WO2008151665A1

WO2008151665A1 - Sol-gel process

Info

Publication number: WO2008151665A1
Application number: PCT/EP2007/055829
Authority: WO
Inventors: Lorenzo Costa; Lucia Gini; Gabriele Lugli
Original assignee: Degussa Novara Technology S.P.A.
Priority date: 2007-06-13
Filing date: 2007-06-13
Publication date: 2008-12-18
Also published as: TW200911689A

Abstract

In a sol-gel process the sol is modified to the gel by the change of the pH-value of the sol, whereby in order to change the pH-value at least one salt is added to the sol.

Description

Sol-gel process

The invention concerns a sol-gel process to make a gel, an aerogel or a glass monolith.

In general, all sol-gel processes which are used to make a gel, an aerogel or a glass monolith, start with the preparation of the sol.

It is well known, that the sol might be prepared in several different ways.

One example of sol preparation is by hydrolizing metal- alkoxides (US 5,264,197).

Alternatively sols can be prepared from mixtures of alkoxides and oxides in fine powder, specially silicon alkoxides and fumed silica (US 4,801,318 and US 5,207,814).

Furtheron from EP 0 974 557 Al it is known that a sol can be prepared from oxides and particulary from silicon oxide in the form of fumed silica only.

This document EP 0 974 557 Al describes a process for fabricating a silicon body, comprising the steps of:

Providing a silica dispersion having at least 500 ppm of dissolved silica; inducing at a pH of about 10.5 or greater, gelation of the dispersion into a gel body and drying the gel body to remove water.

The dispersion of silica is made by dispersing fumed silica in water and the addition of TMAH to raise the pH value from 2 to about 10 and to stabilize the dispersion.

The gelation can be induced by adding a gelling agent, which lowers the pH of the dispersion to the gel point. The gelling agent can be a water-soluble liquid that under goes hydrolysis to consume base, e.g. an ester, amide or an alkyl halide and thereby lowers the pH.

Furtheron the document EP 0 974 557 Al describes that the addition of ammonium salts of weak acids, such as ammonium formate or ammonium citrate tend to cause inhomogeneous gelation.

Therefore it seems to be without any advantage to use ammonium salts of weak acids to lower the pH.

The document EP 0 974 557 Al does not describe the use of TEOS (Tetraethoxysilane) or other alkoxides.

Sols can be differentiated by the precursors used in their preparation; but that is not all, concentration also can play an important role in this respect, since it can characterize the properties of several kind of sols (Journal of Non-Crystalline Solids 71 (1985), 361-371).

Moreover, several other features can be indicated as important for identify sol typology. Among these the one that eventually plays the central role in determining sol- characteristics, particularly its activity, is the value of the hydrogen ions concentration in the sol, or pH of the sol (Journal of Sol-Gel Science and Technology 26, 63-66, 2003, Lorenzo Costa "High purity glass forms by a colloidal sol-gel process; Sol-Gel Science by C. Jeffrey Brinker and George W. Scherer, pages 99-107, Academic Press, 1990).

It is commun practice to prepare acid sols, specially if very fine microstructure of the gel is sought (Journal of Sol-gel Science and Technology 26, 63-66, 2003; Sol-gel science, C. Jeffrey Brinker, George W. Scherer, p. 99-107, Academic Press, 1990) .

Acid sols, particularly silicic acid sols, always contain alkoxides precursors, since silica does not hydrolize under acid conditions (US 10/332096) . Such acid sols, either containing only silicon-alkoxides, or alternatively fumed silica and silicon alkoxides (in a kind of composite sol), are prepared at pH around 2, since for silicon alkoxides at this pH hydrolysis prevails over silanols condensation.

It is commun practice, under these conditions, after hydrolysis is completed and before the sol is poured into moulds, to condition the same for gelation in a reasonable time, typically from one to a few hours, by rising its pH value from 2 to vicinity of 4. This is accomplished with the use of a free base, which is forcibly dilute and slowly added to a well stirred sol in order to avoid local conditions of pH non-homogeneity, that would result into local spots of irreversible microgelation .

