FR3018701A1 - MELAMINE-UREA-FORMALDEHYDE (MUF) PARTICLES CONTAINING A POLYMER HAVING A TG LESS THAN 75 ° C - Google Patents

Abstract

The present invention relates to a particle having a core formed in whole or in part of at least one polymer having a glass transition temperature below 75 ° C, encapsulated in a shell formed of at least one melamine-urea-formaldehyde polymer. It also aims to use such particles to convey a marking material. Finally, it aims at the application of the particles thus charged for purposes including traceability and / or authentication of a support incorporating them.

Description

The present invention relates to the field of core-shell particles used to convey at least one molecule of interest, in particular a marking material.

Marking is an important tool in the context of trademark protection, product traceability and the fight against counterfeiting, for example for the paper, perfume, pharmacy or spirits industries. To authenticate reliably and quickly, a product and through this, to assess whether a given product is or not a counterfeit, it is most often carried out a distinctive and invisible marking of genuine products. The most commonly used marking materials are coloring molecules, luminescent and / or conductive molecules. This ideally unfalsifiable marking is very often visually characterizable at a given wavelength. It is often preferred to use so-called optical markers such as fluorescent molecules, for example, which are invisible to the naked eye, but instead reveal themselves in response to UV exposure. In general, these molecules are immobilized at the level of a support in a form associated with an ancillary material which has a vehicle function for them and allows them to be immobilized in a permanent manner at the level of the support. This material also makes it possible to protect them during storage and makes them compatible with use in a wide variety of media. For example, this ancillary material carrying the marker may be introduced into an ink, paint or varnish which is then deposited on the surface of the support to be treated. However, the marked hydrophobic character of these marking molecules, or even their fragility, significantly limit the conditioning modes that can be envisaged for conveying them. Thus, the document US Pat. No. 6,863,923 describes a printing paste based on a pre-condensate based on polyorganosiloxanes obtained by a sol-gel process and constituting a matrix in which is included at least one coloring, luminescent, conductive and / or catalytic element. . This paste may be applied to a support in the form or pattern desired and densified by heat treatment, at a temperature below the glass transition temperature of the matrix thus formed. Nevertheless, the densification temperature to be applied in this process is of the order of 4000 ° C., which constitutes a serious limitation from the point of view of the useful molecules of interest, for example of usable chromophores. For its part, the document WO 2004/048451 proposes hydrophilic polymers that can be used as inks for security and anti-fraud applications, and in which fluoresor proteins of proteorhodopsin are integrated. However, the manufacture and preservation of these inks require low temperatures, of the order of 4 ° C, hardly compatible with industrial applications.

Consequently, there remains a need for a useful technology for conveying molecules of interest and immobilizing them at the level of a support, and which is devoid of the aforementioned limitations. More specifically, there is a need for a material capable of being loaded with active, including marking, according to an easy and non-limiting implementation technology.

There is also a need for a useful material as a vehicle with respect to a molecule of interest and whose immobilization at the level of the support treated does not require operating conditions specific to each support. The present invention aims precisely to meet these needs. Thus, according to one of its aspects, the present invention relates to particles having a core formed wholly or partly of at least one polymer having a glass transition temperature of less than 75 ° C, encapsulated in a shell formed of at least one melamine-urea-formaldehyde polymer. Unexpectedly, the inventors have indeed found that the core-shell architecture particles in accordance with the invention are particularly advantageous for conveying in their core molecules of interest, in particular a marking material and in particular an optical marker For the purposes of the invention, an optical marker is a molecule which under the action of a stimulus, for example a pH change, a temperature change, or a UV or IR spectral exposure, generates a new chromatic effect, by For example, a color change or a luminescent effect of a definite duration. This optical marker may especially be chosen from organic or inorganic coloring agents, phosphors, fluorophores, thermochromic and photochromic.

The invention also relates to a method for preparing particles according to the invention comprising at least the steps of: (i) having an organic phase containing at least one polymer having a glass transition temperature below 75 ° C. in the solute state, (ii) having an aqueous phase containing at least the melamine, urea, and formaldehyde monomeric entities for forming the melamine-ureaformaldehyde (MUF) polymer, (iii) bringing into the presence of the phase organic and aqueous phase under conditions conducive to the formation of an emulsion of organic phase type in aqueous phase, (iv) perform a heat treatment of the emulsion to activate and / or stimulate the polymerization of monomeric entities and thus form said MUF polymeric bark particles whose core contains the polymer having a glass transition temperature of less than 75 ° C, and (v) isolating said particles the.

