GB2210034A - Production of gels and filaments - Google Patents

Abstract

A liquid precursor for a clear gel, possibly a filament, is formed by reacting an alkyl silicate with an aluminium hydroxychloride. Solvents for the reaction system are ethanol, propylene glycol N, N-dimethylformamide or mixtures thereof.

Description

PRODUCTION OF GELS AND FILAMENTS DESCRIPTION This invention relates to the preparation of liquid precursors from which clear coherent gels may be obtained. The gel may be in the form of a filament, prepared from the gelling precursor by processes such as extruding, spinning or drawing. On firing, the filament converts to a ceramic fibre.

British Patent Application 2,173,179A describes the preparation of a solution from which a clear coherent gel can be obtained, by reacting an aluminium hydroxyhalide of the formula A12(OH) n 6-n where x is C1, Br or I and n is 5 or less, or a polymer hydrate, hydroxy complex or zirconium complex thereof, with water and a lower alkyl silicate. European Patent Application 01197,686 describes a precursor solution, based on a complex of an aluminium chlorhydrate and a polyol, from which a filament may be drawn or a clear, coherent gel obtained.British Patent Application 2,184,430A describes the preparation of filaments and ceramic fibres 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 may be in the form of an aluminium chlorhydrate propylene glycol complex.

To obtain clear transparent monolithic materials from the clear coherent gels whose preparationiis described in British Patent Application No. 2,173,179A, or in European Patent Application No.

0,197,686 the ethanol solvent must be removed. Rapid removal of the ethanol solvent results in the crumbling of the gel into grains of a few mm volume.

Solvent removal must therefore be slow. If a liquid of low vapour pressure is added to the ethanol solvent, the rate of removal of solvent is reduced. A low molecular weight diol such as ethylene glycol or propylene glycol is suitable. A mixture of ethanol with ethylene glycol or propylene glycol must be used to ensure that the solution remains homogeneous.

Alternatively, N,N-dimethylformamide may be the material of low vapour pressure added to the ethanol solvent. A mixture of ethanol and N,N-dimethylformamide must be used to ensure that the solution remains homogeneous. Ethylene glycol, propylene glycol and N,N-dimethylformamide are bidentate ligands and it is thought that this property helps to reduce stress during drying, thus preventing cracking as drying proceeds.

According to the present invention a method for the preparation of a liquid precursor from which a clear coherent gel may be obtained comprising reacting water, a lower alkyl silicate and an aluminium hydroxychloride of the general formula A12(OB)nC16 or a polymer-or a polyol complex thereof in a water-miscible solvent is characterised in that:: (i) when n is 5 the solvent is a mixture of ethanol and propylene glycol, or (ii) when n is 4 or when the aluminium hydroxychloride is a polymer, the solvent is ethanol a a itre of ethanol and propylene glycol, or (iii) when the aluminium hydroxychloride is a polyol complex, the solvent is a mixture of ethanol with either propylene glycol or N,N-dimethylformamide, or is ethanol when the lower alkyl silicate is either tetraethoxysilane or tetramethoxysilane, the clear coherent gel being optionally in the form of a filament convertible to a ceramic fibre on firing.

Gels fdr binding refractory grains may be obtained from solutions of aluminium halohydrates, preferably chlorhydrates, 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 1,356,248 and in United States Patent 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 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. All these solutions require a gel-inducing accelerator for useful binding of refractory grains. Normally, this gellation-inducing accelerator is basic.A dilute solution of ammonia, optionally containing ammonium acetate to control the pH, is usually used. It is important to note that the formation of filaments and/or clear coherent gels from the liquid precursor solutions whose preparation is described in the present invention 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.

The aluminium hydroxychlorides used in the present invention have the general formula A12(OH)nC16 n or a polymer or hydrate thereof, where n is 4 or 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 1;006,384.

Suitable aluminium hydroxychlorides are available under the U.R. Registered Trade Marks of CHLORHYDROL and tiICRO-DRY. In these materials, n is 5 in the formula Al2 (OH) nCl 6-n Aluminium hydroxychlorides of the formula A12(OH)5Cl.mH20 where m is a number less than 4, are available and may be used. Usually it is convenient to dissolve the aluminium hydroxychloride in water. An aqueous solution of the aluminium hydroxychloride A12(OH)4C12 (n is 4 in the formula Al2(OH)nCl6-n) is is available and may be used. An aqueous solution of the aluminium hydroxychloride polymer A113(OH)24C115('aluminium sesquichlorhydrate') is also available and may be used.

