Substrate with photocatalytic coating

SUBSTRATE WITH PHOTOCATALYTIC COATING

The present invention relates to substrates provided with a photocatalytic coating, the process for obtaining such a coating and its different applications. It relates more particularly to coatings comprising semiconductor materials based on metal oxide, in particular titanium oxide, which are capable, under the effect of radiation of adequate wavelength, of initiating radical reactions causing oxidation of organic products. The coatings thus make it possible to confer new functionalities on the materials which they cover, in particular anti-fouling, fungicidal, bactericidal properties, possibly combined with hydrophilic, anti-fogging, optical, etc. properties.

Very diverse substrates can be envisaged, in particular those used in the field of vehicles or buildings, such as glazing, facade materials, cladding, roofing, floor, such as tiles, slates, slabs, tiles, and more generally any material used in construction. These materials can thus be glass, metal, glass-ceramic, ceramic, cement, brick, wood, stone or material reconstituted from these natural materials, plastic material, fibrous material of the mineral wool type, in particular for filtration processes, etc.

They can also be stored in transparent materials, used in particular as glazing, such as glass substrates or in flexible or rigid plastic such as polyester or acrylate substrates such as polymethyl methacrylate (PMMA). Substrates can also be classified in the category of non or very porous (glass) or in the category of (relatively) porous materials such as tiles, ceramics. 2 One can also consider “mono-material” substrates such as glass substrates, or substrates comprising a superposition of materials, layers, such as facades which are provided with a coating of the coated facade type. International patent applications WO97 / 10186 are already known and

WO97 / 10185 coatings containing TiO₂ crystallized anatase with photocatalytic properties, coatings obtained from the thermal decomposition of suitable organometallic precursors and / or from TiO particles₂

"Pre-crystallized" and coated in a mineral or organic binder. The object of the invention is therefore to improve these types of coating so that their photocatalytic performance is prolonged over time, faced with aging conditions which may be encountered in the various applications envisaged.

The invention therefore aims to improve these types of coatings, in particular by retaining or increasing their photocatalytic properties while increasing their durability, in particular mechanical or chemical.

The invention firstly relates to a process for obtaining a substrate provided on at least part of its surface with a photocatalytic coating, crystallized particles of an oxide of a metal A with photocatalytic properties being incorporated into said coating using a mineral binder comprising at least one oxide of a metal B also having photocatalytic properties in the crystallized state. This binder can also optionally comprise at least one oxide of a metal M lacking photocatalytic properties and / or at least one Si compound of the silicon oxide SiO type₂. This method consists in depositing the coating from dispersion (s) in the liquid phase containing: on the one hand said crystallized particles of metal oxide A, on the other hand, at least one precursor compound of metal oxide B of the mineral binder, and optionally a precursor compound of the oxide of the metal M and / or of the compound of Si, and this in a relative proportion defined by a ratio A / (B + M + Si) of between 60/40 and 40 / 60. This ratio corresponds to that of the weights of the metals A on the one hand, and B and optionally of the metal M and silicon Si on the other hand, entering respectively in the composition of the oxide A in the form of particles and in that of (of) precursor (s) of oxide B, and optionally of oxide M and of the Si compound of the SiO type₂.

Advantageously, the conditions for depositing / treating the coating are chosen such that the inorganic binder, and very particularly the oxide of B which is part of it, is at least partially crystallized in the final coating.

Preferably, the oxides of metals A and B are chosen from at least one of the following oxides: titanium oxide, zinc oxide, tin oxide, tungsten oxide. A particularly preferred embodiment consists in choosing the oxides of A and B both in the form of titanium oxide (titanium dioxide), the crystalline anatase form of which is highly photocatalytic.

M-type oxides lacking intrinsic photocatalytic properties are, for example, aluminum or zirconium oxide.