The change of the pH-value represents a delicate step for maintaining control of process conditions, as it is well known, that substantial pH modification by adding a dilute base will clearly increase the sol volume, and this increase of the volume must be predicted and compensated in an accurate programmation for maintaining the sol characteristics. Moreover good stirring of the sol while adding the base is one necessary requirement, but furtheron avoiding as much as possible to dissolve gases into the sol is another necessary, even if conflicting, requirement.

Further difficulties may be the non compatibility of alcaline bases with the Sol-gel process, leaving for the purpose only amines, most frequently ammonia, the volatility of which often is another problem.

On the other hand, also the so called "alkaline sols" (or high pH sols) have a substantial role in Sol-gel technology. Typical "alkaline sols" are made with only silicondioxide powders of adeguate purity and morphology. Examples of silicon dioxide powders suitable for the purpose are the so called "fumed silica", "pyrolytic silica" like AEROSIL^® etc. To hydrolyze silica a pH value superior to about 11,5 is required, typically around 12,5 achievable with amines and/or ammonia.

In the case of "alkaline sols" the alkoxide is not required for the process of gelation, since at the pH of "alkaline sols" oxides usually hydrolize, particulary silica.

It is necessary instead, after enough hydrolysis is achieved, to lower the pH to obtain gelation of the sol.

The operation is the opposite of that described for the acid sol analogy. To achieve the gelation it is now necessary to lower the pH value of the sol, from about 12,5 to about 10.

This is achieved by the addition of water-soluble hydrolysable esters, typically methyl or ethyl formate to a dispersion of fumed silica (see Debra A. Fleming et al . in US 4.775.401 or US 5.240.488).

Other gelling agents like esters or amides are known from EP 0 974 557 Al.

Again the operation of changing the pH value for the all mass of the sol, specially with high volumes, for the same reasons previously discussed, remains a critical process- step .

The US 5,379,364 describes a "pH - increasing ingredient" and a "pH - decreasing ingredient" used to reach a pH of about 9.5 in the sol prior to gelation, where "pH- increasing ingredient is at least one quaternary ammonium hydroxide " (claim 10), or (alternatively) where " pH - increasing ingredient is at least one compound selected from the group consisting of tetra-methyl-ammonium hydroxide and tetra-ethyl-ammonium-hydroxide, and where pH- reducing ingredient consist essentially of water soluble aliphatic ester of an acid selected from the group consisting of formic acid, lactic acid, and glycolic acid.

According to the present invention it has been found that it is possible to avoid all the problems connected with the modification of pH in the sol, even for large volumes of sol and large modification of pH, using high concentrated solutions of suitable salts or even the dry of the suitable salts, that will provoke the rapid change of pH, without causing microgelations in the sol itself and without appreciably modifying its volume, so simplifying relevant procedures of the process and increasing process efficiency.

Moreover, "suitable salts" often have the function of pH buffer in aqueous solutions. In those cases, when the buffered pH value coincides with the desired value of pH of the sol, the most desired condition for the process is verified.

The subject of the invention is a sol-gel process, whereby a sol is modified to a gel by the change of the pH-value of the sol, characterized in that in order to change the pH- value from a starting pH-value to a final pH-value at least one suitable salt is added to the sol.

The suitable salt can be added to increase or to rise the pH-value. In another form of the invention the suitable salt can be added to lower the pH-value.

The starting pH-value can be lower than 3, preferred 0.5 to 2.9. Then the final pH-value can be 3.5 to 5.

If an alkaline dispersion of the silica is used the final pH-value can be 8.0 to 9.5, whereas the starting pH-value can be 10.5 or greater.

The suitable salt can be a salt of a weak acid and of a weak base. In another form of the invention the suitable salt can be a salt of a strong acid and a weak base. In a further form of the invention the suitable salt can be a salt of a weak acid and a strong base.

