Zinc sulfide-based interference pigments and their synthesis process

Abstract

Zinc sulfide-based interference pigments and their synthesis process. The present invention relates to interference pigments comprising a layer of a zinc sulfide-based material. The pigments advantageously comprise a core with a low refractive index and at least one layer with a high refractive index comprising zinc sulfide. The invention further proposes a process for synthesizing these pigments by heterogeneous precipitation of zinc sulfide onto a powdered support, the precipitation being obtained by titration of a sodium sulfide solution at a fixed pH, upon the addition of a zinc saline solution to a reaction medium containing a dispersion of the powdered support, at moderate temperature. Figure for the abstract: Fig. 1.

Description

Title of the invention: Zinc sulfide-based interference pigments and their synthesis process technical field

[0001] The present invention relates to interference pigments comprising at least one zinc sulfide layer, as well as a method for synthesizing these pigments. The invention finds application particularly in the cosmetic, decorative, and food fields. Previous art

[0002] Generally, white interference pigments with effects are obtained by depositing a layer of a material with a high refractive index on the surface of semi-transparent platelet particles of mica, glass or alumina with a lower refractive index.

[0003] The most commonly used high index material is titanium dioxide, and the need remains to offer alternative pigments comprising a high refractive index material to generate physical colors. Description of the invention

[0004] The present invention addresses this need and provides zinc sulfide-based pigments, a compound with the advantage of being suitable for a wide variety of industrial applications. The proposed pigments comprise a core coated with a thin layer of zinc sulfide.

[0005] The invention in particular proposes interference pigments comprising a core of low refractive index and at least one layer of a zinc sulfide-based compound whose thickness is chosen to generate silvery-white or coloured reflections by interference.

[0006] The invention further proposes a liquid-based deposition method for submicron layers of a zinc sulfide (ZnS) compound onto a micrometer-sized support. The synthesis process can advantageously follow a heterogeneous nucleation and homogeneous growth mechanism, resulting in the deposition of a zinc sulfide layer whose chemical composition, crystallinity, crystallography, density, and thickness can be controlled. It is unexpected, under mild chemical conditions and in a liquid-based process, to successfully deposit a homogeneous zinc sulfide layer with controlled thickness, crystallinity, crystallography, and chemical composition onto a powdered support such as mica, for example. This process makes it possible to manufacture zinc sulfide-based pigments using a process carried out in water, at moderate temperature and pH. Brief description of the drawings

[0007] [Fig.1] The [Fig.1] is an elementary EDX map of a pigment according to the invention.

[0008] [Fig.2] Fig.2 is a scanning electron microscopy image of a pigment according to the invention.

[0009] [Fig.3] Fig.3 is an X-ray diffractogram of a pigment according to the invention. Description of the implementation methods

[0010] A first object of the invention relates to an interference pigment, monolayer or multilayer, comprising a core having a refractive index from 1.00 to 2.10, covered with at least one layer of a material comprising zinc sulfide, said layer having an average physical thickness from 10 nm to 350 nm.

[0011] The interference pigment of the invention is advantageously a powder made up of particles each of which includes a core covered with at least one layer of a material comprising zinc sulfide.

[0012] Its dimension is an average number dimension of a population of particles, advantageously equal to the D50 of the population of particles constituting the pigment. In the case of the present invention, the value of D50 is calculated from the particle size distributions obtained by conventional methods such as laser diffraction or image analysis obtained by scanning electron microscopy (SEM) or transmission electron microscopy (TEM).

[0013] The pigment can thus have a dimension, the dimension being able to be defined as D50, ranging from 1 micron to 2000 microns, preferably ranging from 10 microns to 500 microns, while the core can have a dimension ranging from 1 micron to 2000 microns, preferably from 10 microns to 500 microns.

[0014] The average physical thickness of the layer of a material comprising zinc sulfide can range, for example, from 20 nm to 340 nm, from 30 nm to 330 nm, from 40 nm to 320 nm, from 50 nm to 310 nm, from 60 nm to 300 nm, from 70 nm to 290 nm, from 80 nm to 280 nm, from 90 nm to 270 nm, from 100 nm to 260 nm or from 110 nm to 250 nm. The physical thickness, also called optical thickness, can be measured by any method known to those skilled in the art.