The aqueous solution of the aluminium hydroxychloride A12(OH)4C12 ('aluminium dichlorhydrate') has the following properties: Aluminium 10.1% w/w Chlorine 13.13% w/w Al:Cl atomic ratio 1.01 : 1 pH 3.40 The aqueous solution of the aluminium hydroxychloride polymer ('aluminium sesquichlorhydrate') has the following properties: Aluminium 11.0% w/w Chlorine 11.2% w/w Al:Cl atomic ratio 1.29 : 1 pH 3.70 Aluminium hydroxychloride polymers such as aluminium sesquichlorhydrate may be obtained by the hydrolysis of aluminium chlorides, preferably under basic conditions.

The present invention uses a complex formed from an aluminium hydroxychloride and a polyol containing 2 to 6 carbon atoms and at least 2 hydroxy groups. The preferred complexes are formed from aluminium hydroxychlorides or chlorhydrates represented by the formula Al2 (OH) nCl 6-n' where n is 1 - 5 and polyols selected from the group comprising 1,2-propylene glycol l,l,l-trimethylolpropane 1,3-butane diol 1,2,3-trihydroxypropane 2-methyl 2,4-pentane diol 2,2-dimethyl 1,3-dihydroxypropane The preferred aluminium chlorhydrate is a chlorhydrate in which n is 5. The preparation of the preferred aluminium chlorhydrate complexes is described in British Patent 1,159,658 and in United States Patent 3,420,932. The complexes are obtained from aluminium chlorhydrate. British Patent 1,009,959 describes the preparation of aluminium chlorhydrate organic complexes by reacting an aluminium compound of the formula Al2 (OH) nCl n 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 chlorydrate organic complexes may also be directly prepared from aluminium as described in British Patent 1,267,959, also as in United States Patent 3,507,859.

Aluminium metal is treated with a polyhydroxy compound having two 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 A12(OH) Cl (H20) 4.9-5.1 0.9-1.1 0.7-1.1

0.7-1.3 may be prepared.

The exact structure of the complexes has not been determined unambiguously. They may be monomers and/or polymers containing oxo links. The preferred complexes contain 1,2-propylene glycol. Organic complexes of aluminium chlorhydrate are available under the U.K. Registered Trade Mark REHYDROL.

The lower alkyl silicates preferred in the present invention are organic silicates containing at least one C1 - C6 alkyl group. One example of a class of suitable lower alkyl silicates is the tetra [C1 - C6 alkoxy] silanes, for instance tetramethoxysilane and tetraethoxysilane. The four alkoxy groups may be the same, for convenience. The preferred lower alkyl silicate is ethyl silicate, which is traditionally prepared by treating tetrachlorosilane with ethanol.

In the industrial preparation the ethanol used inevitably contains some water (industrial spirit) hence the product obtained consists of a mixture of tetraethoxysilane (ethyl orthosilicate) 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 (Si02) equivalent to about 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. Other methods for the preparation of tetraethoxysilane and ethoxy polysiloxanes are given in European Patent No.

0,004,730; Canadian Patent 1,172,649; United States Patent No. 4,211,717 and British Patent 2,017,129.

Tetraethoxysilane and tetramethoxysilane may be prepared by treating tetrachlorosilane with the appropriate alcohol. The alcohol must be anhydrous to obtain a high yield of the alkoxysilane and the use of a hydrogen chloride acceptor is advantageous, especially in the preparation of tetramethoxysilane.

Other procedures for the preparation of tetraethoxysilane and tetramethoxysilane are known and may be used.

The ethanol solvent may be the U.K. 74 over proof I.M.S. or the U.K. 64 over proof I.M.S. (I.M.S.

is industrial methylated spirit). 'Absolute' alcohol may also be used.

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.

Ceramic fibres made according to the invention may be used to manufacture å lining element for the hot-face surface of a furnace following the procedures set out in British Patents 1,466,241 and 1,481,751. They may also be used in the construction of the thermal insulating units and in the procedures described in British Patents 1,548,866 and 1,548,867 also in United States Patents 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 1,555,459 and in United States Patent 4,344,753. Vacuum forming techniques can be applied to the aggregated filaments or fibres.

Transparent monolithic materials made according to the present invention may be used as sight-glasses for furnaces.

Precursor filaments from which ceramic fibres may be obtained are conveniently formed by drawing filaments from the liquid precursor. A simple method of assessing the ability of a precursor solution to form filaments is to insert into and pull out of the solution a glass rod 10mum in diameter.

