FR3027606A1 - PROCESS FOR PRODUCING A LUMINESCENT BLACK INK AND LUMINESCENT BLACK INK THUS OBTAINED - Google Patents

Abstract

A method of producing a black luminescent ink comprising the following successive steps: - providing carbon-based particles, - impregnating the carbon-based particles with a supercritical fluid, in the presence of a luminescent element.

Description

Process for producing a black luminescent ink and black luminescent ink thus obtained.

TECHNICAL FIELD OF THE INVENTION The invention relates to a process for producing a black luminescent ink and also relates to a black luminescent ink thus obtained. State of the art An increasing number of products are subject to fraudulent reproduction.

In order to detect imitations, so-called "safety" fluorescent inks are increasingly used. Fluorescence remains the cheapest form of labeling and the most widespread in industry. The presence of a fluorescent element, in a black ink, makes it possible to prove the authenticity of a product. Products marked with inks can be, for example, tickets, or official documents (stamp, driving license ...). The production of a tracer both black and fluorescent is therefore particularly interesting for anti-counterfeiting marking. The black inks are formed of functionalized nanoparticles or carbon particles, doped with a fluorescent element, also called fluorophore. Classically, the marking of black particles by a fluorophore is performed during the synthesis of said particles. The fluorophore is generally encapsulated in the particle.

However, the carbonaceous particles of black inks absorb all or almost all optical radiation, including excitation energy, which annihilates the phenomena of fluorescence, by "quenching", significantly decreasing, or even totally, the fluorescence of the fluorophores.

The fluorescence intensity emitted by the printed patterns on the substrate and therefore very low or zero, and therefore impossible to authenticate. The synthesis of fluorescent nanoparticles based on carbon has also been described in the article by Kainz et al. (Chem Mater 2011, 23, 3606-3613).

The nanoparticles have a cobalt core and a carbon shell. They are functionalised on the surface by pyrene / BODIPY hybrid groups. The functionalization is carried out either by covalent bond or non-covalent binding of the "n-n stacking" type. The nanoparticles obtained are fluorescent.

However, in the case of non-covalent bonds, the Bodipy molecules are dissociated from the nanoparticles at the first wash. In the case of nanoparticles functionalized via covalent bonds, the presence of pyrene makes it possible to move the Bodipy molecules away from the surface of the carbon nanoparticles and to reduce the quenching phenomena. However, nearly a third of the fluorescence is nevertheless lost. In addition, this synthesis process is complex and expensive because it requires a first organic synthesis of the pyrene / BODIPY hybrid groups and then the functionalization of the Co / C nanoparticles.

There is therefore a real need for the development of an anti-counterfeiting marking technology for black inks. Object of the Invention The object of the invention is to overcome the drawbacks of the prior art and, in particular, of to propose a process of elaboration, simple and easy to implement, making it possible to form a luminescent black ink. This object is achieved by a process for producing a black luminescent ink comprising the following successive steps: - providing carbon-based particles, - impregnating the carbon-based particles with a supercritical fluid, in the presence of an element luminescent. This object is also achieved by a black luminescent ink, obtained according to the method, the ink being formed of nanoparticles based on carbon, the nanoparticles comprising a luminescent element. BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention given by way of nonlimiting example and represented in the accompanying drawings, in which: FIG. a graphical representation of the particle size distributions obtained according to different embodiments of the invention; FIGS. 2 and 3 represent emission and fluorescence spectra of doped particles produced according to different embodiments of the method of the invention; DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The method of producing a black luminescent ink comprises the following successive steps: providing carbon-based particles, impregnating the carbon-based particles with a supercritical fluid, in the presence of a luminescent element. The ink is black. By black ink is meant that the ink is formed of carbon-based particles. By carbon-based is meant that the particles comprise from 50% to 90% by mass of carbon. According to an alternative embodiment, the particles could consist of carbon.

The presence of carbon in the particles imparts a black color to the ink. The particles absorb all or almost all the optical radiation, as well as the excitation energy. The ink is both black and luminescent.

Luminescent means that the black carbon-based particles comprise luminescent elements: they are doped by the luminescent elements. The particles are capable of emitting a detectable luminescent signal after excitation. Luminescent means that the ink is photoluminescent: it is fluorescent and / or phosphorescent. Fluorescent means an element having the property of being able to absorb light and to re-emit it at a longer wavelength. The emission ceases when the light excitation ceases. By phosphorescent is meant an element having the property of being able to absorb light and to re-emit it at a longer wavelength. The emission persists for some time when the light excitation has stopped.

