Sol-gel process for manufacturing hollow or solid beads or beads with pores

Abstract

Method for forming beads (1) by sol-gel method, comprising: preparation of a sol-gel solution, comprising a molecular precursor, the sol-gel solution being such that it gels beyond a threshold molar fraction of water, said threshold molar fraction depending on the molecular precursor; from the sol-gel solution (2), formation of liquid drops in a medium; the medium comprising water in liquid or vapor form; following step b), gelling of the drops, so that the drops gradually solidify in the medium, forming beads, collection of the beads; the method being characterized in that the sol-gel solution comprises a molar fraction of water lower than said threshold molar fraction; the medium providing additional water making it possible to exceed the threshold molar fraction of water, so that the drops gel upon contact with the ambient medium.

Description

Title of the invention: Sol-gel process for manufacturing hollow or solid beads or beads with pores PREVIOUS ART

[0001] Sol-gel processes are known to allow the production, at low temperature, of ceramics or glasses with high purity and good homogeneity in comparison with conventional high-temperature processes.

[0002] Document WO2021 / 140129 describes a method for forming sol-gel microdisc-type supports, in particular for applications related to cell culture. The method consists of depositing droplets of a sol-gel solution on a support. The droplets flatten on the support and then solidify by gelation / drying.

[0003] The inventor proposes an innovative and simple to implement method, allowing collective production of sol-gel beads or microbeads. The method makes it possible to simultaneously obtain a large number of sol-gel beads or microbeads, while controlling the shape and diameter of the beads, and this at a lower cost. The beads formed can be used in the field of cell culture, but also for different applications, as described below. The beads can be hollow, and form bubbles, or solid. The beads can also have pores. Presentation of the invention

[0004] A first object of the invention is a method for forming beads by sol-gel method, comprising: a. preparation of a sol-gel solution, comprising a molecular precursor, the sol-gel solution being such that it gels beyond a threshold molar fraction of water, said threshold molar fraction depending on the molecular precursor; b. from the sol-gel solution (2), formation of liquid drops in a medium, the medium comprising water in liquid or vapor form; c. following step b), gelling of the drops (12), so that the drops gradually solidify in the medium, forming beads, d. collection of marbles.

[0005] The sol-gel solution may comprise a molar fraction of water lower than said threshold molar fraction; the medium providing additional water making it possible to exceed the threshold molar fraction of water, so that the drops gel on contact with the ambient medium.

[0006] The molecular precursor may in particular be a silicon alkoxide, the threshold molar fraction of water being 1.125%, the sol-gel solution comprising a fraction molar fraction of water greater than 0.05% and less than 1.125%, the molar fraction of water being given relative to the molecular precursor. The molar fraction of water may be less than 1.1% or 1.05% or 1% or 0.9%.

[0007] According to one embodiment, the medium is a gaseous medium comprising water vapor, the method comprising, following step c), a movement of the drops in the gaseous medium, to a receptacle, the duration of the movement of the drops in the gaseous medium being adjusted so that the drops reach the receptacle in the solidified state. The gaseous medium may be air. The relative humidity is greater than 30% or 40%.

[0008] According to one embodiment, the medium is an aqueous liquid medium.

[0009] The molar fraction of water may be greater than a first limit molar fraction, so as to generate a formation of predominantly solid beads, the first limit molar fraction being 0.65%. The first limit molar fraction depends on the precursor. It is established beforehand.

[0010] The molar fraction of water may be less than a second limit molar fraction, so as to generate a formation of predominantly hollow beads, the second limit molar fraction being 0.5%. The second limit molar fraction depends on the precursor. It is established beforehand.

[0011] The aqueous liquid medium may comprise a gelling catalyst agent. The gelling catalyst agent comprises an amine function. The gelling catalyst agent may comprise an amine function. The gelling catalyst agent may be chosen from: ammonia or aminosilane.

[0012] The sol-gel solution may comprise an aminosilane, for example of the APTES and / or APTMS type, forming a gelling catalyst agent, according to a molar fraction of between 0.005% and 0.3%, preferably between 0.03% and 0.1%, the molar fraction being defined relative to the molecular precursor.

[0013] During step b) the diameter of each drop may be less than 10 mm or 2 mm or 1 mm.

[0014] During step b), the diameter of each droplet may be greater than 100 nm.

[0015] A second object of the invention is a ball made of sol-gel material, obtained in applying a method according to the first object of the invention.

[0016] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0017] [Fig.l] shows schematically a ball resulting from the invention.

