Method for preparing coated zinc oxide particles by flame spray pyrolysis - Patents.com

Abstract

The present invention relates to a method for preparing coated zinc oxide particles by flame spray pyrolysis technique, to coated zinc oxide particles and compositions containing said particles. The present invention also relates to specific zinc oxide particles derived from such a method, to compositions containing such particles and also to uses thereof.

Description

[Technical field]

[0001] The present invention relates to a method for preparing coated zinc oxide particles by a flame spray pyrolysis technique, to coated zinc oxide particles and compositions comprising said particles.

[0002] The present invention also relates to particular zinc oxide particles derived from such a process, compositions containing such particles and also uses thereof. [Background technology]

[0003] Zinc oxide is used in many applications (cosmetics, paints, dyes, electronic components, rubber, etc.) notably due to its optical properties, but in particular its light absorbing and / or light scattering properties are used to protect surfaces from UV radiation and / or to convert ambient light into electricity.

[0004] However, zinc oxide has the disadvantage of being unstable, especially over time. More particularly, zinc oxide converts to zinc hydroxide or even Zn in the presence of water, either from the composition containing it or from atmospheric moisture. 2+ Such decomposition can result in partial or even complete solubilization of zinc oxide in water and can serve to significantly reduce or even eliminate the desirable properties of zinc oxide.

[0005] This instability is particularly problematic when zinc oxide is used in photoprotective cosmetic compositions: in fact, UV protection decreases as zinc oxide decomposes.

[0006] It has been assumed that zinc oxide can be coated with silica using the sol-gel method or else fluoro compounds can be grafted onto zinc oxide. However, these solutions are not entirely satisfactory. Zinc oxide coated with silica via the sol-gel method generally has worse optical properties than uncoated particles. As for the grafting technique, the use of fluoro compounds can be harmful to the environment and dangerous to users.

[0007] It is also known to use flame spray pyrolysis (FSP) to prepare zinc oxide particles.

[0008] Flame spray pyrolysis or FSP is a recently well-known method originally developed to synthesize ultrafine powders of single or mixed oxides of various metals (e.g. SiO2, Al2O3, BO3, ZrO2, GeO2, WO3, Nb2O5, SnO2, MgO, ZnO) with controlled morphology and / or deposit them on various substrates, starting from various metal precursors, generally in the form of organic or inorganic, preferably flammable, sprayable liquids, which are sprayed into a flame, which, upon combustion, releases, among other things, nanoparticles of the metal oxides sprayed by the flame itself on these various substrates. The principle of this method was recalled, for example, in a recent (2011) publication by Johnson Matthey entitled "Fiber-assisted synthesis of nanoparticles of metal oxides in a flame-assisted synthesis of various metal ... Many different FSP processes and reactors are also described, for example, in the following patents or patent applications: U.S. Pat. No. 6,233,663; ....

[0009] However, this method, when applied to zinc oxide preparations, can be further perfected, in particular to improve the stability of the zinc oxide particles over time and more particularly their resistance to water. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Pat. No. 5,958,361 [Patent Document 2] U.S. Patent No. 2,268,337 [Patent Document 3] WO 01 / 36332 [Patent Document 4] U.S. Patent No. 6,887,566 [Patent Document 5] International Publication No. 2004 / 005184 [Patent Document 6] U.S. Patent No. 7,211,236 [Patent Document 7] International Publication No. 2004 / 056927 [Patent Document 8] International Publication No. 2005 / 103900 [Patent Document 9] International Publication No. 2007 / 028267 [Patent Document 10] U.S. Patent No. 8,182,573 [Patent Document 11] International Publication No. 2008 / 049954 [Patent Document 12] U.S. Patent No. 8,231,369 [Patent Document 13] International Publication No. 2008 / 019905 [Patent Document 14] US Patent Application Publication No. 2009 / 0123357 [Patent Document 15] US Patent Application Publication No. 2009 / 0126604 [Patent Document 16] US Patent Application Publication No. 2010 / 0055340 [Patent Document 17] International Publication No. 2011 / 020204 [Non-patent literature]

[0011] [Non-Patent Document 1] “Flame Spray Pyrolysis: a Unique Facility for the Production of Nanopowders”, Platinum Metals Rev.,2011,55,(2),149-151 Summary of the Invention [Problem to be solved by the invention]

[0012] There is therefore a real need to develop a process for preparing zinc oxide particles which makes it possible to endow said particles with good stability over time, very particularly good resistance to water, whilst retaining good optical properties with regard to absorption and / or scattering of light, more particularly ultraviolet light. [Means for solving the problem]

[0013] These aims are achieved by the present invention, one of the subjects of which is, inter alia, a method for preparing coated zinc oxide particles, in particular of the Zn-M oxide type, comprising at least the following steps: a. preparing a composition (A) by adding one or more zinc precursors to a flammable solvent or mixture of flammable solvents; and then b. forming a flame in a flame spray pyrolysis device by injecting composition (A) and an oxygen-containing gas until zinc oxide aggregates are obtained; c. injecting into the flame a composition (B) comprising one or more precursors of element M and one or more solvents until a (inorganic) organic, preferably inorganic, coating layer containing at least one element M and at least one oxygen atom is obtained on the surface of the zinc oxide aggregates, said element M being selected from the elements from the 4th, 13th and 14th columns of the Periodic Table of the Elements; The method is characterized by comprising:

[0014] It has been observed that the process according to the invention makes it possible to obtain zinc oxide particles coated with a layer of inorganic material based on element M which are particularly stable over time and have good resistance to water.

[0015] Moreover, unlike conventional coating methods, the method according to the invention has the advantage of preserving good intrinsic properties of the core despite the presence of the coating: in fact, due to the specific properties of the coating layer, it is possible to reduce the proportion of zinc oxide for a given particle mass, without nevertheless reducing and / or adversely affecting the properties of said zinc oxide.

[0016] The method of the present invention therefore allows for the production of stable zinc oxide particles whilst avoiding the inconveniences resulting from the increased amounts of particles that have traditionally been required to maintain the good optical properties of zinc oxide.

[0017] These zinc oxide particles, in particular of the Zn-M oxide type, comprise a core 1 and one or more upper coating layers 2 covering said core 1, and (i) the core 1 consists of zinc oxide, preferably in a crystalline state; (ii) the upper coating layer 2 covers at least 90% of the surface of the core 1, preferably covers the entire surface of the core 1, and comprises one or more (inorganic) organic, preferably inorganic, compounds containing one or more elements M and one or more oxygen atoms; (iii) said element M is selected from the elements from the 4th column, the elements from the 13th column, and the elements from the 14th column of the Periodic Table of the Elements; and - when the element M is silicon, (zinc / silicon) 粒子 the molar atomic ratio is strictly less than 2; and - when said element M is different from silicon, (zinc / M) 粒子 The molar atomic ratio is in the range of 0.1 to 10; and - The BET specific surface area of ​​the particles is 1 m 2 / g~350m 2 / g is understood.

