FR2753980A1 - New coated particles of anatase titanium di:oxide, having anti-UV properties - Google Patents

Abstract

Particles of anatase TiO2 of maximum particle size 100 nm, which are coated with a primary layer of a Ce compound and/or Fe; and a secondary layer of a metallic oxide, hydroxide or oxohydroxide. The particles have a surface area, BET, of at least 70 m<2>/g and a density in the order of 2.5. Also claimed, are their method of preparation and use.

Description

PREPARATION PROCESS
ANTI-UV COATING COMPOSITION
BASED ON TITANIUM DIOXIDE
The present invention relates to a process for preparing an anti-UV coating composition, in particular a stain or resin composition, using a dispersion of anatase titanium dioxide particles having characteristics suitable for this use.

In the field of construction, materials are often degraded by the sun, whose rays in the UV field can lead to an alteration of the properties of the material, and in general of its aesthetic appearance. For example, the color of a material such as wood is changed after exposure to the sun leading to a graying of the wood. To avoid this, the materials are frequently covered with a protective layer such as a stain or a varnish comprising anti-UV agents.

It is known, for example, to use organic compounds of the benzotriazole family. However, these organic products have drawbacks.

On the one hand, there are increasingly severe toxicological restrictions on their use, and, on the other hand, they themselves degrade fairly quickly under the action of UV which leads to detachment or flaking of the particles. coatings and loss of their properties.

In order to avoid this problem of degradation of organic anti-UV agents, it is known to use titanium dioxide in coating compositions.

In this application, the titanium dioxide is, in general, of the rutile crystalline type because the latter has weaker photocatalytic properties than anatase. This point is essential, because the photocatalytic titanium dioxide degrades in the presence of light the organic compounds which are in contact with it. This property has major drawbacks in coating compositions since the latter always have a formulation based on latex-type binders, surface-active agents, anti-foaming agents, etc., which are predominantly organic.

Titanium dioxide of the crystalline anatase type, being known for its high photocatalytic properties, is therefore not used in this application. However, in certain aspects it presents more interesting advantages than rutile type titanium dioxide. In particular, its size is easily controlled: it is therefore easy to obtain particles of anatase titanium dioxide of small size, and which, incorporated in coating formulations, are invisible. In contrast, rutile titanium dioxide often imparts a whitish appearance to formulations. And, in transparent coating compositions such as stains or varnishes, a bleaching of the material is observed during their application.

On the other hand, blocking the photocatalytic properties of anatase crystalline-type titanium dioxide is difficult. It has already been proposed to block this property by depositing layers of metal salts, oxides or hydroxides such as silica or alumina on the surface of the particles. However, this treatment is not always sufficient, and when the treated titanium dioxide is used in organic formulations, photocatalytic degradation of the components of the formula is observed.

An aim of the present invention is therefore to provide, for the preparation of the coating compositions, the use of a titanium dioxide of the crystalline anatase type which does not degrade the other components of the formulation or the substrate.

Another aim is to provide, for the preparation of the coating compositions, the use of a titanium dioxide which results in a transparent formulation.

For this purpose, the invention relates to a process for preparing an anti-UV coating composition, in which particles of anatase titanium dioxide of size of at most 100 nm are used, said particles being coated at least in part. by depositing at least one cerium compound.

The invention also relates to a composition for coating, comprising:
- as an anti-UV agent: particles of anatase titanium dioxide with a size of at most 100 nm, said particles being coated at least in part by a deposit of at least one cerium compound,
- and at least one binder.

These dispersions of titanium dioxide particles lead to coating compositions which are stable over time, are non-toxic and to coatings which are not very permeable to humidity.

Other details and advantages of the invention will emerge more clearly on reading the description and the examples.

The invention therefore relates firstly to a process for preparing an anti-UV coating composition, in which particles of anatase titanium dioxide with a size of at most 100 nm are used, said particles being coated at least in part by a deposit of at least one cerium compound.

The average size of these particles is generally at most 100 nm, preferably at least 20 nm, even more preferably between 20 and 80 nm. This size is measured by transmission electron microscopy (TEM).

The nature of the crystalline phase of this titanium dioxide is predominantly anatase. Majority "means that the level of anatase in the titanium dioxide particles of the coating is greater than 50% by mass. Preferably, the particles of the coating have a level of anatase greater than 80%. The degree of crystallization and the nature of the crystalline phase are measured by X-ray diffraction.