[0015] The pigment of the invention is an interference pigment that can be colored or white. This pigment can further generate one or more reflections of different colors, the color of the reflection(s) being able to differ from the color of the pigment itself. The reflections can thus be silvery-white or another color. In the cosmetics field, for example, a distinction is made between white pearlescent pigments with silvery-white reflections, white pearlescent pigments with colored reflections, and colored pearlescent pigments with Silvery-white reflections and colored mother-of-pearl with colored reflections. These reflections can result in pearlescent and / or iridescent effects during the transmission and reflection of light through them, caused by optical interference phenomena. The generated interference color results from the reinforcement or destruction of reflected light rays at certain wavelengths. A layer of the material containing zinc sulfide deposited on a substrate with a lower refractive index can generate destructive and constructive waves, which in turn generate color. Destructive interference at a given wavelength occurs if the reflections from the two surfaces—air / material and material / substrate—are completely out of phase.For example, minimum reflection occurs for incident light perpendicular to a wavelength X for a layer of zinc sulfide material with refractive index N and thickness e, when N*e=(nl)*X / 2, where n is an integer. When the material layer is illuminated by white light, all wavelengths except X appear in the reflection. Enhancement at a given wavelength occurs if the reflections from the two surfaces of the zinc sulfide material layer are in phase with each other. Thus, for incident light perpendicular to the free surface of the layer, this occurs when N*e=(2n-l)*X / 4.

[0016] The layer of material comprising zinc sulfide is preferably an interference layer. For the purposes of this invention, an "interference layer" is defined as a layer of a material whose optical thickness is capable of generating an optical color when deposited on a given substrate or on a layer of another material.

[0017] The material comprising zinc sulfide is preferably predominantly composed of zinc sulfide. By "predominantly composed of zinc sulfide" is meant a material comprising more than 50% by mass of zinc sulfide relative to the mass of the material.

[0018] In one embodiment, the material comprising zinc sulfide is essentially made up of zinc sulfide.

[0019] The term "essentially composed of zinc sulfide" means a material comprising at least 90% by mass of zinc sulfide, preferably at least 99% by mass of zinc sulfide relative to the mass of said material.

[0020] The material comprising zinc sulfide is a solid solution of zinc sulfide, i.e., a material forming a single crystalline phase comprising zinc sulfide. Those skilled in the art will be able to characterize the number of crystalline phases of the material, for example by X-ray diffraction.

[0021] The material comprising zinc sulfide may be so-called "doped" zinc sulfide, that is to say, a material essentially consisting of zinc sulfide and at least one chemical element, the chemical element preferably being in the form of a metallic ion.

[0022] Thus, in another embodiment, the material comprising sulfide is a material essentially consisting of zinc sulfide and at least one metal ion chosen from Fe2+, Cu2+, Mn2+, Ag+, Au3+, Eu3+, Al3+, Ce3+ and In3+.

[0023] The term "essentially composed of zinc sulfide and at least one metal ion" means a material comprising at least 90% by mass, preferably at least 99% by mass, of a mixture of zinc sulfide and metal ion(s) relative to the mass of the material. In this embodiment, the chemical element, which is preferably a metal ion, is advantageously present in such a quantity that it forms a solid solution, i.e., a single crystalline phase, with the zinc sulfide.

[0024] The layer of a material comprising zinc sulfide preferably has a refractive index ranging from 2.30 to 2.90, for example from 2.35 to 2.80, from 2.40 to 2.70, from 2.45 to 2.65, from 2.50 to 2.50. The refractive index of the layer of a material comprising zinc sulfide is preferably close to 2.40.

[0025] In a particular embodiment, the zinc sulfide represents between 1% and 100% by mass of the pigment. The interference pigment of the invention can therefore comprise between 1% and 70% by mass of zinc sulfide relative to the pigment, advantageously from 15% to 65% by mass, more advantageously from 20% to 60% by mass, and even more preferably from 25% to 55% by mass of the pigment. In a particularly preferred embodiment, the zinc sulfide represents from 30% to 60% by mass, preferably from 30% to 50% by mass, and even more preferably from 35% to 45% by mass of the pigment.

[0026] The core of the pigment itself preferably has a refractive index less than or equal to 2.20, preferably ranging from 1.00 to 2.10. It consists of one or more materials each having a refractive index less than or equal to 2.20, preferably ranging from 1.00 to 2.10, the refractive index being able to be measured by any method known to a person skilled in the art.

[0027] The chemical nature of the pigment core can vary. Said core can be a gas, such as air for example, or a solid substrate which may be porous or non-porous.

[0028] When the core is a porous substrate or when the core is a gas, the interference pigment of the invention can be obtained by depositing the layer of material comprising zinc sulfide on a solid support core, which is then dissolved totally or partially to obtain a gaseous or porous core, respectively.

[0029] In a first embodiment, the core of the pigment of the invention is a solid substrate, advantageously non-porous. This substrate can take different forms; for example, it may be in the form of platelets or beads. In the case where the substrate is platelet and powdery, the size of the substrate will, for example, be the The average length of a platelet population, where length is the largest dimension of the platelets. The substrate can also be defined by its thickness, with thickness representing the average thickness of said population.

[0030] The substrate may be transparent, semi-transparent, or opaque. Thus, the pigment of the invention may be an interference pigment with a metallic effect or a mother-of-pearl, depending on the nature of the substrate used. In a particular embodiment, a semi-transparent substrate will be used.

