Process for preparing colored particulate material by heterogeneous germination

Abstract

The invention relates to a method for preparing colored materials by heterogeneous nucleation of metallic nanoparticles, said nanoparticles having optical properties based on the surface plasmon phenomenon. The invention also relates to said colored materials obtained, as well as compositions comprising them. In particular, the method for preparing the colored particulate material by heterogeneous nucleation includes, notably, mixing at room temperature a suspension comprising: - at least one salt of a metallic element, said metallic element having a plasmonic effect, - at least one reducing agent, and - at least one particulate substrate, said mixture of the suspension forming a colored particulate material.

Description

Title of the invention: Method for preparing a colored particulate material by heterogeneous germination technical field

[0001] The present invention relates to the field of material coloring. In particular, the invention relates to a process for preparing colored materials by heterogeneous nucleation of metallic nanoparticles, said nanoparticles having optical properties based on the surface plasmon phenomenon. The invention also relates to the colored materials obtained by the process, as well as compositions comprising them. State of the art

[0002] The use of a metal in nanoparticle form can impart a different color to a suspension or solid substrate comprising said nanoparticles than the original color of the bulk metal (i.e., a metal not in nanoparticle form). Indeed, when a metallic nanoparticle is subjected to an electromagnetic field with a wavelength much greater than its size, the free electrons in the conduction band located on the surface of said nanoparticle experience the same field and oscillate collectively and in phase. When the incident ground frequency corresponds to the natural frequency of these oscillations, a resonance phenomenon occurs, called surface plasmon resonance. This resonance can occur, in particular, in the visible, ultraviolet (UV), and infrared ranges.These are referred to as metallic elements exhibiting a plasmonic effect, these metallic elements being in nanometric form.

[0003] The plasmon resonance frequency is influenced by various parameters, namely: • the nature of the metal; • the size and shape of the nanoparticles; • the distribution of nanoparticles, including the inter-particle distance; and • the optical properties of the substrate or surrounding medium, including in particular the refractive index.

[0004] Indeed, the refractive index allows for the modulation of color. Color is perceived differently depending on the refractive index. Thus, the color of an object will not be perceived in the same way if the object is present in air or in water, for example.

[0005] Interestingly, it is possible to modulate these different parameters in order to to vary the color of the nanoparticles throughout the visible spectrum, or even to shift the resonance frequency into the UV or near-infrared.

[0006] To achieve this, it is known to use so-called preformed nanoparticles exhibiting surface plasmon resonance to color materials. This technique is very promising and offers significant advantages over traditional coloring methods. Indeed, it makes it possible to generate a variety of colors, including vibrant colors, without resorting to pigments or dyes that pose a risk to human health and the environment, which is particularly suitable and useful for certain applications such as tableware, cosmetics, jewelry, watchmaking, food processing, and medicine.

[0007] However, the use of these preformed nanoparticles presents certain drawbacks. For example, the production of a powdered material, i.e., a particulate material, with specific colors requires precise control of the concentration of both the colloidal suspensions and the preformed nanoparticles. Indeed, the size and interactions between the preformed nanoparticles within the suspension generate several constraints, such as: • an increase in viscosity; • a decrease in suspension stability; • difficulties in dispersing the material within the suspension.

[0008] For all these reasons, the concentration of preformed nanoparticles in colloidal suspensions must be finely controlled and mastered, thus limiting the concentration of the suspension in preformed nanoparticles and indirectly the intensity of the color of the material, consequently obtaining a good production yield.

[0009] The use of preformed nanoparticles therefore requires the implementation of complex processes, comprising many steps, using reagents that are often toxic to human health and / or the environment and requiring costly and energy-intensive infrastructure.

[0010] By way of example, patent FR3096685 proposes a new process for preparing, from a substrate, a colored micrometric particulate material (i.e. in powder form) by means of at least one gold salt or at least gold nanoparticles, based on the principle of heterogeneous germination.

[0011] Heterogeneous nucleation specifically promotes the nucleation and growth of nanoparticles on the surface of a solid substrate, which acts as a catalyst for the reaction. In this way, the solid substrate in the form of particles is colored by the formation of colored nanoparticles on its surface, creating a colored particulate material. This process is particularly advantageous because it reduces the probability of demixing during the shaping of the material. the final application.

[0012] However, the use of the process (FR3096685) has the drawback of being limited to certain types of particulate substrates, thus reducing its potential for industrial application. Consequently, this process cannot be universally adapted or transposed to a wide range of supports. Indeed, the process is not, for example, suitable for coloring large volumes of particulate substrate. In fact, achieving heterogeneous nucleation catalyzed by heat treatment systematically requires adapting the parameters between the process developed at laboratory scale and the process at industrial scale in order to maintain the same conditions for the entire volume of particulate substrate. These constraints increase, on the one hand, the development time for new ranges of colored materials and, on the other hand, limit its implementation to certain types of particulate substrate.Furthermore, the need to heat the reaction medium necessarily implies costly and energy-intensive infrastructure.

[0013] Moreover, the growing awareness of climate challenges associated with the global objective of reducing greenhouse gas emissions encourages manufacturers to explore alternative processes that are simple, environmentally friendly, universal and effective for coloring materials.

