JPH07100611B2 - Method for producing modified titania sol - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for producing a titania sol, and more particularly to a method for producing a titania sol which has excellent dispersibility and is stable in a wide pH range.

Technical Background of the Invention and Problems Thereof In recent years, titanium oxide particles have been increasingly used for their chemical properties. For example, titanium oxide can be used as a surface coating material for cosmetics or plastics by utilizing the blocking power of ultraviolet rays, or as an antireflection coating material by utilizing a high refractive index, or by combining these effects. It is used as a functional hard coat material.

As described above, titanium oxide is used in many applications, but in any case, titanium oxide is required to have many functions. For example, when titanium oxide is used as a cosmetic material, not only the effect of blocking ultraviolet rays, but also smoothness, texture and transparency are required. Further, when titanium oxide is used as the coating material, in addition to transparency and high refractive index, further excellent film forming properties, adhesion, film hardness, mechanical strength, abrasion resistance, etc. are required.

In order to use titanium oxide particles for various purposes in this way,
Titanium oxide particles are required to have various characteristics according to their respective uses, but it is preferable that they are ultrafine particles regardless of their use. Moreover, when the titanium oxide particles are required to have transparency, it is required that the particles not only have a uniform particle shape and size but also be highly dispersed in the medium at the time of compounding. It is particularly preferable to use highly dispersed colloidal titania (titania sol) in order to produce titanium oxide particles satisfying these characteristics.

As a method for producing such a titania sol, a method obtained by neutralizing metatitanic acid conventionally obtained by hydrolyzing an aqueous titanium salt solution and then peptizing with a gel obtained by washing, or an aqueous titanium salt solution is used. A method of obtaining a sol obtained by deionizing with an ion exchange resin or the like is known.

However, the titania sol obtained by the above manufacturing method has the following problems.

The dispersed particles of the sol are spherical or pseudo-spherical aggregates, and since the particle size is also non-uniform, they become cloudy and inferior in transparency.

It is possible to obtain a sol having a small particle size and excellent transparency, depending on the conditions for forming the sol, but in this case, a large amount of acid is required as a stabilizer.

The sol is stable only in the acidic region (pH 3 or less), and when it is neutral or alkaline, precipitation or gelation occurs, so the range of use is limited.

Even if the sol is mixed with an organic solvent such as alcohol or the solvent is replaced with an organic solvent, it becomes unstable and precipitates, which makes it difficult to use as a hard coat agent for the surface of plastic or the like.

The titania particles obtained from the titania sol have a high specific surface area and a strong surface activity, and therefore may promote the deterioration of the resin when used in a mixture with a resin or the like.

The present inventors have conducted extensive studies to solve the above problems, and if the titania sol is produced by a specific method, the problems associated with the conventional titania sol can be solved,
The inventors have found that a titania sol having excellent performance is obtained, and that the modified titania particles obtained from this titania sol have various excellent properties, and have completed the present invention.

OBJECT OF THE INVENTION The present invention is intended to solve the problems associated with the prior art as described above, and is excellent in dispersibility and wide.
A method for producing a modified titania sol that is stable in the pH range, does not aggregate, is stable even when concentrated to a high concentration, and does not cause precipitation even when mixed with an organic solvent and has excellent coatability. It is intended to be provided.

SUMMARY OF THE INVENTION The method for producing a modified titania sol according to the present invention is characterized by comprising the following steps (a) and (b).

(A) a step of preparing a titania sol precursor by adding hydrogen peroxide to a hydrous titanic acid gel or sol and dissolving the hydrous titanic acid in the presence of an inorganic compound to heat the solution. A step of adding a silicon compound or a zirconium compound to the titania sol precursor and allowing the silicon compound or the zirconium compound to act on the titania sol precursor.

DETAILED DESCRIPTION OF THE INVENTION The method for producing the modified titania sol according to the present invention will be specifically described below.

The method for producing a titania sol according to the present invention comprises (a) a step of preparing a titania sol precursor and (b) a step of causing a silicon compound or a zirconium compound to act on the titania sol precursor. (A) and (b) will be described.