[0018] [Fig.2] shows a first embodiment of the invention.

[0019] [Fig. 3] shows schematically the main steps of the first embodiment of the invention.

[0020] [Fig.4] shows a second embodiment of the invention.

[0021] [Fig. 5] shows schematically the main steps of the second embodiment of the invention.

[0022] Figures 6A to 6D are images of beads formed during a first series of tests, implementing the second embodiment.

[0023] [Fig.7] is an image of bubbles (hollow balls) formed during a second series of tests, implementing the second embodiment.

[0024] Figures 8A and 8B are images of beads formed during a third series of tests, implementing the first embodiment. PRESENTATION OF SPECIAL EMBODIMENTS

[0025] [Fig. 1] shows an example of a ball 1 according to the invention. The ball is made from a sol-gel material. It has a diameter ¢, less than or equal to 20 mm, and preferably less than or equal to 10 mm, and preferably less than or equal to 1 mm. The diameter $ is preferably between 100 nm and 1 mm, and more preferably between 1 pm and 1 mm or between 10 and 20 pm and 100 pm or 500 pm.

[0026] The bead may be intended for applications related to cell culture, as a substitute for the glass or polymer beads described in connection with the prior art. Other types of application may be envisaged, as described at the end of the description.

[0027] The method can be used to form solid beads or hollow beads. A hollow bead corresponds to a bubble, extending around a central gaseous part.

[0028] The beads according to the invention are obtained by implementing a sol-gel type process, abbreviation of solution-gelation. This is a chemical process known to those skilled in the art, making it possible to manufacture, at low temperature, glasses or ceramics. Such a process involves the use of a sol-gel solution, formed: - a molecular precursor, for example an organometallic compound or a metal salt; - possibly an organic solvent; - water; - an acid or basic catalyst.

[0029] In the presence of water, or water vapor, a network of oxides is formed, by means of hydrolysis-condensation reactions, forming a gel. The latter then undergoes drying, to eliminate the solvent present in the gel. The drying can be of the evaporative type, at a pressure lower than or equal to atmospheric pressure, so as to form a dry gel, usually designated by the term xerogel, in the form of a monolithic solid.

[0030] The molecular precursor may for example be an organometallic compound of metal or metalloid, for example a metal alkoxide of formula M(OR)n, where M is a metal or a metalloid, and R is an organic group, for example alkyl. - The metal M can be for example a transition metal, or a lanthanide: it can be Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ra, W, Re, Os, Ir, Pt, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Yb, Al, Ga, In, Ge, Sn, Pb. - The metalloid element can be chosen from Si, Se, Te. - R can be an alkyl group, for example containing between 1 and 10 carbon atoms, or a phenyl group. - n is a natural integer corresponding to the number of ligands bound to M, which corresponds to the valence of M.

[0031] The molecular precursor is placed in a solution, for example an alcoholic solution. The organic solvent may be an aliphatic or aromatic monoalcohol, or a diol.

[0032] The sol-gel solution may also comprise a catalyst, and / or water, or compounds making it possible to act on the porosity, for example a surfactant.

[0033] According to one embodiment, which particularly concerns cell culture applications, the sol-gel solution comprises a functionalization compound, in particular an organic compound, the function of which is to form a grafting agent. By grafting agent is meant a molecule or a functional group capable of promoting attachment, by grafting, of a chemical or biological element to the surface of the xerogel resulting from the implementation of the sol-gel process. The grafting agent can then be collagen, or polylysine. It is then possible to use a functionalization compound comprising an epoxy function, the latter being conducive to the formation of chemical bonds with amine functions.

[0034] The sol-gel solution may also comprise an active ingredient conferring particular properties to the beads that it is desired to form. These may, for example, be optical properties, for example a particular color, in which case the sol-gel solution may comprise an ink. It may also be an ability to generate fluorescence light. In the latter case, the sol-gel solution comprises fluorescent agents. The sol-gel solution may comprise agents conferring light scattering properties, for example metal oxide particles, for example titanium oxide particles. This makes it possible to obtain light-scattering beads. These may also be organic particles, carbon black.

[0035] The sol-gel solution may include agents for adjusting the dielectric or electrical properties or the light reflection properties. The beads may contain electrically conductive particles, and thus be used to form electromagnetic shielding.

[0036] The sol-gel solution may comprise a compound whose optical properties are modified in the presence of a chemical or biological species. The beads formed from the solution may then be used in a sensor of said chemical or biological species.