The suitable salt can be added to the sol after having been solved in a solvent. In a preferred feature of the invention the suitable salt can be added to the sol in order to increase the pH-value.

The quantity of the suitable salt can be calculated by the dissociation constant, confirmed by titration in an experimental way. The salt can be added in any concentration, if used in solution. The solvent can be water. The aqueous solution can contain Ethanol or Acetone.

The suitable salt apparently does not have any impact on the polymerisation process except the change of the pH- value, which is important for the start of the polymerisation .

In a preferred feature of the invention the sol-gel process is a method using TEOS as one educt, which is hydrolysed to result in a sol, whereby the pH-value of the sol an acidic one. In a preferred feature of the invention the pH-value of the sol is lower than 3, especially between -0.5 and 2.9.

Furtheron the sol can contain silica, which can be produced pyrogenically . These sol-gel processes are known methods, except the use of the gelling agent. The method according to the invention can be used in the sol-gel process of the type described in US 5,207,814. These methods for preparing monoliths of an aerogel of at least one oxide comprises the following operating steps:

(a) mixing an alkoxide of at least one metal with water in the presence of an acidic catalyst to form a mixture; (b) hydrolyzing the mixture obtained in step (a) to form a hydrolysate;

(c) adding an oxide of at least one metal as a fine powder having a relatively high specific surface area to the hydrolysate of step (b) to form a colloidal solution;

(d) causing the colloidal solution of step (c) to become a gel;

(e) washing the gel;

(f) drying the gel at values of temperature and pressure which are higher than the critical pressure and temperature values of the solvent used for the washing to form said monolith.

According to the invention the suitable sol can be added in the step (d) of the process according to the US 5,207,814 and/or PCT/EP 2003/014759.

Furtheron the sol can be made by the addition of an tetraalkylorthosilan to a dispersion of fumed silica as it is known from WO 2006/094874. According to WO 2006/094874 the process for the production of monoliths, in particular of glass, by means of the invert sol-gel process comprises the following steps:

(a) dispersion of a pyrogenically prepared oxide of a metal and/or metalloid to form an aqueous or water- containing dispersion;

(b) addition of metal alkoxide and/or metalloid alkoxide to the dispersion, which is optionally hydrolysed by means of water before the addition;

(c) mixing of the components to form a homogeneous colloidal sol; (d) optional removal of coarse contents from the colloidal sol;

(e) gelling of the colloidal sol in a mould;

(f) replacement of the water contained in the aerogel by an organic solvent;

(g) drying of the aerogel;

(h) heat treatment of the dried gel.

A "suitable salt" can be defined according to simple criteria referred to the sol: first it can be compatible with the sol and its planned objectives, typically avoiding to contaminate it with undesirable cationic species; secondly, at equilibrium in aqueous solution, it can exibit a pH-value suitable to modify the pH of the sol in the desired direction. For example: if the need is to rise the pH-value of the sol before gelation, the "suitable salt" solution should have a pH value higher than the original pH of the sol. Viceversa, in the opposite case, where the need instead would be to lower the sol's pH, the "suitable salt" solution should have a pH value lower than the pH of the original sol. The suitable salt can be added in the form of a powder or in the form of a concentrated solution in water. This has the advantage that the volume of the reaction mixture remains nearly unchanged.

Identified suitable salts for this invention are listed in table 1, table 2 and table 3. The suitable salt has to fulfil two general criteria:

First, it must be compatible with the sol and its planned objectives,

Secondly, it must exhibit a pH-value suitable to modify the pH of the sol in the desired direction.

The suitable salt according to the invention can be: A: All the salts of weak acids and weak bases. Since metal cations are already excluded by the first of the two general criteria, to this class will belong only the ammonium and/or tetraalkyl ammonium salts of weak acid.

B: All the salts of strong acids and weak bases.

Since metal cations are excluded by the first general criteria, will belong to this class only the salts of ammonium and/or tetraalkyl ammonium and mineral acids.