[0014] There is therefore a need for a new material coloring process designed to minimize its impact on the environment while maintaining optimal efficiency and adaptable to a wide range of supports and thus overcoming the disadvantages of the prior art. Summary of the invention

[0015] To meet this need, the invention proposes a new process that overcomes the aforementioned drawbacks, notably by being adaptable to a wide range of substrates and exhibiting low energy consumption while maintaining optimal coloring efficiency. In particular, the invention provides a new process for preparing a colored particulate material, optionally capable of changing color under the influence of a stimulus. This process is simple, economical, guarantees optimal color stability, and possesses significant modularity, allowing access to a wide range of colors and types of colored materials, and minimizing solvent transfer.

[0016] In the context of the invention, the substrate is a particulate material, that is to say a material in powder form, on which the metallic ions forming colored nanoparticles after nucleation will preferentially nucleate and grow, forming a colored particulate material, the latter comprising at least one nanoparticle on its surface, said nanoparticle having plasmonic properties.

[0017] Thus, the invention relates to a method for preparing a colored particulate material by heterogeneous germination, comprising the implementation of a single step a) of mixing at room temperature a suspension, said suspension comprising: • at least one salt of a metallic element, said metallic element exhibiting a plasmonic effect, • at least one reducing agent, and • at least one particulate substrate.

[0018] Such a process makes it possible to form a colored particulate material while overcoming the drawbacks of the prior art. Given that the process is carried out at room temperature, the process according to the invention advantageously offers low energy consumption while exhibiting an acceptable reaction time, thus reducing operational costs and greenhouse gas emissions compared to an equivalent prior art process.

[0019] Indeed, the inventors have surprisingly discovered that heterogeneous germination carried out at room temperature, i.e., at a temperature between 19 and 25 °C, makes it possible to achieve heterogeneous germination at low energy cost while maintaining optimal staining efficiency. To achieve this, the inventors have notably identified at least one reducing agent of interest.

[0020] Also, according to a preferred embodiment, the reducing agent is chosen from the group consisting of sodium tetrahydruroborate (NaBH4), hydroquinone, tetrabutylammonium borohydride (TBH4), hydrazine, propane, glucose, sucrose, citric acid, ascorbic acid, citrate, triethanolamine (TEA), hydrolamine and mixtures thereof.

[0021] Preferably, the reducing agent is triethanolamine (TEA), which is particularly well-suited for carrying out heterogeneous germination at room temperature, i.e., a reaction that does not require heat catalysis while offering an acceptable reaction time. Alternatively, it can also be used at higher temperatures. Thus, TEA can be used at temperatures between 1°C and 100°C.

[0022] Thus, according to one variant, the invention also relates to a method for preparing a colored particulate material comprising a single step a) of mixing at a temperature between 1°C and 100°C a suspension comprising: • at least one salt of a metallic element, said metallic element exhibiting a plasmonic effect; • Triethanolamine (TEA); and • at least one particulate substrate.

[0023] The process according to the invention thus makes it possible to obtain a spectrum of varied colors and presents a significant modularity, thus offering the possibility of coloring a wide variety of materials with a wide variety of colors.

[0024] Advantageously, the process according to the invention is suitable for coloring all types of material with all types of salts of metallic elements, said metallic elements having a plasmonic effect.

[0025] According to a particularly advantageous embodiment of the invention, the salt of a metallic element, said metallic element having a plasmonic effect, is chosen from a gold salt, a silver salt, a copper salt, an aluminum salt, a magnesium salt, an indium salt, a nickel salt, a gallium salt, a cobalt salt, an iron salt, a palladium salt, a ruthenium salt, a rhodium salt, a platinum salt and mixtures thereof.

[0026] A metallic element salt is a salt in which the metallic element is in the oxidation state. For example, a gold (+III) salt is a salt in which gold is in the oxidation (+III) state.

[0027] According to one embodiment, the gold salt (+III) is chosen from tetrachloroauric acid HAuC14, potassium tetrachloroaurate KAuC14 and their mixture, and preferably KAuC14.

[0028] According to one embodiment, the silver salt (+1) is chosen from AgNO3, AgClO4, Ag(acac), AgCl, Ag2SO4 and their mixture, and preferably AgNO3.

[0029] According to one embodiment, the copper salt (+11) is chosen from copper chloride (CuCl2), copper acetate (Cu(CH3COO)2), copper sulfate (CuSO4), Cu(acac)2, Cu2O (+1), Cu(OH)2, Cu(NO3)2 and their mixture, and preferably CuCl2.

[0030] In the context of the invention, all types of particulate materials can be used as a particulate substrate for coloring. Thus, the particulate substrate can be chosen from organic materials, inorganic materials, and hybrid materials.

[0031] The process according to the invention is thus suitable for coloring all types of materials, offering a wide variety of choices for the materials to be colored. The process is therefore versatile, simple, and efficient, while also having low energy consumption.

[0032] When the particulate substrate is an inorganic substrate, said substrate may be chosen from all known mineral substances. Preferably said substrate is chosen from the group consisting of silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorption pigments and mixtures thereof.

