soluble in water. Ethanol is an example of a suitable alcohol solvent. The preferred magnesium salt is the salt of an inorganic acid, for instance a halide or a nitrate. Of these salts, magnesium chloride is preferred. Hydrates of magnesium chloride are preferred because these are well defined crystalline material.

The said settable precursor liquid, with solvent evaporation if necessary, sets to a clear coherent gel which on firing forms a ceramic fibre or a monolithic material. To obtain a precursor liquid suitable for forming a filament, the hydrolysis reaction is preferably carried out at the reflux temperature of the solvent system. Preferably the settable precursor liquid has the oxide stoichiometry required for cordierite 2 Mg0. 2Al2O3 . 5Si02. It is important that the magnesium salt, the lower alkyl silicate and the aluminium hydroxyhalide complex do not react to form an insoluble precipitate.

The invention provides a cordierite ceramic fibre. It also provides settable precursor liquids from which cordierite ceramic fibres may be prepared. The very low coefficient of expension of cordierite makes it useful as a ceramic fibre material.

According to a fourth aspect of the invention a process for the preparation of a ceramic fibre and/or a monolithic material comprises preparing a homogeneous solution by hydrolysing an organic silicate containing at least one C1-C6 alkyl group in the presence of a magnesium salt and an aluminium hydroxyhalide organic complex formed from an aluminium hydroxyhalide and a polyol containing 2 to 6 carbon atoms and at least two hydroxy groups both being soluble in water and the solvent required for the hydrolysis reaction, forming from the solution a filament or clear coherent gel, evaporating solvent if necessary, then firing to form a ceramic fibre or a monolithic material.

The solvent required for the hydrolysis rection is preferably a C-C4 monohydric alcohol, for instance ethanol. The preferred organic silicate is ethyl silicate. Preferably the solution has the oxide stoichiometry required for cordierite 2MgO . 2Al203. 5Si02. The invention therefore provides a process for the preparation of a cordierite ceramic fibre and/or monolithic material.

In all of the aspects of the invention, to be able to form filaments and/or clear coherent gels, the solution must be homogeneous. The amount of water and the amount of solvent are therefore important. The clear coherent gels and filaments may be dried and fired to remove any remaining organic material, to give monolithic materials and fibres. It is understood that during evaporation of the solvent, hydrolysis and condensation-polymerisation reactions cause a steady increase in the viscosity to occur and at a particular point, filaments can be formed from the solution; ultimately a clear and coherent gel which is rigid is obtained. The reaction temperature is preferably 60 C or higher, especially if an alkyl silicate is present, when the reaction temperature is preferably the reflux temperature of the solvent-system.Depending on the reaction temperature and the reaction time filaments may be obtained before the solution sets to a clear and coherent gel. It is important to note that the formation of filaments and/or clear coherent gels does not need the gelation-inducing accelerators necessary for binding refractory grains when using the solutions whose preparation is described in British Patent Nos. 1,356,248 and 2,004,263, or in United States Patent Nos. 3,975,202 and 3,979,215.It is also important to note that the formation of filaments and/or clear coherent gels from homogeneous solutions obtained by hydrolysing lower alkyl silicates, for instance ethyl silicate, in the presence of magnesium salts and aluminium hydroxyhalides or their organic complexes formed from polyols containing 2 to 6 carbon atoms and at least two hydroxy groups does not require the gelling agent comprising light magnesia dispersed in a mixture of water and glycerol described in West German Offenlegungschrift 2,154,764.

Precursor filaments may conveniently be formed by drawing filaments from the solution. A simple method of assessing the ability of a solution to form filaments is to insert into and pull out of the solution a glass rod 1Omm in diameter. Other methods of obtaining filaments are known and these may be used if desired The lower alkyl silicates preferred in the present invention are organic silicates containing at least one C1-C6 alkyl group. Ethyl silicate is the preferred organic silicate. One example of a class of suitable lower alkyl silicates is the tetra [C1-C6 alkoxy] silanes, for instance tetraethoxysilane and tetraisopropoxysilane. The four alkoxy groups may be the same, for convenience.

The preferred lower alkyl silicate, ethyl silicate is traditionally prepared by treating tetrachlorosilane with ethanol. When anhydrous ethanol is used, the product is tetraethoxysilane (ethyl orthosilicate) Si(EOt)4 with hydrogen chloride as by-product. In the industrial preparation, the ethanol used invariably contains some water (industrial spirit), hence the product obtained consists of a mixture of tetraethoxysilane and ethoxypolysiloxanes (ethyl polysilicates) which are formed by condensation-polymerisation reactions resultant on water being present and being catalysed by the hydrogen chloride by-product. Reaction conditions are chosen to yield a product which will give on ignition silica (SiO2) equivalent to 40% w/w, which corresponds to a mixture of ethoxysiloxane oligomers with an average of 5 silicon atoms.This material is known as technical ethyl silicate -40, or ethyl silicate -40. It is the preferred ethyl silicate in the present invention because it contains oligomeric material which enhances filament formation. Other methods for the production of mixtures of tetraethoxysilane and ethoxypolysiloxanes are given in British Patent 2,017,129; European Patent 0,004,730; Canadian Patent 1,172,649 and United States Patent No. 4,211,717.