C: All the salts of weak acids and strong bases. Strong bases are here intended alkaline metals. All the alkaline metals are excluded in general by the first criterium, however there may be special cases, when the presence of an alkaline metal is not uncompatible with the sol objectives.

In these cases, even if unfrequent, the salt of alkaline metals and weak acids should be considered "suitable salts".

Table 1:

A: WEAK ACID AND WEAK BASE

NAME FORMULA pKal pH solution (approximate)

Ammonium Formate CH₅NO₂ 3,75 6,5

Ammonium Acetate C₂H₇NO₂ 4,76 7,0

Ammonium Propionate C₄H₁₁NO₂ 4, 87 7,1

Ammonium Butirate C₅H₁₃NO₂ 4, 82 7,0

Ammonium Hexanate C₆H₁₅NO₂ 4,85 7,1

Ammonium 2 - Ethylhexanate CsH₁₉NO₂ 4, 7,0

Ammonium Caprilate C₈H₁₉NO₂ 4,89 7,1 Ammonium Pivalate C₅Hi₀NO₂ 5,03 7,1

Ammonium Palmitate C₁₈H₃₉NO₂

Ammonium Stearate C₁₆H₃₅NO₂

Ammonium C₇H₁₅NO₂ 4,90 7,1 Cyclohexancarboxylate

Ammonium benzoate C₇H₉NO₂ 4,19 6,7

Ammonium Saliciate C₇H₉NO₃ 2,98 6,1

Ammonium Oleate C₁₈H₃₇NO₂

Ammonium Lactate C₃H₉NO₂ 3,08 6,2

Ammonium Oxalate (mono) C₂H₅NO₄ 1,23 5,2

Ammonium Malonate (mono) C₃H₇NO₄ 2,83 6,0

Ammonium Succinate (mono; C₄H₉NO₄ 4,16 6,7

Ammonium Maleate (mono) C₄H₇NO₄ 1,83 5,5

Ammonium Malate (mono) C₄H₉NO₅ 3,40 6,3

Ammonium Fumarate (mono) C₄H₇NO₄ 3,03 6,1

Ammonium Tartrate (mono) C₄H₉NO₆ 3,22 6,2

Ammonium C₈H₁₅NO₄

Cyclohexanbicarboxylate (mono)

Ammonium Phthalate (mono) C₈H₉NO₄ 2,89 6,1

Ammonium Terephthalate (mono) C₈H₉NO₄ 3,51 6,4

Ammonium Citrate (mono) C₆H₁₁NO₇ 3,14 6,2

Ammonium Citrate (dibasic) C₆H₁₄N₂O₇ 4,2

Ammonium Citrate (tribasic) Ammonium C10H19N3O8 1,70 5,5

Ethylendiamminotetraacetate (mono)

The series of the weak acids, that can form "suitable salts" is more extended, of course, than examples in table 1 illustrate. To better identify the weak acids series it has been somewhat organized in the following:

1) Salts of organic acids monocarboxylic, aliphatic, or aromatic. Typical examples of these acids are: Formic Acid, Acetic Acid, Propionic Acid, Hexanoic Acid, 2Ethylhexanoic Acid, Caprilic Acid, Versatic Acid, Palmitic Acid, Cyclohexancarboxylic Acid, Cyclohexaneacetic Acid, Benzoic Acid, Salicylic Acid, etc .

2) Salts of organic acids monocarboxylic, aliphatic unsatuturate as, for example oleicacid.

3) Salts of organic acid monocarboxylic containing in the aliphatic chain one OH group as, for example, in the lactic acid.

4) Salts or organic acids dicarboxylic, aliphatic, or aromatic, as for example, Oxalic Acid, Malonic Acid,

Succinic Acid, Malic Acid, Fumaric Acid, Tartaric Acid, Cyclohexandicarboxylicacid, Phthalic Acid.

5) Salts of organic acid, aliphatic, or aromatic, tri and tetracarboxylic as, for example Citric Acid and the Ethylendiamminotetraacetic Acid.