[0033] According to another aspect, the invention also relates to a colored particulate material obtained by the process according to any of the embodiments previously described.

[0034] Finally, according to a last aspect, the invention relates to a coloured composition comprising at least one coloured particulate material according to the invention, and at least one solvent in which said coloured particulate material is dispersed. Detailed description of the invention

[0035] Definitions

[0036] For the purposes of this invention, "stabilizing agent" means charged molecules that enable electrostatic stabilization when adsorbed onto the surface of nanoparticles, or polymers that enable steric repulsions between nanoparticles.

[0037] By "structuring agent" in the sense of the invention, we mean any molecule capable of adsorbing itself on the surface of nanoparticles and thus promoting the growth of certain crystalline faces of the nanoparticle, thereby enabling the formation of anisotropic particles.

[0038] By "heterogeneous germination" in the sense of the invention, we mean the reduction of a metal ion on the surface of a solid support, acting as a catalyst for germination. This is in contrast to homogeneous germination where the nuclei form spontaneously in the reaction medium.

[0039] For the purposes of this invention, "particulate substrate" or "particulate material" refers to a finely divided material, similar to a powder, comprising a collection of particles. This substrate is in powder form, onto which the salts of metallic elements are deposited, forming nanoparticles by nucleation. Advantageously, the largest dimension of this particulate substrate is at most 1 mm.

[0040] By "largest dimension" in particular of a nanoparticle or of the particulate substrate, for the purposes of the invention, means the largest distance separating two points located on the external contour of said nanoparticle or said particulate substrate.

[0041] For the purposes of this invention, "ambient temperature" means a temperature between 19 and 25 °C.

[0042] For the purposes of this invention, "acceptable reaction time" means a reaction time of at most 30 minutes, preferably less than 15 minutes. The reaction is considered complete when the observed color has stabilized.

[0043] Process for preparing a colored particulate material

[0044] The present invention therefore relates to a process for preparing a colored particulate material by heterogeneous germination with low energy consumption, implemented in a single step.

[0045] The process according to the invention thus comprises a step a) of mixing at room temperature a suspension, said suspension comprising: • at least one salt of a metallic element, said metallic element exhibiting a plasmonic effect; • at least one reducing agent; and • at least one particulate substrate.

[0046] Said one-step process forming a colored particulate material from said particulate substrate. Said colored particulate material comprising at least one nanoparticle formed on the surface of said colored material.

[0047] According to one aspect of the invention, the particulate substrate used in the process according to the invention can be pre-colored either natively or by the presence of nanoparticles formed on its surface. These nanoparticles can be deposited according to the process of the present invention. Furthermore, the process according to the invention can be repeated several times, and thus comprise several cycles, each cycle allowing the formation of a colored particulate material. This is particularly advantageous for obtaining a colored particulate material exhibiting the desired coloration.

[0048] By way of example, a first cycle can be implemented from a gold salt, then a second cycle from a silver salt, etc.

[0049] The process according to the invention thus makes it possible to prepare a colored particulate material by heterogeneous germination while overcoming the drawbacks of the prior art. Indeed, the process according to the invention exhibits low energy consumption, thereby reducing operating costs and greenhouse gas emissions compared to an equivalent prior art process, while maintaining similar coloring efficiency. The colorimetric parameters of the colored particulate material are thus similar to those of a colored particulate material obtained by a prior art process, in particular the process according to FR3096685.

[0050] The process according to the invention is simple, insofar as the formation of nanoparticles on the surface of the particulate material is carried out in a single step at room temperature. Indeed, implementation at room temperature offers numerous advantages, notably better control of the parameters related to nucleation during heterogeneous nucleation. Conversely, when implemented at higher temperatures, nucleation can be too rapid, thus requiring adjustments, for example, the addition of an extra temperature stabilization step, before reaching the final target temperature. Consequently, at room temperature, these constraints are eliminated.

[0051] Furthermore, the process according to the invention is also suitable for coloring materials on both laboratory and industrial scales, regardless of the volume of materials to be colored. In this way, the process according to the invention offers better reproducibility and time savings compared to the processes described in art. previous. Indeed, this allows us to overcome the influence of temperature on the nucleation and growth of nanoparticles on the surface of the particulate substrate, thus facilitating its transposition to industrial scales.

[0052] According to a particular object of the invention, the suspension of step a) comprises water and / or an organic solvent.

[0053] According to a particular object of the invention, the suspension of step a) is an aqueous suspension. Preferably, said suspension of step a) comprises at least 5% water by mass relative to the total mass of said suspension, more preferably at least 10% water by mass relative to the total mass of the suspension.

[0054] According to another object of the invention, the suspension of step a) is an organic suspension. When the suspension of step a) is an organic suspension, said suspension preferably comprises alcohol such as ethanol.

[0055] According to another embodiment of the invention, the suspension of step a) may comprise a mixture of water and organic solvent in a ratio between 5 / 95 and 95 / 5, preferably between 60 / 40 and 40 / 60.

[0056] According to a particularly preferred embodiment of the invention, the mixing in step a) is carried out at room temperature, i.e., at a temperature between 19°C and 25°C. Thus, the heterogeneous germination carried out during step a) is performed without heat treatment. In this way, the reaction can be carried out efficiently, regardless of the volume of material to be colored.