[0018] More particularly, the coated zinc oxide particles of the present invention degrade only slightly over time in the presence of water, even when formulated in an aqueous composition.

[0019] It has also been observed that the zinc oxide particles prepared according to the invention have good optical properties in terms of light absorption and / or light scattering, more particularly they have high UV absorption and low or high visible scattering, allowing uses such as sunscreens and / or modification of the visual appearance, while enjoying the benefits based on their resistance in the presence of water.

[0020] Moreover, the compositions containing coated zinc oxide particles according to the present invention showed good shielding power, especially against long and short UV-A rays.

[0021] Furthermore, compositions including the coated zinc oxide particles of the present invention have particularly high transparency, which may prove advantageous, especially when the composition is applied onto the skin and the coating is allowed to dry.

[0022] Moreover, the coated zinc oxide particles according to the present invention do not require a hydrophobic coating and can therefore be used across a broad range of formulations (e.g. in fully aqueous and / or surfactant-free formulations), further reducing the risk of inappropriate deposits (on the edges of the basin, on the walls of pipes or on rocks) when the resulting formulation is finally placed in water (basin drain, lake water or sea water).

[0023] The accompanying drawings are schematic diagrams, not necessarily to scale, and are intended, inter alia, to illustrate the principles of the invention. In these figures, Figs. 1-2, identical elements (or parts of elements) are identified by identical reference symbols. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 illustrates a cross-sectional view of a zinc oxide particle according to one embodiment of the present invention. [Diagram 2] FIG. 2 illustrates a cross-sectional view of a zinc oxide particle also covered with an additional coating layer according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Other objects, features, aspects and advantages of the invention will become more apparent on reading the description and the examples that follow.

[0026] In this specification, unless otherwise indicated, - the term "at least one" is equivalent to and interchangeable with the term "one or more"; - the expression "..." is equivalent to and can be substituted for the expression "to the extent of...", implying the inclusion of a limit; - the expression "keratinous substances" denotes in particular human keratinous fibres such as skin and also hair; - The core (1), also called the "central part", - the upper coating layer (2), also called the "outer layer", "shell" or "coating"; - the expression "(inorganic) organic compound" is equivalent to "organic or inorganic compound"; - "Alkyl" means "alkyl group", i.e. C1-C 10 is understood to mean in particular C1-C8, more in particular C1-C6 and preferentially C1-C4 linear or branched hydrocarbon-based radicals, such as methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl or tert-butyl, an "aryl" group is understood to mean a monocyclic or polycyclic fused or non-fused carbon-based group containing 6 to 22 carbon atoms, at least one ring of which is aromatic, preferentially an aryl group is phenyl, biphenyl, naphthyl, indenyl, anthracenyl or tetrahydronaphthyl, preferably phenyl; - An "arylate" group is one or more -C(O)O - is understood to mean an aryl group containing a carboxylate group, for example a naphthalate or naphthenate, - "complexed zinc" is understood to mean zinc forming a "metal complex" or "coordination compound" in which the metal ion corresponding to the central atom, i.e. zinc, is chemically bound to one or more electron donors (ligands); - "ligand" is understood to mean a coordinating organic chemical group or compound, i.e. one that comprises at least one carbon atom and is capable of coordinating to a metal, in particular a Zn atom, preferably Zn(II), which, when coordinated or complexed, results in a metal compound (internal complex or chelate) with a predetermined number of electrons, which corresponds to the principle of coordination sphere - see Ullmann's Encyclopedia of Industrial Chemistry, "Metal complex dyes", 2005, p. 1-42. More particularly, the ligand is an organic group that comprises at least one group that is electron donating via inductive and / or mesomeric effects, more particularly an organic group that carries at least one amino, phosphino, hydroxy or thiol electron donating group, or the ligand is a persistent carbene, in particular of the "argengo" type (imidazol-2-ylidene) or comprises at least one carbonyl group.As ligands, mention may be made more particularly of: i) those containing at least one phosphorus atom -P< (i.e. phosphines, for example triphenylphosphine), ii) bidentate ligands of formula RC(X)-CR'R''-C(X)-R''', where R and R'''' are identical or different and represent a linear or branched (C1-C6) alkyl group and R' and R'' are identical or different and represent a hydrogen atom or a linear or branched (C1-C6) alkyl group, preferentially R' and R'' represent a hydrogen atom and X represents an oxygen or sulfur atom or an N(R) group, where R represents a hydrogen atom or a linear or branched (C1-C6) alkyl group), such as acetylacetone or a β-diketone, iii) ligands of formula [HO-C(O)]AC(O)-OH, where A is a substituted or unsubstituted aryl group when n has the value zero. a monovalent radical, or when n is 1 or more a polyvalent radical, which is saturated or unsaturated, cyclic or acyclic and aromatic or non-aromatic, based on a hydrocarbon containing 1 to 20 carbon atoms, optionally interrupted by one or more heteroatoms and / or optionally substituted, inter alia, by one or more hydroxyl groups, preferably A represents a monovalent (C1-C6) alkyl group or a polyvalent (C1-C6) alkylene group, optionally substituted by one or more hydroxyl groups, and n represents an integer between 0 and 10 (both inclusive), preferably n is 0 to 5, e.g. 0, 1 or 2) (poly)hydroxycarboxylic acid ligands and their deprotonated forms, such as lactic acid, glycolic acid, tartaric acid, citric acid, maleic acid and arylates, e.g. naphthalates, and iv) C2-C aryl groups containing 2 to 5 hydroxyl groups. 10 polyol ligands, especially ethylene glycol, glycerol, even more especially the ligands carrying carboxy, carboxylate or amino groups, in particular the ligands being selected from acetate groups, (C1-C6)alkoxylate groups, (di)(C1-C6)alkylamino groups and arylate groups, such as naphthalate or naphthenate groups, The term "fuel" is understood to mean a liquid compound that is burned in a chemical reaction that uses dioxygen and energy to generate heat: combustion. In particular, the liquid fuel is selected from protic solvents, in particular alcohols such as methanol, ethanol, isopropanol, n-butanol, aprotic solvents, in particular those selected from esters such as methyl esters and acetates such as 2-ethylhexyl acetate, acids such as 2-ethylhexanoic acid (EHA), acyclic ethers such as ethyl ether, methyl tert-butyl ether (MTBE), methyl tert-amyl ether (TAME), methyl tert-hexyl ether (THEME), ethyl tert-butyl ether (ETBE), ether tert-amyl ether (TAEE), diisopropyl ether (DIPE), cyclic ethers such as tetrahydrofuran (THF), aromatic hydrocarbons such as xylene or arenes, those derived from non-aromatic hydrocarbons and mixtures thereof.