Preferably, in step (i), the organic phase contains at least one polymer having a glass transition temperature of less than 50 ° C. According to a particular embodiment of the invention, the heat treatment of step (iv) consists of heating at a temperature of between 28 ° C. and 100 ° C., preferably between 50 ° C. and 99 ° C.

According to a preferred embodiment of the invention, the method further comprises a dialysis step (vi). The invention also relates to a method for charging a molecule of interest to the core of a particle according to the invention, characterized in that it comprises the impregnation of said particle in supercritical CO2 medium containing one or more molecule (s) interest. In the remainder of the text, the expressions "between ... and ...", "ranging from ... to ..." and "varying from ... to ..." are equivalent and mean to mean that terminals are included unless otherwise stated.

Unless otherwise indicated, the expression "comprising / including a" shall be understood as "comprising / including at least one".

Particles The particles have a core-shell structure in which the melamine-urea-formaldehyde (MUF) shell contains at least one polymer having a glass transition temperature of less than 75 ° C, preferably less than 50 ° C.

Preferably, the particles according to the invention are water-dispersible. The particles according to the invention may be of nano or micron size and / or advantageously have a mean size of between 50 nm and 200 μm. Preferably, the average particle size is between 1 μm and 20 μm. At the nanometer scale, the particle size is preferably between 50 nm and 200 nm. This size can be determined by scanning electron microscopy. The size of the factors of the invention may in particular be adjusted by the parameters chosen for the polymerization. A prolonged duration of the polymerization will result in increasing the size of the particles.

In general, the particles according to the invention are non-porous. Advantageously, the polymer having a glass transition temperature below 75 ° C. and the MUF are combined in a polymer / MUF weight ratio ranging from 1% to 10 ° A.

The melamine-urea-formaldehyde (MUF) polymer The melamine-urea-formaldehyde (MUF) is a thermosetting polymer belonging to the family of aminoplasts, polymers that are very widespread and widely used in many fields, for example in the wood industry. as adhesives, in the manufacture of electrical equipment or kitchen utensils as coating including food grade, in the manufacture of varnish, paint ... Such a material is advantageous for several reasons: - it does not is not colored which makes it possible to use it as surface material, - it has a very high hardness and rigidity as well as a very high resistance to abrasion, - it has a very good thermal and chemical resistance, - it has a very good resistance to light.

According to the process of the invention, the melamine-urea-formaldehyde polymer is formed by a polymerization or condensation reaction between melamine (M), urea (U) and formaldehyde (F), around of the heart of interest. This synthesis, particularly illustrated in example 1 below, clearly falls within the skills of those skilled in the art. Advantageously, the units (M), (U) and (F) are used in proportions by weight equal to 2.5 / 1 / 8.5. The mechanical properties of the particle are in particular proportional to the proportion of formaldehyde. The MUF particles can in particular be obtained according to the method described by Liu, X. et al (Liu, X, et al., Macromolecular Materials and Engineering, 2009, 294, 389-395). According to a particular embodiment, the particle bark may also include silica or organo-silica. Their presence in the reaction medium during the polymerization of the monomeric entities dedicated to form the MUF will lead to their incorporation into the bark. The presence of these ancillary materials is particularly advantageous for conferring, if necessary, surface functionalities that are ancillary to the particles according to the invention via specific coupling agents, such as, for example, trimethoxy silane.

Polymer having a glass transition temperature of less than 75 ° C. The polymer forming all or part of the core of the particles preferably has a glass transition temperature of between 30 ° C. and 75 ° C., preferably between 30 ° C. and 50 ° C. ° C.

In the context of the present invention, the term "glass transition temperature" or "Tg" means the change of state of the polymer or of the polymerized material, under the action of the temperature, causing significant variations in its mechanical properties. . Below this temperature, the polymer is said to be vitreous (solid state) and exhibits the behavior of an elastic solid body. Above this temperature, the polymer shows a behavior of plastic solid (viscoelastic state), following the weakening of intermolecular bonds (Van der Waals force ...).