[0057] Advantageously, the heterogeneous germination carried out during step a) does not require heat treatment, drastically reducing the energy consumption required to implement the process.

[0058] However, according to a variant of the present invention, when the reducing agent is TEA, the process can be carried out at a temperature between 1°C and 100°C. Preferably, the process is carried out at room temperature. However, it can be carried out at a temperature above 25°C for certain specific particulate substrates. The solvent is preferably aqueous, but according to a variant, it can be a hydroalcoholic solvent.

[0059] Preferably, step a) of mixing, regardless of the embodiments previously described, is carried out under agitation, preferably under mechanical agitation, in particular by means of a paddle mixer or a magnetic stir bar, thus facilitating the heterogeneous germination reaction and the contact of the metallic element salts with the particulate substrate.

[0060] According to another particularly preferred object, the mixture of step a) comprises at least one salt of a metallic element, said metallic element having a plasmonic effect, said salt is chosen from the group consisting of a gold salt, a salt of silver, a salt of copper, an aluminum salt, a magnesium salt, an indium salt, a nickel salt, a gallium salt, a cobalt salt, an iron salt, a palladium salt, a ruthenium salt, a rhodium salt, a platinum salt and mixtures thereof.

[0061] Even more preferably, the mixture in step a) comprises at least one salt of a metallic element exhibiting a plasmonic effect selected from a gold salt, a silver salt, a copper salt and mixtures thereof.

[0062] In the context of the present invention, the metallic element salts are the precursors of the nanoparticles which will be formed by heterogeneous germination on the particulate substrate or possibly on a colored particulate material according to the process of the invention, in order to obtain a colored particulate material according to the present invention.

[0063] According to another particular embodiment, the mixture in step a) comprises at least two distinct metallic element salts, each exhibiting a plasmonic effect. According to a variant of the invention, the mixture in step a) comprises at least two distinct metallic element salts, each being added to said mixture successively and each exhibiting a plasmonic effect. In other words, a first metallic element salt may be added to color the particulate substrate and a second metallic element salt may be added to color the particulate material colored with the first salt, in order to obtain a colored particulate material, the latter comprising at least two distinct colored nanoparticles.

[0064] Preferably, the mixture in step a) comprises at least one gold salt and one silver salt. Said salts may be added successively or simultaneously.

[0065] According to another particular embodiment, the mixture in step a) comprises at least three distinct metallic salts exhibiting a plasmonic effect. When it comprises three distinct salts, the mixture in step a) preferably comprises at least one gold salt, one silver salt, and one copper salt. These salts may be added successively or simultaneously.

[0066] Advantageously, the association and combination of several salts of distinct metallic elements, each exhibiting a plasmonic effect, makes it possible to broaden the range of colors that can be achieved. Consequently, by extension, it broadens the range of colored particulate material obtained using the process according to the invention.

[0067] According to one embodiment of the invention, the mixture in step a) preferably comprises at least one reducing agent selected from the group consisting of sodium tetrahydruroborate (NaBH4), hydroquinone, tetrabutylammonium borohydride (TBH4), hydrazine, propane, triethanolamine (TEA), boranes, organic acids, amines, sugars and mixtures thereof.

[0068] More preferably, the mixture of step a) comprises at least one reducing agent selected from sodium tetrahydruroborate (NaBH4), hydroquinone, tetrabutylammonium borohydride (TBH4), hydrazine, propane, glucose, sucrose, citric acid, ascorbic acid, citrate, triethanolamine (TEA), hydrolamine and mixtures thereof.

[0069] When the reducing agent is chosen from among the amines, it is preferably chosen from Triethanolamine (TEA) and hydrolamine.

[0070] When the mixture in step a) includes at least one reducing agent selected from sugars, this is preferably selected from glucose or sucrose.

[0071] When the mixture in step a) includes at least one reducing agent selected from organic acids, this is preferably selected from ascorbic acid and its derivatives and citric acid and its derivatives.

[0072] According to another embodiment, step a) includes citrate as a reducing agent.

[0073] Preferably, the mixture in step a) comprises a molar / mass ratio of said reducing agent and particulate substrate of between 10 and 6500 (mol / gram), more preferably between 50 and 820 (mol / gram). Conversely, a homogeneous germination reaction is likely to occur.

[0074] Preferably, the largest dimension of the particulate substrate to be stained is between 1 µm and 1 mm. Indeed, the heterogeneous nucleation carried out during step a) is particularly effective on such particulate substrates with such dimensions. Conversely, the nanoparticles will not be able to settle on the surface of the substrate. Furthermore, if the substrate is larger than 1 mm, it will not be possible to keep the different elements in suspension with moderate agitation.

[0075] The substrate on which the heterogeneous germination reaction will occur, i.e., the particulate substrate to be colored, can be in any form, including platelets such as polyhedra or flakes, or beads. The process according to the invention is therefore universal and can color many types of particulate substrate.

[0076] The particulate substrate of the mixture in step a) can be an organic, inorganic, or hybrid material.

[0077] When the particulate material is inorganic, it is chosen from silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorption pigments and mixtures thereof.