[0027] The fuel may optionally be selected from liquefied hydrocarbons such as acetylene, methane, propane or butane, and mixtures thereof.

[0028] Method for preparing coated zinc oxide particles The preparation method according to the present invention comprises a step (a) of preparing a composition (A) containing one or more zinc precursors and one or more flammable solvents.

[0029] The zinc precursors and flammable solvents that can be used according to the present invention can be selected from zinc precursors and flammable solvents conventionally used in flame spray pyrolysis.

[0030] Preferably, the zinc precursor contained in composition (A) comprises one or more zinc atoms optionally complexed with one or more ligands containing at least one carbon atom.

[0031] More preferentially, said ligands are chosen from acetate groups, (C1-C6)alkoxylate groups, (di)(C1-C6)alkylamino groups and arylate groups, such as naphthalate or naphthenate groups.

[0032] Preferably, the flammable solvent is selected from protic flammable solvents, aprotic flammable solvents and mixtures thereof, more preferentially from alcohols, esters, acids, acyclic ethers, cyclic ethers, aromatic hydrocarbons or arenes, non-aromatic hydrocarbons and mixtures thereof, even better from 2-ethylhexyl acetate, 2-ethylhexanoic acid (EHA), ethyl ether, methyl tert-butyl ether (MTBE), methyl tert-amyl ether (TAME), methyl tert-hexyl ether (THEME), ethyl tert-butyl ether (ETBE), ether tert-amyl ether (TAEE), diisopropyl ether (DIPE), tetrahydrofuran (THF), xylene and mixtures thereof.

[0033] More preferentially, the flammable solvent is selected from aprotic flammable solvents containing at least 3 carbon atoms and mixtures thereof, and even more preferentially from xylene, tetrahydrofuran, 2-ethylhexyl acetate, 2-ethylhexanoic acid (EHA) and mixtures thereof.

[0034] Advantageously, the content of zinc precursor in composition (A) is between 1% and 60% by weight, preferably between 15% and 30% by weight, relative to the total weight of composition (A).

[0035] The preparation method according to the present invention further comprises the step (b) of injecting the composition (A) and an oxygen-containing gas into a flame spray pyrolysis (FSP) device to form a flame.

[0036] In this step (b), the composition (A) and the oxygen-containing gas are advantageously injected into the flame spray pyrolysis apparatus by two separate injections, in other words, the composition (A) and the oxygen-containing gas are injected separately, i.e., the composition (A) and the oxygen-containing gas are not injected using a single nozzle.

[0037] More specifically, composition (A) is transported through one tube and an oxygen-containing gas (also called "dispersed oxygen") is transported through the other tube, the inlets of the two tubes being positioned such that the oxygen-containing gas creates a negative pressure to suck up composition (A) by the Venturi effect and convert it into droplets.

[0038] Step (b) may optionally further comprise the additional injection of a "premix" mixture comprising oxygen and one or more combustible gases. This "premix" mixture, also called "supported flame oxygen", allows the generation of a supporting flame intended to ignite and sustain the flame arising from composition (A) and the oxygen-containing gas (i.e., "dispersed oxygen").

[0039] Preferably, in step (b), composition (A), the oxygen-containing gas and, if optionally present, the "premix" mixture are injected into a reaction tube, also called "enveloping tube". Preferably, this reaction tube is made of metal or quartz. Advantageously, the reaction tube has a height of at least 30 cm, more preferentially at least 40 cm and even better still at least 50 cm. Advantageously, the length of said reaction tube is between 30 cm and 300 cm, preferably between 40 cm and 200 cm, more preferentially between 45 cm and 100 cm and even better still, this length is equal to 50 cm.

[0040] The mass ratio between the mass of the solvent present in composition (A) on the one hand and the mass of the oxygen-containing gas on the other hand is defined as follows: first, the amount of oxygen-containing gas (also called oxidizer compound) is calculated so that the aggregates formed by composition (A), i.e. the combustible solvent and zinc precursor on the one hand, and the oxygen-containing gas on the other hand, can react together in a combustion reaction in stoichiometric ratio (so that there is no excess or deficiency of the oxidizer compound).

[0041] Starting from this calculated amount of oxygen-containing gas, also called "calculated oxidant", a new calculation is carried out according to the formula: injected oxidant = calculated oxidant / φ, where φ is preferably between 0.30 and 0.9, more preferentially between 0.4 and 0.65, in order to deduce therefrom the amount of oxygen-containing gas to be injected, also called "injected oxidant".

[0042] This method is defined, inter alia, by Turns, S. R. An Introduction to Combustion: Concepts and Applications, 3rd ed.; McGraw-Hill: New York, 2012.

[0043] The preparation method according to the present invention further comprises a step (c) of injecting into the flame formed in step (b) a composition (B) comprising one or more solvents and one or more precursors of element M, said element M being selected from the elements from the 4th column, the elements from the 13th column and the elements from the 14th column of the Periodic Table of the Elements.

[0044] In other words, the process of the present invention is a continuous mode and the flame formed in step (b) is maintained.

[0045] In step (c) of the preparation method of the present invention, compositions (A) and (B) are injected separately and simultaneously. In other words, composition (A) is transported by one tube and composition (B) is transported by the other tube. The distance between the outlets of the two tubes is preferably at least 30 cm, more preferentially at least 40 cm.

[0046] Preferably the flame formed in step (b) is at a temperature of 2000° C. or greater over at least a part of the flame.

[0047] At the site of injection of composition (B) into the flame formed in step (b) and maintained in step (c), i.e. during the process, the temperature is preferably between 200°C and 600°C, more preferentially between 300°C and 400°C.

[0048] Advantageously, in step (c), composition (B) is injected through a spray ring placed above said reaction tube as described above, and in particular where the injection of composition (A) takes place.

[0049] Preferably, the element M precursor comprises at least two M atoms and several M-carbon covalent bonds. More preferentially, the element M precursor comprises at least three M atoms and several M-carbon covalent bonds.

[0050] The element M that can be used according to the invention is selected from the elements from the 4th column (titanium column), the 13th column (boron column) and the 14th column (carbon column) of the Periodic Table of the Elements.