Table 2:

Bj_ STRONG ACIDS AND WEAK BASES

NAME FORMULA pkb Moles/1 pH solution (approximate; Ammonium Chloride H₄Cl N 4, 74 0, 10 5, 10

4, 76 1, 00 4, 63

Ammonium Sulfate 0, 10 5,50

Ammonium Phosphate H6NO4P 0, 20 4,20 monobasic

Ammonium Phosphate H9N2O4P 8,00 Dibasic

Ammonium Nitrate 0, 10 5,43

Ammonium Iodide 0,10 4, 60

Ammonium Fluoride

Ammonium Chloride

Table 3:

C: WEAK ACIDS AND STRONG BASES

NAME pkal Moles/1 pH solution (approximate)

Na or K Acetate 4,74 1,00 9,4

4.74 0, 10 8,9

Na or K Formate 3.75 1, 00 8,9 3,75 0,10 8,4

Na or K Henanate 4, 85 1, 00 9,4 4,85 0,10 8,9

Na or K Citrate 3, 14 1, 00 8, 6 3,14 0,10 8,1 Na or K NDTA

Na(HO₂CCHs)₂ N (CH₂) 2N(CH₂CO₂H₂) 2 1,70 1,00 7,9 K (HO₂CCH₂) ₂N (CH₂) ₂N (CH₂CO₂H₂) ₂ 1,70 0,10 7,4 In still another embodyment, where sols doped with metal cations under specific formulations are gelled by rising the sol pH using a free base, only opaque glass-ceramics can be obtained.

Under identical conditions, but rising the pH of the doped sol with "suitable salts", all the samples result in good transparent glasses with the level of dopant metal as programmed. In this case different materials (more desirable) are obtained with the proper use of "suitable salts": Also these relevant results are pursued as still another application of the present invention.

This invention is concerned primarily with the control of pH in the sol and particularly with large modifications of pH in the same, typically required when the sol is conditioned for gelation, before poring it into moulds.

The pH control of the sol, according to the present invention, achieved through the use of salts of appropriate formulation, offers important and general improvements to sol-gel processes.

EXAMPLES

Example 1

Gelation of "ALKOXIDE" SOL comparing the addition of free- base versus Ammonium Acetate

300.0 g of aqueous HCl (0.01N) were transferred in a 1 1 erlenmayer flask and maintained in good agitation with a magnetic stirrer. The temperature of the solution was 24 ⁰C. 100 g of TEOS (tetraethyl ortho silicate) were added gradually to the solution. The temperature was risen slowly to 30 ⁰C at the end of TEOS addition. The sol was sonicate by ultrasonic waves for degassing and homogenization and then cooled to room temperature. From this sol three parts, 94.3 g each, were transferred to three equal containers, originating three new samples of the same sol, labelled A, B, C, to the purpose of comparing different procedures for modifying pH of the sol before gelation.

SAMPLE A STANDARD PROCEDURE (free base added)

- The starting pH of the sol was 1.87.

To the sol 6.69 ml of Ammonia Solution 0,14M were added, drop by drop for a total of 0,937 mMoles with stirring.

- The time required was 12 minutes, to change the pH-value from the starting pH-value to the final pH-value.

- The final pH was 4.34.

- An uniform gelation was obtained in less than 5 hours.

SAMPLE B NEW PROCEDURE APPLYING THE CURRENT INVENTION

- The starting pH of the sol was 1.87.

- To the sol 0,22 ml of Ammonium Acetate Solution 10,16 M was transferred with stirring for a total amount of 2.24 mMoles.

- The final pH-value was 4.34.

- The time required was less than 5 seconds to change the pH-value from the starting pH-value to the final pH- value .

- A very uniform gelation was obtained in less than 5 hours .

SAMPLE C EXPERIMENTAL TRANSFER OF 0,937 m Moles

Of free - base (Ammonia Solution 0,14M) for comparison

- The starting pH of the sol was 1.87. - To the sol 6,69 ml Ammonia Solution 0,14M was transferred with stirring.