[0031] The solid substrate may advantageously be chosen from natural micas, synthetic micas, alkali-earth carbonates such as calcium carbonate, alkali-earth sulfates such as barium sulfate, natural pearls such as guanine or hypoxanthine, alumina, aluminum, silica, borosilicate, pearlite, an organic polymer (such as a plastic), and a metal oxide such as zinc oxide or bismuth oxychloride. The substrate may consist essentially of one or more of these materials, without excluding the possible presence of impurities.

[0032] The core of the interference pigment of the invention is preferably free of zinc sulfide. It may nevertheless include zinc sulfide as an impurity, for example an amount less than 0.1% by mass of the mass of the core.

[0033] In a second embodiment, the core of the pigment of the invention may comprise a gas or be composed of a gas such as air, so that the pigment takes the form of so-called "core-shell" particles, the core of which is hollow and the shell comprises at least one interfering layer of a material comprising zinc sulfide. As explained previously, such a core may be obtained from a solid support that is partially or totally removed by physical or chemical means after the application of the shell formed by the layers applied to the support.

[0034] The pigment of the invention can be monolayer, in the sense that it comprises a single layer covering the core, or multilayer.

[0035] According to a first embodiment, the pigment can therefore be a single-layer pigment comprising a single layer of a material containing zinc sulfide. "A single layer" is understood to mean a layer of homogeneous composition, which can be applied in one or more manufacturing process steps, preferably in a single step. The pigment of the invention can alternatively be multilayer, that is to say, comprise at least two layers of different compositions, including at least one layer of a material containing zinc sulfide and one layer not containing zinc sulfide. Preferably, the core does not comprise several layers of different materials, and its composition is homogeneous.

[0036] The pigment of the invention may comprise, regardless of the number of layers of different materials that may cover the core, a single or more layers of a material containing zinc sulfide.

[0037] The pigment may include, for example, two layers of a material containing zinc sulfide, of which a first layer is made of zinc sulfide and a second layer is made of doped zinc sulfide, as described above, separated from each other by an intermediate layer of low refractive index, including, for example, silica.

[0038] In addition to the zinc sulfide material, the pigment may comprise at least one material with a refractive index ranging from 2.30 to 2.90, such as, for example, a material selected from the group consisting of Fe₂O₃, FeTiO₃, Cr₂O₃, and / or Fe₃O₄. This material may be incorporated into the zinc sulfide-based layer material or be in the form of a separate layer from the zinc layer. Thus, the zinc sulfide layer may comprise, in addition to zinc sulfide, at least one material with a refractive index ranging from 2.30 to 2.90. According to one embodiment, the pigment comprises a first layer made of zinc sulfide, which is coated with and in contact with a second layer made of a material other than zinc sulfide with a refractive index ranging from 2.30 to 2.90.

[0039] In a second particular embodiment of the invention, the interference pigment is multilayered and comprises at least one alternation of a first layer of a material comprising zinc sulfide having a refractive index from 2.30 to 2.90, and a second layer, contiguous to the first, of a material having a refractive index from 1.00 to 2.10, the difference in refractive index between said first layer and said contiguous second layer being greater than or equal to 0.3, and preferably ranging from 0.3 to 1.5, more preferably being on the order of 1.3.

[0040] In this description, "on the order of" or "close to" means a value that is equal to plus or minus 10% of the given numerical value, or a value taking into account measurement uncertainties.

[0041] The refractive index of the layer of a material whose refractive index ranges from 1.00 to 2.10, preferably ranges from 1.00 to 1.60.

[0042] In the interference pigment of the invention, the layer of a material comprising zinc sulfide can be coated on one of its surfaces with a continuous or semi-continuous layer of metallic nanoparticles, for example gold, silver or copper nanoparticles, the nanoparticles having for example at least an average dimension less than or equal to 500 nm, preferably less than or equal to 100 nm.

[0043] A silicon dioxide layer with a thickness ranging from 1 nm to 1 micron can be intercalated between the core and the zinc sulfide layer, and be in contact with both the core and the zinc sulfide layer. It can also be treated afterward to generate porosity within the silica layer.

[0044] A multilayer interference pigment of the invention may comprise a stack of several layers comprising at least one layer comprising zinc sulfide having a refractive index from 2.30 to 2.90, at least one layer of a material having a refractive index from 1.00 to 2.10, and at least one layer of a material not comprising zinc sulfide and having a refractive index from 2.30 to 2.90, the order and thickness of the different layers being chosen to generate colored reflections.

[0045] The pigment of the invention may include an outer protective layer. This outer protective layer is advantageously transparent and without coloring function, thus differing from an interference layer comprising a zinc sulfide-based material. The outer protective layer may be obtained by a surface treatment with a chemical compound fulfilling a function of interest. The protective layer may, for example, facilitate the formulation of the pigment in solvents or oils, or protect against UV radiation.