[0078] By way of non-limiting example, the mixture in step a) may comprise an inorganic particulate substrate such as silica, quartz, feldspar, limestone, kaolin, metal oxides, aluminate, alumina, zirconium dioxide, non-oxides, ultra-refractory ceramics such as borides, carbides, refractory metal nitrides, silicon or magnesium reinforced ceramics, metals such as aluminum, copper, agent, steel and their combinations.

[0079] Thus, very advantageously, the process according to the invention is suitable for coloring a wide variety of materials.

[0080] According to another embodiment, the particulate substrate can be organic, preferably cellulose.

[0081] Advantageously, the process according to the invention is carried out at room temperature, thus enabling the coloring of organic and / or thermosensitive materials, thereby offering a versatile alternative process suitable for coloring thermosensitive materials.

[0082] In the context of the present invention, the particulate substrate may be colorless, transparent, semi-transparent, opaque, or colored. Indeed, said substrate may be, depending on a particular purpose, natively colored or colored by the process according to the invention. Such an embodiment is particularly advantageous when it is desired to repeat the process according to the invention. In this respect, the process according to the invention then comprises several successive cycles during which the colored material obtained during the first cycle is used as a substrate during the second cycle, and so on.

[0083] Also, according to one aspect of the invention, the particulate material of step a) may comprise particles coated on the surface with a layer containing at least one metal oxide or silicon dioxide. Advantageously, said metal oxide or silicon dioxide layer facilitates the adhesion of the nanoparticles formed on the surface of the particulate material by heterogeneous nucleation.

[0084] According to another object, step a) is carried out in less than 30 minutes, more preferably in less than 15 minutes. Thus, the process according to the invention advantageously has lower energy consumption than the prior art process while offering similar execution time and coloring efficiency.

[0085] According to another embodiment of the invention, the mixture in step a) may also include at least one stabilizing agent. When the suspension also includes at least one stabilizing agent, this is selected from citrate, malate, succinate, citric acid, polyvinyl alcohol, polyacrylic acid, poly(ethylene glycol) (PEG), amino derivatives such as diethylamine, sulfur derivatives such as thiols, triphenylphosphine-based ligands, dendrimers, amino surfactants such as cetyltrimethylammonium bromide (CTAB), sodium do-decyl sulfate (SDS), PVP (poly-n-vinylpyrrolidone), polyelectrolytes, NMP-type monomers, and mixtures thereof.

[0086] According to another embodiment, the mixture in step a) may also include at least one structuring agent. This advantageously allows growth to be directed onto certain crystalline faces and thus the shape of the nanoparticles formed on the surface of the material to be modulated. Controlling the shape of the nanoparticles is an important parameter since it allows the plasmon resonance of the metallic elements to be exploited and therefore the final color of the colored material to be controlled.

[0087] When the mixture in step a) comprises at least one structuring agent, this agent is preferably chosen from citrate, malate, succinate, polyvinylpyrrolidone (PVP), surfactants, and mixtures thereof. More preferably, the surfactants are chosen from cetyltrimethylammonium bromide (CTAB), diethylamine (DEA), ethylenediaminetetraacetic acid (EDTA), and mixtures thereof.

[0088] According to another aspect, the process according to the invention is carried out in a single step, but it may nevertheless include additional steps, in particular an isolation step. This step allows the colored material in solid form to be separated from the liquid phase.

[0089] Thus, according to one embodiment of the invention, the process according to the invention comprises carrying out the following steps: a. a mixture at room temperature of a suspension comprising: • at least one salt of a metallic element exhibiting a plasmonic effect; • at least one reducing agent, and • at least one particulate substrate, b. isolation of the colored particulate material obtained in step a), said colored particulate material having at least one nanoparticle formed on its surface by heterogeneous nucleation.

[0090] Preferably, step b) of isolating the colored particulate material from step a) comprises the following substeps: i. solid / liquid separation of the mixture from step a), to isolate the colored particulate material from the liquid phase, said liquid phase comprising the free elements in suspension; and ii. drying to obtain the colored particulate material in dry form.

[0091] Preferably, substep i) of solid / liquid separation is carried out using at least one solid / liquid separation technique selected from filtration, sedimentation, centrifugation, evaporation, lyophilization and combinations thereof.

[0092] When substep i) of solid / liquid separation is carried out by means of at least two solid / liquid separation techniques, these are carried out successively, which advantageously reduces the duration of the isolation step.

[0093] According to one embodiment, step b) of isolating the colored particulate material may further include at least one additional substep of washing or rinsing the colored particulate material. This step is preferably carried out after separation and before drying.

[0094] The washing or rinsing step is preferably carried out with water and / or an organic solvent. When the step includes washing with water and an organic solvent, this can be carried out simultaneously or successively.

[0095] Thus, according to a particularly preferred embodiment, the process according to the invention comprises the following successive steps: a. mixture according to any one of the embodiments described above, and b. isolation of the colored particulate material obtained at the end of step a) from the liquid phase, the latter advantageously comprising the following sub-steps: i. solid / liquid separation of the mixture from step a) so as to isolate the colored particulate material from the liquid phase, and ii. Optionally, a washing step using water and / or an organic solvent to wash the isolated particulate material; and iii. drying of the particulate material and recovery of colored particulate material in dry and powdery form.