One of the essential characteristics of the titanium dioxide particles used in the process according to the invention is that these particles are covered at least in part by a deposit of a cerium compound.

Preferably, this cerium compound is a cerium oxide precursor, that is to say a cerium compound which can decompose into cerium oxide, in particular under the effect of calcination. This is the case, for example, with cerium oxyhydroxides.

According to the process of the invention, the titanium dioxide particles used are such that the ratio by weight of the cerium compound, expressed as CeO2, to the titanium dioxide is at least 0.01% by weight, preferably at most 50% by weight, even more preferably between 0.1 and 25%. This quantity of cerium compound is measured on the particles of the dispersion by X-ray fluorescence.

According to a preferred variant of the process according to the invention, the titanium dioxide particles used can also be coated at least in part with a layer of at least one metallic compound other than cerium in simple or mixed form.

The term “mixed” is understood to mean a metal compound based on at least two different elements (silicoaluminates, etc.).

This metallic compound other than cerium can be chosen from silica, zirconia, oxides or hydroxides of aluminum, zinc, titanium or tin. Silica and aluminum (hydr) oxides are the preferred compounds of the invention.

In general, the ratio by weight of compounds other than cerium, expressed as metal oxides, to titanium dioxide is at most 100% by weight, preferably at most 60% by weight.

If the particles are coated with silica, the content of silica relative to titanium dioxide is generally at most 100% by weight, preferably between 5 and 70%.

If the particles are coated with an aluminum compound, the content of this compound, expressed as A1203, relative to the titanium dioxide is generally at most 20% by weight, preferably between 2 and 15% .

According to a first embodiment, the particles used in the process according to the invention are in the form of a dispersion.

By dispersion is meant here and for the entire description any system consisting of fine particles of colloidal dimensions based on oxide and / or hydrated oxide (hydroxide) of titanium, in suspension in a liquid phase, in particular aqueous. It will be noted that the titanium can be either completely in the form of colloids, or simultaneously in the form of ions and colloids, without however the proportion represented by the ionic form exceeding approximately 10% of the total of the titanium in the colloidal dispersion. . According to the invention, use is preferably made of colloidal dispersions in which the titanium dioxide is completely in colloidal form.

The liquid phase of this dispersion can be aqueous or organic depending on whether it is desired to prepare an aqueous or organic coating composition. The organic liquid phase is generally chosen from those compatible with the solvent of the coating compositions. It can be for example:
- an aliphatic (white spirit type), aromatic (xylene type) or terpene (terebentin type) hydrocarbon,
- acetates (butyl acetate type),
- ketones (acetone type),
- alcohols (butanol type),
or a mixture of these solvents.

The titanium dioxide particles in dispersion preferably have a particle dispersion index of at most 1.3.

The dispersion index is determined by the formula:
84 - 16
1 =
2e50
in which:
- 84 is the particle diameter for which 84% of the particles have a
diameter less than 84
- e16 is the diameter 'of the particles for which 16% of the particles have a
diameter less than e16,
- e50 is the average diameter of the particles.

The dispersion index is measured by TEM.

In general, these titanium dioxide dispersions have a proportion of solid suspended in the dispersion of between 1 and 30% by weight.

According to a second embodiment, the titanium dioxide particles can be used in powder form.

Such a powder can be obtained by removing water from a dispersion of titanium dioxide particles as described above, for example by atomization.

Usually, the powder particles have a specific surface area of at least 40 m2 / g. This specific surface is a surface measured by the BET method.

By surface measured by the BET method is meant the specific surface determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established from the BRUNAUER - EMMETT - TELLER method described in the periodical The
Joumal of the American Society ", 60, 309 (1938).

The titanium dioxide particles which can be used in the process according to the invention can be prepared by any method known to those skilled in the art and in particular by precipitation of metal compounds - of cerium or other than cerium - from precursor salts of these compounds. metallic particles on the surface of the starting anatase titanium dioxide particles in aqueous dispersion.

This process can be carried out by introducing the precursors of the metal compounds - generally in the form of aqueous solutions of salts - into the aqueous dispersion of titanium dioxide particles and then by modifying the pH to obtain the precipitation of these compounds on the particles of titanium dioxide. titanium dioxide.

Thus, it is possible to precipitate at the surface of starting titanium dioxide particles a cerium compound from a cerium salt such as cerium acetate, cerium sulfate, cerium chloride or nitrate. of cerium, Cerium acetate being the preferred salt.