Other methods of obtaining the filaments are known and may be used if desired. The filaments may be converted to ceramic fibre by firing in air. One procedure for achieving the slow drying rate necessary for conversion of the transparent coherent gel formed from the liquid precursor into a monolithic material is to cover the gelowith aluminium foil pierced with several lmm diameter holes.

The invention is illustrated by the following examples.

EXAMPLE 1 A solution of technical ethyl silicate (2.0g) in varying amounts of ethanol was added to a vigorously stirred solution of aluminium sesquichlorhydrate (7.45g). The resulting solutions were allowed to stand at ambient temperature until gelation occurred. Table I gives the gelation time and the characteristics of the resultant gel. The solutions have the oxide stoichiometry 2.27 A1203: 2 Si02.

TABLE I Effect of ethanol on gel formation from aluminium sesquichlorhydrate and technical ethyl silicate.

Vol. of ethanol Gelation time Gel.characteristics added (cm3 ) (days) 5.0 2 Cloudy 7.0 3 Slightly cloudy 9.0 4 Clear, coherent 11.0 5 Clear, coherent 13.0 6 Clear, coherent 17.0 8 Clear, coherent 21.0 8 Clear, coherent 25.0 12 Clear, coherent All gels were coherent gels. Clear, coherent gels were formed from clear, homogeneous solutions.

EXAMPLE 2 A solution of technical ethyl silicate (lO.OOg) in varying amounts of ethanol was added to a vigorously stirred solution of aluminium sesquichlorhydrate (37.08g). The mixture was brought to reflux temperature and refluxed. Gelation occurred about 5 minutes after the time of refluxing. The results are given in Table II. The solutions have the oxide st)ichiometry of 2.26 A1203 : 2 SiO2 TABLE II Effect of ethanol under reflux conditions.

Vol.of ethanol Time to reach Reflux time Gel characteristics added (cm3) Reflux (min.) (min.) 25 7 4 Cloudy 55 6 13 Cloudy 65 7 9 Slightly turbid 85 8 14 Slightly turbid 105 9 10 Cloudy All gels were coherent gels.

These results show that the reaction between technical ethyl silicate and aluminium sesquichlorhydrate to form a clear coherent gel is best carried out at ambient temperature.

EXAMPLE 3 A solution of technical ethyl silicate (2.0g) in varying amounts of a mixed solvent comprising 25% v/v propylene glycol and 75% v/v ethanol was added to a vigorously stirred solution of aluminium sesquichlorhydrate (7.45g) . The resulting solutions were allowed to stand at ambient temperature until gelation occurred. Table III gives the gelation time and characteristics of the resultant gel. The solutions have the oxide stoichiometry 2.27 A1203: 2 Si02.

TABLE III Effect of solvent on gel formation from technical ethyl silicate and aluminium sesquichlorhydrate.

Volume of Solvent Gelation time Gel characteristics added (cm 3 ) (days) 5.0 25 Cloudy 7.0 6 Cloudy 9.0 7 Slightly cloudy 11.0 14 Slightly cloudy 13.0 12 Clear and coherent 17.0 19 Clear and coherent 21.0 22 Clear and coherent 25.0 33 Clear and coherent All gels were coherent gels. Clear, coherent gels were formed from clear, homogeneous solutions.

EXAMPLE 4 A solution of technical ethyl silicate (2.00g) in varying amounts of ethanol was added to a vigorously stirred solution of aluminium dichlorohydrate (8.12g). The resulting solutions were allowed to stand at ambient temperature until gelation occurred. Table IV gives the gelation time and the characteristics of the resultant gel. The solutions have the oxide stoichiometry 2.27 A1203 : 2 Si02.

TABLE IV Effect of ethanol on gel formation from aluminium dichlorhydrate and technical ethyl silicate.

Volume of ethanol Gelation time Gel characteristics added (cm3) (days) 5.0 2 Cloudy 7.0 3. Slightly cloudy 9.0 5 Slightly cloudy 11.0 9 Slightly cloudy 13.0 13 Clear, coherent 17.0 12 Clear, coherent 21.0 8 Clear, coherent 25.0 10 Clear, coherent All gels were coherent gels. Clear, coherent gels were formed from clear, homogeneous solutions.

EXAMPLE V A solution of technical ethyl silicate (lO.llg) in varying amounts of ethanol was added to a vigorously stirred solution of aluminium dichlorhydrate (40.38g). The mixture was brought to reflux temperature and refluxed. Gelation occurred about 5 minutes after the time of refluxing.

The solutions have the oxide stoichiometry 2.27 A1203: 2 SiO2 . The results are given in Table V and show that the reaction between technical ethyl silicate and aluminium dichlorhydrate to form a clear coherent gel is best carried out at ambient temperature.