The authors of the present invention have succeeded by this process in reconciling two constraints hitherto difficult to reconcile, namely photocatalytic performance and durability and more precisely they have succeeded in prolonging the photocatalytic properties of the coating over time. Indeed, it has turned out that the catalytic effect of the coating is probably mainly obtained by the particles which are incorporated therein which are already crystallized, already catalytically active at the start. The temptation is then to maximize the amount in the coating. But surprisingly, both too high and too low amounts of particles have not been found suitable for the dual purpose sought (namely both photocatalytic properties and sufficient durability), the adjustment of the ratio discovered by the invention being all the less easy to do since the level and the type of particles of oxide of A probably influence the morphology of the binder comprising oxide of B, the evolution of the more or less pronounced photocatalytic nature and of its maintenance over time of the coating not being linear as a function of this or that parameter. The process according to the invention has therefore discovered that it is possible to select an A / (B + M + Si) ratio from a range which manages to best reconcile a satisfactory level of photocatalytic activity and the maintenance of this high level of photocatalytic activity over time. The reasons are not fully explained, at least for the photocatalytic performance of the coating. It can be argued that it is advantageous that the mineral binder also contributes to the activity of the coating. It should also be noted that the coatings obtained tend to have excellent optical properties, in particular a high light transmission and a very low level of blur.

The precursors mentioned above of oxide B and optionally of oxide (s) M are advantageously organometallic compounds capable of decomposing into oxide under the effect of an appropriate treatment, in particular heat. With regard to the precursor of the compound of Si, in particular of the precursor of SiO₂, a compound of the family of silicon al oxides (silanes) can be used.

Advantageously, the method according to the invention uses crystallized particles of the oxide of A (in particular TiO₂ predominantly crystallized in anatase form) in the form of agglomerates of crystallites, preferably having agglomerates of average size of approximately 5 to 80 nm and of crystallites of average size of approximately 5 to 20 nm (in particular 5 to 10 nm), in dispersion in a liquid phase, in particular in colloidal suspension in an aqueous medium or in dispersion in at least one organic solvent. These sizes correspond to the “diameters” of the agglomerates and crystallites in question, by assimilating their shapes to spheres (even if this is not necessarily the case, and that in particular, the agglomerates in question can also be approximately lenticular in shape or in stick shape). Rather than speaking of agglomerates of crystallites, one can in fact use a more correct terminology, namely that the agglomerates are particles, and the crystallites what one can denote by the term of crystal coherence domain. As a first approximation, we can consider that the same agglomerates are found in the final coating, having undergone little or no structural or dimensional modification. In fact, it has been observed that when the process for obtaining the photocatalytic coating involves a heat treatment (detailed below), this treatment led to structurally modify these particles, with a significant increase in the size of the crystallites. For example, when the crystallites of TiO₂ are initially of the order of 5 to 10 nm, they are generally in the final coating rather of the order of 10 to 20 5 nm: their size has approximately doubled (factor 1.5 to 2.5). A detailed description of these particles can be found for example in the application.

WO97 / 10185 cited above or in application WO / FR97 / 02068 of 18 November

1997, published under the number WO98 / 23549 or the application FR-2 681 534. Preferably, the organo-metallic compound (s) precursor (s) of oxide of B, and optionally l 'oxide, are chosen from the family of tetraalkoxides of form M (OR)₄, with M the metal in question and R a carbon radical of the linear or branched alkyl type, all identical or different, having in particular from 1 to 6 C. Mention may thus be made of tetrabutoxide or titanium tetraisopropoxide. They can also be chosen from trialcoxides of the MR '(OR) type₃, with R and R 'radicals identical or different from the type of those of the tetraalkoxides mentioned above, or from halides, in particular titanium chlorides. These precursors being highly hydrolyzable and reactive, it is preferable to put them in solution with at least one chelating / stabilizing agent, for example of the β-diketone type such as acetylacetone (2,4-pentanedione), benzoylacetone (1-phenyl- 1, 3 butanedione), di-isopropylacetylacetone, or alternatively acetic acid diethanolamine or compounds of the glycol family such as ethylene glycol or tetraoctylene glycol. The concentration of precursor in the solution is then adjusted (to a given level of dry extract for example) by carrying out the appropriate dilutions using organic solvent (s).

According to a first variant of the invention, the mineral binder of the coating according to the invention comprises only the metal oxide B, the ratio A / (B + M + Si) mentioned above then becoming more simply the ratio A / B. According to a second variant, the mineral binder comprises metal oxide B, of the TiO type₂, and a silicon compound, of SiO type₂, the ratio A / (B + M + Si) then becoming A / (B + Si).

The simplest embodiment of the method according to the invention consists in depositing the coating from a dispersion containing the precursor (s) and from a dispersion containing the particles, which are premixed into a dispersion. single before projection on the substrate or immersion of the latter therein, although it is also possible to make a deposit from several, in particular two distinct dispersions, without premixing. 6 A first type of deposition technique is called “hot” deposition, that is to say that during the dispersion / substrate contact, the latter is at a temperature high enough to allow thermal decomposition of the precursor (s). ): this is the pyrolysis type technique in the liquid phase. A second type of technique is called “cold” deposition, that is to say that during the dispersion / substrate contact, it is at ambient temperature or at least at a temperature too low to cause the decomposition of the (of) precursor (s): these are the sol-gel type techniques, with a deposition mode of the “hardened”, “cell-coating”, laminar or “spray-coating” type.