[0096] According to another embodiment of the invention, step b) of isolating the colored particulate material can be repeated several times, preferably from 1 to 5 times, more preferably from 1 to 3 times. Each repetition forming a cycle.

[0097] Thus, the process preferably comprises an isolation step a) and an isolation step b), said isolation step comprising the following successive substeps: i. solid / liquid separation of the mixture from step a) so as to isolate the colored particulate material from the liquid phase; ii. washing of the particulate material from the previous step; iii. solid / liquid separation of the mixture from the previous step; and iv. drying to obtain the colored particulate material comprising at least one nanoparticle formed on its surface by heterogeneous nucleation.

[0098] The successive washing steps advantageously allow the removal of excess organic compounds, for example the reducing agent, the stabilizing agent, the structuring agent present in the suspension, which are likely to cause problems on the final colouring.

[0099] Preferably, drying is carried out using an oven.

[0100] According to a particular object of the invention, at least one nanoparticle is formed on the surface of the colored particulate material, the latter having a larger dimension between 2nm and 100nm.

[0101] Advantageously, the nanoparticle formed on the surface of the colored particulate material has a substantially hemispherical shape. Since the substantially hemispherical shape is the most thermodynamically stable, it is preferred for improving the stability of the color. However, according to certain embodiments, it is possible to modulate the shape of the nanoparticles formed on the surface of the colored particulate material according to the knowledge of those skilled in the art.

[0102] According to another aspect of the invention, the particulate substrate may be porous. The use of such porous substrates advantageously increases the accessible and available surface area for depositing the metallic element salts that form the nanoparticles by nucleation. In this context, the nanoparticle formed on the surface of the colored particulate material advantageously has a longitudinal shape, such as a rod shape, particularly when the pore size allows it. The use of porous particulate material thus makes it possible to modulate the shape of the nanoparticles to obtain a wider range of colors.

[0103] According to another embodiment, the method according to the invention comprises carrying out the following steps: a. mixture at room temperature of a suspension comprising: • at least one salt of a metallic element exhibiting a plasmonic effect; • at least one reducing agent; and • at least one particulate material, forming the colored particulate material; b. Optionally, washing the mixture, c. addition of at least one salt of a metallic element distinct from the mixture of step a) into the mixture from step a) or b); d. isolation of the coloured particulate material from the previous step, said coloured particulate material having at least two distinct nanoparticles formed on its surface by heterogeneous nucleation.

[0104] According to another embodiment of the invention, the process according to the invention includes a pretreatment step of the particulate substrate, located before step a), in order to prepare and condition the particulate substrate to be colored by means of the process according to the invention. This pretreatment step is carried out by means of a heat treatment and / or an alkaline treatment, which activates the surface charges of the substrate, thereby improving the nucleation of the nanoparticle, as well as its attachment to the substrate. This pretreatment step is followed by cooling to room temperature and then filtration, followed by drying to obtain a powder, said powder comprising a multitude of particulate substrates, suitable for to be colored using the process according to the invention.

[0105] Activating the charges on the surface of the particulate substrate allows in particular to promote electrostatic interactions between the particulate substrate and the nanoparticles during the heterogeneous germination of step a), but also to increase the grafting rate of metallic element salts exhibiting a plasmonic effect on the particulate substrate.

[0106] According to another object of the invention, the method according to the invention consists of carrying out the following successive steps: a. Mixture at room temperature of a suspension comprising: • at least one salt of a metallic element, said metallic element exhibiting a plasmonic effect; • at least one reducing agent; and • at least one particulate substrate, b. isolation, and optionally washing, of the colored particulate material from step a), said colored particulate material having at least one nanoparticle formed on its surface by heterogeneous nucleation.

[0107] According to another object of the invention, the process may also include a further step, after obtaining the colored particulate material, in which a stimulus is applied to said colored particulate material. Such a stimulus may, in particular, modify the color of the colored particulate material.

[0108] Preferably, said stimulus is an external stimulus such as heat treatment, exposure to UV radiation, the use of a laser, or spontaneous or induced rehydration. This will make it possible to act on the shape, size of the nanoparticles and the surrounding optical index and consequently their color, thus modifying the color of the colored particulate material.

[0109] Coloured particulate material

[0110] According to another aspect, the invention relates to a colored particulate material obtained by the process according to any of the embodiments previously described.

[0111] Preferably, the colored particulate material according to the invention comprises on its surface at least one nanoparticle formed by heterogeneous germination, preferably a multitude of nanoparticles formed by heterogeneous germination.

[0112] According to another embodiment, the coloured particulate material comprises at least one nanoparticle on its surface, said nanoparticle having a spherical, spheroidal or anisotropic shape such as rods, cubes, triangles, bipyramids,....

[0113] According to a particular embodiment, at least one nanoparticle formed on the surface of the colored particulate material according to the invention has a larger dimension between 2 and 100nm, preferably between 10 and 50nm.

[0114] Colourful composition

[0115] Finally, according to a last aspect, the invention relates to a coloured composition comprising at least one coloured particulate material according to the invention, and at least one solvent in which said coloured particulate material is dispersed.