[0046] The outer protective layer may be organic or mineral in nature, and hydrophilic or hydrophobic. This layer may, for example, be an inorganic layer such as silica or cerine, or a polymer such as PMMA, polystyrene, or polyvinyl chloride.

[0047] The pigment of the invention may advantageously undergo an additional hydrophobic treatment to facilitate its dispersion in a fatty phase, for example, an oily phase. Such a treatment involves applying a hydrophobic surfactant to all or part of the pigment's surface. This surfactant may advantageously be selected from amino acids, metallic soaps, esters, silicone or fluorinated compounds, acrylic compounds, lipids, or a mixture of at least two of these compounds.

[0048] The amino acid-type surfactant may be, for example, glycine, alanine, sarcosine, proline, hydroxyproline, aspartic acid, glutamic acid, or lysine, or a derivative such as, for example, an acylated amino acid comprising a fatty acid, saturated or unsaturated, having from 1 to 22 carbon atoms, preferably from 8 to 20 carbon atoms. Such an acylated amino acid-type surfactant may be, for example, stearoyl glutamic acid, lauroyl glutamic acid, lauroyl aspartic acid, myristoyl glutamic acid, stearoyl lysine, lauroyl lysine, myristoyl lysine, and palmitoyl proline, or one of their salts, such as a sodium, potassium, calcium, magnesium, or aluminum salt.The agents particularly preferred in salt form are preferably sodium myristoyl glutamate, disodium stearoyl glutamate, sodium lauroyl aspartate, dilauramido-glutamide lysine, sodium palmitoyl sarcosine, magnesium palmitoyl glutamate and disodium cocoyl glutamate.

[0049] As metallic soap type surfactants, aluminium myristate and magnesium stearate may be advantageously cited.

[0050] As an ester-type surfactant, advantageous examples include isostearyl sebacate, dextrin and fatty acid esters such as dextrin stearate, dextrin isostearate, dextrin palmitate, or polyglyceryl-2 tetraisostearate.

[0051] As a silicone surfactant, advantageous examples include methicone, hydrogenodimethicone, dimethicone, tetramethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, tetraethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, isobutyltrimethoxysilane, decyltrimethoxysilane, octyltrimethoxysilane, octyltriethoxysilane, octadecyltriethoxysilane and hexadecyltriethoxysilane.

[0052] As a fluorinated surfactant, advantageous examples include perfluorohexyl-ethyltriethoxysilane, trifluoropropyltrimethoxysilane, trifluoropropyltriethoxysilane, tridecafluorooctyltrimethoxysilane or tridecafluorooctyltriethoxysilane.

[0053] As an acrylic-type surfactant, we can mention (co)polymers comprising ethyl(meth)acrylate, butyl(meth)acrylate, octyl(meth)acrylate, decyl(meth)acrylate, lauryl(meth)acrylate, myristyl(meth)acrylate, palmityl(meth)acrylate, stearyl(meth)acrylate, oleyl(meth)acrylate or linoleyl(meth)acrylate groups.

[0054] Examples of lipid surfactants include phospholipids, lecithin, triglycerides, fats, oils, and waxes, advantageously such agents being of natural origin. The outer protective layer may also include a UV-blocking material such as cerium-doped silica. A UV-protective layer has the advantage of stabilizing the color and / or reflections of the pigment over time, particularly throughout the service life of formulations and manufactured products containing the pigment.

[0055] Examples of pigments of the invention include the following stacks of layers, the slash denoting the separation between two different layers, and the parentheses denoting a single layer comprising several materials:

[0056] Core / ZnS

[0057] Core / ZnS / low index material / ZnS

[0058] Core / ZnS / low index material / ZnS, the stacking of the three ZnS / low index / ZnS layers being repeatable several times successively

[0059] Core / ZnS doped with a metal ion

[0060] Core / SiO2 / ZnS

[0061] Core / SiO2 / ZnS / low index material / ZnS

[0062] Core / ZnS / high index material different from ZnS

[0063] Core / (ZnS+high index material other than ZnS)

[0064] Core / ZnS / ZnS doped with a metal ion

[0065] Core / (ZnS+ZnS doped with a metal ion)

[0066] Core / high index material different from ZnS / ZnS

[0067] Core / high index material different from ZnS / low index material / ZnS,

[0068] In these examples, the high-index material other than ZnS has a refractive index ranging from 2.30 to 2.90, and can be, for example, Fe2O3, FeTiO3, Cr2O3, Fe3O4, and the low-index material preferably has a refractive index ranging from 1.00 to 1.60. The described stacks can be covered with a continuous or semi-continuous layer of metallic nanoparticles, for example, gold, copper or silver nanoparticles, but also with a non-interference protective layer.