The polymers that are suitable according to the invention also prove to be advantageously compatible with an impregnation process in a supercritical medium and more particularly in a supercritical CO2 medium. Thus, the polymer forming the core of the particles according to the present invention can be advantageously chosen from polyethylene glycols, polyvinylpyrrolidones, polyethylmethacrylates, polyvinyl alcohols, polystyrenes, latices, polyethylenes, polypropylenes and their mixtures. In particular, the polymer forming the core of the particles is chosen from polyethylene glycol, polyoxyethylene stearate, poly (vinylidene co-vinyl chloride), poly (styrene-co-maleic anhydride), polyvinylpyrrolidone, poly (vinyl butyral-co- vinyl alcohol-co-vinyl acetate), poly (maleic anhydride-act-1-octadecene), polyvinyl chloride and polyvinyl alcohol. Preferably, it is a polyethylene glycol, in particular with a molecular weight ranging from 400 g.mo1-1 to 40,000 g.mo1-1 and in particular about 6000 g.mo1-1. The present invention further provides a process for the preparation of particles comprising the following steps: (i) having an organic phase, in particular immiscible with water, containing at least one polymer having a temperature of glass transition less than 75 ° C, especially in the solute state, (ii) having an aqueous phase containing at least the melamine, urea and formaldehyde monomer entities, intended to form the melamine-ureaformaldehyde (MUF) polymer (Iii) contacting the organic phase and the aqueous phase under conditions conducive to the formation of an organic phase emulsion in aqueous phase, (iv) thermally treating the emulsion to activate and / or to stimulate the polymerization of the monomeric entities and thus to form said particles with polymeric MUF bark and whose core contains the polymer having a temperature of 30 ° C. vitreous temperature below 75 ° C, and (v) isolate said particles.

Organic Phase The process according to the invention firstly requires an organic phase containing the polymer (s) having a glass transition temperature below 75 ° C to be encapsulated.

The choice of the organic solvent or mixture of organic solvents is therefore advantageously chosen with regard to the nature of the polymer having a glass transition temperature below 75 ° C., to be treated according to the invention. Moreover, it is also chosen in view of its lack of solubility in an aqueous medium to precisely obtain a corresponding emulsion when it comes into contact with the aqueous phase.

This choice is within the skill of those skilled in the art. The solvent may or may not be volatile and is preferably chosen from dichloromethane, cyclohexane, toluene or heptane. The organic phase may comprise from 1% to 20% by weight of polymer (s) having a glass transition temperature of less than 75 ° C relative to its total weight. Aqueous phase The process according to the invention also requires the provision of an aqueous phase.

This aqueous phase contains at least melamine (M), urea (U), and formaldehyde (F), as defined above. The aqueous phase may comprise water and optionally further contain a water-soluble polymer. By "water-soluble polymer" is meant in the context of the present invention a liquid compound at room temperature and miscible with water (miscibility in water greater than 50% by weight at 25 ° C and at atmospheric pressure). The water-soluble polymers that can be used in the compositions according to the invention may or may not be volatile. Among the water-soluble polymers that can be used in the compositions in accordance with the invention, mention may be made especially of polyvinyl alcohol (PVA), polyoxazoline, polyvinylpyrrolidone (PVP), gelatin, cellulose and hydroxyalkylcellulose, chitosans, and chitin.

In particular, polyvinyl alcohol makes it possible to obtain particles of micrometric size, because of its ability to stabilize larger particles. As is apparent from the foregoing, the MUF and the polymer (s) having a glass transition temperature below 75 ° C are contacted in controlled amounts so that the particles have a polymeric weight ratio (s). ) / MUF ranging from 1% to 10 ° A. Surfactant According to a preferred variant of the invention, the emulsion is formed in the presence of at least one surfactant Advantageously, the surfactant (s) is (are) present (s) initially in the aqueous phase. The surfactants that are suitable for the invention may be chosen from any of the classes of surfactants (anionic, cationic, nonionic or amphoteric). More specifically, they may be chosen from the surfactants conventionally used in the emulsification processes of the organic phase under consideration. Thus, to obtain emulsions of organic phase type in water, surfactants having a hydrophilic / lipophilic balance (HLB) greater than 14 are chosen.

The surfactants that may be used according to the invention are advantageously non-amphoteric and may be more particularly chosen from anionic, cationic and nonionic surfactants. Advantageously, the surfactant (s) are solubilized beforehand in the aqueous phase. The surfactants are used in the process according to the invention in a reduced amount and preferably between 0.1% and 10% by weight, expressed relative to the weight of the aqueous phase. The surfactant is, for example, chosen from sodium dodecyl sulphate (SDS), cetyltrimethylammonium bromide, poloxamers as sold under the trade names Pluronic® by the company BASF, and the surfactants marketed under the names IGEPAL® and Tritons® X. According to a preferred embodiment of the invention, the surfactant used is sodium dodecyl sulphate (SDS).