Step (a) First, a hydrous titanic acid gel or sol is prepared by a conventionally known method according to a conventional method. Hydrous titanate gel is
For example, it can be obtained by adding an alkali to an aqueous solution of a titanium salt such as titanium chloride or titanium sulfate for neutralization.
The hydrous titanic acid sol can be obtained by passing an aqueous solution of a titanium salt through an ion exchange resin to remove anions. In order to prepare these sols or gels, conventionally known methods can be widely used, not limited to the above methods. The hydrous titanic acid as used herein means the hydrated titanium oxide or the hydroxide of titanium obtained by the above method.

Next, hydrogen peroxide is added to the hydrous titanic acid gel or hydrous titanic acid sol or mixture thereof obtained as described above to dissolve the hydrous titanic acid to prepare a uniform aqueous solution. At this time, it is preferable to heat or stir as necessary. At this time, if the concentration of hydrous titanic acid is too high, it takes a long time to dissolve the hydrous titanic acid, and further undissolved gel precipitates or the resulting aqueous solution becomes too viscous, which is not preferable. . For this reason
TiO ₂ concentration is about 10% by weight or less, preferably about 5% by weight
The following is desirable.

If the amount of hydrogen peroxide to be added is 1 or more in H ₂ O ₂ / TiO ₂ weight ratio, hydrous titanic acid can be completely dissolved. When the H ₂ O ₂ / TiO ₂ ratio is less than 1, the hydrous titanic acid is not completely dissolved and unreacted gel or sol remains, which is not preferable. Also, the larger the H ₂ O ₂ / TiO ₂ ratio, the higher the dissolution rate of hydrous titanic acid, and the reaction ends in a short time.However, if hydrogen peroxide is used in excess, unreacted hydrogen peroxide will be removed from the system. It will be left in a large amount, which will adversely affect the next step, which is not preferable. Therefore, H ₂ O ₂ / T
It is preferable to use hydrogen peroxide in an amount such that the iO ₂ ratio is about 1 to 6, preferably about 2 to 6, and when hydrogen peroxide is used in such an amount, hydrous titanic acid is produced in about 0.5 to 20 hours. Dissolve completely.

The reaction temperature at this time is 50 ° C. or higher, preferably 70 ° C. or higher.

Next, a predetermined amount of an inorganic compound is mixed with an aqueous solution (aqueous titanic acid solution) in which hydrous titanic acid is obtained as described above, and the mixture is heated to 60 ° C. or higher, preferably 80 ° C. or higher to hydrolyze the titanic acid. By doing so, a titania sol precursor in which uniform titania particles are dispersed can be obtained.

The inorganic compound used in the present invention is preferably Al
Group III of the periodic table such as, Group IV of Ti, Zr, Si, Sn, etc.,
Compounds of one or more elements selected from Group V such as V and Sb, Group VI such as W and Group VIII such as Fe are used. As the form of the compound, an oxide, a hydroxide, an oxyacid, an oxyacid salt or the like is used. These may be added in solid form or mixed as an aqueous solution. As a preferred method, a gel or sol of these inorganic compounds is used. When a sol is used, the average particle size of the dispersed particles is preferably about 30 mμ or less, preferably about 15 mμ or less.

For example, in the case of silicon, silica gel, silica sol or a citric acid solution is used. The silicic acid solution here is a solution of a silicic acid low polymer obtained by dealkalizing an alkaline silicate aqueous solution by an ion exchange method or the like.

As the mixing amount of the inorganic compound increases, the long-term stability and light resistance of the titania sol finally obtained increase, and a high-concentration sol can be obtained. However, as the mixing amount increases, long-term stability and light resistance will not increase, etc.
It is not preferable because the effect of adding the inorganic compound is not increased. Moreover, when the mixing amount is small, the particle size of the obtained titania particles tends to increase. However, if it is too small, problems such as non-uniform particle size and aggregation of particles occur.

Considering the above, as the amount of the inorganic compound to be mixed, the ratio of the value obtained by converting the weight of the titanium in the aqueous titanic acid solution into TiO ₂ and the value obtained by converting the weight of the inorganic compound into the oxide (MOx) TiO ₂ The / MOx (weight ratio) is preferably in the range of 0.25 to 200. The method for mixing the aqueous solution of titanic acid and the inorganic compound is not particularly limited, and a predetermined amount of the aqueous solution of titanic acid and the inorganic compound may be mixed at a time, or the aqueous solution of titanic acid and a part of the inorganic compound are mixed first. And then heat
As the reaction proceeds, the rest of both may be added.