[0037] The sol-gel solution may comprise an agent having an influence on the electrical conductivity. This may for example be conductive particles, for example metal particles

[0038] The sol-gel beads obtained can have a variable density, generally less than 2. The density can be less than 1.5, or even less than 1, due to the presence of pores in the beads or the formation of hollow beads, taking the form of bubbles, i.e. hollow spherical envelopes.

[0039] It is also possible to seek to increase the density, for example by adding metal oxides with a density greater than that of glass or by adding dense particles, for example lead, gold, tungsten: this may for example be titanium oxide (density 4.23 g / cm3), or alumina oxide (density 3.95 g / cm3), tin oxide (density 6.95 g / cm3) or zirconia oxide (density 5.68 g / cm3).

[0040] According to one embodiment, described below, the method makes it possible to obtain porous beads, comprising a multitude of pores distributed in a solid matrix. This type of bead has a high opacity.

[0041] It is known that a sol-gel solution undergoes gelation in the presence of a certain molar fraction of water, called the threshold molar fraction. By molar fraction of water is meant the molar fraction relative to the molecular precursor. The threshold molar fraction depends on the precursor. It can be determined by a person skilled in the art, for example experimentally, by gradually modifying the molar fraction of water in the sol-gel solution and determining the molar fraction from which gelation is obtained.

[0042] An important aspect of the invention is that the sol-gel solution, comprising the molecular precursor, comprises a molar fraction of water lower than the threshold molar fraction. This makes it possible to have a solution which cannot gel spontaneously. The quantity of water in the sol-gel solution is insufficient to lead to a hydrolysis / condensation cycle sufficient for the formation of a gel. The solution is in a state which can be considered metastable, in which the precursors are partially hydrolyzed. An additional supply of water is necessary to allow the triggering of the hydrolysis / condensation cycle sufficient to obtain gelation.

[0043] In the following examples, the precursor is TMOS (Tetramethoxysilane, knowing that it could be another type of silicon alkoxide, for example TEOS (tetraethyl orthosilicate). Regarding TMOS, the inventors have determined that the threshold molar fraction of water amounts to 1.125%. Below this molar fraction, the solution does not have enough water to enter into a gelation process.

[0044] The molar fraction of 1.125% previously described is to be considered as a limit. Preferably, the molar fraction of water is less than or equal to 1.1% or 1.05%, or 1%, or 0.95%, or 0.9%, so as to deviate from the limit. The molar fraction of water is preferably greater than 0.1% or 0.2%.

[0045] Preferably, the sol-gel solution comprises a compound, forming a gelling catalyst agent, comprising an amine function. This is preferably an aminosilane, for example APTES (3-Aminopropyl)triethoxysilane or APTMS (3-Aminopropyl)trimethoxysilane). These are silica precursors comprising an amine function. The molar fraction of aminosilane is preferably between 0.005% and 0.3% (relative to the molecular precursor), and preferably between 0.03% and 0.13%, and more preferably between 0.03% and 0.1%. The addition of an aminosilane makes it possible to accelerate gelling. The presence of the gelling catalyst promotes rapid gelling as soon as the quantity of water becomes sufficient. First embodiment

[0046] A first embodiment of a process for manufacturing a ball, using a sol-gel process, is shown diagrammatically in [Fig. 2]. The main steps of this process are shown diagrammatically in [Fig. 3].

[0047] Step 100: formation of drops

[0048] A sol-gel solution 2, as previously described, is introduced into a reservoir 5 of a dispenser 3, of the nebulizer or sprayer type, making it possible to form drops 12, and preferably drops calibrated in volume. The drops are formed in a medium 4 contained in an enclosure 7. In this embodiment, the medium is a gas comprising water vapor. Preferably, the medium 4 is air (or mainly composed of air) comprising water vapor, the relative humidity preferably being greater than 30% or 40%.

[0049] The drops 12 are preferably microdrops, the volume of which is between 105 nL and a few ml or tens of ml. The diameter of the drops 12 is preferably between 100 nm and 5 mm, or between 100 nm and 1 mm. According to one possibility, the diameter can reach 10 mm or 20 mm.

[0050] The dispenser 3 may be a commercial spray or nebulizing device. The dispenser may be configured to form drops of calibrated size. The drops are formed successively or simultaneously.

[0051] The dispenser allows the formation of drops 12 of sol-gel solution. The geometric characteristics, in particular the diameter, of the drops depend on the choice of the nozzle of the dispenser 3 as well as the viscosity of the sol-gel solution 2 and the flow rate. It is usually considered that the lower the flow rate, the smaller the diameter of the drops.