[0051] In other words, the element M is preferably selected from titanium, zirconium, boron, aluminum, gallium, indium, thallium, carbon, silicon, germanium, tin and lead, more preferentially from titanium, zirconium, aluminum, carbon, silicon and tin, and even better from silicon, aluminum and titanium. Advantageously, the element M is selected from silicon and aluminum, and even better still, the element M is silicon.

[0052] Even more preferentially, the element M precursor is chosen from hexadimethyldisiloxane, tetraethoxysilane, 1,2-bis(triethoxysilyl)ethane, 1,2-bis(trimethoxysilyl)ethane, methoxytrimethylsilane and mixtures thereof.

[0053] According to a first particular embodiment of the invention, the element M is preferably an element from the fourth row, and even better still, the element M is titanium.

[0054] According to another particular embodiment of the invention, the element M is preferably an element from the thirteenth column, and even better still, the element M is aluminium.

[0055] According to another particular embodiment of the invention, the element M is an element from the fourteenth column, and even better still, the element M is silicon.

[0056] In the method according to the invention, (Zn / M) 注入 The molar atomic ratio can be calculated and corresponds to the ratio of the amount in moles of zinc atoms implanted in step (b) on the one hand to the amount in moles of element M implanted in step (c) on the other hand.

[0057] In particular, when element M is silicon, (zinc / silicon) 注入 The molar atomic ratio is preferably strictly less than 2.5, more preferentially less than or equal to 2, even better still less than or equal to 1.5, and even more preferentially in the range ranging from 0.1 to 1.5, and even better still from 0.5 to 1.

[0058] When element M is different from silicon, (Zn / M) 注入 The molar atomic ratio is preferably in the range ranging from 0.1 to 10, more preferentially from 0.2 to 5.

[0059] Preferably, nitrogen is bubbled through the composition (B) of the present invention before injection in step (c).Therefore, the injection rate of the composition (B) can be controlled by pressure determination known to those skilled in the art, for example, by the method defined by Scott, DW; Messerly, JF; Todd, SS; Guthrie, GB; Hossenlopp, IA; Moore, RT; Osborn, AG; Berg, WT; McCullough, JP, Hexamethyldisiloxane: chemical thermodynamic properties and internal rotation about the siloxane linkage, J.Phys.Chem., 1961, 65, 1320-6.

[0060] According to a particular embodiment of the present invention, the composition (B) as described above is adjusted to a temperature ranging from 25°C to 70°C, more preferentially from 30°C to 60°C, before injection in step (c).

[0061] Preferably, the content of element M precursor in composition (B) injected in step (c) of the method according to the invention is between 1% and 60% by weight, more preferentially between 5% and 30% by weight, relative to the total weight of composition (B).

[0062] Composition (B) comprises one or more solvents.

[0063] Preferably, the solvent present in composition (B) is chosen from polar protic solvents other than water, more preferentially from (C1-C8)-alkanols. Even more preferentially, composition (B) comprises ethanol.

[0064] Preferably, the solvent present in composition (B) is chosen from solvents that are flammable at the flame temperature of step (c), preferably at temperatures between 200° C. and 600° C., more preferentially between 300° C. and 400° C. Even better, the solvent present in composition (B) has a boiling point above room temperature (25° C.), more preferentially between 50° C. and 120° C.

[0065] Preferably, the content of solvent present in composition (B) injected in step (c) of the method according to the invention is between 40% and 99% by weight, more preferentially between 50% and 98% by weight and even better still between 70% and 95% by weight, relative to the total weight of composition (B).

[0066] The preparation method according to the present invention optionally comprises: - a treatment step (d1) comprising introducing the zinc oxide particles obtained after step (c) into an alkaline bath having a pH between 7 and 11, preferentially between 7.5 and 9, and / or - a step (d2) of calcining the zinc oxide particles obtained after step (c) or at the end of the treatment step (d1). It may further include.

[0067] When the treatment step (d1) is present, (i) the treatment preferably lasts for between 10 and 600 minutes, more preferentially between 40 and 300 minutes, and / or (ii) the pH of the alkaline bath preferably varies between 7 and 11, more preferentially between 7.5 and 9, and / or (iii) the temperature is preferably room temperature, i.e. 25° C.; and / or (iii) The content of particles obtained after step (c) in the alkaline bath is preferably between 0.5 and 100 g of particles per liter of alkaline bath, more preferentially between 1 and 10 g of particles per liter of alkaline bath.

[0068] When the calcination step (d2) is present, (i) the calcination preferably lasts for between 60 and 400 minutes, more preferentially between 60 and 180 minutes, and / or (ii) The temperature is preferably in the range of 100°C to 600°C, more preferentially 100°C to 300°C, even more preferentially 130°C to 250°C.

[0069] According to a particular embodiment of the invention, the particles obtained by the preparation method according to the invention are doped. According to this embodiment, composition (A) further comprises one or more precursors of an element D different from element M, D being selected from fluorine, yttrium, vanadium, scandium, zirconium, hafnium, iron, copper and tungsten.

[0070] Zinc Oxide Particles Another subject of the invention is a zinc oxide particle, in particular of the Zn-M oxide type, comprising a core 1 and one or more upper coating layers 2 covering said core 1, (i) Core 1 consists of zinc oxide, preferentially in the crystalline state; (ii) the upper coating layer 2 covers at least 90% of the surface of the core 1, preferably the entire surface of the core 1, and comprises one or more (inorganic) organic, preferably inorganic, compounds containing one or more elements M and one or more oxygen atoms; (iii) said element M is selected from the elements from the 4th column, the elements from the 13th column, and the elements from the 14th column of the Periodic Table of the Elements; and - when the element M is silicon, (zinc / silicon) 粒子 the molar atomic ratio is strictly less than 2, preferably ranging from 0.1 to 1.5, more preferentially from 0.5 to 1; and - when said element M is different from silicon, (zinc / M) 粒子 the molar atomic ratio is in the range of 0.1 to 10, preferably in the range of 0.1 to 5; and - The BET specific surface area of ​​the particles is 1 m 2 / g~350m 2 / g It is understood that the zinc oxide particles are

[0071] The particles according to the invention preferably comprise a core 1 consisting of zinc oxide in the crystalline state. The crystalline state of the core 1 and also its composition can be determined, for example, by conventional X-ray diffraction methods.

[0072] Advantageously, the core 1 of the particle according to the invention consists of one or more aggregates of crystalline primary zinc oxide particles, in other words, it consists of several microcrystals of zinc oxide.

[0073] Preferably, the zinc oxide particles are obtained by the preparation process of the invention as defined above.

[0074] The zinc oxide particles according to FIG. 1 consist of zinc oxide in a crystalline state and contain one or more aggregates of primary zinc oxide particles and have a diameter D m Includes Core 1.