TABLE V Effect of ethanol solvent under reflux conditions.

Vol.of ethanol Time to reach Reflux time Gel character added (cm3) reflux time (min) istics 25.0 8 4 Cloudy 35.0 5 7 Cloudy 45.0 6 7 Cloudy 55.0 11 8 Cloudy 65.0 7 10 Slightly turbid 85.0 7 17 Slightly turbid 105.0 10 25 Slightly turbid 125.0 10 36 Cloudy All gels were coherent gels.

EXAMPLE 6 3.28g aluminium chlorhydrate A12(OH)5C1 (CHLORHYDROL or MICRO-DRY) was dissolved in 3.28 cm3 distilled or deionised water. To this solution was added with vigorous stirring a solution of 2.00g technical ethyl silicate in varying amounts of a mixed solvent containing 25% v/v propylene glycol and 75% v/v ethanol. The resulting solutions were allowed to stand at ambient temperature until gelation occurred.

Table VI gives the gelation time and characteristics of the resultant gel. The solutions have the oxide stoichiometry 2.27 A1203 : 2 Si02.

TABLE VI Effect of solvent on gel formation from cluminium chlorhydrate and technical ethyl silicate.

Volume of solvent Gelation time Gel characteristics added (cm)3 (days) 5 9 Cloudy 7 13 Cloudy 9 11 Slightly turbid 11 15 Slightly turbid 13 18 Slightly turbid 17 22 Clear, coherent 21 30 Clear, coherent All gels were coherent gels. Clear, coherent gels were formed from clear, homogeneous solutions.

EXAMPLE 7 13.44g of an aluminium chlorhydrate propylene glycol complex sold under the Trade Mark of REHYDROL as REHYDROL 11 was dissolved in a solution comprising 5.00g technical ethyl silicate dissolved in varying amounts of a solvent comprising 25% v/v propylene glycol and 75% v/v ethanol. In some cases, varying amounts of water were also added. The resulting solutions were allowed to stand at ambient temperature until gelation occurred. Table VII gives the gelation time and characteristics of the resultant gel. The solutions have the oxide stoichiometry 2.99 A1203 : 2.0 Si02.

TABLE VII Effect of solvent on gel formation from technical ethyl silicate and aluminium chlorhydrate propylene glycol complex.

Vol.of solvent Vol.of water Gelation Gel character added (cm3) added (cm3) time (days) istics 17.5 Nil 12 Clear, coherent 22.5 Nil 17 Clear, coherent 27.0 Nil 20 Clear, coherent 17.5 0.75 11 Clear, coherent 17.5 3.3 | 10 Clear, coherent 17.5 5.5 13 Slightly turbid All gels were coherent gels. Clear, coherent gels were formed from clear, homogeneous solutions.

EXAMPLE 8 13.44g of an aluminium chlorhydrate propylene glycol complex sold under the Trade Mark of REHYDROL as REHYDROL II was dissolved in a solution consisting of 5.00g technical ethyl silicate and varying amounts of a solvent comprising 50% v/v ethanol and 50% v/v N,N-dimethylformamide. In some cases, varying amounts of water were also added. Gelation occurred about 5 minutes after the time of refluxing.

The solutions have the oxide stoichiometry 2.99 Al203 : 2 Si02. Table VIII gives the gelation time and characteristics of the resultant gel.

TABLE VIII Gel formation from technical ethyl silicate and aluminium chlorhydrate propylene glycol complex using 50% v/v ethanol and 50% v/v N,N dimethylformamide as solvent.

Volume of sol- Vol. of Water Time to reach Reflux Gel characteristics vent added (cm3) reflux (mins) time (mins) added (cm3) 27.0 1.5 6 93 Clear, coherent 27.0 2.5 3 104 Clear, coherent 27.0 3.5 7 97 Clear, coherent 27.0 4.5 3 82 Cloudy, 27.0 5.0 9 67 Cloudy, 27.0 5.5 5 65 slightly turbid 27.0 6.5 4 72 cloudy 27.0 7.5 2 95 Cloudy 17.0 2.5 7 36 Clear, coherent 19.0 2.5 4 50 Clear, coherent 21.0 2.5 8 69 Clear, coherent 23.0 2.5 3 67 Clear, coherent 25.0 2.5 3 84 Clear, coherent 29.0 2.5 3 104 Clear, coherent 31.0 2.5 3 133 Slightly turbid All gels were coherent gels. Clear, coherent gels were formed from clear homogeneous solutions.