[0069] According to a particular embodiment, the pigment of the invention differs from a pigment comprising zinc sulfide and a platelet glass substrate having an average thickness of less than 1 pm. More specifically, the pigment of the invention may differ from a pigment in which the glass substrate is made of ECR ​​glass of composition: SiO2 (63-70%), Al2O3 (3-6%), CaO (4-7%), MgO (1-4%), B2O3 (2-5%), Na2O (9-12%), K2O (0-3%), TiO2 (0.1-4%), ZnO (1-5%).

[0070] The pigment of the invention is also preferably different from a pigment comprising a glass platelet substrate, a zinc sulfide layer and a semi-transparent metallic layer, the glass comprising 65-75% by mass of silicon oxide; 2-9% by mass of aluminium oxide; 0.0-5% by mass of calcium oxide; 5-12% by mass of sodium oxide; 8-15% by mass of boron oxide; 0.1-5% by mass of titanium oxide; and 0.0-5% by mass of zirconium oxide.

[0071] Finally, the pigment of the invention may differ from a platelet-based reflective pigment having an internal support comprising zinc sulfide with a thickness ranging from 50 nm to 1000 nm and comprising two opposing principal surfaces and at least one lateral surface. In this reflective pigment, an outer metallic layer having a thickness ranging from 10 nm to 150 nm partially covers the internal support, such that the lateral surface of the internal support is not covered by the outer metallic layer. This outer metallic layer is, for example, selected from aluminum, copper, silver, gold, platinum, palladium, nickel, cobalt, tin, niobium, chromium, and titanium.

[0072] The pigment of the invention can be synthesized by physical or chemical deposition of zinc sulfide on a solid support.

[0073] In the case of a physical deposit, the layer of material comprising the zinc sulfide can be deposited on the solid support by an atomic layer deposition process.

[0074] In the case of chemical deposition, the pigment can be obtained by heterogeneous precipitation of zinc sulfide onto the support, from a zinc saline solution and a sodium sulfide solution. Alternatively, it can be synthesized by bubbling gaseous H2S or by thermal decomposition of a sulfur precursor in place of sodium sulfide, in the presence of the support. For example, a semi-transparent powdered support such as mica is used, but any type of support, such as one of the solid substrates described previously in the context of the description of the pigment of the invention, is suitable.

[0075] Another method for synthesizing the pigment of the invention uses a homogeneous precipitation method. This synthesis method consists of slowly releasing one of the sulfur-containing reagents by raising the temperature during the reaction. The released reagent is distributed homogeneously throughout the entire volume of the solution, which allows for maintaining a low and uniform supersaturation over time. This makes it possible to generate a zinc sulfide layer on a solid support in a controlled manner. According to this method, a suspension of solid particles, a zinc salt, and a precipitating agent that is inactive at room temperature are mixed. The precipitating agent, which allows the generation of hydrogen sulfide by thermal decomposition, is, for example, thioacetamide using water as the reaction solvent, or thiourea, preferably using a heavy alcohol such as a polyol as the solvent.

[0076] In one embodiment, the heterogeneous precipitation of zinc sulfide can be carried out by titrating a sodium sulfide solution at a fixed pH into the reaction medium comprising solid particles and a dilute zinc saline solution. The sodium sulfide solution is added continuously, and the quantity is regulated by a titrator according to a predetermined pH. The objective is to maintain conditions of low supersaturation at all times, allowing for the heterogeneous nucleation of zinc sulfide on the solid particles and the formation of a continuous and homogeneous layer of zinc sulfide on their surface. The titrator containing the sodium sulfide solution is used to compensate for the decrease in pH during the addition of the zinc saline solution by maintaining a predetermined fixed pH, referred to as the steady-state pH.The steady-state pH is chosen to form a continuous, homogeneous, compact layer of zinc sulfide of controlled thickness on the solid particles. Depending on the amount of zinc salt added, the thickness of the zinc sulfide layer increases until it reaches the optimal physical thickness conditions for generating interference colors.

[0077] A second object of the invention relates to a process for synthesizing a pigment, for example an interference pigment, which consists of the heterogeneous precipitation of a material comprising zinc sulfide onto solid particles. The material comprising zinc sulfide may be zinc sulfide or a solid solution of zinc sulfide and a metallic ion as described previously.

[0078] This process may include:

[0079] - a first step in preparing an aqueous dispersion of solid particles, the dispersion having a pH ranging from 2 to 8 and a temperature close to boiling, and - a second step of coating the solid particles comprising the addition, in said aqueous dispersion, of an aqueous solution of a zinc salt, such as zinc nitrate, and an aqueous solution of sodium sulfide, the pH of the reaction medium obtained being maintained between 2 and 7 to obtain a suspension of particles coated with the material comprising zinc sulfide.