[0075] The zinc oxide particles according to FIG. 1 have a thickness d m It also includes an upper coating layer 2 having

[0076] The zinc oxide particles according to Figure 2 correspond to the zinc oxide particles according to Figure 1, further comprising an additional coating layer 3 different from coating layer 2. Said additional coating layer 3 covers at least 90% of the surface of upper layer 2, preferably completely covers upper coating layer 2.

[0077] Number average diameter D of core 1 m can be determined, for example, by transmission electron microscopy (abbreviated as TEM). Preferably, the number-average diameter D m is in the range spanning from 3 to 1000 nm, more preferentially from 6 to 50 nm, even more preferentially from 10 to 30 nm.

[0078] The zinc oxide particles according to the present invention comprise one or more upper coating layers covering at least 90% of the surface of the core.

[0079] The degree of coverage of the core by the upper coating layer can be determined, for example, using a visual analysis of the TEM-BF or STEM-HAADF type coupled with a STEM-EDX analysis.

[0080] Each of the analyses is carried out on a statistical particle count, in particular at least 20 particles. The particles are deposited on a metal grid made of a metal different from zinc and different from any other metal forming part of the particle, whether in the core or in the upper coating layer. For example, the grid is made of copper (unless it is desired to use copper in the manufacture of the particles).

[0081] Visual analysis of the TEM-BF and STEM-HAADF images allows one to infer, based on contrast, whether the coating completely surrounds the core of the particle. By analyzing each of the 20 (or more) images, it is possible to infer the degree of coverage of the core, and then take the average to determine the average degree of coverage.

[0082] STEM-EDX analysis makes it possible to verify whether the coating indeed contains mainly or exclusively the metal M. For this purpose, measurements must be carried out on the edge of the particles (on at least 20 particles). These measurements then reveal the metal M.

[0083] STEM-EDX analysis also allows to verify whether the cores actually contain metallic zinc. For this, measurements must be carried out in the center of the particles (on at least 20 particles). These measurements then reveal metallic zinc and metallic M.

[0084] Preferably, the upper coating layer 2 completely covers the surface of the core 1 .

[0085] Number average thickness d of the upper coating layer 2 m The number average thickness d may also be determined by transmission electron microscopy. m is in the range spanning from 1 to 30 nm, more preferentially from 1 to 15 nm, even more preferentially from 1 to 6 nm.

[0086] Advantageously, the upper coating layer 2 is amorphous.

[0087] Preferably, the upper coating layer 2 consists of one or more oxides of element M. As mentioned above, element M is selected from elements from column 4 (titanium column), column 13 (boron column) and column 14 (carbon column) of the Periodic Table of the Elements.

[0088] In other words, the element M is preferably selected from titanium, zirconium, boron, aluminum, gallium, indium, thallium, carbon, silicon, germanium, tin and lead, more preferentially from titanium, zirconium, aluminum, carbon, silicon and tin, and even better from silicon, aluminum and titanium. Advantageously, the element M is selected from silicon and aluminum, and even better still, the element M is silicon.

[0089] According to a first particular embodiment of the invention, the element M is preferably an element from the fourth row, and even better still, the element M is titanium.

[0090] According to another particular embodiment of the invention, the element M is preferably an element from the thirteenth column, and even better still, the element M is aluminium.

[0091] According to another particular embodiment of the invention, the element M is an element from the fourteenth column, and even better still, the element M is silicon.

[0092] Very particularly preferably, the upper coating layer 2 consists of silicon oxide SiO2, aluminum oxide Al2O3 and / or titanium oxide TiO2, more preferentially consisting of silicon oxide SiO2.

[0093] The zinc oxide particles according to the present invention have a (zinc / M) 粒子 It contains zinc and element M in a molar atomic ratio.

[0094] This ratio corresponds to the ratio between, on the one hand, the amount in molar of element M present in the particles according to the invention and, on the other hand, the amount in molar of zinc atoms present in the particles according to the invention.

[0095] This ratio can be determined by spectroscopic measurements according to one of the following two methods: according to the first method, the powder is spread and subjected to an X-ray fluorescence measurement test using an X-ray spectrometer, from which the metal ratio is deduced; according to the other method, the particles of the invention are dissolved beforehand in acid. Then, on the material obtained, an elemental analysis is carried out by ICP-MS (Inductively Coupled Plasma Mass Spectroscopy), from which the metal ratio is deduced.

[0096] In particular, when element M is silicon, (zinc / silicon) 粒子 The molar atomic ratio is preferably less than or equal to 1.5, more preferentially said ratio is in the range ranging from 0.1 to 1.5, even more preferentially from 0.5 to 1.

[0097] When element M is different from silicon, (Zn / M) 粒子 The molar atomic ratio is preferably in the range of 0.1-5.

[0098] Preferably, the sum of the content of zinc oxide and the content of element M oxide is at least equal to 99% by weight relative to the total weight of the core 1 and of the upper coating layer 2 .

[0099] The number-average diameter of the particles according to the invention can also be determined by transmission electron microscopy. Preferably, the mass-average diameter of the particles according to the invention is in the range ranging from 3 to 1000 nm, more preferentially from 10 to 100 nm, preferably from 15 to 70 nm.

[0100] The particles according to the present invention have a BET specific surface area of ​​1 m 2 / g~350m 2 / g, more preferentially 1m 2 / g~200m 2 / g, and even more preferentially 30-100m 2 / g.

[0101] The zinc oxide particles may optionally further comprise an additional coating layer 3 overlying the upper coating layer 2 and preferably comprising one or more hydrophobic organic compounds.

[0102] The hydrophobic organic compound is more preferentially chosen from silicones, in particular silicones comprising at least one fatty chain, carbon-based derivatives comprising at least 6 carbon atoms, in particular fatty acid esters, and mixtures thereof.

[0103] The additional coating layer 3 can be produced by liquid or solid methods.

[0104] Via a liquid process, hydroxyl functional groups on the surface of the particles are reacted with reactive functional groups of the compounds that form the coating (typically silanol functional groups of silicones or acid functional groups of carbon-based fatty materials).

[0105] Through the solid method, the particles are contacted with a liquid or pasty compound that contains a hydrophobic material, and then after contacting, the mixture is dried and the mixture is broken down, for example by milling.

[0106] Another subject of the present invention relates to a composition, preferably a cosmetic composition, comprising one or more zinc oxide particles as defined above and preferably obtained by the process according to the invention.

[0107] The composition according to the invention is advantageously an aqueous composition.