[0052] The nozzle of the dispenser 3 is arranged facing a receptacle 10, at a distance from the latter. The receptacle may be a solid plate, preferably a hydrophobic plate, or a liquid, for example water or an oil, for example a silicone oil. In the example shown, the receptacle 10 corresponds to the bottom of the enclosure 7.

[0053] Step 110: gelation and drying.

[0054] Following step 110, the drops 12 formed by the dispenser 3 are directed towards the receptacle 10, through the air 4 extending between the dispenser 3 and the receptacle 10.

[0055] Given the molar fraction of water in the soil, previously mentioned, the solution does not gel before being dispersed in the form of drops 12 in the air. Gelation is initiated upon contact of the drops and the moisture contained in the air. The higher the relative humidity, the faster the gelation. Under the effect of gelation, the drops 12 solidify, which leads to the formation of beads 1.

[0056] The air temperature may be room temperature, i.e. 20°C. Generally, the temperature may preferably be between 10°C and 60°C. It is considered that a high temperature accelerates gelation.

[0057] The temperature and / or pressure in the enclosure 7 can be adjusted, so as to promote the gelling of the drops 12. The movement of the drops towards the receptacle 10 can be spontaneous, for example by gravity, which corresponds to the preferred embodiment. The movement of the drops 12 towards the receptacle 10 can be forced, for example by driving the gaseous medium towards the receptacle, resulting in a movement of the drops to the receptacle. The current can also be opposed to the spontaneous movement of the drops: this makes it possible to increase the duration of movement of the drops in the gaseous medium. The longer the movement duration, the more advanced the gelling and drying are when the drops reach the receptacle.

[0058] During the movement between the distributor 3 and the receptacle 10, the drops undergo gelling and drying, which causes them to gradually solidify: they then form beads. The duration of the movement of the drops 12 between the distributor 3 and the receptacle 10 is adjusted so that the drops reach the receptacle when the gelling phase has advanced sufficiently for the drops are in a sufficiently solid state so as not to spontaneously deform or break when they reach the receptacle. Thus, and this is an important aspect of the invention, the drops are not deformed, or only negligibly so, upon contact with the receptacle.

[0059] Unlike the method described in WO2021 / 140129, the beads reaching the receptacle do not deform on the latter. The main difference between the method described in WO2021 / 140129 and the method which is the subject of the invention lies in the duration of the movement of the drops 12 between the dispenser 3 and the receptacle 10. In WO2021 / 140129, the distance between the dispenser and the receptacle is sufficiently small so that the drops, reaching the receptacle, can be deformed under their own weight, so as to flatten. In WO2021 / 140129, the distance between the dispenser and the receptacle is thus preferably less than 10 cm. On the contrary, in the method which is the subject of the invention, the drops reach the receptacle in the form of balls, solid or hollow, having been solidified so as to no longer be deformable under their own weight, that is to say in the absence of an external constraint.

[0060] The distance between the nozzle of the dispenser 3 and the receptacle is determined according to the experimental conditions, in particular: composition of the soil solution, relative humidity, temperature, size of the drops.

[0061] The gaseous medium 4 may comprise one or more compounds promoting gelling, for example an ammonia vapour, or a gas comprising an amine function, for example methylamine. The enclosure 7 may also be saturated with water vapour. Accelerating the gelling process makes it possible to reduce the travel time of the drops between the distributor 3 and the receptacle 10. This makes it possible to reduce the distance between the distributor 3 and the receptacle 10: the method can be implemented using a more compact device.

[0062] It has been found that the addition of an aminosilane to the sol solution, for example APTES and / or APTMS, as previously mentioned, makes it possible to obtain beads with a regular shape. The addition of an aminosilane promotes condensation during spraying. In particular, the sol solution must have a pH between 7 and 9 thanks to the addition of aminosilane in order to be in a state considered metastable and to gel in a few seconds during spraying.

[0063] Step 120. extraction

[0064] During this step, the beads 1 deposited on or in the receptacle 10 are extracted. Step 120 may include a complementary drying phase of the material forming each bead. The use of a solid and preferably hydrophobic receptacle facilitates the recovery of the beads, by avoiding the formation of OH bonds between the beads resulting from the implementation of the method, and the receptacle 10.