A heat treatment after the dispersion / substrate contacting phase is necessary in the case of “cold” deposits, to harden the coating and ensure complete decomposition of the precursors. However, it is also advantageous in the case of “hot” deposits, since it can improve the cohesion of the coating and promote at least partial crystallization of the binder resulting from the decomposition of the precursor (s). This treatment is in particular carried out at at least 400 ° C, for example at more than 450X, in particular around 500 to 550 ° C; more particularly when the substrate is able to withstand this type of treatment, which is the case for glass matrix, ceramic or glass-ceramic substrates.

The invention also relates to a substrate provided on at least part of its surface with a photocatalytic coating incorporating crystallized particles of an oxide of a metal A with photocatalytic properties using at least one mineral binder partially crystallized comprising an oxide of a metal B also exhibiting photocatalytic properties in the crystallized state, in particular a substrate obtained according to the method described above. It is characterized by a high porosity, in particular greater than 40% and preferably between 45 and 65%. This porosity could have been calculated indirectly, by measuring the layer index, in comparison with what it would have to be if the material were completely dense. This indirect method is fairly representative of the porosity and the surface morphology of the layer, since the index measurement also takes into account, at least in part, the level of surface roughness of the layer. 7 (Other indirect methods also exist, in particular those consisting in measuring the weight of the coating deposited per unit area of ​​the substrate, reduced to a given coating thickness).

In fact, this high porosity has many advantages. First of all, it makes it possible to drop the refractive index of the material, to modulate its optical appearance. In the case of a TiO-based coating₂ (i.e. with TiO crystallized particles₂ mainly anatase and a Ti0-based binder₂, possibly combined with SiO₂), dropping its index to values ​​of at most 2, in particular of the order of 1, 4 to 1, 8, preferably of approximately 1, 7 to 1, 8, makes it possible to attenuate very strongly its well-known reflective appearance. Furthermore, the porosity of the coating is linked to a high surface roughness, hence a large developed surface of the coating promoting its photocatalytic activity.

Finally, this roughness, which is probably in two different orders as described in the aforementioned patent WO98 / 23549, gives the coating a reinforced and durable hydrophilic character, which gives it marked anti-rain, anti-fog properties (by spreading drops of water in invisible film), and promotes the elimination of mineral soiling by entrainment by rainwater. Surprisingly, this significant porosity did not result in an excessive embrittlement of the coating on the mechanical level.

The subject of the invention is also a substrate provided on at least part of its surface with a coating with photocatalytic properties incorporating crystallized particles of an oxide of a metal A with photocatalytic properties using a binder less partially crystallized, comprising at least one oxide of a metal B also having photocatalytic properties in the crystallized state, and optionally at least one oxide of a metal M lacking photocatalytic properties and / or a silicon compound of the silicon oxide type, in particular obtained according to the method described above. It is characterized by a relative proportion A / (B + M + Si) between 60/40 and 40/60, by weight reduced to the weight of the metals (and possibly Si) respectively entering into the composition of the particles of the oxide of A and in the composition of the oxide of B, and optionally of the oxide of M and of the compound of Si of the mineral binder. 8 It should be noted that in the invention, the envisaged substrate may have a certain porosity, when it is a question of tiles for example, or a fibrous appearance.

(mineral insulation wool for example). When we say that the substrate is provided with the photocatalytic coating, it should therefore be understood that this is deposited on its surface, but that it can also impregnate it over a certain depth if it is porous / fibrous. This is the reason why the quantity of coating exposed can be expressed either by its thickness on the substrate, when it is non-porous, for example when it is a glass substrate, or by an amount of material per unit. surface, especially when the substrate has a certain porosity.