Further, a method in which the entire amount of the inorganic compound and a part of the titanic acid aqueous solution are first mixed and heated, and then the remaining titanic acid aqueous solution is added may be used.

Further, the mixing timing of the inorganic compound does not necessarily have to be after the hydrous titanic acid has been dissolved in hydrogen peroxide, and may be mixed in the stage of gel or sol before being dissolved in hydrogen peroxide. They may be mixed when preparing the acid gel or sol. In short, it is necessary that the above-mentioned inorganic compound be present when the aqueous solution of titanic acid is hydrolyzed. When an aqueous solution of titanic acid in which an inorganic compound does not coexist is heated and hydrolyzed, it becomes a titania sol even if the titania concentration is dilute, but the resulting titania sol is extremely unstable. For example, it is no longer present as a sol. It cannot be done and a precipitate forms.

In the present invention, when a titanium compound is used as the inorganic compound to be mixed with the titanic acid aqueous solution, a hydrous titanic acid gel or sol of the starting material of the present invention may be used, or the titanic acid aqueous solution of the present invention is not added with the inorganic compound. It is also possible to use the titania sol obtained by heating and hydrolyzing as it is. In these cases, it is preferable to completely dissolve the hydrous titanic acid gel or sol with hydrogen peroxide to prepare an aqueous titanic acid solution, and then mix the titanium compound.

The titania sol precursor thus obtained is also a titania sol in which titania particles are dispersed in a dispersion medium.

Step (b) Then, a silicon compound or a zirconium compound is added to the titania sol precursor obtained as described above, and the silicon compound or the zirconium compound is allowed to act on the titania sol precursor to obtain the titania sol according to the present invention.

The titania sol precursor, when prepared, contains titania (TiO ₂ +
The concentration of the inorganic oxide) is usually about 0.01 to 10% by weight, but in the present invention, the titania sol precursor can be used as it is, or in some cases, can be concentrated and used. However, when the titania in the titania sol precursor has a high concentration, the titania sol precursor is unstable because aggregation of the titania particles is likely to occur. Therefore, the titania concentration in the titania sol precursor is preferably about 7% by weight or less.

(1) Addition of Silicon Compound The silicon compound added to the titania sol precursor is a silicic acid solution obtained by dealkalizing an aqueous alkali silicate solution with a cation exchange resin, and is obtained by neutralizing an alkali silicate with an acid. A silica sol or a solution or dispersion of a silicon compound such as an alkoxide such as ethyl silicate or a hydrolyzate thereof is used. Commercially available silica sol can also be used, and in these cases, the particle size of silica is preferably about 50 mμ or less.

As a method of adding the solution or dispersion of the silicon compound to the titania sol precursor, while heating the solution of the titania sol precursor, a method of gradually adding the solution or dispersion of the silicon compound, the solution of the titania sol precursor and the silicon compound. Alternatively, any method of heating after mixing with the dispersion at once may be used, and it should be selected according to the titania concentration, the silica concentration in the solution or dispersion of the silicon compound. When the titania concentration is 1% by weight or less, which is relatively dilute, there is no problem in mixing both at once, but when the titania concentration is 1% by weight or more, silica aggregates the titania. In some cases, if the silica concentration is high, aggregation and polymerization of silica alone will occur, so a method of gradually adding is preferable.

The temperature at the time of addition or mixing is usually preferably heated to about 60 ° C. or higher in order to accelerate the reaction between the silicon compound and titania. However, when alkoxides such as ethyl silicate are used, their hydrolysis rate is high and silica colloidal particles are easily generated in the mixed solution.Therefore, alkoxide is gradually added at a relatively low temperature of about 40 ° C or lower. After completion of the addition, the temperature is raised to about 60 ° C or higher to complete the reaction.

The pH of the mixed solution at the time of adding the silicon compound is preferably neutral or alkaline from the viewpoint of stability of the titania sol precursor and the formed titania sol, and is usually in the range of about 6-10.

As described above, after adding the solution or dispersion of the silicon compound, the addition amount of silica when the silicon compound is converted to SiO ₂ , and the weight ratio SiO ₂ / TiO of TiO ₂ in the titania sol precursor. ₂ is required to be at least 0.03 in order to exert the effect of adding the silicon compound. However, as the amount added increases gradually, the original properties of titania (UV shielding, high refractive index) are impaired, so SiO ₂ / TiO ₂
The upper limit of the _two values is preferably about 0.7.