[0065] The balls 1 can be recovered using a recovery support, preferably flexible, for example a cloth. It can be a porous nylon filter. The mesh of the recovery support can be optimized to retain the balls 1 while allowing the elimination of balls of too small a size or debris, the latter passing through the recovery support. For example, when the diameter (or the largest diagonal) of the balls is equal to 600 μm, the particle size of the recovery support can be 400 μm. When the diameter of the balls is 200 μm, the particle size can be 150 μm.

[0066] Step 130: Washing

[0067] The beads recovered during step 120, placed on the recovery support, are washed, for example by a bath in a washing solution, for example isopropanol, making it possible to eliminate residual acids present in the sol-gel solution or any unreacted precursors. The washing solution may be an aqueous solution, for example an aqueous solution comprising 50% by mass of isopropanol. The method may successively comprise several baths, for example two or three successive baths.

[0068] Step 140: Post-wash drying

[0069] Following step 130, the beads are dried. Drying may be carried out at room temperature or at a higher temperature, for example up to 100°C or above. The drying temperature may be lowered if a partial vacuum is formed around the beads. During drying, the beads may be placed on the recovery support and the assembly is placed in an oven.

[0070] Optionally, the balls are subjected to a post-drying heat treatment. Second embodiment

[0071] A second embodiment of a method for manufacturing a ball, by sol-gel process, is shown diagrammatically in [Fig.4]. The main steps of this method are shown diagrammatically in [Fig.5]. This is a different configuration from that described in WO2021 / 140129 since the drops are not formed in a gaseous medium, but in a liquid.

[0072] The method follows the following steps:

[0073] Step 200: formation of drops

[0074] Step 200 is similar to step 100 described in connection with the first embodiment, except that the drops are formed in an aqueous liquid medium 6 contained in an enclosure 6. The meeting of the two liquids (sol-gel solution resulting from the dispenser and liquid medium 6) results in atomization of the sprayed sol-gel solution, which forms fine microdroplets. The size of the droplets may be similar to what was described in step 100. It is considered that the size of the nozzle allows the formation of beads whose diameter is preferably between 1 pm and 300 pm.

[0075] Step 210: gelation and drying.

[0076] The drops formed in the medium immediately undergo gelation, the hydrolysis being initiated by the water present in the medium. The aqueous medium 6 can be left at room temperature or heated, for example to a temperature of 40°C, or even up to 60°C. It is considered that a high temperature promotes gelation. The aqueous medium is for example water, or predominantly water (volume fraction of water > 80% or > 90%).

[0077] Preferably, the aqueous medium comprises a compound carrying an amine function, for example ammonia, the volume fraction relative to water possibly being between 0.1 and 5%, preferably 0.1% - 1% or 2%. The addition of the compound carrying the amine function makes it possible to accelerate gelation. In the absence of such a compound, the formation of small silica particles, of the order of 100 nm, has been observed.

[0078] Under the effect of gelling, the drops 12 harden and take the form of balls 1. The solid balls resulting from the gelling sediment and accumulate at the bottom of the enclosure.

[0079] As indicated in step 110, it was found that the presence of an aminosilane (APTES and / or APMS) makes it possible to obtain beads whose shape is regular.

[0080] Step 220: Recovery

[0081] The aqueous medium is filtered by a recovery support, for example a sieve, the mesh size of which depends on the size of the beads formed.

[0082] Step 230: washing

[0083] The recovered beads are washed, for example in a crystallizer containing isopropanol.

[0084] Step 240: Drying

[0085] Following washing, the beads can be dried, for example in an oven at 100°C for a period which may exceed 1 hour or 2 hours.

[0086] Another notable aspect of the invention is that the concentration of water in the sol-gel solution can affect the structure of the beads obtained following gelation, and this regardless of the second embodiment: - When the molar fraction of water is greater than a first limit molar fraction, here equal to 0.65%, while being less than or equal to 1.125%, and preferably less than or equal to 1.05% or 1%, the process allows the majority of solid beads to be obtained. - When the molar fraction of water is less than a second limit molar fraction, here equal to 0.5%, while preferably being greater than 0.05% or 0.1% or 0.2%, the process allows the majority of hollow beads to be obtained. The inventors attribute the formation of hollow beads to the fact that the amount of water in the sol-gel solution causes very rapid hydrolysis when the additional water resulting from the liquid medium is added. Since hydrolysis is exothermic, local heating leads to the formation of a gas. This results in the formation of hollow beads.

[0087] When the molar fraction of water is between the first limit molar fraction (0.65%) and the second limit molar fraction (0.5%), the beads obtained have pores. Beyond the first limit molar fraction, the beads may have pores.