The coating according to the invention, whether obtained by the process described above, and / or that it conforms to the coatings whose intrinsic characteristics have been described above, advantageously has the following structure (very particularly in the case where the oxides of A and B are both TiO-based₂): crystallized particles of dimensions between 5 and 80 nm having crystal coherence domains of dimensions between 5 and 20 nm (with the conventions described above), and an inorganic binder at least partially in the form of grains which are placed around crystallized particles, in the inter-particle interstices, and which are of an average size between 5 and 25 nm, preferably 10 to 20 nm. These “grains” of approximately spherical shape are not completely crystallized, however probably partially crystallized on a very small scale which is difficult to measure and thus “coats” the particles and bind them together. Advantageously, the substrate according to the invention is provided with the photocatalytic coating of the invention comprising particles of Ti0₂ essentially in anatase form and a mineral binder combining Ti0₂ partially crystallized and SiO₂ . Preferably, the coating has an index of at most 2, in particular between 1, 5 and 1, 9 or between 1, 6 and 1, 9 or between 1, 6 and 1, 8. According to one embodiment, at least one layer is interposed between the substrate and the photocatalytic coating, layer (s) which can have different functions (optical function, barrier to species liable to migrate from the substrate such as alkalis, anti-static, layer adhesion, ...). 9 These may be layers based on Si compounds, such as Si, SiO₂,

SiON, SiOC, Si₃NOT₄, or based on metal oxide possibly doped (Sn0₂ : F,

SnO₂ : Sb, ...).

The substrates provided with such coatings have already been mentioned in the preamble to the present application. They may include a transparent material of the glass type, plastic material, in particular to form part of glazing fitted to buildings or vehicles, or screens of television type devices, computer, touch screens for example, of any laminated glazing or

"Monolithic" (that is to say comprising only a single glass or a single sheet of plastic material). It is also advantageous to incorporate a transparent substrate provided with the coating of the invention in an insulating multiple glazing structure, either so that the coating is on an internal face of the glazing, or on an external face of the glazing. It can be conventional insulating glazing with one or more interlayer gas slides, for example of the type sold by Saint-Gobain Vitrage under the name BIVER or CLIMALIT D, CONTRATHERM, CONTRASONOR, CONTRARISC, or those called “vacuum” where the intermediate gas slide is replaced by the vacuum, as described for example in patent EP-645,516. In the latter case, in particular, it is particularly advantageous to have the coating on the external face, on the face of the insulating glass facing outwards, to avoid the formation of fogging due to its hydrophilic nature.

The coating of the invention is also advantageous for glass walls of freezers / refrigerators. In fact, many materials can serve as substrates for the coating according to the invention, for example materials made of metal, ceramic, plastic, cement and all the materials mentioned above used in architecture, for applications as a facade material, siding, roofing such as tiles, slates. It can also be materials equipping floors or interior or exterior residential walls, such as slabs, tiling. It is also possible to deposit the coating on fibrous materials of the mineral wool type for thermal and / or acoustic insulation, or fibers of the textile reinforcement type, these fibrous materials can find application in filtration processes for example: thus exploiting the anti-fouling, bactericidal, fungicidal, anti-fogging properties of the coating of the invention as required.

The subject of the invention is also the dispersion in the liquid phase described in particular above and which can be used to manufacture the photocatalytic coating according to the invention. Said dispersion comprises in particular a solvent chosen from water, ethylene glycol, ethanol, propylene glycol and their mixtures.

Preferably, the nature of the crystalline phase of the titanium dioxide particles of the dispersion according to the invention is mainly the anatase crystalline form. "Mostly" means that the anatase level of the titanium dioxide particles of the coating is greater than 50% by mass. Preferably, the particles of the coating have an anatase level greater than 80%. The crystallization rate and the nature of the crystalline phase are measured by X-ray diffraction. The dispersions according to the invention are generally obtained by mixing a dispersion of the titanium dioxide particles and solutions of the precursor compounds and / or of a silicon compound. Depending on the nature of the compounds used, additives such as co-solvents, surfactants or stabilizers can also be added during this mixing. The mixture can also be improved by stirring the dispersion with ultrasound.

Other details and advantageous characteristics of the invention appear from the following description of nonlimiting exemplary embodiments, using the following figures: FIG. 1: a very schematic representation of the structure of a photocatalytic coating according to the invention,

B figure 2: a picture obtained by scanning electron microscopy

(SEM) of the surface of a photocatalytic coating according to the invention.

A first series of examples relates to the deposition of a coating 3 known as “anti-fouling” essentially based on titanium oxide on a transparent substrate 1. 11 The substrate 1 is made of clear flat soda-lime glass 15 x 40 cm² and of

4 mm thick. It goes without saying that the invention is not limited to this specific type of glass. The glass may also not be flat, but curved.