The preparation of isopropyl polysilicate mixtures is described in British Patent Specification Nos. 948,132; 948,136 and 948,137. The preparation of the other alkyl orthosilicate-polysilicate materials is described in United States Patent No. 3,244,739 also in British Patent Nos. 992,111 and 993,249.

The aluminium hydroxyhalides used in the present invention have the general formula Al2(0H)nX6.n or a polymer or hydrate thereof where X is Cl, Br or I and n is less than 6. In the preferred aluminium hydroxyhalides X is Cl and n is 5. Methods of making the aluminium hydroxychlorides include: treating aluminium metal with dilute hydrochloric acid solution, treating aluminium hydroxide with hydrochloric acid under pressure, electrolytically from aluminium metal and dilute hydrochloric acid solution as described in British Patent No. 1,006,384.

Suitable aluminium hydroxychlorides are available under the U.K. Registered Trade Marks of CHLORHYDROL and MICRO-DRY. In these materials, n is 5 in the formula Al2(OH)nCI6n. Aluminium hydroxychlorides of the formula Al2(OH)5Cl. mH2O where m is a number less than 4, are available and may be used. Usually it is convenient to dissolve the aluminium hydroxyhalide in water.

The present invention uses an aluminium hydroxyhalide organic complex formed from an aluminium hydroxyhalide and a polyol containing 2 to 6 carbon atoms and at least 2 hydroxy groups. The preferred aluminium hydroxyhalide organic complexes are formed from aluminium halohydrates or hydroxyhalides as represented by the formula: A6(OH)nX6n where X is Cl, Br or I and n is 1-5 and polyols selected from the group comprising: 1,2-propylene glycol 1 , 1 , 1 -trimethylolpropane 1,3-butane diol 1,2,3-trihydroxypropane 2-methyl 2,4-pentanediol 2,2-dimethyl 1,3-dihydroxypropane.

The preferred aluminium halohydrate is a chlorhydrate in which n is 5. The preparation of the preferred aluminium chlorhydrate organic complexes is described in British Patent No. 1,159,658 and in United States Patent No. 3,420,932. The complexes are obtained from aluminium chlorhydrate. British Patent No. 1,009,959 describes the preparation of aluminium chlorhydrate organic complexes by reacting an aluminium compound of the formula Al2Cl6n(OH) where n is 1-5 with a hydroxylic reagent selected from aliphatic alcohols containing at least two hydroxy groups, the reaction being carried out in the presence of at least sufficient water to dissolve all the aluminium compound, followed by removal of water.Aluminium chlorhydrate organic complexes may also be directly prepared from aluminium as described in British Patent No. 1,267,959 also as in United States Patent No. 3,507,859. Aluminium metal is treated with a polyhydroxy compound having 2 carbon atoms each linked to a hydroxy group (e.g. 1,2-propylene glycol) and either or both aqueous aluminium chloride and hydrochloric acid solution, then drying the resulting solution. By this procedure, complexes of the type

may be prepared.

The exact structure of the aluminium chlorhydrate organic complexes is not known with certainty. They may be monomers or they may be polymeric materials containing oxo links. The preferred aluminium chlorhydrate organic complexes are complexes of 1,2-propylene glycol. Organic complexes of aluminium chlorhydrate are available under the U.K. Registered Trade Mark of REHYDROL.

The preferred alcohol solvent is ethanol. The ethnol may be the U.K. 74 over proof l.M.S. or the U.K. 64 over proof l.M.S. (I.M.S. is industrial methylated spirit). Conveniently, the solvent may be allowed to evaporate at ambient temperature until the solution will form a filament, or the solvent may be evaporated under reduced pressure at a temperature higher than ambient temperature.

The calcia-silica and calcia-alumina filaments provided by the invention may be used in this drawn form, for example in sound insulation. Alternatively, the filaments and the other filaments provided by the invention may be fired to give refractory ceramic fibres. The firing temperature and firing time required depend on the composition of the filament.

Fibres made according to the present invention may be used as thermal insulating materials, also in the construction of thermal insulating units based on fibrous materials. In some constructions, the filaments may be used as prepared and subsequently fired to remove any remaining organic material. The spinel ceramic fibres nd the cordierite ceramic fibres made according to the invention may be used to manufacture a lining element for the hot-face surface of a furnace following the procedures set out in British Patents Nos. 1,466,241 and 1,481,571. They may also be used in the construction of the thermal insulating units and in the procedures described in British Patents Nos. 1,548,866 and 1,548,867 also in United States Patents Nos. 4,194,036 and 4,324,602.Ceramic fibres made according to the invention may also be used in the manufacture of the two-layer heat insulating liner for furnace construction which is described in British Patent No. 1,555,459 and in United States Patent 4,344,753. Vacuum forming techniques may be applied to the appropriate aggregated fibres.