[0116] Preferably, the solvent is chosen from alcohols such as ethanol, esters such as ethyl acetate, or an aqueous solvent. Examples

[0117] Example 1: Colouring process according to the invention using gold salts

[0118] In this example, the inventors colored a particulate substrate with gold nanoparticles.

[0119] To achieve this, the inventors implemented the following process: a. Mixing, under stirring, at room temperature, a suspension in a 50 mL flask, the suspension comprising: • 2 g of substrate, namely glass frit; • 10 mL of distilled water; • 1.037 mL of concentrated TEA at 50 mM; and • 0.518 mL of KAuCl4 concentrated at 10 mM The mixing in step a) was carried out until the color of the suspension was stabilized, i.e. for 15 min. a. Isolation of the colored particulate material, comprising the following sub-steps: i. Centrifuge the mixture from step a) at 4000 rpm for 3 minutes to isolate the colored particulate material and the liquid phase, ii. Wash the colored material from the previous step with water, iii. Centrifuge the mixture from the previous step at 4000 rpm for 3 minutes to isolate the colored particulate material and the liquid phase. iv. Drying by means of an oven at 80°C of the coloured particulate material from the previous step, so as to obtain a coloured particulate material comprising at least one nanoparticle formed on its surface by heterogeneous germination, in powder form.

[0120] The colored particulate material obtained is then heat-treated (800°C) to obtain the final color. Thus, the glass frit (in powder form) comprising the gold nanoparticles formed on its surface is melted and then cooled to obtain colored enamel from the colored particulate material (i.e., the glass frit).

[0121] The colorimetric parameters are measured using a spectrophotometer, according to the Lab color model. This model, well known to those skilled in the art, called Lab, allows for more precise measurement and quantification of colors over a very broad spectrum. Thus, L stands for Luminosity, and a and b denote the chromatic components.

[0122] The colorimetric parameters of the colored enamel according to example 1 are as follows: • L* = 44.15; • a = 40.18; and • b = 12.90.

[0123] This example thus demonstrates that the process according to the invention makes it possible to color a particulate material by heterogeneous germination at room temperature.

[0124] Example 2: Colouring process according to the invention using gold salt and silver salt

[0125] In this example, the inventors colored a particulate substrate with gold nanoparticles, then with silver nanoparticles. The process is as follows: a. Mix under stirring, at room temperature, in a 50 mL flask, for 15 minutes, the suspension comprising: • 2 g of substrate, namely the glass frit • 10 mL of distilled water; • 1.037 mL of concentrated TEA at 50 mM; and • 0.518 mL of KauCl4 concentrated at 10 mM b. Mix, while stirring, at 80°C, for 15 minutes, a suspension comprising: • the colored particulate material having at least one gold nanoparticle, serving as a new substrate; • 0.375mL of 50mM concentrated citrate; • 0.187 mL of AgNO3 concentrated at 10 mM; and • 600pL of concentrated ascorbic acid at 50mM. c. isolation of the colored particulate material obtained, said isolation comprising the following sub-steps: i. Centrifugation at 4000rpm for 3 minutes of the mixture obtained in order to isolate the colored particulate material and eliminate the liquid phase; ii. Washing the colored particulate material with water; iii. Centrifugation at 4000rpm for 3 minutes of the mixture from the previous step in order to isolate the colored particulate material and remove the liquid phase; iv. Drying of the coloured particulate material separated from the liquid phase, said coloured particulate material comprising at least one gold nanoparticle and at least one silver nanoparticle.

[0126] Here again, as in example 1, a heat treatment step at a temperature of 800°C makes it possible to transform the glass frit into colored enamel.

[0127] The colorimetric parameters of the enamel are then measured using a spectrophotometer, according to the Lab color model: • L* = 29.32; • a = 35.96; and • b = 20.61

[0128] Example 3: Colouring process according to the process of the invention of a large volume with gold salts

[0129] This example aims to demonstrate that the process can be implemented with a larger volume, without affecting the coloring efficiency of the process according to the invention.

[0130] The process used in this example includes the implementation of the following steps: a. Mixing, under stirring, at room temperature, for 15 minutes, a suspension in a 500 mL flask, the suspension comprising: i. 10g of substrate, namely glass frit ii. 100 mL of distilled water; iii. 5.183 mL of TEA concentrated to 50 mM; and iv. 2.592de KauCl4 concentrated at OrnM b. isolation of the colored particulate material from the previous step by implementing the following sub-steps: i. Centrifugation at 4000rpm for 3 minutes of the mixture obtained in order to isolate the colored particulate material and removal of the liquid phase; ii. Washing the colored material with water; iii. Centrifugation at 4000rpm for 3 minutes of the mixture from the previous step in order to isolate the colored particulate material and removal of the liquid phase; iv. Drying of the colored particulate material comprising at least one gold nanoparticle formed on its surface by heterogeneous nucleation.

[0131] Here again, a heat treatment step of the colored particulate material is implemented in order to obtain the colored enamel.