(2) Addition of zirconium compound Examples of the zirconium compound added to the titania sol precursor include acidic salts such as zirconium chloride and zirconium sulfate, basic salts such as zirconyl ammonium carbonate and zirconyl salts such as zirconyl oxychloride. Aqueous solutions or dispersions are used.

The method of adding such a zirconium compound to the titania sol precursor may be the same as in the case of the silicon compound, and the titania sol precursor may be mixed with an aqueous solution or dispersion of the zirconium compound at once, or in some cases, gradually. You may add.

Zirconium compound added to titania sol precursor
ZrO ₂ / TiO ₂ (weight ratio) in terms of ZrO ₂ is at least 0.05 or more and preferably about 0.9 or less for the same reason as in the case of the silicon compound.

The solid content concentration of the titania sol precursor at the time of addition, temperature,
The pH is the same as in the case of silicon compounds.

The silica or zirconia-modified titania sol thus obtained has a pH of 6 to 6 if necessary.
After adjusting to 10, starting materials such as silica or zirconia or salts derived from pH adjustment during the process can be removed by conventional means such as ultrafiltration and ion exchange. further,
Depending on the application, the titania sol modified with silica or zirconia is concentrated.
Known methods such as an evaporation method and an ultrafiltration method can be adopted.

When the titania sol precursor is added with an aqueous solution or dispersion of a silicon compound, or an aqueous solution or dispersion of a zirconium compound as described above, the oxide or hydroxide of silicon is formed on the surface of the titania particles contained in the titania sol precursor. Or zirconium oxide or hydroxide is adsorbed, or the titania particles react with these oxides or hydroxides to cause an interaction,
Some changes occur in the properties of the titania particles. Such changes in the properties of the titania particles include, for example, a slight decrease in the surface activity of the titania particles. Therefore, the titania particles obtained from the titania sol produced by the present invention do not accelerate the deterioration of the resin when used as a filler for a resin, for example, and when the titania particles are blended in a cosmetic product, an ultraviolet ray is generated. Therefore, the color tone of the particles does not change.

Further, the titania particles obtained from the titania sol produced according to the present invention are, like the titania particles obtained from the titania sol precursor, the properties that the titania particles originally have, that is, the ultraviolet shielding effect, the properties such as a high refractive index are almost impaired. It is not. Moreover, the titania sol produced by the present invention has excellent dispersibility as well as excellent long-term stability and light resistance as the titania sol precursor. It is also stable in a wide pH range, can be concentrated to a high concentration of 30% by weight or more, and has excellent properties such that it is stable even if the proportion of the organic solvent in the dispersion medium is high. ing.

Effects of the Invention The titania sol produced by the present invention and the titania particles obtained from this titania sol have the following excellent properties.

(1) Titania sol has excellent dispersibility, long-term stability, and light resistance, and is stable in a wide pH range.

(2) Titania sol can be concentrated to a high concentration.

(3) When the dispersion medium is replaced with an organic solvent, the titania sol is stable even if the proportion of the organic solvent in the dispersion medium is increased.

(4) The titania sol is superior in ultraviolet shielding effect and transparency as compared with the conventional titania fine particle dispersion liquid.

(5) When titania particles obtained from titania sol are mixed with a resin and used as a filler or as a coating agent, deterioration of the resin is prevented and the durability and weather resistance of the resin are improved. Therefore, for example, when blended with a plastic sheet for food packaging, it can be stored for a long period of time as compared with conventional packaging materials.

(6) When the titania particles obtained from the titania sol are used as a surface coating agent for a transparent substrate such as glass and plastic, it has excellent adhesion to the substrate, transparency, and a high refractive index excellent in ultraviolet shielding effect. A coating film is obtained.

(7) When titania particles obtained from titania sol are blended in a cosmetic product, a cosmetic product having an excellent ultraviolet shielding effect and a feeling of finishing can be obtained, and its deterioration (color tone change, etc.) can be prevented.

The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

Example 1 (Preparation of titania sol precursor) Titanium sulfate was dissolved in pure water to obtain an aqueous solution containing 0.4% by weight of TiO ₂ . While stirring this aqueous solution, 15% aqueous ammonia was gradually added to this aqueous solution to obtain a white slurry liquid of pH 8.5. The slurry is filtered and then washed,
A cake of hydrous titanic acid gel having a solid content concentration of 9% by weight was obtained.