Filaments and fibres made according to the present invention may be used as components of concrete aggregates and of refractory aggregates. Calcia-silica and/or calcia-alumina filaments or fibres are preferred as components of concrete aggregates. Incorporation of the filaments or fibres into concrete aggregates and refractory aggregates improves the properties of objects made from these aggregates.

The first and second aspects of the invention are illustrated by the following examples.

EXAMPLE I 5.24 g calcium chloride dihydrate CaCI2. 2H2O crystals were dissolved in a mixture of 20 cm3 ethanol and 2 cm3 water. To this solution was added 0.5 cm3 of 0.2 M hydrochloric acid (HCI) solution and 209 technical ethyl silicate (silica equivalent 40% by weight). The mixture was refluxed for 90 minutes. After refluxing, filaments about 20cm long could be pulled from the resulting solution. Filaments could be pulled from the solution for up to 75 minutes after the refluxing ceased. A filament of composition 20% CaO; 80% SiO2 by weight was given. The ethanol can conveniently be the U.K. 74 over proof industrial methylated spirit.

The filaments gave a calcia-silica fibre after air-drying to remove solvent and then firing for 2 hours at about 800"C.

EXAMPLE 2 1009 aluminium chlorhydrate (A12(OH)5CI- sold under the Registered Trade Mark MICRO-DRY and 23.31 g magnesium chloride hexahydrate MgCI2.6H2O crystals were dissolved in 200 cm3 of water. 100 cm3 of water was removed by distillation under reduced pressure giving a solution from which filaments 17cm long could be drawn. The solution gelled on standing, giving a clear coherent and rigid gel.

The filaments and gel were fitted for two hours at 1000 C. Examination by X-ray diffraction showed that the crystalline phase in the ceramic fibres formed was spinel and that the crystalline phase formed from the gel was spinel.

EXAMPLE 3 1009 aluminium chlorhydrate (Al2(OH)5CI- sold under the Registered Trade Mark MICRO-DRY and 11.959 calcium chloride dihydrate CaCI2.2H2O crystals were dissolved in 200 cm3 of water.

About 112cm3 of water was removed by distillation under reduced pressure giving a solution from which filaments 17cm long could be drawn. The solution gelled on standing giving a clear coherent and rigid gel.

The filaments and gel were fired for two hours at 1000 C. Examination by X-ray diffraction showed that the crystalline phase in the ceramic fibre formed was gamma-alumina and that the crystalline phase formed from the gel was gamma-alumina.

The following further example illustrates the third and fourth aspects of the invention.

EXAMPLE 4 37.68g of magnesium dichloride hexahydrate MgCI2.6H2O crystals and 69.619 of technical ethyl silicate (silica equivalent 40% by weight) were dissolved in 200cm3 ethanol and to this solution was added 50g of the aluminium chlorhydrate propylene glycol complex sold under the Registered Trade Mark REHYDROL as REHYDROL 11. 140 cm3 of solvent were removed by distillation under reduced pressure giving a solution from which filaments 15cm long could be drawn. The solution gelled on standing giving a clear, coherent and rigid gel. The solution has the oxide stoichiometry required for cordierite, 2Mg0. 2Al203. 5SiO2. The ethanol can conveniently be the U.K. 74 over proof industrial methylated spirit.

The filaments and gel were fired for two hours at 1000"C. Examination by X-ray diffraction showed that the filaments had formed cordierite fibres and the gel had also been converted to cordierite.

Claims (8)

1. A method of preparing a filament which is characterised by the use, as a precursor solution, of a calcium or a magnesium salt dissolved in a lower alkyl silicate and/or an aluminium hydroxyhalide.

2. A method according to Claim 1 wherein the lower alkyl silicate is in the form of a hydrolysate.

3. A method according to Claim 1 or Claim 2 wherein the said salts are chlorides.

4. A method according to any preceding claim wherein the precursor solution contains a magnesium salt, an alkyl silicate and an aluminium hydroxyhalide characterised in that the aluminium hydroxyhalide is in the form of an organic complex.

5. A method according to Claim 4 wherein the organic complex is an aluminium chlorohydrate propylene glycol complex.

6. A method according to Claim 4 or Claim 5 wherein the solution has the oxide stoichiometry of cordierite.

7. A modification of the method according to any preceding claim wherein a clear coherent gel instead of a filament is prepared.

8. A method of preparing a ceramic fibre by firing a filament prepared according to any of Claims 1 to 6.