Between the coating 3 and the substrate 1, there is a thin layer (optional) 2 based on silicon oxycarbide denoted SiOC in order to constitute a barrier to the diffusion to alkali which is detrimental to the photocatalytic property of the coating, and / or a layer with an optical function deposited for example in a known manner by a vapor phase pyrolysis technique

(CVD) and about 50 nm thick. Two different “cold” deposition methods were used, namely deposition by soaking in “cell-coating” with a rate of emptying of the bath of the order of 5 to 30 cm / minute, and deposition by “spray -coating ", that is to say by cold liquid spraying. These well-known techniques are detailed in the patent applications referenced above, to which reference may be made. The deposits are made from a dispersion mixing two initial solutions / dispersions 1 and 2:

^ solution 1: this is the solution containing the organometallic precursor of the TiO-based mineral binder₂. It is titanium isopropylate Ti (O CH (CH)₂)₄, stabilized with acetyl acetonate CH₃-CO-CH₂-CO-CH₃, in solution in ethanol,

"dispersion 2: it is the liquid phase in ethylene glycol containing the photocatalytic crystallized particles, with the following characteristics:

^•→ specific surface of the particles:> 350 m²/ g

* → - particle sizes: "40 nm" → - sizes of the crystallites which constitute the particles: 7 nm

^ crystal phase: more than 80% anatase

The composition of the dispersion resulting from the mixture of solution 1 and of dispersion 2 is adjusted to obtain the Ti ratio₍₂₎/ Ti_{1) desired, i.e. the ratio of the weight of titanium (2) from the particles of dispersion 2 to the weight of titanium (1) from the precursors of solution 1. (By convention, the ratio can also be that of the weight of titanium oxide coming from the particles on the weight of titanium oxide coming from the metal precursor by assuming that 100% of the precursor is converted into oxide: this amounts to the same).

Examples 1 to 7 relate to deposition by soaking in cell-coating, with comparable deposition conditions, that is to say with the same rate of emptying of the bath (6 cm / minute), and the same concentration of titanium (i.e. 3% solids, converted to oxide weight) from the precursor in solution 1.

At the end of the deposition, the substrates are subjected to a heat treatment at around 450-500 ° C. for at least 30 minutes. Table 1 below groups together for each of them:

* → - the Ti ratio₍₂₎/ Ti₍₁₎ explained above, without unity,

"→ - the thickness (e) of the coating (3) in nm,

'→ - the light transmission value T_L in%, measured according to the Illuminant

D₆₅, '→ the “blur” value in%, measured by the ratio of the diffuse transmission to the integrated light transmission over the entire visible range.

13 TABLE 1

Ti₍₂₎/ Ti₍₁₎ e (nm) T_L (%) blurry (%)

EXAMPLE 1 0/100 20 88.5 0.88

EXAMPLE 2 20/80 20 to 30 89.6 0.27

EXAMPLE 3 40/60 40 to 50 89.5 0.76

EXAMPLE 4 50/50 40 to 60 90.2 0.50

EXAMPLE 5 60/40 60 90.5 0.61

EXAMPLE 6 80/20 30 to 40 89.7 0.44

EXAMPLE 7 100/0 30 to 40 90.3 1 <img file="WO9944954A1_D0001.tif" />

Example 8 was produced from the dispersion conforming to that used for Example 4, but deposited on the substrate by cold spraying, by a spray nozzle called “airless” under a pressure of 0.7 bar (0 , 7.10⁵Pa). The layer obtained, after heat treatment at 450-500 ° C for at least 30 minutes, has a thickness of approximately 35 to 60 nm, with a value of T_L 88.6% and 0.6% blur with the same conventions as in Table 1. The preceding examples 1 to 8 were evaluated in terms of photocatalytic activity, before and after a treatment aimed at simulating accelerated aging of the coating.

The measurement of the photocatalytic activity is carried out as follows:

^•→ - O - test carried out on approximately 15 cm² coating, * → - Θ - weighing of the sample and measurement of the thickness of the substrate, of the T_L and blur.