Emulsion The two phases, organic and aqueous, are brought into contact under conditions conducive to the production of a stable emulsion at room temperature, with stirring. Stirring is maintained at a slow rate when adding the polymer (s) having a glass transition temperature below 75 ° C and then increased to vigorously stir and produce the stable emulsion. Advantageously, the stirring is maintained at 300 rpm and then increased to 500 rpm to obtain the stable emulsion. This agitation, usually mechanical, is extended by a sufficient time to obtain the expected emulsion. According to a particular embodiment, stirring is maintained for 10 minutes.

At the end of this step, an emulsion, formed of droplets of organic phase dispersed in an aqueous phase, is obtained. It is within the abilities of those skilled in the art to adjust the size of the droplets by modulating the nature of the organic solvent, the surfactant (s) retained (s), and the water-soluble polymer, but also by modulating the stirring speed used.

The method according to the invention then comprises a step of heating the emulsion, with stirring, for a time sufficient to transform the droplets of the emulsion in the form of particles. The units M, U, and F polymerize to form the MUF which is organized around the core of interest to form the expected particles.

The heating considered in the process according to the invention can be carried out at a temperature ranging from 28 ° C. to 100 ° C., preferably ranging from 50 ° C. to 99 ° C. According to a particular embodiment, the emulsion obtained is heated at 86 ° C. for 180 minutes with continuous stirring at 500 rpm with the addition of 10 ml of water every 60 minutes to compensate for the amount of water evaporated during the treatment. Thermal. The method according to the invention further comprises a step of isolating the particles obtained in the previous step. They can in particular be isolated by tangential or frontal filtration of the liquid medium containing it. The particles of smaller sizes can in particular be isolated by ultrafiltration. These particles advantageously undergo centrifugation in an alcoholic medium.

Finally, the particles are preferably dialyzed for several days. Application of the particles according to the invention as a vehicle for at least one molecule of interest As mentioned above, a particular interest of the particles according to the invention is their compatibility with a mode of impregnation of the polymer forming their core in a CO2 medium. supercritical. A process for impregnating a polymer with a supercritical solvent indeed offers the significant advantage of being free from organic solvents, used for traditional impregnation methods and which are likely to pose toxicity problems more or less large scale. The inventors have thus found that the core-shell architecture particles according to the invention lend themselves to this charge phenomenon by impregnation in supercritical CO2 medium. Supercritical CO2 is particularly interesting for adsorption impregnation in view of its high diffusivity, its high density, and its low viscosity. Unexpectedly, the material forming the bark of the particles does not impede the impregnation of the polymer, forming the core of the particles, with a molecule of interest in supercritical CO2 medium. Thus, another object of the present invention relates to a method useful for charging at least one molecule of interest in the core of a particle according to the invention comprising impregnating said particle in supercritical CO2 medium containing one or more molecules (s). ) of interest. More specifically, the present invention also provides a method useful for preparing a particle according to the invention charged with at least one molecule of interest, comprising at least the steps of: - disposing of said molecule of interest in supercritical CO2 medium, bringing said CO2 medium containing said molecule into contact with at least one particle according to the invention under conditions conducive to the impregnation of the polymer forming the core of said particle with said molecule, and recovering the charged particles as a molecule of interest .

For example, said CO2 medium containing said molecule can be brought into contact with at least one particle according to the invention under a pressure of between 200 bar and 350 bar, preferably between 250 bar and 300 bar. According to a particular embodiment, said CO2 medium containing said molecule is brought into contact with at least one particle according to the invention at a temperature between 70 ° C. and 90 ° C., preferably equal to 80 ° C., more particularly during a duration of between 45 minutes and 75 minutes, preferably equal to 60 minutes. According to another particular embodiment, the placing in the presence of said CO2 medium containing said molecule with at least one particle according to the invention is carried out in the presence of a co-solvent, for example tetrahydrofuran (THF). Molecule of interest The particles considered according to the invention can be further loaded in their core with at least one molecule of interest.

These molecules of interest may be any compound having an affinity with the polymer having a glass transition temperature below 75 ° C forming their core. In this respect, the particles according to the invention are particularly advantageous for transporting hydrophobic molecules.

In view of the field of application more particularly considered according to the invention, namely that of the marking, the particles according to the invention prove to be particularly interesting for conveying a great diversity of markers, and in particular of optical markers. Thus, according to the invention, a molecule of interest may be a marking material and preferably an optical marker.