Preferably, this treatment is carried out at a temperature of at least 20 "C, for example 25" C.

Likewise, when the particles also have a layer of a metal compound other than cerium on their surface, said particles can be obtained by precipitation, on the surface of particles of titanium dioxide - previously treated with a cerium compound - d 'a metal compound other than cerium from a precursor metal salt, oxide or hydroxide in single or mixed form.

When the compound other than cerium is silica, it can be precipitated from a silicon salt such as an alkali silicate.

When the compound other than cerium is an aluminum compound, it can be precipitated from an aluminum salt such as aluminum sulphate, sodium aluminate, basic aluminum chloride, aluminum hydroxide diacetate.

The precipitation of the silica and the aluminum compound can be carried out at acidic or basic pH. The pH is controlled by the addition of an acid such as sulfuric acid or by the simultaneous and / or alternate introduction of an alkaline silicon compound and an acidic aluminum compound. Preferably, during this treatment, the pH is between 8 and 10.

Preferably, this treatment is carried out at a temperature of at least 50 ° C.

It may be advantageous to introduce dispersants into the precipitation media. These dispersants can be chosen from polycarboxylic acids and their salts, such as alkali metal or ammonium salts. It is possible, for example, to use: substituted or unsubstituted polyacrylic acids, acrylic copolymers (the latter can comprise monomer units such as sulfonic acid derivatives, for example 2-acrylamido 2-methyl propanesulfonic acid) or polyvinylpyrrolidone (PVP).

In general, the dispersants have a molecular weight of between 1000 and 400,000. For example, polyvinylpyrrolidone (PVP) of molecular weight 40,000 or polyethylene glycol of weight 4000 can be used.

These dispersants are generally used in an amount of at least 1% by weight relative to the titanium dioxide, preferably at most 35%.

Generated, after precipitation, the particles obtained are recovered by separation from the liquid medium, and they are redispersed in another liquid medium so as to obtain a dispersion of titanium dioxide particles.

This liquid medium can be acidic or basic, preferably it is a basic solution having a pH of the order of 8-9.

If the liquid medium is organic, it is preferable to treat the particles so as to account for them and promote their dispersion in an organic medium. This compatibilization can be carried out for example by atomization of the aqueous dispersion in the presence of a fatty acid such as stearic acid or of a metal salt of a fatty acid.

It is also possible to graft a hydrophobic chain, for example a trialkoxysilane, to the surface of the particles in aqueous dispersion. The dispersion of compatibilized particles is then mixed with an organic medium so as to cause the particles to migrate into the organic phase.

The organic dispersion can also be obtained by bringing an aqueous dispersion of titanium dioxide particles into contact with the desired organic solvent and then heating so as to remove the water by distillation. Such a process can only be implemented in the case where the organic solvent chosen has a boiling point higher than that of water and is soluble in water.

The starting anatase titanium dioxide particles to be treated with a cerium compound, and optionally another metal compound other than cerium, can be obtained by any preparation process known to those skilled in the art making it possible to obtain dioxide particles. of anatase titanium less than 100 nm in size.

According to a preferred embodiment, these particles are obtained by a so-called solution or wet preparation process (thermolysis, thermohydrolysis or precipitation of a titanium salt without calcination of the particles obtained) as opposed to oxidation or high pyrolysis processes. temperature of a titanium salt or calcination of particles from a solution process.

They may, for example, be particles of titanium dioxide obtained by the process described in application EP-A-0 335773.

It may in particular be the preparation process which consists in hydrolyzing at least one titanium compound A in the presence of at least one compound B chosen from:
(i) acids which have:
- either a carboxyl group and at least two hydroxyl groups
etlou amines,
- or at least two carboxyl groups and at least one group
hydroxyl and'or amine,
(ii) organic phosphoric acids of the following formulas:

where, n and m are integers between 1 and 6, p is
an integer between 0 and 5, R1, R2, R3 identical or different
representing a hydroxyl, amino, aralkyl, aryl, alkyl or hydrogen group,
(iii) compounds capable of liberating sulfate ions in an acidic medium,
(iv) salts of the acids described above,
and in the presence of anatase titanium dioxide seeds having a size of at most 5 nm and in a weight ratio expressed as TiO2 present in the seeds / titanium present before introduction of the seeds into the hydrolysis medium, expressed as TiO2 of between 0.01% and 3%.