[0108] The coated zinc oxide particles of the present invention may be present in dry form (powder, flakes, plates), as a dispersion, or as a liquid suspension, or even as an aerosol. The coated zinc oxide particles of the present invention may be used as is or may be mixed with other ingredients.

[0109] The composition according to the invention may be in various galenical forms, thus in the form of a powder (powdered) composition or a liquid composition, in the form of a milk, a cream, a paste or an aerosol composition.

[0110] The composition according to the invention is in particular a cosmetic composition. That is to say, the multi-layer material according to the invention is in a cosmetic medium. The term "cosmetic medium" means a medium suitable for application to keratinous materials, in particular human keratinous materials such as the skin, said cosmetic medium generally consisting of water or a mixture of water and one or more organic solvents or a mixture of organic solvents. Preferably, the composition comprises water in a content of in particular 5% to 95% by weight relative to the total weight of the composition.

[0111] The term "organic solvent" refers to an organic substance capable of dissolving other substances without chemical modification. Examples of organic solvents that can be used in the compositions of the present invention may include, for example, lower C2-C6 alkanols, such as ethanol and isopropanol, polyols and polyol ethers, such as 2-butoxyethanol, propylene glycol, propylene glycol monomethyl ether and diethylene glycol monoethyl ether and monomethyl ether, as well as aromatic alcohols, such as benzyl alcohol or phenoxyethanol, and mixtures thereof.

[0112] If present, the organic solvent is preferably present in a proportion of from 0.1% to 40% by weight, more preferentially from 1% to 30% by weight and even more particularly from 5% to 25% by weight relative to the total weight of the composition.

[0113] The compositions of the invention may contain a fatty phase and may be in the form of a direct or inverse emulsion.

[0114] The content of zinc oxide particles present in the composition of the invention is preferably in the range from 0.1% to 40% by weight, more preferentially from 0.5% to 20% by weight, even better still from 1% to 10% by weight and even more preferentially from 1.5% to 5% by weight relative to the total weight of the composition.

[0115] According to a specific embodiment of the present invention, the composition according to the present invention can also be in the form of anhydrous composition, for example in the form of oil.The term "anhydrous composition" is intended to mean a composition that contains less than 2% by weight of water, preferably less than 1% by weight of water, and even more preferentially less than 0.5% by weight of water relative to the total weight of the composition, or even a composition that does not contain water.In this type of composition, the water that may be present is not added during the preparation of the composition, but corresponds to the residual water provided by the mixed components.

[0116] The compositions according to the invention may be prepared according to techniques known to those skilled in the art, in particular as simple or multiple emulsions (abbreviated as oil-in-water or O / W, water-in-oil or W / O, oil-in-water-in-oil or O / W / O or water-in-oil-in-water or W / O / W), for example in the form of a cream, a milk or a cream gel or else in the form of a powder or an aerosol composition.

[0117] Another subject of the invention is a composition according to the invention, preferably a cosmetic composition, for use in protecting the skin, preferably human skin, against visible radiation (i.e. wavelengths between 400 nm and 800 nm) and / or ultraviolet radiation (i.e. wavelengths between 100 nm and 400 nm), UV-A radiation (i.e. wavelengths between 320 nm and 400 nm) and / or UV-B radiation (i.e. wavelengths between 280 nm and 320 nm). The composition according to the invention allows efficient screening of a broad spectrum of solar radiation, in particular UV-A radiation (including long-wave UV-A radiation), while at the same time being particularly stable over time under UV exposure.

[0118] The composition according to the invention may optionally comprise one or more additional UV screening agents other than the zinc oxide particles according to the invention, selected from hydrophilic, lipophilic or insoluble organic UV screening agents, and / or one or more mineral pigments. It will preferentially be composed of at least one hydrophilic, lipophilic or insoluble organic UV screening agent.

[0119] Another subject of the invention is the use of zinc oxide particles as defined above, preferably obtained by the process according to the invention, in - formulating cosmetic or pharmaceutical compositions intended in particular to protect the skin, in particular human skin, against visible and / or ultraviolet radiation or to modify the appearance of the skin, in particular human skin, - Formulating paints, varnishes and / or stains; or - Producing coatings for electronic devices or products, in particular to obtain moisture-resistant electronic components For use.

[0120] The coated zinc oxide particles of the present invention are preferably protective against UVA and UVB radiation, which may in particular improve the total screening rejection of UV radiation while maintaining good total transmission in the visible range and excellent transparency in the visible range (400-780 nm).

[0121] How to apply Another subject of the invention is a method for treating keratinous materials, and in particular human keratinous materials such as the skin, by applying to said material a composition as defined above, for example by spraying, preferably by 1 to 5 successive applications with drying between the layers.

[0122] The composition of the present invention can be used in a single application or multiple applications. When multiple applications of the composition of the present invention are intended, the content of particles of the oxide of element M of the present invention is generally lower than when a single application of the composition is intended.

[0123] For the purposes of the present invention, the term "single application" means a single application of the composition, which can be repeated several times a day, with an interval of at least one hour between each application, if desired, or can be applied once a day.

[0124] For the purposes of the present invention, the term "multiple applications" refers to the application of the composition several times, typically 2 to 5 times, with a few seconds to minutes between each application. Each multiple application may be repeated several times a day, spaced an hour or more apart, or may be performed once a day, if desired.

[0125] There may also be a method of successive application, such as a saturated single application, i.e. a single application of a cosmetic composition comprising a high concentration of zinc oxide particles coated with silicon oxide according to the present invention, or else multiple applications of a cosmetic composition (lower concentration) comprising one or more zinc oxide particles coated with silicon oxide according to the present invention. In the case of multiple applications, several successive applications of a cosmetic composition comprising one or more zinc oxide particles coated with silicon oxide according to the present invention may be repeated with or without a delay between applications.

[0126] According to one embodiment of the present invention, multiple applications are performed on the keratinous material with a drying step between successive applications of the cosmetic composition comprising the zinc oxide particles coated with silicon oxide according to the present invention. The drying step between successive applications of the cosmetic composition comprising one or more zinc oxide particles coated with silicon oxide according to the present invention can be performed in open air or artificially, for example using a hot air drying system such as a hair dryer.

[0127] Another subject of the present invention is the use of one or more zinc oxide particles coated with silicon oxide according to the invention as defined above, as a UVA and UVB screening agent for protecting keratinous materials, especially human keratinous materials such as the skin.

[0128] The following examples serve to illustrate the invention and are not limiting in nature. EXAMPLES

[0129] Example 1: 1.1 First, a composition (A) of zinc naphthenate (500 mM) in xylene was prepared.

[0130] Next, uncoated zinc oxide particles P1 (outside the present invention) were prepared by a conventional FSP preparation method Prep 1 using the previously prepared composition (A).