EXAMPLE 9 13.44g of an aluminium chlorhydrate propylene glycol complex sold under the Trade Mark of RERYDROL as REHYDROL II was dissolved in a solution consisting of 5.00 g technical ethyl silicate dissolved in 27 cm3 of a solvent comprising varying amounts of ethanol and N,N-dimethylformamide.

2.5 cm3 water was added and the resulting solution was brought to reflux temperature and refluxed. Gelation occurred about 5 minutes after refluxing. Table IV gives the characteristics of the resultant gels. The solutions have the oxide stoichiometry 2.99 A1203: 2 Si02. TABLE IX Effect of solvent composition on gel formation from technical ethyl silicate and aluminium chlorhydrate-propylene glycol complex.

SOLVENT COMPOSITION Time to reach Reflux Gel cheracteristics reflux (mins) time (mins) Ethanol (% v/v) D.M.F. (%v/v) NIL 100 7 17 Clear, coherent 20 80 3 32 Clear, coherent 40 60 3 64 Clear, coherent 60 40 4 78 Clear, coherent 80 20 4 74 Slightly turbid 100 NIL 3 83 Slightly turbid D.M.F. = N,N-dimethylformamide All gels were coherent gels. Clear coherent gels were formed from clear, homogeneous solutions.

EXAMPLE 10 lOOg of aluminium chlorhydrate-propylene glycol complex sold under the Trade Mark REHYDROL as REHYDROL II was dissolved in a solution of 53.04g tetraethoxysilane in 200 cm3 ethanol. After removal of about 118 cm 3 solvent by distillation, filaments could be drawn and a clear coherent gel was formed on standing. The filaments were converted to a ceramic fibre on firing to 10000C in air.

The solution has the oxide stoichiometry 2.99 A1203 : 2 SiO2. DTA and XRD examination of the filament and the ceramic fibre formed on firing gave the following results: DTA - Endotherm at ca 250 C - loss of volatiles Broad exotherm at 500-6000C - oxidation of organic material Sharp exotherm peak at ca - formation of 980 0C crystallites X-ray diffraction (XRD) showed the formation of crystallites to start at 9000 C during the conversion of the filament to a ceramic fibre by firing in air.The formation of mullite, together with gamma alumina is first seen by XRD in the material fired to 10000C. On firing to 1100 C, XRD shows that more mullite plus gamma alumina is present and above this temperature only mullite is seen, the conversion to a mullite ceramic fibre being substantially complete at 1300 0C.

EXAMPLE 11 lOOg of aluminium chlorhydrate - propylene glycol complex sold under the Trade Mark of REHYDROL as REHYDROL II was dissolved in a solution of 38.76g tetramethoxysilane in 200 cm3 ethanol After removal of about 110cm3 solvent by distillation, filaments could be drawn and a clear coherent gel was formed on standing. The filaments were converted to a ceramic fibre on firing in air to 1000 0C. The solution has the oxide stoichiometry 2.99 A1203 : 2 Si02.

DTA and XRD examination of the filament and the fibre formed on firing gave the following results: DTA - Endotherm at ca 260 C - loss of volatiles Broad exotherm at 400-6000C - oxidation of organic material Sharp exotherm peak at - formation of ca 9800C crystallites.

XRD showed that during the conversion of the filaments to a ceramic fibre by firing in air, the formation of crystallites started at 9000C. The formation of mullite, together with gamma alumina is first seen by XRD in the material fired to 1000 0C. On firing to 11000C XRD shows that more mullite plus gamma alumina is present and above this temperature only mullite is seen, the conversion to a mullite ceramic fibre being substantially complete at 13000C.

Claims (4)

1. A method for the preparation of a liquid precursor from which a clear coherent gel may be obtained comprising reacting water, a lower alkyl silicate and an aluminium hydroxychloride of the general formula Al2 (OH) nCl n or a polymer or a polyol complex thereof in a water-miscible solvent is characterised in that: (i) when n is 5 the solvent is a mixture of ethanol and propylene glycol, or (ii) when n is 4 or when the aluminium hydroxychloride is a polymer, the solvent is ethanol or a mixture of ethanol and propylene glycol, or (iii) when the aluminium hydroxychloride is a polyol complex, the solvent is a mixture of ethanol with either propylene glycol or N,N-dimethylformamide, or is ethanol when the lower alkyl silicate is either tetraethoxysilane or tetramethoxysilane.

2. A method as claimed in Claim 1 wherein the clear coherent gel is in the form of a filament convertible to a ceramic fibre on firing.

Methods substantially as described in the Examples.

4. Gelled products, fired or otherwise, produced by a method as claimed in any of the preceding claims.