[0080] The homogeneity of the zinc sulfide layer deposited on the surface of the particles can be controlled by various physico-chemical parameters such as temperature, salt solution concentration and pH.

[0081] The concentration of the solid particle dispersion that was prepared in the first step ranges, for example, from 1 g / L to 20 g / L or from 5 g / L to 15 g / L.

[0082] The second step is a coating step of the solid particles, which is preferably carried out under stirring of the reaction medium. The temperature of the reaction medium during the coating step can range from 20°C to 100°C, preferably from 60°C to 90°C.

[0083] The solid particles can be selected from natural micas, synthetic micas, alkali-earth carbonates such as calcium carbonate, alkali-earth sulfates such as barium sulfate, natural pearls such as guanine or hypoxanthine, alumina, aluminum, silica, borosilicate, pearlite, an organic polymer (such as a plastic), or a metal oxide such as zinc oxide or bismuth oxychloride. Their size can range from 1 micron to 2000 microns, preferably from 10 microns to 500 microns. The size is defined in the same way as the size of the substrate described above in the context of the first object of the invention and can be measured by scanning electron microscopy (SEM) or transmission electron microscopy (TEM). The particles are preferably platelet-shaped, but can take any other form.

[0084] Regarding the aqueous saline zinc solution, it is preferably dilute and acidified. Its concentration can range from 0.001 M to 10 M or from 0.01 M to 0.1 M, and its rate of addition to the aqueous dispersion of solid particles can range from 0.001 mL / min to 10 mL / min, or from 0.1 mL / min to 1 mL / min. The zinc salt is preferably zinc nitrate.

[0085] The aqueous sodium sulfide solution is a dilute solution which can have a concentration between 0.5 M and 3 M. The pH of the reaction medium is preferably maintained between 2 and 7 throughout the synthesis by controlled addition of the dilute sodium sulfide solution.

[0086] The process of the invention may include a third step, which is a washing and drying step consisting of centrifuging the suspension of particles coated with zinc sulfide, redispersing the particles in ethanol, and then drying them at a temperature ranging from 20°C to 80°C for 12 to 24 hours.

[0087] It may be necessary to perform an annealing step on the zinc sulfide-coated particles to densify the zinc sulfide layer and intensify the color of the interference pigment. The process of the invention may therefore include a step in which the zinc sulfide-coated particles obtained after the third coating step or the fourth washing and drying step as described above undergo heat treatment. The zinc sulfide-coated particles may be placed, under air or argon, at a temperature up to the thermal decomposition temperature of ZnS. The synthesis process of the invention may include a step of coating the zinc sulfide-coated particles with an organic or mineral matrix to form a protective layer. The terms coating and encapsulation may be used interchangeably.

[0088] This encapsulation step, when it consists of coating with silica, can be carried out by a sol-gel process known to those skilled in the art. According to a variant of this process, the dried zinc sulfide-coated particles are dispersed in a citric acid solution brought to a basic pH. This solution is injected into a water-ethanol reaction medium (for example, 25 / 75 v / v). A silica precursor, for example, tetraethyl orthosilicate (abbreviated TEOS), is then added to the mixture, and the solution is left under stirring for 24 hours at room temperature. The suspension is then centrifuged, washed with ethanol, and dried for 24 hours in an oven at 80°C.

[0089] Finally, the synthesis process of the invention may include a step of total or partial removal of the core consisting of solid particles, so as to obtain a solid porous substrate, or a gaseous core. Once the layer of material comprising the zinc sulfide has been deposited on the solid particles at the end of the second step, the removal of the core may be carried out chemically or physically. For example, calcium carbonate particles may be used, coated with zinc sulfide, and then the calcium carbonate dissolved with an acid without damaging the structure of the layer of material comprising the zinc sulfide.

[0090] The interference pigment of the invention can be incorporated into various manufactured products such as food products, paints, inks, dyes, plastics and cosmetics.

[0091] In a particular embodiment, the interference pigment described above may be used in cosmetic products, such as cosmetic products of skincare or makeup.

[0092] A third object of the invention therefore relates to a cosmetic composition, in particular a cosmetic skincare or makeup composition, comprising the interference pigment as described above.

[0093] Examples of makeup products include eyeshadows, nail polishes, eyeliners, lipsticks, eyebrow products, liquid and compact foundations, pressed powders, and loose powders. Skincare products include, for example, white or tinted creams and lip balms.

[0094] These products may be liquid or solid, contain water or be anhydrous. They are, for example, in the form of aqueous gels, water-in-oil or oil-in-water emulsions.

[0095] Cosmetic products may include, in addition to the interference pigment of the invention, at least one cosmetic ingredient known to those skilled in the art, chosen in particular from solvents, oils, pigments other than the pigment of the invention, lacquers, colorants, waxes, cosmetically active compounds, surfactants, UV filters, gelling agents, and thickeners. Those skilled in the art may choose the cosmetic ingredients based on their general knowledge.