[0131] The same composition (A) and a composition (B) containing hexadimethyldisiloxane and ethanol in a ratio of 3:1 were then used to prepare zinc oxide particles P2 coated with silicon dioxide by the preparation method Prep 2 according to the present invention (invention).

[0132] The parameters for the Prep 1 method are as follows: - Ratio (composition (A) / O2) = 5 mL / min liquid and 7 L / min gas (O2). To adjust the oxygen flow rate, use φ = 0.48.

[0133] The parameters for the Prep 2 method are as follows: - Ratio (composition (A) / O2) = 5 / 7, i.e. 5 mL / min liquid and 7 L / min gas (O2). To adjust the oxygen flow rate, φ = 0.48 is used.

[0134] In this Prep 2 method, a 40 cm high temperature quartz tube is used. In addition, nitrogen is first bubbled through composition (B). When composition (B) is injected, nitrogen is bubbled through the tube to allow evaporation of hexadimethyldisiloxane (HMDSO) and (Zn / Si). 注入 A stream of nitrogen heated to 25° C. is adjusted so that ratio=1.

[0135] 1.2 After the particles were prepared, it was observed that the resulting zinc oxide particles were crystalline.

[0136] Furthermore, the particles obtained according to the method Prep 2 according to the invention are coated with silicon dioxide and have a (Zn / Si) of 1. 粒子 It has an atomic ratio.

[0137] The BET specific surface area of ​​the particles according to method Prep 2 is 74 m 2 / g.

[0138] The particles according to method Prep 2 have a number average diameter equal to 40 nm.

[0139] 1.3 Water resistance rating: A first aqueous suspension S1 was prepared from particles P1 and water at a content of 200 mg P1 / L water.

[0140] Similarly, a second aqueous suspension S2 was prepared from particles P2 and water at a content of 200 mg P2 / L water.

[0141] Each of the suspensions S1 and S2 was then placed in an ultrasonic bath for 10 minutes at a power of 20 W.

[0142] Then, the fraction of suspension S1 and the fraction of suspension S2 were adjusted to pH=5 using a nitric acid solution.

[0143] Then, for each suspension, conventional anodic stripping voltammetry techniques were used to measure the Zn present in the suspension as a function of time and against the amount of zinc introduced. 2+ The content is measured.

[0144] The results are collated in the table below.

[0145] [Table 1]

[0146] It should be noted that the coated zinc oxide particles P2 obtained according to the preparation method Prep 2 according to the invention have a significantly better water resistance than the uncoated zinc oxide particles P1 obtained according to the comparative preparation method Prep 1. The uncoated particles P1 are largely expelled in less than one hour and therefore cannot be used in compositions containing water or when the coating comes into contact with water.

[0147] Example 2: An aqueous suspension S3 was prepared from zinc oxide particles sold by BASF under the name Z-COTE HP1 (oxide and triethoxycaprylylsilane) and water at a content of 200 mg of commercial ZnO / L water. In other words, the particles Z-COTE HP1 are coated with a layer of triethoxycaprylylsilane, thus providing hydrophobicity and protection from water. The BET specific surface area of ​​the particles is 15.8 m 2 / g.

[0148] Suspension S3 was then placed in an ultrasonic bath at a power of 20 W for 10 min.

[0149] The Zn present in Suspension S2 (pH=8) and Suspension S3 (pH=8) prepared in Example 1 above was then measured using a conventional anodic stripping voltammetry method for each suspension. 2+ The content is measured as a function of time and as a function of the amount of zinc introduced.

[0150] The results are collated in the table below.

[0151] [Table 2]

[0152] The coated zinc oxide particles P2 obtained according to the preparation method Prep 2 according to the invention have a particularly high BET specific surface area (74 m for the particles derived from Prep 2). 2 / g, whereas the commercial compound has a 2 It should be noted that, despite the solubility of 0.01 g / g, the particles have significantly better water resistance than commercially available zinc oxide particles.

[0153] 2.4. Evaluation of optical properties Aqueous solutions S'1 and S'2 were prepared from particles P1 and P2 prepared in Example 1, and aqueous solution S'3 was prepared from zinc oxide particles sold under the name Z-COTE HP1. In each of the solutions, the content of particles represents 0.3% by weight, while the remainder is a water / propylene glycol (50 / 50) mixture.

[0154] The transmittance is obtained by exposing a sample to a beam of light and then taking the ratio of the transmitted intensity to the incident intensity. T=I / I0

[0155] The results are collated in the table below.

[0156] [Table 3]

[0157] These results show that the transmittance of the uncoated zinc particles (P1), the particles according to the invention (P2) and the commercial particles is similar in the UVA radiation zone, even though the zinc concentration in the particles according to the invention (ZnO / SiO2, P2) is half that of the comparative uncoated zinc oxide particles (ZnO, P1).

[0158] Furthermore, the zinc oxide particles according to the invention (P2) and the uncoated particles (P1) have a high level of transmittance in the visible spectral range, which is significantly better than the commercially available products.

[0159] Therefore, the zinc oxide particles of the present invention make it possible to obtain better water resistance and better transparency in the visible spectral range while retaining good optical properties.

[0160] Example 3: Composition (A) and an aluminum precursor (aluminum tri-sec-butoxide, C 12 H 27 Alumina-coated zinc oxide particles P3 were prepared by the preparation method Prep 2 according to the present invention using a composition (C) containing alumina (AlO3) and xylene in a ratio of 3:1.

[0161] The parameters for the Prep 2 method are as follows: - Ratio (composition (A) / O2) = 5 / 7, i.e. 5 mL / min liquid and 7 L / min gas (O2). To adjust the oxygen flow rate, φ = 0.48 is used.

[0162] In this Prep 2 method, a 40 cm high temperature quartz tube is used. In addition, nitrogen is first bubbled through composition (C). When composition (C) is injected, nitrogen is bubbled through the composition (C) to allow evaporation of aluminum tri-sec-butoxide and (Zn / Al). 注入 A stream of nitrogen heated to 40° C. is adjusted so that ratio=1.

[0163] After the particles were prepared, it was observed that the resulting zinc oxide particles were crystalline.

[0164] Moreover, the particles obtained according to the method Prep 2 according to the invention are coated with Al2O3 and have a (Zn / Al) of 1. 粒子 It has an atomic ratio.

[0165] The BET value of the particles according to method Prep 2 is 60m 2 / g.

[0166] The particles have a size: 36 nm.

[0167] Example 4: The same composition (A) and a tin precursor (tin ethylhexanoate, C) in xylene 16 H 30 Zinc oxide particles P4 coated with tin dioxide were prepared by the preparation method Prep 2 according to the present invention using a composition (D) containing tin dioxide (Tin dioxide) and ...