As specified above, the invention therefore also relates to the use of a particle according to the invention and loaded in its core with at least one marking material, as a marking tool. The particles according to the invention have the advantage of not harming the reactivity of the molecule of interest that they carry in their heart. In the case of an optical marker, the latter remains characterizable although located in the heart of said particle. These optical markers may especially be chosen from organic or inorganic coloring agents, phosphors or fluorophores, thermochromes, fluorophores, or photochromes. Among the organic or inorganic coloring agents, mention may be made in particular of: azo dyes, for example N-ethyl-N- (2-hydroxyethyl) -4- (4-nitrophenylazo) aniline as sold under the name Disperse Red 1 by Sigma Aldrich, Disperse Red 19 dye marketed by Sigma Aldrich, 4- (4-nitrophenylazo) aniline as sold under the name Disperse Orange 3 by Sigma Aldrich, 244- (2-Chloro). 4-nitrophenylazo) -Nethylphenylamino] ethanol as sold under the name Disperse Red 13 by the company Sigma Aldrich, and the Sudan Red 7B dye as marketed by Sigma Aldrich; triazine dyes, for example 4,41,4 "-s-triazine-2,4,6-triyl-tribenzoic acid, anthraquinone dyes, for example 1-amino-9,10-dihydro-9 Sodium 10-dioxo-4- (2,4,6-trimethylanilino) anthracene-2-sulphonate as sold by Sigma Aldrich under the name Acid Blue 129, 1-aminoanthraquinone, Alizarin Red S for example sold by the company Sigma Aldrich, or the Blue Foron marketed by the company Clariant - sulphidic dyes, for example the cyclic anhydride of 2-sulphobenzoic acid, or phenol red for example marketed by Sigma Aldrich and carbonyl dyes, for example 1,4-bis (pentylamino) anthraquinone, as sold under the name Oil Blue N by the company Sigma Aldrich, As the phosphor or fluorophore that can be envisaged according to the invention, it is especially possible to mention rhodamine-B-isothiocyanate (RBITC) fluorescein isothiocyan atte (FITC) fluorescein; rhodamine; eosin; pyranine; aminoG; Texas red; the bodipy; cyanine; nanocrystals of ZnO, ZnS, CdSe, InGaP, InP, Si, Ge, GaAs, GaP, GaAsP; rare earth ion doped sulfide, phosphate or vanadate oxide matrices, such as Y 2 O 3: Eu, Y 2 O 25: Eu, BaMgAl 1 O 6 17: Eu, GdBO 3: Eu, YGdBO 3: Eu, YPVO 4: Eu, Gd 2 O 3: Tb , Gd2025: Tb, Y3A15012: Tb, Y25i05: Ce, LaPO4: Tb, Ce; semiconductor matrices or transition metal-doped oxides, such as ZnS: Mn, ZnS: Au, ZnS: Al, ZnS: Ag, ZnO: Ag, ZnO: Cu, ZnO: Mn, Zn25iO4: Mn , Al 2 O 3: Cr, Al 2 O 3: Ti. As for the thermochromic compound (s), it (s) can be chosen from diacetylenic compounds, crystal violet lactone or vanadium dioxide V02. Among the photochromic compounds, there may be mentioned benzopyrans, naphthopyrans, spirobenzopyrans, spironaphthopyrans, spirobenzoxazines, spironaphthoxazines, fulgides and fulgimides.

The present invention also relates to a support material comprising a particle as defined above, and loaded in its core with at least one molecule of interest. Advantageously, it may be an ink or a varnish. The particles according to the invention are then preferably loaded at their core in at least one marking material and more particularly optical. Other characteristics, advantages and modes of application of the particles and of the preparation method according to the invention will become more apparent from the exemplary embodiments of the invention and the examination of the attached figures, presented by way of illustration and not limitation of the In the field of the invention and in which: - Figure 1 shows MUF capsules incorporating PEG, scanning electron microscopy, the capsules having a size between 100 nm and 2 pm; FIG. 2 represents MUF capsules incorporating PEG and loaded by an optical marking molecule after supercritical CO2 impregnation. The particles are intact and have retained the same dimensions as before the incorporation of the optical marking molecule.