Preferentially, the carbon-based particles are hybrid particles formed of a metal and a carbon element. These particles are denoted M / C. Hybrid M / C particles are black.

The carbon-based particles are preferentially synthesized according to a process comprising the following steps: providing a first aqueous solution containing metal ions; adding to the first solution a second solution containing the carbon precursor, so as to form aggregates based on carbon, - subjecting the aggregates to a heat treatment, the heat treatment being carried out at a temperature greater than or equal to 100 ° C and less than or equal to 400 ° C, and preferably at a temperature greater than or equal to 100 ° C and less than or equal to 300 ° C, so as to form the carbon-based particles. Preferably, the carbon precursor is a blowing agent, which avoids using both a carbon precursor and a blowing agent.

Alternatively, the solution could include a carbon precursor and a blowing agent. Porogenic agent is understood to mean an element making it possible to create pores within the particles so as to confer a certain porosity on the particles.

Preferably, the blowing agent is chosen from cyclodextrins and their derivatives. Cyclodextrins are cyclic oligosaccharides. They are composed of 5 to 12 glucose monomers.

The cyclodextrin molecules are in the form of a truncated cone formed of a hydrophobic cavity and activated hydroxyl groups. The cyclodextrin molecules advantageously have good solubility in water. The cylcodextrin may be p-cyclodextrin or γ-cyclodextrin.

By cyclodextrin derivatives is meant cyclodextrins having one or more substitutions on one or more glucose units. It may be, for example, the substitution of a hydroxyl group by an alkyl group, linear or branched, optionally substituted, and having from 1 to 20-I carbon atoms. One or more hydroxyl groups may also be substituted with amines, themselves optionally substituted. The modification of cyclodextrins may make it possible to improve their solubility in water in particular. The pore size of the particles is determined by the size of the cyclodextrin ring, i.e. by the number of glucose monomers. The pore size is thus easily adjustable and controllable. They are, advantageously, all substantially of the same size. The porosity is homogeneous. Even more preferably, the blowing agent is p-cyclodextrin. P-cyclodextrin has 7 units of glucose. P-cyclodextrin has, for example, an internal cavity of 6.0-6.5 Å. These molecules make it possible to obtain porous particles having pores with a diameter of less than 2 nm, and preferably less than 1 nm. The number of pores in the particles will be determined by the ratio between the concentration of metal precursors, ie the metal ions, and the concentration of cyclodextrin in solution.

The molar ratio between the metal precursor and the carbon precursor is between 1 and 10, and preferably of the order of 5. The metal is selected from copper, cobalt, zinc, manganese, nickel, palladium, vanadium, tungsten, platinum, molybdenum, iron and chromium. Preferably, the carbon-based particles are Cu / C particles. According to another embodiment, they could be in Co / C.

Advantageously, the solution is stirred by any suitable means, for example by stirring with a magnetic bar. The reagents are thus homogeneously distributed in solution, which makes it possible to obtain monodisperse particles in size and having pores regularly distributed in the structure.

Advantageously, during the formation of the particles, the pH of the solution is between 7 and 13, and even more advantageously between 8 and 9. Advantageously, the bringing into contact with the metal and carbon-based precursors is carried out at room temperature and at ambient pressure. By ambient temperature means a temperature of the order of 15-30 ° C, and more particularly, of the order of 20-30 ° C. By ambient pressure is meant a pressure of the order of 1 bar. The process is easily industrialized. Advantageously, this process involves few steps, does not require the production and manipulation of inverse micelles and their destabilization. Advantageously, the use of an aqueous solution makes it possible to reduce the waste treatment costs resulting from the process and is inexpensive. According to a particular embodiment, before the heat treatment step, the solution is filtered, so as to recover the aggregates formed. The aggregates are then heat-treated to form the crystallized carbon-based particles.

During the heat treatment, the water is advantageously evaporated. The heat treatment is advantageously carried out for a period of between 30 minutes and 6 hours. Such a time makes it possible to obtain well-crystallized particles.