This process for preparing the particles therefore comprises several steps and, firstly, a step for preparing the starting solution comprising a titanium compound A, a compound B as defined above and titanium dioxide seeds.

This starting solution, intended to be hydrolyzed, is preferably completely aqueous; optionally, another solvent can be added, for example an alcohol, provided that the titanium compound A and the compound B used are then substantially soluble in this mixture.

As regards the titanium compound A, a compound chosen from among halides, oxyhalides, titanium alkoxides, sulfates and more particularly synthetic sulfates is generally used.

The term “synthetic sulphates” is understood to mean solutions of titanyl sulphates produced by ion exchange from very pure titanium chloride solutions or by reaction of sulfuric acid with a titanium alkoxide.

Preferably, the operation is carried out with titanium compounds of the titanium halide or oxyhalide type. The titanium halides or oxyhalides more particularly used in the present invention are the fluorides, chlorides, bromides and iodides (respectively oxyfluorides, oxychlorides, oxybromides and oxyiodides) of titanium.

According to a particularly preferred embodiment, the titanium compound is titanium oxychloride TiOCI2.

The amount of titanium compound A present in the solution to be hydrolyzed is not critical.

The initial solution also contains at least one compound B as defined above. By way of nonlimiting examples of compounds B coming within the scope of the present invention, there may be mentioned in particular:
- hydroxypolycarboxylic acids, and more particularly hydroxydi- or hydroxytricarboxylic acids such as citric acid, maleic acid and tartaric acid.

- (polyhydroxy) monocarboxylic acids, such as for example acid
glucoheptonic and gluconic acid, - poly (hydroxycarboxylic acids), such as for example tartaric acid, - mono dicarboxylic acids and their corresponding amides, such as for example
aspartic acid, asparagine and glutamic acid, - monocarboxylic amino acids, hydroxylated or not, such as for example lysine,
serine and threonine, - methylene aminotriphosphonate, ethylenediaminotetraphosphonate
methylene, methylene triethylenetetraaminohexaphosphonate,
methylene tetraethylenepentaaminoheptaphosphonate,
methylene pentaethylenehexaaminooctaphosphonate, - methylene diphosphonate; 1.1 'ethylene; 1,2 ethylene; 1.1 'propylene;
1.3 propylene; 1.6 hexamethylene; 2,4 dihydroxypentamethylene - 2,4
diphosphonate; 2,5 dihydroxyhexamethylene - 2,5 disphosphonate; the 2.3
dihydroxybutylene - 2,3 diphosphonate; 1 hydroxybenzyl - 1,1 'diphosphonate; the
1 aminoethylene 1-1 'diphosphonate; hydroxymethylene diphosphonate; 1
hydroxyethylene 1,1 'diphosphonate; 1-hydroxypropylene 1-1 'diphosphonate; 1
hydroxybutylene 1-1 'diphosphonate; read 1 hydroxyhexamethylene -1.1 '
diphosphonate.

As already indicated, it is also possible to use as compound B all the salts of the aforementioned acids. In particular, these salts are either alkali metal salts, and more particularly sodium salts, or ammonium salts.

These compounds can also be chosen from sulfuric acid and ammonium and potassium sulphates, etc.

Preferably, the compounds B as defined above are hydrocarbon compounds of the aliphatic type. In this case, the length of the main hydrocarbon chain preferably does not exceed 15 carbon atoms, and more preferably 10 carbon atoms.

The amount of compound B is not critical. In general, the molar concentration of compound B relative to that of titanium compound A is between 0.2 and 10% and preferably between 1 and 5%.

Finally, the starting solution comprises seeds of titanium dioxide used in a specific manner.

Thus, the titanium dioxide seeds used in the present invention must first of all have a size of less than 8 nm, measured by X-ray diffraction. Preferably, titanium dioxide seeds having a size between 3 and 5 are used. nm.

Then, the weight ratio of the titanium dioxide present in the seeds to the titanium present in the hydrolysis medium before introduction of the seeds - that is to say provided by the titanium compound A - and expressed as TiO2 is between 0.01 and 3%.

This ratio can preferably be between 0.05 and 1.5%. The meeting of these two conditions on the seeds (size and weight ratio) associated with the process as described above makes it possible to precisely control the final size of the titanium dioxide particles by associating a particle size with a number of seeds. It is thus possible to obtain particles whose size varies between 5 and 100 nm.