[0096] In a particular embodiment, the cosmetic composition comprises an oily phase in which the interference pigment is dispersed, which includes an outer protective layer of a hydrophobic nature.

[0097] The invention is described in more detail by the following example. Unless otherwise stated, the temperature is between 20°C and 25°C, and the pressure is atmospheric pressure.

[0098] Example 1: Synthesis of an interference pigment according to the invention

[0099] The equipment used includes a titrometer, a peristaltic pump and a temperature-controlled reactor coupled to an agitation system.

[0100] 1-Preparation of the dispersion of platelet particles in the reactor An acidic aqueous dispersion of mica particles (commercial reference of the company Topy industries, PDM-2OL) at a concentration close to 10 g / L is heated under stirring to a temperature close to boiling.

[0101] 2-Preparation of the saline solution based on Zn2+ An acidified saline solution based on Zn2+ with a concentration of 0.03M is placed in a peristaltic pump to be injected at a rate of 0.5 mL / min into the reactor.

[0102] 3-Titrating solution and titrator conditions. A 1 M Na2S solution is prepared and serves as the sulfur-containing agent for the synthesis of the ZnS layer. The static acidic pH set on the titrator is greater than 3. 4-Coating by germination and growth of zinc sulfide Moderate stirring and a temperature close to boiling were maintained throughout the synthesis during the addition of the saline solution to the reactor. The static, steady-state pH, controlled by the pre-selected titrator, was maintained throughout the synthesis by controlled addition of the dilute sodium sulfide solution. In this case, 100 mL of saline solution was added.

[0103] 5-Washing Once the reaction was complete, the zinc sulfide-coated mica particles were centrifuged and redispersed in ethanol. They were then dried for 12 hours in an oven at 80°C.

[0104] 6-Observation A continuous and homogeneous layer of zinc sulfide nanoparticles is observed on the mica particles at the end of the synthesis. The coverage rate and the thickness of the ZnS layer are controlled by the volume of zinc salt introduced into the reaction medium. In this example, the addition of 100 mL of solution results in a white powder with a yellow / orange sheen. Microstructural analyses by transmission and scanning electron microscopy made it possible to visualize respectively the deposited thickness of the ZnS layer ([Fig.1]) which is approximately 75 nm as well as its surface homogeneity ([Fig.2]). X-ray diffraction analyses have shown the crystallinity as well as the crystallographic nature of ZnS ([Fig.3]).

[0105] Example 2: Cosmetic formulas containing the interference pigment of the invention

[0106] Cosmetic formulas for makeup are prepared, particularly intended for application on the skin and / or lips. These formulas comprise an interference pigment according to the invention.

[0107] Liquid lipstick INGREDIENTS % by mass MINERAL OIL 5 PASTY FATS 10 RED IRON OXIDES 2 ZINC OXIDE 2 ORGANIC LAKES 2 INTERFERENCE PIGMENT OF THE INVENTION 2 SILICA 6 MICA 4 ESTER OILS Qsp 100 Anhydrous lip balm [Tables2] INGREDIENTS % by mass SILICA 5 POLYETHYLENE WAX 5.5 CANDELILLA WAX 3 SHEA BUTTER 1.5 IRON OXIDES 5 ORGANIC PIGMENTS (LACKS) 2 INTERFERENCE PIGMENT OF THE INVENTION 3 POLYDECENE HYDROGEN Qsp 100 Lip balm INGREDIENTS % by mass NYLON 8 SILICA 9 POLYETHYLENE 17 GLYCERIN 14 INTERFERENTIAL PIGMENT OF THE INVENTION 1 ISONONYL ISONONANOATE Qsp 100 Compact face powder [Tables4] INGREDIENTS % by mass MICA 50 SILICA 10 NYLON 8 MAGNESIUM STEARATE 2 SORBIC ACID 0.1 IRON OXIDES 10 INTERFERENCE PIGMENT OF THE INVENTION 5 GLYCOLS 2 ISONONYL ISONONANOATE Qsp 100 PRESERVATIVES Qs Emulsion foundation INGREDIENTS % by mass OILS ESTERS 6.5 MINERAL OIL 3.5 CAPRYLIC / CAPRIC TRIGLYCERIDES 2.2 BEESWAX 0.8 METHYL POLYMETHACRYLATE 1.1 INTERFERENCE PIGMENT OF THE INVENTION 3 IRON OXIDES (black, red and yellow) 10 SILICA 2 WATER q.s. 100 Anhydrous eyeshadow [Tableauxô] INGREDIENTS % by mass SILICA 15 SYNTHETIC FLUORPHLOGOPITE 10 IRON OXIDES 8 INTERFERENCE PIGMENT OF THE INVENTION 15 OILS AND WAXES Qsp 100 Serum INGREDIENTS % by mass GLYCOL 3 GELDING POLYMER 3 MINERAL OIL 2 POLYETHYLENE GLYCOL 1.5 INVENTIONAL INTERFERENCE PIGMENT 4 PRESERVATIVES Qs PERFUME CONCENTRATE 0.3 WATER Qsp 100 Loose scented powder [Tables8] INGREDIENTS % BY MASS MICA 33.5 SILICA (AND) LAUROYL LYSINE 10 INVENTIONAL INTERFERENCE PIGMENT 20.5 CALCIUM ALUMINUM BOROSILICATE 16.5 CORN STARCH (AND) AQUA 11 CAPRYLYL GLYCOL 1 PENTYLENE GLYCOL 1 Preservatives and fragrances QS Fluid skincare cream INGREDIENTS % BY MASS INVENTIONAL INTERFERENCE PIGMENT 0.5 Polyurethane-35 4 Oils 12 Pastes 2 Fatty Alcohol 1.3 Steareth-21 1 Steareth-2 0.5 Urea 10 Aqueous Phase Gelling Agent 0.5 Phenoxyethanol 0.35 Acrylates / C10-C30 Alkyl Acrylate Crosspolymer 0.3 Xanthan Gum 0.1 Hyaluronic Acid 0.2 Polyglycerol 17 Glycerol 3.9 Glycols 3.6 Preservatives and Perfumes qs Water qsP 100