[0088] An advantage of the second embodiment is that it allows the formation of porous beads, i.e. beads comprising pores, and this is all the more so when the water concentration is greater than 0.5% or 0.65%. This makes it possible to obtain beads having a certain opacity. Such beads can be used as an opacifying agent. First series of tests

[0089] A first series of tests was carried out to produce solid beads, the soil drops being sprayed into an aqueous medium, according to the second embodiment. The experimental conditions were: - Distributor 3: PulsaJet gun and UniJet 6501 nozzle (orifice: 0.66mm) supplier Spraying Systems; - Enclosure 7: IL glass test tube, containing a mixture of water and ammonia, at room temperature (20°C) - Precursor: Tetramethoxysilane (TMOS) 98% (Evonik). - Catalyst: HCl (35% vol.) (Thermofisher) in sol solution and ammonia (0.25% by volume) in water.

[0090] 10 mL of TMOS was poured into a 50 mL beaker, kept stirring at room temperature. A solution of 1 mL of deionized water was prepared to which 20 μL of HCl was added. The solution was slowly poured into the beaker containing the TMOS (10 mL). Since the hydrolysis of TMOS is exothermic, the water + HCl mixture was poured at a rate of 2.5 mL / min.

[0091] 1.5 mL of APTES was then added to the beaker. The sol solution therefore contains 0.83% water and 0.1% APTES (molar fractions of TMOS).

[0092] The sol-gel solution, the preparation of which is described in the previous paragraph, was pressurized (1 bar) in a tank 5 connected to the sprayer. The nozzle was immersed, vertically, in the test tube filled with water, to a depth of 3 cm below the water surface. The flow rate was 0.23 L / min. The sol-gel solution was expelled into the water bath. The resulting jet from the gun was atomized by the water present in the test tube.

[0093] It was found that the expelled drops reacted instantly with the water to form solid beads which sedimented, by gravity, at the bottom of the test tube. The beads formed had an average diameter of 45 pm, but could vary from 20 pm to 60 pm.

[0094] The water was poured through a 30 μm sieve (Haver and Boecker). The recovered beads were placed in a crystallizer containing isopropanol diluted to 50% (mass fraction) in deionized water, so as to obtain a wash. Following the wash, the crystallizer was placed in an oven at 100 °C for 2 hours to dry the beads.

[0095] The density of each bead was 1.0, a value to be compared to the density of a “traditional” sol gel (1.7) or glass (2.3). The low density of each bead is due to the formation of pores in the beads, inducing opacity of the latter by Mie diffusion.

[0096] [Fig.6A] shows the beads, in isopropanol, observed by optical microscopy (magnification 100). [Fig.6B] shows the beads, dry, observed by optical microscopy (magnification 40). [Fig.6B] allows the opacity of the beads to be appreciated, due to the diffusion of light by the pores formed inside the beads.

[0097] The pressure in the sol-gel solution reservoir was increased to 2 bars, i.e. a flow rate of 0.31 L / minute. Microspheres with an average diameter of 25 μm were obtained. [Fig.6C] shows the beads, in isopropanol, observed by optical microscopy (magnification 100).

[0098] The pressure in the sol-gel solution reservoir was reduced to 0.4 bar, i.e. a flow rate of 0.12 L / minute. Microspheres with an average diameter of 95 μm were obtained. [Fig.6D] shows the beads, in isopropanol, observed by optical microscopy (magnification 100).

[0099] Figures 6A, 6C and 6D are directly comparable, since they were acquired using the same observation method. They make it possible to assess the possibility of varying the size of the beads as a function of the spraying parameters. Second series of tests

[0100] During these tests, the second embodiment was implemented. The experimental conditions were: - Distributor 3: PulsaJet gun and UniJet nozzle 650067 (orifice: 0.53mm) supplier Spraying Systems; - Enclosure 7: IL glass test tube, containing a mixture of water and ammonia, at room temperature (20°C) - Precursor: Tetramethoxysilane (TMOS) 98% (Evonik). - Catalyst: HCl (35% vol.) (Thermofisher) in sol-gel solution and ammonia (0.25% by volume) in water.

[0101] 10 mL of TMOS was poured into a 50 mL beaker, kept stirring at room temperature. A solution of 0.5 mL of deionized water was prepared to which 10 μL of HCl was added. The solution was slowly poured into the beaker containing the TMOS (10 mL). Since the hydrolysis of TMOS is exothermic, the water + HCl mixture was poured at a rate of 2.5 mL / min.