[0132] The colorimetric parameters of the colored enamel, according to the Lab model measured using a spectrophotometer, are as follows: • L* = 40.15; • a = 45.01; and • b = 14.74

[0133] Example 4: Colouring process according to the process of the invention of a large volume with gold and silver salts

[0134] In this example, the inventors colored a particulate substrate with gold nanoparticles, then with silver nanoparticles in a large volume. The process is as follows: a. Mix the suspension in a 500 mL flask at room temperature, stirring for 15 minutes. The suspension will include: • 10 g of substrate, namely • 100 mL of distilled water; • 5.183 mL of 50 mM concentrated TEA; and • 2.592 mL of KauCl4 concentrated at 10 mM b. Mix, while stirring, at 80°C, for 15 minutes, a suspension comprising: • the colored particulate material having at least one gold nanoparticle, serving as a new substrate; • 1.873 mL of 50mM concentrated citrate; • 0.937 mL of 10 mM concentrated AgNO3; and • 600 pL of ascorbic acid concentrated at 50mM. c. isolation of the colored particulate material obtained, said isolation comprising the following sub-steps: i. Centrifugation at 4000rpm for 3 minutes of the mixture obtained in order to isolate the colored particulate material and eliminate the liquid phase; ii. Washing the colored particulate material with water; iii. Centrifugation at 4000rpm for 3 minutes of the mixture from the previous step in order to isolate the colored particulate material and remove the liquid phase; iv. Drying of the coloured particulate material separated from the liquid phase, said coloured particulate material comprising at least one gold nanoparticle and at least one silver nanoparticle.

[0135] Finally, an additional heat treatment step at a temperature of 800°C allows a support to be colored with said colored particulate material.

[0136] The colorimetric parameters of the colored enamel, according to the Lab model measured using a spectrophotometer, are as follows: L* = 44.94; a = 42.58; and • b = 27.53

[0137] Example 5 - Verification of the reproducibility of the process according to the invention

[0138] The objective of this example is to demonstrate that the method according to the invention allows to obtain reproducible results. For this, the process described in example 3 was carried out 4 times, the colorimetric parameters of the colored material obtained were measured using a spectrophotometer.

[0139] The results are presented in Table 1 below:

[0140] [Tables 1] L* ab Sample 1 40.15 45.01 14.74 Sample 2 43.9 41.15 11.59 Sample 3 44.83 42.81 10.93 Sample 4 45.38 43.66 10.56

[0141] The results demonstrate that the process according to the invention makes it possible to obtain reproducible coloring results.

Claims

Demands

1. A process for preparing a colored particulate material by heterogeneous germination, comprising carrying out a step a) of mixing at room temperature a suspension, said suspension comprising: • at least one salt of a metallic element, said metallic element having a plasmonic effect, • at least one reducing agent, and • at least one particulate substrate.

2. A method according to the preceding claim, characterized in that the suspension comprises at least 5% water by mass relative to the total mass of said suspension.

3. A process according to any one of the preceding claims, characterized in that the reducing agent is selected from the group consisting of sodium tetrahydroborate (NaBH4), hydroquinone, tetrabutylammoniumborohydride (TBH4), hydrazine, propane, glucose, sucrose, citric acid, ascorbic acid, citrate, triethanolamine (TEA), hydrolamine and mixtures thereof.

4. A method according to any one of the preceding claims, characterized in that the salt of a metallic element is selected from the group consisting of a gold salt, a silver salt, a copper salt, an aluminum salt, a magnesium salt, an indium salt, a nickel salt, a gallium salt, a cobalt salt, an iron salt, a palladium salt, a ruthenium salt, a rhodium salt, a platinum salt, and mixtures thereof.

5. A method according to any one of the preceding claims, characterized in that the largest dimension of the particulate substrate is between 100m and 1mm.

6. A method according to any one of the preceding claims, characterized in that the particulate substrate is selected from an inorganic particulate substrate of the group consisting of silicates, glasses, metal oxides, rare earth oxides, metals, frits, enamels, glazes, ceramics, absorption pigments, and mixtures thereof.

7. A method according to any one of the preceding claims, characterized in that the duration of step a) is at most 30 minutes.

8. A method according to any one of the preceding claims, characterized in that that the process also includes a step b) of isolating the colored particulate material obtained in step a), said colored particulate material having at least one nanoparticle formed on its surface by heterogeneous germination.

9. A method according to the preceding claim, characterized in that step b) of isolating the colored particulate material successively comprises the following substeps: i. solid / liquid separation of the mixture from step a), to isolate the colored particulate material from the liquid phase; and ii. drying to obtain the colored particulate material in dry form.

10. A method according to the preceding claim, characterized in that the solid / liquid separation is carried out by means of at least one solid / liquid separation technique selected from filtration, sedimentation, centrifugation, evaporation, freeze-drying and combinations thereof.

11. A method according to any one of claims 8 to 10, characterized in that step b) comprises an additional substep of washing the colored particulate material.

12. A method according to any one of the preceding claims, characterized in that the largest dimension of the nanoparticle present on the surface of the colored particulate material is between 2 and 100nm.

13. A process according to any one of the preceding claims, characterized in that the process includes a step prior to step a), of pretreating the particulate substrate by means of a heat treatment and / or an alkaline treatment.