Seeds of titanium dioxide in anatase form are used so as to induce precipitation of titanium dioxide in anatase form. Generally, due to their small size, these germs tend to be in the form of poorly crystallized anatase. The seeds are usually in the form of an aqueous suspension consisting of titanium dioxide. They can generally be obtained in a known manner by a process for neutralizing a titanium salt with a base.

The next step consists in carrying out the hydrolysis of this starting solution by any means known to those skilled in the art and in general by heating. In this last case,
The hydrolysis can preferably be carried out at a temperature greater than or equal to 70 ° C. It is also possible to work firstly at a temperature below the boiling point of the medium and then to maintain the hydrolysis medium in level at the same time. boiling temperature.

Once the hydrolysis has been carried out, the titanium dioxide particles obtained can be recovered by separating the precipitated solid from the mother liquors before being redispersed in an aqueous liquid medium so as to obtain a dispersion of titanium dioxide. This liquid medium can be acidic or basic. It is preferably an acid solution, for example an aqueous solution of nitric or hydrochloric acid.

Preferably, the preparation process according to the invention relates to transparent coating compositions such as compositions for stain or varnish. By transparent coating is meant a coating through which the treated support can be seen. It may or may not be colored.

The invention also relates to a composition for coating comprising:
- as an anti-UV agent: particles of anatase titanium dioxide with a size of at most 100 nm, said particles being coated at least in part with a layer of at least one cerium compound,
- at least one binder.

This composition can be based on an aqueous or organic solvent. The organic solvents are of the type of those mentioned above.

The titanium dioxide particles, in the form of a dispersion or a powder, have the characteristics defined above.

The binder can be chosen from water-soluble polymers, alkyd, aminoplast, acrylic or styrene-acrylic, vinyl, cellulosic, polyurethane, epoxy-type latexes, etc. These binders are suitable both for coating compositions in aqueous phase as well as in organic phase. .

According to a preferred variant, it is possible to use acrylic latexes of small size, i.e. of size less than 100 nm, such as the RHODOPAS ULTRAFINE latexes sold by Rhône-Poulenc, in particular as a stain.

The composition can also include a surfactant.

The surfactant is preferably a nonionic surfactant, for example ethoxylated with an ethoxylation number of between 3 and 12, for example a propoxylated surfactant (PO). The amount of surfactant can be between 1 and 120% by weight relative to the titanium dioxide expressed on a dry basis.

This composition can comprise any type of agent conventionally used in the coating application, such as thickeners, biocidal agents, anti-UV agents (HALS or organic absorbers), coloring pigments, coalescing agents.

In general, the content of said composition of titanium dioxide particles is such that the content of the dry coating, obtained from the composition, of titanium dioxide is at most 10% by weight, preferably between 1 and 8%.

This composition can in particular be used for any transparent coating, in particular as a stain or a varnish.

The following examples illustrate the invention without, however, limiting its scope.

EXAMPLES
Dispersion 1 - Preparation of a dispersion according to the invention with a treatment based on CeO2, SiO2, Al203
Step 1 - Hydrolysis
The following are added successively to 1300 g of a titanium oxychloride solution at 1.73 moVkg:
- 121 g of 36% hydrochloric acid,
- 15.14 g of citric acid,
- 1562 g of purified water,
- 5.72 g (2% / TiO2) of anatase germs having a size between 5 and 6 nm.

The mixture is brought to a boil and is kept there for 3 h.

Titanium 2 - Particle recovery and re-dispersion
The solution is then filtered and the particles obtained are washed with water until complete elimination of the chlorides. They are then redispersed at pH 9 (controlled by the addition of sodium hydroxide). The dry extract is 20% by weight.

The dispersion obtained is stable. The particle size measured by TEM is 45 nm. X-ray diffraction analysis indicates that the particles are based on titanium dioxide only in the anatase form. The particles have a specific surface
BET of 300 m2 / g.

Step 3 - Treatment of particles with a cenum compound
A solution 1, which is an aqueous solution of cerium acetate obtained by dissolving 13.56 g of cerium acetate in 750 g of purified water, is used.

1159 g of the dispersion obtained at the end of step 2 are introduced into a 3-liter reactor provided with stirring. Then 240 g of purified water are added. The pH of the dispersion is adjusted to 9 by adding sodium hydroxide. The temperature is maintained at 25 "C.