"→ © - spray deposit of a palmitic acid solution (8g of acid for 11 of chloroform) distance glass / spray 20 cm, vertical substrate, 3 to 4 successive passages, * → - 0 - weighing of the sample after deposition of palmitic acid to evaluate the thickness of acid deposited in nanometers,

"-» - θ - measure of blur and T_L after deposit,

"→ © - measurement of the variation of the blur as a function of the irradiation time under UVA (≈ 30 W / m²), 14

"→ - θ - graphical determination of the time after which the blurring has decreased by 50%: time called T_1/2 (disappearance),

* - * - © - evaluation of the photocatalytic activity of the coating at the rate of disappearance v (in nm / h) defined by: v (nm / h) = [thickness of palmitic acid (nm)] / [2 xt_1/2 (disappearance) (h)]

The aging of the coatings consists in subjecting them to mechanical abrasion as follows:

* → - sample size 7 cm x 15 cm

* → applied load: 600 grams

"→ surface of the abrasion felt: 1.5 cm²

"→ - n cycles (1 cycle = 1 round trip of felt carrying arm and load), with n = 200 and 500.

Table 2 below groups together for each of the examples:

*-*^• the speed of disappearance v1 before abrasion,

*^•* - the disappearance speed v2 after 200 cycles,

* -&gt; the disappearance speed v3 after 500 cycles

TABLE 2

V1 (0 cycles) V2 (200 cycles) V3 (500 cycles)

EXAMPLE 1 20 10 0

EXAMPLE 2 40 5-10 0

EXAMPLE 3 35 - "10

EXAMPLE 4 70 10-20 "10

EXAMPLE 5 35 - “5

EXAMPLE 6 20 1-2 0

EXAMPLE 7 20 0 0

EXAMPLE 8 40 - 7 <img file="WO9944954A1_D0002.tif" />

Furthermore, the analyzes showed that the coatings of Examples 3, 4 and 5 had a structure probably close to that, very simplified, represented in FIG. 1 representing the glass substrate 1, the layer of SiOC 2 and the coating 3. This coating comprises the particles, aggregates 15 of crystallites 5 between which come together grains 4 of TiO₂ amorphous or slightly crystallized which form the mineral binder of the coating

FIG. 2, referring more specifically to example 4, is a photograph obtained by scanning electron microscopy giving an indication of the surface appearance of example 4. a fairly rough surface, allowing the coating to offer a large developed surface

Note that Example 4 has a refractive index of about 1.65

As a first approximation, we can therefore consider, knowing that the refractive index of TiO₂ "Solid" is 2.4, that the porosity of the coating is approximately (2.4-1, 65) / (2.4-1), that is to say approximately 54% The coating according to Example 4 also has a high hydrophilicity exposed to UN / .A rays. for 20 minutes to activate it, put it in the dark and measure the contact angle with water φ regularly: this contact angle remains less than 10 ° for at least 20 days in the dark. From these data the following conclusions can be drawn: these are examples 3, 4, 5 and 8 and, in particular example 5 where the ratio Ti₍₂₎/ Ti₍₁₎ is 50/50 which combine all the desired properties, namely

* - »- a high T_L, weak blur, refractive index less than 2, giving the coating a very favorable optical appearance, * - *^• a satisfactory photocatalytic activity, which is still present even after mechanical attack, which proves the durability of the coatings, and which allows these coatings to be used in real conditions, for example on glazing outdoors, having a "lifetime" acceptable These are in fact the only examples produced which still exhibit photocatalytic activity, even moderate, after 500 abrasion cycles.

A second series of examples relates to the same type of coating, using the same "solution 1" and "dispersion 2". The deposition conditions are identical to those of examples 1 to 7 above, with the difference that now the speed of emptying of the bath is higher, equal to 24 cm / minute Another difference concerns the substrate. it is the same glass, but previously covered with a first 50 nm SiOC layer deposited by CVD and then with a second layer of tin oxide doped with fluorine SnO₂ : F of 450 nm, deposited by pyrolysis of powder in a known manner. And in this series 16 of examples, the way to evaluate the quantity Q of photocatalytic coating is not the measurement of its thickness but the quantity of material per unit area of ​​the substrate, expressed in μg / cm².

The photocatalytic activity of the examples is measured before any abrasion test, this is the value V₁ explained above. The durability of the coating is evaluated qualitatively by simple ragging: "++" means that the coating resists very well, "+" means that it still resists correctly and "-" that the coating is (almost) gone after ragging.