- The time which was required was less than 5 seconds to change the pH-value from the starting pH-value to the final pH-value.

- An instant gelation occurred without control, resulting in a heterogeneous mixture of liquid and gel.

- No accurate pH determination was possible.

The results of Example 1 illustrate the advantages that the procedure based on the current invention has over the well

- optimized, standard procedure. Among these advantages there are:

- A shorter time; standard procedure (sample A) , requires a time for pH modification that is more than two orders of magnitude longer than that required by the new procedure based on the current invention (sample B) ; No acceleration is possible for procedures based on addition of free - base, as evidenced by Sample C.

- A smaller added volume; standard procedure requires an increasing of sol volume that is 30 times that required by the new invention.

- Process reliability, the procedure made possible by the new invention does not implies critical control of process parameters like rate of addition and proper dilution of the active agent, but only the control of the total amount added, a significant simplification of the process that implies a lower probability of deviation from specifications due to human and/or random errors . Example 2

Gelation of "Alkoxide" sol using Ammonium Citrate Dibasic or Ammonium Citrate Tribasic

400 g of aqueous HCl 0,01N were transferred in a 1 1 erlenmayer flask and maintained in good agitation with a magnetic stirrer. The temperature of the solution was 25 ⁰C. To the solution 133.3 g of TEOS were added gradually. The temperature was risen slowly to 31 ⁰C at the end of TEOS addition. The sol was sonicated for degassing and homogeinization, then cooled to room temperature. From this sol were weighted two parts of 254,13 g each and transferred into two equal containers, and labelled SAMPLE A and SAMPLE B.

SAMPLE A - Test with Ammonium Citrate Dibasic

- The starting pH of the sol was 2.01.

- To the sol 0,75 ml Ammonium Citrate solution 5,0OM, equivalent to 3,75 mMoles added with stirring.

- The time required was less than 5 seconds to change the pH-value from the starting pH-value to the final pH- value. The final pH-value is reached when the polymerisation starts.

- The final pH was 4.05.

- Very uniform gelation in less than 5 hours.

SAMPLE B - Test with Ammonium Citrate Tribasic

- The starting pH of the sol was 2.01.

- To the sol 0.62 ml Ammonium Citrate solution 4,98 M/l, equivalent to 2.59 mMoles total amount of the suitable salt, was added with stirring. - The time required was less than 5 seconds to change the pH-value from the starting pH-value to the final pH- value. The final pH-value is reached when the polymerisation starts.

- The final pH was 4.15.

- A very uniform gelation occurres in less than 5 hours.

The results of Example 2 verify the conclusion of Example 1 with a different salts of Table 1. In this case Ammonium Citrate Dibasic and Ammonium Citrate Tribasic are producing results very similar to those obtained in example 1 with Ammonium Acetate.

Example 3

Gelation of "Alkoxide" sol using Ammonium Phosphate monobasic and dibasic

A solution of ammonium phosphate monobasic (NH₄:H2PO₄) was prepared as it follows:

59 , 331 6 g of NH₄ I H₂PO₄ , reagent grade , from Aldrich were dissolved in 160,893Og of water, doubly-ionized, doubly distilled at room temperature. Nominal concentration: 3,11 M; measured pH: 3,75.

A silicon alkoxide sol was prepared as it follows: 60Og of aqueous HCl 0,01N were transferred in a 21 erlenmayer flask and maintained in good agitation. Temp, of the solution was 24 ⁰C. 266, 6g of TEOS were added gradually. The temperature was risen slowly to 30° C at the end of TEOS addition.

Sol was sonicated for degassing and homogenization, then cooled to roomtemperature . From this sol were weighted 3 parts of 260,32 g each and transferred into 3 equal containers, labelled respectively A, B end C. In container A the aqueous solution of NH₄: H₂PO₄ is added drop by drop, with good stirring, to reach the pH of 3,75.

Total volume of salt solution added: 33,6 ml.

Total salt added: 1,045 x 10^"1 moles.