Solution 1 is then introduced continuously with a flow rate of 2 ml / min. The pH is regulated to 9 by simultaneous introduction of a 5 M aqueous sodium hydroxide solution and a 1 M aqueous phosphoric acid solution. Maturing is then carried out for 1 hour at 25 ° C., still with stirring.

Step 4 - Processing of particles by silicon and aluminum compounds
The following solutions are used:
- solution 2: aqueous solution of sodium silicate obtained by dissolving 108.78 g of sodium silicate in 100 g of water (sodium silicate with 355 g / l of SiO2 and SiONa2O molar ratio = 3.3)
- solution 3: aqueous solution of sodium aluminate obtained by dissolving 39.28 g of sodium aluminate in 40 g of water (sodium aluminate 24% by weight

1159 g of the dispersion obtained at the end of step 2 are introduced into a 3-liter reactor provided with stirring. Then 240 g of purified water are added. The pH of the dispersion is adjusted to 9 by adding sodium hydroxide. The temperature is maintained at 25 "C.

Solution 1 is then introduced continuously with a flow rate of 500 mllh. The pH is regulated to 9 by simultaneous introduction of a 5 M aqueous sodium hydroxide solution and a 1 M aqueous phosphoric acid solution. Maturing is then carried out for 1 hour at 25 ° C., still with stirring.

Step 4 - Processing of particles with silica and alumina
The following solutions are used:
- solution 2: aqueous solution of cerium silicate obtained by dissolving 108.78 9 of sodium silicate in 100 g of water (sodium silicate at 355 g / l of SiO2 and SiO / Na2O molar ratio = 3.3)
- solution 3: aqueous solution of sodium aluminate obtained by dissolving 39.28 g of sodium aluminate in 40 g of water (sodium aluminate at 24% by weight of Al203 and 19% by weight of Na2O)
The reaction medium resulting from step 3 is heated to 90 ° C. (temperature rise in 35 min) and solution 2 is introduced at a flow rate of 2 ml / min. The pH is regulated to 9 by the introduction of H2SO4 3M A cure time of 1 hour at 90 ° C is carried out.

Still at 90 ° C. and at pH 9, solution 3 is introduced at a flow rate of 2 ml / min, then left to mature for 1 hour. The reaction medium is then cooled to ambient temperature.

The dispersion obtained is centrifuged. The cake obtained is washed with purified water and then this cake is redispersed in aqueous medium at pH 9.

The dry extract of the dispersion is 25% by weight.

The particle size measured by TEM is 45 nm. The cerium content is 25% by weight, expressed as Ce2O relative to TiO2, the SiO2 content is 12% by weight relative to TiO2 and that of Al203 is 4% by weight relative to TiO2.

Then, to this dispersion is added lgepal HOP89 which is an ethoxylated alkyl phenol surfactant marketed by Rhone-Poulenc. The level of this agent is 20% by weight relative to the dry weight of the dispersion.

Dispersion 3 - Preparation of a dispersion according to the prior art with surface treatment based on SiO2, Al2o3
Step 1 - Hydrolysis
The same hydrolysis is carried out as in step 1 of Example 1.

Step 2 - Particle recovery and re-dispersion
The same step 2 is carried out as in Example 1.

Step 3 - Processing of the particles with silica and alumina
The following solutions are used:
- solution 4: aqueous solution of sodium silicate at 26% by weight of SiO2
- solution 5: aqueous solution of sodium aluminate at 25.6% by weight of Al203
1172 g of the dispersion obtained at the end of step 2 are introduced into a reactor provided with stirring. The pH is adjusted. to 9 by the introduction of H2SO4 or
NaOH. 15 g of polyvinylpirrolidone diluted in 300 g of purified water are added to the dispersion.

The reaction medium is then heated to 90 ° C. (temperature rise in 35 min) and 346 g of solution 4 is introduced at a flow rate of 0.2 ml / min. The pH is regulated to 9 by the introduction of H2SO4. 6 M. A ripening time of 1 hour is carried out at 90 "C.

Still at 90 ° C. and at pH 9, 62.5 g of solution 5 are introduced at a flow rate of 2 ml / min, then the mixture is left to mature for 1 hour. The reaction medium is then cooled to ambient temperature.

The dispersion obtained is centrifuged. The cake obtained is washed with purified water and then this cake is redispersed in aqueous medium at pH 9.

The dry extract of the dispersion is 25% by weight.

The particle size measured by TEM is 45 nm. The level of SiO2 is 60% by weight relative to the TiO2 and that of A1203 is 10%.