Claims

Demands

1. Interferential pigment, monolayer or multilayer, comprising a core having a refractive index from 1.00 to 2.10, covered with at least one layer of a material comprising zinc sulfide, said layer having an average physical thickness from 10 nm to 350 nm, characterized in that the core of the pigment comprises a gas or is made of a gas.

2. Interferential pigment according to claim 1, characterized in that the average physical thickness ranges from 20 nm to 340 nm, from 30 nm to 330 nm, from 40 nm to 320 nm, from 50 nm to 310 nm, from 60 nm to 300 nm, from 70 nm to 290 nm, from 80 nm to 280 nm, from 90 nm to 270 nm, from 100 nm to 260 nm or from 110 nm to 250 nm.

3. Interference pigment according to claim 1 or 2, characterized in that the pigment has a dimension ranging from 1 micron to 2000 microns.

4. Interference pigment according to any one of the preceding claims, characterized in that the material comprising zinc sulfide is a material essentially consisting of zinc sulfide, or a material essentially consisting of zinc sulfide and at least one metal ion selected from Fe2+, Cu2+, Mn2+, Ag+, Au3+, Eu3+, Al3+, Ce3+ and In3+.

5. Interference pigment according to any one of the preceding claims, characterized in that the layer of a material comprising zinc sulfide has a refractive index ranging from 2.30 to 2.90, preferably close to 2.

40.

6. Interference pigment according to any one of the preceding claims, characterized in that the pigment is a single-layer pigment comprising a single layer of a material comprising zinc sulfide.

7. Interference pigment according to any one of the preceding claims, characterized in that the pigment is multilayered and comprises at least one alternation of a first layer of a material comprising zinc sulfide having a refractive index from 2.30 to 2.90, and a second layer, contiguous to the first, of a material having a refractive index from 1.00 to 2.10, the difference in refractive index between said first layer and said contiguous second layer being greater than or equal to 0.3, and preferably ranging from 0.3 to 1.5, more preferably being on the order of 1.

3.

8. An interference pigment according to any one of the preceding claims, characterized in that the layer of a material comprising sulfide of zinc is coated on one of its surfaces with a continuous or semi-continuous layer of metallic nanoparticles such as gold, silver or copper nanoparticles.

9. Pigment according to any one of the preceding claims, characterized in that the pigment comprises an outer protective layer, which may be of an organic or mineral nature, and of a hydrophilic or hydrophobic nature.

10. A method for synthesizing an interference pigment according to any one of the preceding claims, which consists of the heterogeneous precipitation of a material comprising zinc sulfide, on solid particles.

11. A process for synthesizing a pigment according to the preceding claim comprising: - a first step of preparing an aqueous dispersion of the solid particles, the dispersion having a pH from 2 to 8 and a temperature close to boiling, and - a second step of coating the solid particles comprising adding, into said aqueous dispersion, an aqueous solution of a zinc salt, such as zinc nitrate, and an aqueous solution of sodium sulfide, the pH of the reaction medium obtained being maintained between 2 and 7 to obtain a suspension of solid particles coated with the material comprising zinc sulfide.

12. Cosmetic composition comprising an interference pigment according to any one of claims 1 to 9.

13. Cosmetic composition according to claim 12, characterized in that it comprises an oily phase in which the interference pigment is dispersed, which includes an outer protective layer of a hydrophobic nature.

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