In container B 33, 6 ml of salt solution were transferred, with good stirring, to the sol, producing a pH of 3,75.

In container C 67,2 ml of salt solution were transferred with good stirring to the sol.

Final pH was 3,75. Total salt transferred: 2,090 x 10^" ¹HIoIeS.

In all three containers a very uniform gel was formed in less than 5 hrs .

A solution of ammonium phosphate dibasic (NH₄) 2HPO₄ was prepared as it follows: 33,7069 g of ammonium phosphate dibasic, reagent grade from Aldrich, were dissolved with 80,8215 g of water (doubly-ionized, doubly-distilled) at roomtemperature nominal concentration of solution: 3,11 M;

pH: 8,23.

The silicon alkoxide sol was prepared with identical procedure as for the previous examples with monobasic ammonium phosphate. From this sol were weighted 2 parts of 259,80 g each and transferred in two identical containers labelled A and B.

In container A the aqueous solution of (NH₄) ₂ HPO₄ is added drop by drop, with good stirring, to reach a pH of 3,65. Total volume of salt solution added: 1,45 ml. Total salt added: 4,51 x 10^~3 moles.

In container B 1,45 ml is transferred with good stirring. The pH of the sol is 3,65. In all two container a very uniform gelation was formed in less than 5 hrs .

The results of Example 3 verify the conclusions of Examples 1 and 2 also with salts belonging to table 2.

Moreover, in the case of monobasic ammonium phosphate, that has a buffer effect around pH 3,75, it was verified that the proper use of the buffering at the desired value of pH constitutes one of the most favourable condition for the relevant process step.

Example 4

Gelation of "Invert" sol comparing Ammonia free base and Ammonium Acetate

24.7 1 of invert sol were prepared in our pilot plant as described.

18.5 1 aqueous suspension of aerosol EG-50 30 % by weight were transferred into the hydrolysis reactor. Concentrated HCl was added, drop-wise, with good stirring until the pH was lowered to 2.88. 6.6 kg of TEOS (7.12 1) was added maintaining mechanical stirring for 1 hour.

The sol fresly prepared was divided into two distinct reactors identified as A and B for comparing different procedures for modifying pH of the sol before gelation.

REACTOR A: Standard Procedure (comparison)

The volume of the sol was 12.35 1.

- The starting pH of the sol was 2.88.

To the sol 163 ml of Ammonia Solution 0,1OM were added, with stirring, very slowly for a total of 16,3 mMoles.

The time required was 42 minutes to change the pH-value from the starting pH-value to the final pH-value. The final pH-value is reached when the polymerisation starts .

- The final pH was 4.94.

- An uniform gelation was obtained in less than 5 hours.

- The aquagels were properly conditioned and supercritically dried. Good aerogels were obtained.

- The aerogels were densified in an oven with standard thermal treatment. Good silica glasses were obtained.

REACTOR B: New procedure according to the invention

- The volume of the sol was 12.35 1.

- The starting pH was 2.88.

- To the sol 2,60 ml of Ammonium Acetate Aqueous Solution 10,16 M were transfered into the reactor, with stirring, for a total of 25,9 mMoles.

- The final pH was 4.94.

- A very uniform gelation was obtained in less than 5 hours .

The aerogels were densified in an oven with standard thermal treatment. Good silica glasses were obtained. This example 4 verifies with large volumes of sol the applicability of the procedure of modifying sol pH by use of "suitable salts".

Also it confirms the advantages of the procedure based on the current invention over the standard procedure with respect to time of the operation, added volume to the sol and process reliability.

Example 5

Metal doping of SOL-GEL glasses

An "alkoxide" sol was prepared by transferring in a 1 1 erlenmayer flask 300 g of HNO3 aqueous solution 0,001 M., 0,303 moles of aluminum as Al (NU3)3 x 9 H₂O where than added with the purpose of preparing an aluminum doped SOL- GEL glass. To the flask was then added 100 g of TEOS, drop- wise, with good stirring. About 400 ml of sol were obtained. The sol was than divided into two identical containers, labelled A, B, with the purpose of comparing different procedures for modifying the sol pH.