To 5.55 kg of this cake, 6.06 kg of 33% hydrogen peroxide and 1 part of pure water
After adding 3.4 kg, heat at 80 ° C for 5 hours to obtain TiO _2.
25 kg of a 2.0% by weight solution were obtained. This titanic acid aqueous solution is
It was transparent yellowish brown and had a pH of 8.1.

Next, 130 g of silica sol having a particle size of 7 mμ and a concentration of 15% by weight, 9 kg of the above titanic acid aqueous solution, and 191 kg of pure water were mixed, and then heated at 95 ° C. for 60 hours. The solution was yellowish brown at first, but became a milky white transparent colloidal solution after 60 hours.

The colloidal solution thus obtained was concentrated by a vacuum evaporation method to obtain 18.1 kg of a titania sol precursor having a solid content of 1.1%.

(Treatment with silicon compound) A sodium silicate solution having a SiO ₂ concentration of 23.8 wt% and a SiO ₂ / Na ₂ O molar ratio of 3.0 was diluted with pure water to prepare 3.2 wt% of diluted sodium silicate, which was previously prepared. The column was passed through a cation exchange resin (Diaion SK-1B manufactured by Mitsubishi Kasei Kogyo Co., Ltd.) layer to obtain a silicic acid solution having a SiO ₂ concentration of 3.0 wt%.

Next, sodium hydroxide was added to 2,000 g of the titania sol precursor to adjust the pH of the titania sol precursor to 9.7, and then the mixture was heated to 95 ° C. While maintaining the titania sol precursor at this temperature, 440 g of the silicic acid solution diluted to 0.5% by weight with pure water was gradually added over 12 hours, and after the addition was completed,
This temperature was maintained for another hour. When the obtained diluted titania sol was concentrated by a vacuum evaporation method, a titania sol as shown in Table 1 was obtained.

Example 2 Example 1 was repeated except that the amount of the added silicic acid solution was 880 g and the addition time was 30 hours.
A titania sol as shown in was obtained.

Example 3 A titania sol as shown in Table 1 was obtained in the same manner as in Example 1 except that 2240 g of the silicic acid solution was added over 38 hours.

Example 4 To 2000 g of the titania sol precursor obtained in Example 1, dilute aqueous ammonia was added to adjust the pH to 10.8. Add ethyl silicate diluted with ethyl alcohol (as SiO ₂
14.5 g by weight) and 19.4 g of 9.4% aqueous ammonia were added from separate addition ports at 30 ° C. over 19 hours. After the addition is complete, add 0.1N hydrochloric acid to the reaction mixture to adjust the pH to 9.2 and then add 60 ° C.
It was heated to aging and aged for 1 hour.

The obtained titania sol was subjected to ultrafiltration to remove ethanol, ammonia and hydrochloric acid in the solvent and then concentrated by vacuum evaporation to obtain titania sol as shown in Table 1.

Example 5 Zirconyl ammonium carbonate (trade name: Zircosol AC,
ZrO ₂ by diluting Daiichi Rare Element Chemical Industry Co., Ltd. with pure water
Was adjusted to 0.5%.

On the other hand, 2000 g of the titania sol precursor obtained in Example 1
The temperature was raised to 5 ° C, and while maintaining this temperature, 630 g of the above zirconyl ammonium carbonate solution was added to this titania sol precursor.
Gradually added over 25 hours. 95 ° C for 1 hour after addition
Held in. The obtained titania sol is subjected to ultrafiltration,
After the ammonium carbonate in the solvent was washed and concentrated by vacuum evaporation, a titania sol as shown in Table 1 was obtained.

Example 6 1.0 kg of the titanic acid aqueous solution obtained in Example 1 and 200 kg of pure water were mixed and then heated at 95 ° C. for 2 hours to obtain 0.01% by weight of titania sol as TiO ₂ . Hydrolysis was performed under the same conditions as in Example 1 except that this sol was used instead of the silica sol of Example 1 to obtain a titania sol precursor.

Using this titania sol precursor, the same silicic acid solution and the same conditions as in Example 1 were used to obtain the titania sol shown in Table 1.