Dispersion 4 according to the prior art with surface treatment based on SiO2
This dispersion of the prior art is a dispersion of particles of rutile titanium dioxide having a solids content of 25% by weight. The particles have an average size of 80 nm and a silica-based surface treatment in a content of 110% by weight relative to TiO2.

Preparation of stains from dispersions 1 to 4
The following components are mixed:
- 407.9 g of water,
- 3.7 g of ThixolE 60 marketed by COATEX which is a thickener,
- 2, g of ClérolE TPE 714 marketed by BEVALOID which is an anti
foam,
- 20 g of DowanolE dpnb marketed by DOW which is a
coalescence,
- 20 g of propylene glycol which is a coalescing agent,
- 523 g of DS 913 marketed by Rhône-Poulenc which is a latex
styrene acrylic,
- 3.3 g of Prnxe gxl which is a biocidal agent, - 3.4 g of 30% NaOH.

Mixing is carried out using a Rayneri stirrer, then 55.06 g of one of the preceding dispersions are added thereto.

Evaluation of the properties of stains
Transparency measurement
A wet 100 µm thick stain film is pulled onto a carton
LENETA (black and white background), giving a dry film with a thickness of 20 µm.

The transparency is evaluated using a DATACOLOR DC 3890 spectrophotometer in specular mode included. The result is expressed in AL (difference between the L of the black background treated with the stain and of an untreated black background). The lower this AL, the better the transparency.

L is the measure of reflectance (light / dark shade) in the intrinsic coloration quantified by means of the chromatic coordinates L, a and b, given in the CIE 1976 system (L, a, b) as defined by the International Commission d 'Lighting and listed in the Collection of French Standards (AFNOR), colorimetric color No. X08-12 (1983).

Anti-UV activity measurement
We apply 3 coats of stain at a rate of 300 g / m2 on wood panels (maritime pine 15 X 18 X 1 cm3). These panels are exposed to UV rays in an accelerated aging device QUVB marketed by the company Q-PANEL operating at 60 "C.

The anti-UV protection can be evaluated both by a visual assessment and a colorimetric measurement through the evolution of the reflectance factor L as defined above. In fact, during accelerated aging, the greater the drop in L, the more the wood turns gray and ages.

The result was expressed in AL which is the difference between the initial L of the wood and the L of the wood after accelerated aging.

The stain not containing titanium dioxide particles is observed to have an aL of 12; the color after aging is very heterogeneous (flaking).

Measurement of photocatalytic activity
The photocatalytic activity is evaluated on the titanium dioxide particles in powder form. For this, the titanium dioxide dispersions are dried in an oven at 120 "C.

The test consists in measuring the speed of photooxidation of isopropanol gas by titanium dioxide according to a test of the same principle as that described in the article by Brick
G., Strickland TH and Zanucci JS, Permanence of Organic Coatings ASTM STP 781, 1982, p. 35-42.

3 ml of isopropanol and 1.5 g of titanium dioxide to be tested are introduced into a Pyrex tube. The latter is immobilized in the form of a pellet on a metal support; the surface area of titanium dioxide exposed to UV is 10 cm2. The tube is then sealed.

The test device consists of a carousel rotating around 3 low-pressure UVA lamps with an emission maximum between 300 and 400 nm. The tube containing the sample is placed in the carousel, the face of the support containing the titanium dioxide to be evaluated on the UVA radiation side. The carousel itself is placed in an oven at 60 "C.

The light power in the UVA received by the titanium dioxide is approximately 30 W / m2.

Isopropanol in the vapor phase adsorbs onto titanium dioxide and reacts with oxygen to form acetone.

The progress of the photodecomposition of isopropanol is measured regularly over a period of 24 hours using a gas chromatograph by monitoring the quantity of O2 remaining in the tube. This progress is translated using the O2 disappearance rate constant expressed in mmoleih / m2.

The higher the photooxidation rate measured by this test, the greater the degradation of the stain.

The results of this test were confirmed by an evaluation of the visual appearance of the stain after 200 cycles of accelerated aging as previously described.

Depending on the photocatalytic activity, a more or less pronounced chalking can be observed.