Table 3 below groups together for Examples 9 to 13 the Ti ratio₍₂₎/ Ti₍₁₎ with the same meaning as in table 1, the Q value, the V value_1t and the crumpling test:

TABLE 3

Ti₍₂₎/ Ti₍₁₎ Q (μg / cm²) V1 (nm / h) Ragging

EXAMPLE 9 0/100 22 18 ++

EXAMPLE 10 20/80 24 128 +

EXAMPLE 11 40/60 23 159 +

EXAMPLE 12 50/50 25 231 +

EXAMPLE 13 100/0 23 222 - <img file="WO9944954A1_D0003.tif" />

The same trend can be seen from this table as for the first series: with the same or almost identical amount of coating deposited, an optimum is found for examples 11 and 12 with Ti ratios₍₂ Ti₍₁₎

40/60 and 50/50. Only example 12 makes it possible to have a photocatalytic activity above 200 and a correct durability.

A third series of examples relates to a coating using TiO particles₂ of the dispersion used in all the previous examples, but with a mixed binder combining TiO₂ and SiO₂.

The solution containing the binder precursors uses: as solvent: ethanol and ethylene glycol in mass proportions 75/25, ^ as stabilizer: acetylacetonate,

"^ As a precursor to TiO₂ : titanium tetrabutoxide (TBT) "^ as a precursor of SiO₂ : tetraethylorthosilicate (TEOS). 17

The relative proportion of TBT and TEOS is adjusted so as to have a TiO rate in the solution₂/ SiO₂ 15/85 by weight (having the convention that all TBT converts to TiO₂ and all the TEOS in SiO₂).

This solution is then added to the dispersion of particles used in the previous examples, in proportions such that the ratio r Ti (particles) / (Ti precursor + Si precursor) is obtained. (The dry extract of the solution is 3%).

The deposition and substrate conditions are identical to those of Examples 9 to 13.

Table 4 below groups together the values ​​of the ratio r explained above, the speed value V1 explained above, the light reflection of coated substrate R_L (%), and the change in T_L (ΔT_L) observed after 500 cycles of the abrasion test described in the context of the first series of examples, as well as the ratio r, which expresses the quantities of Ti and Si by weight of oxide: r1 = TiO₂ (particles) / (TiO₂ binder + SiO₂ binder)

It also indicates Q., the amount of TiO₂ total in the coating (particles and TiO₂ from the titanium precursor) in μg / cm² and Q₂ the quantity (calculated) in total weight of the coating also in μg / cm².

TABLE 4 r r. V, R_L ΔT_L Q ^ Q₂

Example 14 45.1 / 54.9 40/60 8 12.5 0 8.8 18.2

Example 15 55.3 / 44.7 50/50 40 10 1 9.3 16.3

Example 16 65/35 60/40 45 14 4 10.1 15.4 <img file="WO9944954A1_D0004.tif" />

By adjusting the solids content of the solution and the rate of emptying, coatings were re-made by fixing the ratio r at 55.3 / 44.7 and by modulating the quantity of coating deposited.

Table 5 below shows, for these additional examples from Example 15, the quantity Q in μg / cm², the corresponding thickness e in nm when it was measured, the value of V., (nm / h), the value of R_L and of ΔT_L explained above: 18 TABLE 5

QE v₁ RL ΔT_L

Example 15 bis 17 110 42 8.5 1.5

Example 15b 33 - 88 12.1 2.5

Example 15c 49,460 130 11 0.8 <img file="WO9944954A1_D0005.tif" />

From this third series of examples, we can see the interest to add in the binder a material, SiO₂, which does not intervene in photocatalysis but which makes it possible to better homogenize the coating and which tends to reinforce its durability. We also see that if the ratio r is decisive, other parameters can also intervene, in particular the value Q, preferably between 15 and 45 μg / cm² to take into account both the cost of the coating and the impact of its thickness on the optical appearance. It goes without saying that the invention is not limited to these specific examples. It is in particular within the framework of the invention to further improve the photocatalytic activity of the particles 5 by doping them, by introducing the dopants into the crystal lattice or by covering the particles of said dopants, of the Fe, Cu, Ru, Mo type. , Bi, Ta, Nb, Co, Ni, Va, ... as described in the aforementioned patent WO97 / 10185.

It is also possible, within the framework of the invention, to add to the mineral binder other oxides which are not or only slightly photocatalytic in the crystallized state, for example by adding to the dispersion the precursors of other oxides of the SiO type₂, such as tetraethoxysilane TEOS. The A / (B + M + Si) ratio mentioned above, optimal in the 40/60 to 60/40 range, can also, with a lower level of requirement, be envisaged in the 35/65 to 40/60 ranges and 65/35 to 60/40.