[0168] The parameters for the Prep 2 method are as follows: - Ratio (composition (A) / O2) = 5 / 7, i.e. 5 mL / min liquid and 7 L / min gas (O2). To adjust the oxygen flow rate, φ = 0.48 is used.

[0169] In this Prep 2 method, a 40 cm high temperature quartz tube is used. In addition, nitrogen is first bubbled through composition (D). When composition (D) is injected, nitrogen is bubbled through the composition (D) to allow evaporation of tin ethylhexanoate and (Zn / Sn). 注入 A stream of nitrogen heated to 50° C. is adjusted so that ratio=1.

[0170] After the particles were prepared, it was observed that the resulting zinc oxide particles were crystalline.

[0171] Furthermore, the particles obtained according to the method Prep 2 according to the invention are coated with tin dioxide, resulting in a (Zn / Sn) of 1. 粒子 It has an atomic ratio.

[0172] The BET value of the particles according to method Prep 2 is 72m 2 / g.

[0173] Example 5: The same composition (A) and a titanium precursor (titanium isopropoxide, C 12 H 28Zinc oxide particles P5 coated with titanium dioxide were prepared by the preparation method Prep 2 according to the present invention using a composition (E) containing titanium dioxide (O4Ti) and xylene in a ratio of 3:1.

[0174] The parameters for the Prep 2 method are as follows: - Ratio (composition (A) / O2) = 5 / 7, i.e. 5 mL / min liquid and 7 L / min gas (O2). To adjust the oxygen flow rate, φ = 0.48 is used.

[0175] In this Prep 2 method, a 40 cm high temperature quartz tube is used. In addition, nitrogen is first bubbled through composition (E). When composition (E) is injected, nitrogen is bubbled through the composition (E) to allow evaporation of titanium isopropoxide and (Zn / Ti). 注入 A stream of nitrogen heated to 50° C. is adjusted so that ratio=1.

[0176] After the particles were prepared, it was observed that the resulting zinc oxide particles were crystalline.

[0177] Furthermore, the particles obtained according to the method Prep 2 according to the invention are coated with titanium dioxide and have a (Zn / Ti) of 1. 粒子 It has an atomic ratio.

[0178] The BET value of the particles according to method Prep 2 is 40m 2 / g.

[0179] A Raman test was carried out on particles P1 to P5. The Raman peaks of ZnO were observed for particles P1 to P5.

Claims

1. In particular, a method for preparing coated zinc oxide particles of the Zn-M oxide type, comprising at least the following steps: a. preparing a composition (A) by adding one or more zinc precursors to a flammable solvent or mixture of flammable solvents; and then b. forming a flame in a flame spray pyrolysis device by injecting the composition (A) and an oxygen-containing gas until zinc oxide aggregates are obtained; c) injecting into the flame a composition (B) comprising one or more precursors of element M and one or more solvents until an inorganic coating layer consisting of at least one element M and at least one oxygen atom is obtained on the surface of the zinc oxide aggregates, said element M being selected from the elements from the 4th column, the elements from the 13th column and the elements from the 14th column of the Periodic Table of the Elements; The method according to claim 1, further comprising:

2. The zinc precursor comprises one or more zinc atoms optionally complexed to one or more ligands containing at least one carbon atom, preferably the ligands are selected from the group consisting of: acetate, (C １ ~C ６ ) alkoxylate, (di)(C １ ~C ６ 2. The process according to claim 1, characterized in that the aryl group is selected from alkylamino and arylates, such as naphthalates or naphthenates.

3. 2. The method according to claim 1, characterized in that the flammable solvent is chosen from protic flammable solvents, aprotic flammable solvents and mixtures thereof, preferably from alcohols, esters, acids, acyclic ethers, cyclic ethers, aromatic hydrocarbons or arenes, non-aromatic hydrocarbons and mixtures thereof, and more preferentially, the flammable solvent is chosen from aprotic flammable solvents containing at least 3 carbon atoms and mixtures thereof, and even better still from xylene, tetrahydrofuran, 2-ethylhexyl acetate, 2-ethylhexanoic acid (EHA) and mixtures thereof.

4. 2. The method according to claim 1, characterized in that the content of zinc precursor in composition (A) is between 1% and 60% by weight, preferably between 15% and 30% by weight, relative to the total weight of composition (A).

5. 2. The method according to claim 1, characterized in that the flame formed in step (b) and maintained in step (c) is at a temperature of between 200°C and 600°C, preferably between 300°C and 400°C, at the outlet of the tube transporting the composition (B).

6. 2. The method according to claim 1, characterized in that the element M is selected from titanium, zirconium, boron, aluminum, gallium, indium, thallium, carbon, silicon, germanium, tin and lead, preferably from titanium, zirconium, aluminum, carbon, silicon and tin, more preferentially from silicon, aluminum and titanium, even better still from silicon and aluminum, and even more preferentially, the element M is silicon.

7. 2. The method according to claim 1, characterized in that the precursor of element M comprises at least two M atoms and several M-carbon covalent bonds, preferably the precursor of element M comprises at least three M atoms and several M-carbon covalent bonds, and more preferentially the precursor of element M is selected from hexadimethyldisiloxane, 1,2-bis(triethoxysilyl)ethane, 1,2-bis(trimethoxysilyl)ethane and mixtures thereof.

8. The element M is silicon, and (zinc / silicon) 注入 2. The method according to claim 1, characterized in that the molar atomic ratio is strictly less than 2.5, preferably less than or equal to 2, more preferentially less than or equal to 1.5, and even better still in the range extending from 0.1 to 1.5, and even more preferentially from 0.5 to 1.

9. The element M is different from silicon, and (zinc / M) 注入 The method according to claim 1, characterized in that the molar atomic ratio is in the range spanning from 0.1 to 10, preferably from 0.2 to 5.

10. The composition (B) is preferably prepared from polar protic solvents other than water, more preferably from (C １ ~C ８ 2. The method according to claim 1, characterized in that it comprises one or more solvents selected from the group consisting of: ethanol;

11. 2. The method according to claim 1, characterized in that the content of precursor of element M in composition (B) is between 1% and 60% by weight, preferably between 5% and 30% by weight, relative to the total weight of composition (B).

12. 2. The method according to claim 1, further comprising a treatment step (d1) comprising the introduction of the zinc oxide particles obtained after step (c) in an alkaline bath having a pH between 7 and 11 and / or a step (d2) of calcining the zinc oxide particles obtained after step (c) or at the end of treatment step (d1).

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