[0102] 1.0 mL of APTES was then added to the beaker. The sol-gel solution therefore contains 0.4% water and 0.1% APTES (molar fractions relative to TMOS).

[0103] The sol-gel solution, the preparation of which is described in the previous paragraph, was pressurized (2 bar) in a tank connected to the sprayer. The nozzle was immersed, vertically, in the test tube filled with water, at a depth of 3 cm below the water surface. The flow rate was 0.21 L / min. The sol-gel solution was expelled into the water bath. The resulting jet from the gun was atomized by the water present in the test tube.

[0104] It was found that the expelled drops react instantly with water to form solid bubbles with an average diameter of 60 pm, but which can vary from 40 pm to 80 pm. The formation of small solid nanoparticles was also observed, resulting from the formation of "satellite" drops.

[0105] The water was poured through a 30 µm sieve (Haver and Boecker). The collected bubbles were placed in a crystallizer containing isopropanol diluted to 50% (mass fraction) in deionized water, so as to obtain a wash. Following the wash, the crystallizer was placed in an oven at 100 °C for 2 hours to dry the bubbles.

[0106] The average density of each bubble was estimated to be 0.7.

[0107] [Fig.7] shows the beads, in isopropanol, observed by optical microscopy (magnification 40). Third series of tests

[0108] During these tests, the first embodiment was implemented.

[0109] The experimental conditions were: - Distributor 3: PulsaJet gun and UniJet nozzle 650033 (orifice: 0.38 mm) supplier Spraying Systems; - Enclosure 7: IL glass test tube, containing a mixture of water and ammonia, at room temperature (20°C) - Precursor: Tetramethoxysilane (TMOS) 98% (Evonik).

[0110] Catalyst: HCl (35% vol.) (Thermofisher) in sol-gel solution and ammonia (0.25% by volume) in water.

[0111] 10 mL of TMOS was poured into a 50 mL beaker, kept stirring at room temperature. A solution of 1 mL of deionized water was prepared to which 20 μL of HCl was added. The solution was slowly poured into the beaker containing the TMOS (10 mL). Since the hydrolysis of TMOS is exothermic, the water + HCl mixture was poured at a rate of 2.5 mL / min.

[0112] 1.5 mL of APTES was then added to the beaker. The sol solution therefore contains 0.8% water and 0.1% APTES (molar fractions relative to TMOS).

[0113] The sol-gel solution, the preparation of which is described in the previous paragraph, is pressurized (3 bars) in a tank connected to the sprayer.

[0114] The sol-gel solution was expelled into air, the relative humidity of which was 45% higher than room temperature (20°C). The gun was placed in an air enclosure 7, at a height of 3.5 meters above a receptacle (room floor).

[0115] It was found that the expelled drops react instantly with the water present in the air to form solid beads with an average diameter of 100 pm (diameter between 70 pm and 140 pm) moving on the receptacle by gravity. The beads were recovered and placed in a crystallizer containing isopropanol diluted to 50% (mass fraction) in deionized water, so as to obtain a wash. Following the wash, the crystallizer was placed in an oven at 100 °C for 2 hours to dry the bubbles.

[0116] The average density of each ball was estimated to be 1.6.

[0117] [Fig.8A] shows the beads, in isopropanol, observed by optical microscopy (magnification 100).

[0118] The distributor 3 was replaced by a Pulsajet bi-fluid gun, using a feed gas (air) and a J1650-PAj 105-50 nozzle. The feed gas pressure was 1.5 bar. Beads with an average diameter of 30 μm were obtained. [Fig.8B] shows the beads, in isopropanol, observed by optical microscopy (magnification 100). Thus, the modification of the sprayer also makes it possible to modify the characteristics of the beads obtained.

[0119] The beads, whether solid or hollow, resulting from the implementation of the method, can be used for different purposes: for example the manufacture of optical elements (fluorescent beads, diffusing beads, fluorescent beads), or the manufacture of sensors (beads comprising active principles whose optical properties are modified in the presence of a chemical or biological species).

[0120] Although described in connection with a molecular precursor of organosilane type, of TMOS type, the invention can be applied to the manufacture of solid or hollow sol-gel beads, using other types of precursors. It requires a determination, for example experimental, of the threshold water molar fraction, in below which the solution does not gel. Gelation only occurs after the addition of additional water, when each drop of the solution is formed in the medium, gaseous or liquid, containing water molecules. Gelation is all the more accelerated when the sol-gel solution, and / or the gaseous or liquid medium, contains a gelation catalyst, in particular one bearing an amine function.