Comparative Example 1 The hydrous titanic acid gel cake obtained in Example 1 was diluted with pure water to obtain a suspension having a TiO ₂ concentration of 2.0% by weight. To this, 0.1 N hydrochloric acid was gradually added until it became a sol. The obtained sol was stabilized at 80 ° C. for 1 hour, and then water was evaporated under reduced pressure to obtain a concentrated sol by the peptization method.

This sol could be concentrated to 20% by weight, but had a low pH of 1.7, and contained 2.1% by weight of chlorine ions.

Test Example 1 The titania sols obtained in Examples 1 to 4 and Comparative Example 1 were tested for solvent substitution with isopropyl alcohol (IPA) and stability of titania powder obtained from these sols. The result shown in FIG. Each test method is as follows.

(1) Solvent substitution stability Dilute each concentrated sol to 10% by weight with water and use ultrafiltration method until the IPA concentration in the total solvent is 40%, 70% and 90% by weight. Substitution was performed and the presence or absence of precipitate was observed.

In the table, ○ indicates no change, and × indicates generation of gel-like precipitate.

Powder Stability The sol was evaporated to dryness and then dried at 110 ° C. for 12 hours to obtain TiO ₂ powder.

Powder above in 1 ml of 100% solution of squalane or glycerin
After adding 0.2 g and mixing well, the change in color tone was observed after irradiation with ultraviolet rays for 16 hours using an ultraviolet lamp for a weather meter.

Test Example 2 The titania sol obtained in each of Examples 1 and 3 and Comparative Example 1 was taken so as to have a solid content of 20 g, and 70 g of an acrylic aqueous resin ARON 104 (manufactured by Toagosei Kagaku Kogyo Co., Ltd., concentration: 30%)
Mixed well with.

After mixing 1.5 g of this with an isocyanate curing agent Coronate HL (manufactured by Nippon Polyurethane Industry Co., Ltd.), it was applied on a glass plate and dried at 130 ° C. for 30 minutes to obtain a coating film. The weather resistance of this coating film was tested by measuring the yellowing degree (NBS unit) of the above resin with a weather meter (manufactured by Suga Test Instruments Co., Ltd.). The results are shown in Fig. 1.

Test Example 3 The titania sol obtained in Example 1 was adjusted to a TiO ₂ concentration of 0.005% by weight, and the absorbance in the ultraviolet region (about 400 nm or less) and the light transmittance in the visible light region (about 400 nm or more) were measured. As a comparative example, fine particle titania obtained by vapor-phase oxidation of titanium chloride (Aerosil, P-25 manufactured by Degussa Co., Ltd.) was dispersed in water and the TiO ₂ concentration was adjusted to 0.005% by weight. The transmittance was measured. The results are shown in FIG. In the figure is a curve showing the light transmittance of the titania sol of the present invention, 'is a curve showing the light transmittance of the titania sol of the comparative example, is a curve showing the absorbance of the titania sol of the present invention, and'is a comparative example. 3 is a curve showing the absorbance of the titania sol.

A spectrophotometer (manufactured by Hitachi Ltd., Model 330) was used for the measurement.

[Brief description of drawings]

FIG. 1 is a diagram showing the weather resistance of a resin when titania particles obtained from the titania sol produced by the present invention are added to the resin. FIG. 2 is a diagram showing the absorbance and light transmittance of the titania sol of the present invention and a commercially available titania fine particle dispersion liquid.

Continuation of the front page (56) Reference JP-A-50-122493 (JP, A) JP-A-55-132626 (JP, A) JP-A-61-136919 (JP, A) JP-A-62-283817 (JP , A)

Claims (2)

[Claims] 1. A method for producing a stable modified titania sol comprising the following steps: (a) A solution obtained by adding hydrogen peroxide to a hydrous titanic acid gel or sol to dissolve hydrous titanic acid is an inorganic compound. A step of preparing a titania sol precursor by heating in the coexistence of, (b) a step of adding a silicon compound or a zirconium compound to the titania sol precursor and allowing the silicon compound or the zirconium compound to act on the titania sol precursor. 2. The silicon compound or zirconium compound has a SiO ₂ / TiO ₂ (weight ratio) of at least 0.03, or a ZrO ₂ / TiO ₂ (weight ratio) of at least 0.03.
A method according to claim 1 characterized in that it is added to the titania sol precursor in an amount such that it is 5.