SAMPLE A: Standard Procedure

- The volume was 200 ml.

- The initial pH was 0.78.

- To the sol Ammonia Solution 0,14M was added drop by drop until the pH of the sol risen at 2.4.

- Then gelation occurs.

SAMPLE B: Procedure based on current invention

- The volume was 200 ml.

- The initial pH was 0.78.

- To the sol Ammonium Acetate Solution 0,5 M was added to rise the pH to 2.4. - Then gelation occurs.

The aquagels samples A and B were properly conditioned for supercritical drying and dried supercritically . All were densified in oven to glass with the same standard procedure. All the samples A resulted in opaque ceramic material. All the samples B resulted in good glasses.

By elemental analysis, all samples contained between 6,3 % and 6,4 % of aluminium by weigh.

The example 5 provides evidence of substantial advantages of the invention when applied to the synthesis of doped glasses through SOL-GEL process.

Example 6

Gelation of sol type 4S using ammonium acetate

A composite sol, conventionally indicated as 4S was prepared as follows:

In a 4 1 plastic beaker 2700 g of HCl 0,0 IN were weighted and 900 g of TEOS were added. The two-phases liquid was stirred mechanically with a magnetic stirrer and simultaneously sonicated with a 450 watts Branson Sonifier for 15 minutes.

A one-phase clear liquid was obtained. Cooled to roomtemperature excess ethanol from hydrolysis was evaporated at reduced pressure. Added 569,7 g of fumed silica (EG -50 from Degussa) with proper stirring.

The suspension was treated with an IKA-LABORTECHNIK

ULTRATURRAX homogenizer respectively at 5000 r.p.m. for 10 minutes and at 10000 r.p.m for 30 minutes.

After the mechanical homogenization the suspension was centrifuged for 20 minutes at 2000 r.p.m. The obtained sol (4S) was divided into 3 fractions of 811 g each labelled respectively as A, B and C. Fraction A was titrated with aqueous ammonia 0,14 M. A pH of 3,54 was reached after 57,10 g of ammonia solution have been added drop-wise, with stirring, to the sol. Total ammonia added: 7,994 M Moles.

An homogeneous gel was obtained in less than 5 hrs .

Fraction B was titrated with ammonium acetate aqueous solution, 2,8 M. A pH of 3,22 was reached after 2,15g of solution were added drop-wise, with stirring, to the sol.

Total ammonium acetate added 6,02 m Moles.

A very homogeneous gel was obtained in less than 5 hrs.

Fraction C was not titrated; instead 2,15g of ammonium acetate solution 2,8 M were transferred to it. After good stirring, a pH value of 3,22 was recorded.

Aqua-gels were properly conditioned and super-critically dried, good aero-gels were obtained.

Aero-gels were densified in oven with standard thermal treatment; good silica glasses were obtained.

The result of example 6 constitute still another evidence of advantageous applicability of the use of a concentrated solution of a "suitable salt" for pH - modification of a sol again of different characteristics and formulation.

Claims

1. Sol-gel process, whereby a sol is modified to a gel by the change of the pH-value of the sol, characterized in that in order to change the pH-value from a starting pH-value to a final pH-value at least one suitable salt is added to the sol.

2. Sol-gel process according to claim 1, characterized in that the suitable salt is a salt of a weak acid and a weak base.

3. Sol-gel process according to claim 1, characterized in that the suitable salt is a salt of a strong acid and a weak base.

4. Sol-gel process according to claim 1, characterized in that the suitable salt is a salt of weak acid and a strong base.

5. Sol-gel process according to claim 1, characterized in that the suitable salt is added after having been solved in a solvent.

6. Sol-gel process according to claim 1, characterized in that the suitable salt is added to lower the pH- value of the sol.

7. Sol-gel process according to claim 1, characterized in that the suitable salt is added to increase the pH-value .