[0121] It has also been observed that, in particular in the second embodiment: - above a first limit molar fraction of water, the process allows the production of predominantly full beads; - below a second limit molar fraction of water, lower than the first limit fraction, the process allows the production of mainly hollow beads; - between the first limit molar fraction of water and the second limit molar fraction of water, the process allows the production of porous beads.

[0122] When the precursor is TMOS: - the first limit molar fraction of water is 0.65% - the second limit molar fraction of water is 0.5%

[0123] It is considered that by using another organosilane, for example TEOS, the first limiting water mole fraction and the second limiting water fraction can be, as a first approximation, considered to have the same value as those defined for TMOS.

[0124] When another precursor is used, the first limit molar fraction and / or the second limit fraction can be defined experimentally by those skilled in the art, by modifying the molar fraction of water in the sol-gel solution.

[0125] The beads can be integrated into a matrix, for example a polymer, so as to modify the mechanical properties of the matrix. For example, the integration of beads into a polymer can make it possible to increase the compressive strength. This can make it possible to reduce the weight of the polymer. The polymer can for example be a polymer as described in US 11091638. Such a polymer is designated by the term “rheoplex polymer”. Its viscosity increases almost instantly under the effect of an impact. Thus, in the “normal” state, this type of polymer is relatively flexible, while under the effect of an impact, the polymer hardens instantly. The addition of beads makes it possible to improve the compressive strength.

Claims

Claims

1. Method for forming beads (1) by sol-gel method, comprising: a. preparation of a sol-gel solution, comprising a molecular precursor, the sol-gel solution being such that it gels beyond a threshold molar fraction of water, said threshold molar fraction depending on the molecular precursor; b. from the sol-gel solution (2), formation of liquid drops in a medium (6, 7), the medium comprising water in liquid or vapor form; c. following step b), gelling of the drops (12), so that the drops gradually solidify in the medium, forming beads, d. collection of the beads; the method being characterized in that the sol-gel solution comprises a molar fraction of water lower than said threshold molar fraction; the medium providing additional water making it possible to exceed the threshold molar fraction of water, so that the drops gel upon contact with the ambient medium.

2. The method of claim 1, wherein the molecular precursor is a silicon alkoxide, the threshold molar fraction of water being 1.125%, the sol-gel solution comprising a molar fraction of water greater than 0.05% and less than 1.125%, the molar fraction of water being given relative to the molecular precursor.

3. The method of claim 2, wherein the mole fraction of water is less than 1.1% or 1.05% or 1% or 0.9%.

4. Method according to any one of the preceding claims, in which the medium is a gaseous medium comprising water vapor, the method comprising, following step c), a movement of the drops in the gaseous medium, to a receptacle, the duration of the movement of the drops in the gaseous medium being adjusted so that the drops reach the receptacle in the solidified state.

5. The method of claim 4, wherein the gaseous medium is air.

6. The method of claim 5, wherein the relative humidity is greater than 30% or 40%.

7. A method according to any one of claims 1 to 3, wherein the medium is an aqueous liquid medium.

8. A method according to claim 7, wherein the molar fraction of water is greater than a first limit molar fraction, so as to cause the formation of predominantly solid beads, the first limit molar fraction being 0.65%.

9. A method according to claim 7, wherein the molar fraction of water is less than a second limiting molar fraction, so as to cause the formation of predominantly hollow beads, the second limiting molar fraction being 0.5%.

10. A method according to any one of claims 7 to 9, wherein the aqueous liquid medium comprises a gelling catalyst agent.

11. The method of claim 10, wherein the gelling catalyst agent comprises an amine function.

12. A method according to claim 11, wherein the gelling catalyst agent is selected from: ammonia or aminosilane.

13. Method according to any one of the preceding claims, in which the sol-gel solution comprises an aminosilane, for example of the APTES and / or APTMS type, forming a gelling catalyst agent, according to a molar fraction of between 0.005% and 0.3%, preferably between 0.03% and 0.1%, the molar fraction being defined relative to the molecular precursor.

14. A method according to any preceding claim, wherein in step b) the diameter of each drop is less than 10 mm or 2 mm or 1 mm.

15. A method according to any preceding claim, wherein in step b), the diameter of each drop is greater than 100 nm.

16. Ball (1) made of sol-gel material, obtained by applying a method according to any one of the preceding claims.

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