Results

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<tb><SEP> chalking
<tb> Dispersion <SEP> 3 <SEP> 1.1 <SEP> 5 <SEP> 0.66 <SEP> homogeneous <SEP> chalking
<tb> Dispersion <SEP> 4 <SEP> 5.7 <SEP> 8 <SEP> 0.33 <SEP> heterogeneous <SEP> chalking
<tb>
It is observed that the dispersions of titanium dioxide particles treated with cerium exhibit low photocatalytic activity. This makes it possible to avoid chalking of the stain and to keep this stain its initial appearance while providing anti-UV protection.

In addition, these stains have good transparency.

Claims (21)

1. Process for preparing an anti-UV coating composition, characterized in that anatase titanium dioxide particles of size of at most 100 nm are used, said particles being coated at least in part by a deposit of at least one compound of cerium. 2. Method according to the preceding claim, characterized in that the weight ratio of the cerium compound, expressed as CeO2, on titanium dioxide is at least 0.01% by weight, preferably at most 50%. in weight. 3. Method according to any one of the preceding claims, characterized in that the particles are coated at least in part by depositing at least one metal compound other than cerium in simple or mixed form. 4. Method according to the preceding claim, characterized in that the metal compound other than cerium is chosen from silica, zirconia, oxides or hydroxides of aluminum, zinc, titanium or tin. 5. Method according to any one of claims 3 or 4, characterized in that the weight ratio of compounds other than cerium, expressed as metal oxides, on titanium dioxide is at most 100% by weight. 6. Method according to any one of the preceding claims, characterized in that the titanium dioxide particles are used in the form of a dispersion. 7. Method according to the preceding claim, characterized in that the dispersion index of the particles in the dispersion is at most 1.3. 8. Method according to any one of claims 1 to 5, characterized in that the titanium dioxide particles are used in the form of a powder. 9. Method according to the preceding claim, characterized in that the particles have a specific surface area of at least 40 m21g. 10. Method according to any one of the preceding claims, characterized in that the particles are obtained by precipitation at the surface of particles of titanium dioxide starting from a cerium compound from a cerium salt such as l. cerium acetate, cerium sulfate, cerium chloride, cerium nitrate. 11. Method according to any one of claims 3 to 10, characterized in that the particles are obtained by precipitation at the surface of titanium dioxide particles treated with cerium, of a metal compound other than cerium from a metal salt, oxide or hydroxide in simple or mixed form. 12. Method according to the preceding claim, characterized in that a silicon compound is precipitated from a silicon salt such as an alkali silicate. 13. Method according to one of claims 1 1 or 12, characterized in that an aluminum compound is precipitated from an aluminum salt such as alumina sulfate, Sodium aluminate, basic aluminum chloride, aluminum hydroxide diacetate. 14. Method according to any one of claims 10 to 13, characterized in that the starting titanium dioxide particles are obtained by a solution preparation process. 15. Method according to the preceding claim, characterized in that the starting titanium dioxide particles are obtained by hydrolysis of at least one titanium compound A in the presence of at least one compound B chosen from: (i) acids which have: - either a carboxyl group and at least two hydroxyl groups and / or amines, - or at least two carboxyl groups and at least one group hydroxyl and / or amine, (ii) organic phosphoric acids of the following formulas:

(iv) the salts of the acids described above, and in the presence of anatase titanium dioxide seeds having a size of at most 5 nm and in a weight ratio expressed as TiO2 present in the seeds / titanium present before introduction of the seeds in the hydrolysis medium, expressed as TiO2 of between 0.01% and 3%. (iii) compounds capable of liberating sulfate ions in an acidic medium, representing a hydroxyl, amino, aralkyl, aryl, alkyl or hydrogen group, integer between 0 and 5, R1, R2, R3 identical or different where, n and m are integers between 1 and 6, p is a 16. Method according to any one of the preceding claims, characterized in that the anti-UV coating composition is transparent. 17. Composition for coating, characterized in that it comprises: - as an anti-UV agent: particles of anatase titanium dioxide with a size of at most 100 nm, said particles being coated at least in part with a deposit of at least one cerium compound, - at least one binder. 18. Composition according to the preceding claim, characterized in that it comprises at least one surfactant. 19. Composition according to the preceding claim, characterized in that the surfactant is chosen from ethoxylated nonionic surfactants having an ethoxylation number of between 3 and 12. 20. Composition according to any one of claims 17 to 19, characterized in that the content of said composition of titanium dioxide particles is at most 10% by weight, preferably between 1 and 8%. 21. Composition according to any one of claims 17 to 20, characterized in that it is used as a stain or varnish.