EXAMPLE Synthesis of 1VIUF-PEG Particles According to the Invention A solution Si is prepared from 1.2 g of urea (M = 60.06 g.mol1-1, Sigma Aldrich) in 60 ml of distilled water at room temperature. ambient, with stirring (100 rpm), for 5 minutes. At the same time, a solution S2 is prepared from 5 g of polyethylene glycol (PEG) (M. 6000, Sigma Aldrich) in 60 ml of dichloromethane (M = 84.93 g.mol -1, Sigma Aldrich). A solution S3 is also prepared from 7.62 g of melamine and 14 g of formaldehyde (37% by weight solution) in 140 ml of distilled water and then heated at 70 ° C for 25 minutes to obtain a pre-solution. Melanin-formaldehyde polymer MF. In a reaction beaker, the solution 51, 60 ml of polyvinyl alcohol solution (PVA) (6.3% by weight solution), and 60 ml of sodium dodecyl sulphate solution (SDS) (0.5% solution by weight) are added to the solution S3 with stirring at 300 rpm. The 60 mL of PEG solution (solution S2) is slowly added to the above mixture, the stirring speed being increased to 500 rpm. This stirring step is continued for 10 minutes at room temperature to form a stable emulsion, and then the solution is heated to 86 ° C to allow growth of the shell polymer around the PEG.

The reaction is maintained for 180 minutes with continuous stirring with the addition of 10 mL of distilled water every 60 minutes to replace the amount of water evaporated. After 3 hours of reaction, the particles are recovered and washed with ethanol using successive centrifugations lasting 10 minutes at 8000 rpm.

The particles are then dialyzed for 3 days. The particles obtained are characterized by scanning electron microscopy. They have a size between 100 nm and 2 iam. The particles are then stained with supercritical CO2 and then again characterized by scanning electron microscopy. After staining, it is noted that the particles are intact and have the same dimensions as before the staining (Figures 1 and 2).

Claims (16)

REVENDICATIONS1. A particle having a core formed in whole or in part of at least one polymer having a glass transition temperature of less than 75 ° C, encapsulated in a shell formed of at least one melamine-urea-formaldehyde polymer. 2. Particle according to the preceding claim, characterized in that it is hydrodispersible. 3. Particle according to any one of the preceding claims, characterized in that it has a mean size of between 50 nm and 200 μm, preferably between 1 μm and 20 μm or between 50 nm and 200 nm. 4. Particle according to any one of the preceding claims, characterized in that the weight ratio between the polymer having a glass transition temperature of less than 75 ° C and the melamine-urea-formaldehyde polymer varies from 1 ° A to 10 ° A . 15 5. Particle according to any one of the preceding claims, characterized in that the polymer forming all or part of the core has a glass transition temperature of between 30 ° C and 75 ° C, preferably between 30 ° C and 50 ° C. ° C. 6. Particle according to any one of the preceding claims, characterized in that the polymer forming all or part of the core is chosen from polyethylene glycols, polyvinylpyrrolidones, polyethylmethacrylates, polyvinyl alcohols, polystyrenes, latexes, polyethylenes, polypropylenes and mixtures thereof. 7. Particle according to any one of the preceding claims, characterized in that the polymer forming all or part of the core is polyethylene glycol. 8. Process for preparing particles as defined in any one of claims 1 to 7, characterized in that it comprises the following steps: (i) having an organic phase containing at least one polymer having a temperature less than 75 ° C, in the solute state, (ii) having an aqueous phase containing at least the melamine, urea, and formaldehyde monomer entities for forming the melamine-ureaformaldehyde (MUF) polymer, (iii) bringing the organic phase and the aqueous phase into contact with conditions conducive to the formation of an emulsion of organic phase type in aqueous phase, (iv) effecting a thermal treatment of the emulsion to activate and / or stimulate the polymerizing the monomeric entities and thereby forming said MUF polymeric shell particles and having a core containing the polymer having a glass transition temperature below 75 ° C, and (v) isolating said particles. 9. Method according to the preceding claim, characterized in that the heat treatment of step (iv) consists of heating at a temperature between 28 ° C and 100 ° C, preferably between 50 ° C and 99 ° C . 10. Method according to any one of claims 8 to 9, characterized in that it further comprises a step (vi) of dialysis. 11. Particle as defined in any one of claims 1 to 7, characterized in that it is further charged in its core with at least one molecule of interest. 12. Particle according to claim 11, characterized in that the molecule of interest is a marking material and preferably an optical marker. 13. A method for charging a molecule of interest in the core of a particle as defined in any one of claims 1 to 7, characterized in that it comprises the impregnation of said particle in supercritical CO2 medium containing one or more several molecule (s) of interest. 14. Use of a particle according to claim 12 or 13 as a marking tool. 15. Support material comprising at least one particle according to claim 12 or 13. 16. Material according to claim 15, characterized in that it is an ink or a varnish.