If the heat treatment temperature is strictly greater than 300 ° C, the cyclodextrin molecules are calcined. Advantageously, when the temperature of the heat treatment is less than 300 ° C., the cyclodextrin molecules are not denatured. Cyclodextrin molecules are disposed in the pores of the particle. The heat treatment can be carried out under air. It does not require the use of any particular gas.

The particles obtained are crystallized, advantageously spherical, and have a regular porosity: ordered and uniformly sized pores, at the nanoscale, and evenly distributed within the particles. The particles are formed of a carbon element and a metal, the metal being selected from copper, cobalt, zinc, manganese, nickel, palladium, vanadium, tungsten, platinum, molybdenum, iron and chrome. The particles preferably have a diameter of less than or equal to 100 nm, and preferably less than or equal to 50 nm.

Such dimensions make it possible to form, advantageously, a stable ink, that is to say a liquid mixture containing particles in suspension, dispersed homogeneously in the liquid phase. Such dimensions also make it possible, advantageously, not to block, obstruct the devices used for depositing the ink. For example, these dimensions avoid clogging the printer heads if the deposit is made by inkjet. The particles advantageously have pores with a diameter of less than 2 nm. Preferably, the pores have a diameter less than or equal to 1 nm. By less than 2 nm, we mean strictly less than 2 nm. Once synthesized, the particles are then impregnated with a luminescent element, also called phosphor, to form a luminescent black ink. By impregnation is meant the action of penetrating and diffusing the luminescent element in the particles. The luminescent element can be inserted into the pores and / or defects of the black particles. It can be a surface impregnation or volume.

Advantageously, it is a volume impregnation, the luminescent element diffusing through the pores of the particles. The impregnation by volume is advantageously homogeneous. After impregnation, the luminescent element is bonded to the particles. The luminescent element can not be dissociated, or very little, particles by a simple washing for example. The luminescent element is not covalently bonded to the carbon-based particle. The labeling is nevertheless more resistant than in the case of non-covalent bonds at the particle surface.

By supercritical fluid is meant a fluid heated above its critical temperature and compressed above its critical pressure. To obtain the supercritical fluid from the fluid, the chamber is thus heated so as to reach a pressure Ps and a temperature Ts, the pressure Ps being greater than the pressure Pc of the critical point of the fluid, the temperature Ts being greater than the temperature Tc of the critical point of the fluid.

The supercritical fluid is advantageously inert with respect to the carbon-based particles. Preferably, the fluid introduced into the reaction chamber is carbon dioxide. Even more preferentially, it is liquid carbon dioxide. It is a solvent called "green", that is to say non-polluting for the environment. The products obtained by the process, involving carbon dioxide, do not generate polluting discharges aqueous or organic, harmful to the environment. Preferentially, the pressure Ps and the temperature Ts are maintained for a period of at least 15 minutes. This duration makes it possible to achieve homogeneous doping.

Advantageously, the duration is between 15 minutes and 7 hours, and preferably the duration is between 1 hour and 7 hours. The choice of the duration makes it possible to fix the depth of impregnation. In the case where the fluid is carbon dioxide, the pressure Ps is between 100bars and 400bars, and still more preferably between 300bars and 350bars. The temperature Ts is between 50 ° C and 200 ° C, and even more preferably the temperature Ts is between 100 ° C and 170 ° C.

These ranges of pressure and temperature make it possible to position themselves beyond the critical point of the fluid and to transform carbon dioxide into supercritical carbon dioxide. The pressure Pc and the temperature Tc of the critical point are respectively 31 ° C and 74bar for carbon dioxide. The supercritical fluid is then absorbed by the particles, causing at the same time the luminescent element.

The supercritical fluid impregnates, doping the particles and forming luminescent particles. The impregnation of carbon-based particles, rather than polymer-based particles, advantageously makes it possible to work at high temperature without altering the carbon matrix.

Once the particles are impregnated, the pressure and the temperature are lowered back below the critical point of the fluid, to ambient temperature and pressure, so as to eliminate the fluid absorbed by the substrate. By ambient temperature is meant a temperature of the order of 20-25 ° C and at ambient pressure, a pressure of the order of 1 bar. According to an alternative embodiment, a solvent, also called co-solvent, can be placed in the reaction chamber. The solvent, or co-solvent, is any solvent added to the reaction chamber, in addition to the fluid to be converted into supercritical fluid. Preferably, the solvent is an organic solvent. The fluorescent element advantageously has a better solubility in an organic solvent. Preferably, the luminescent marker is a fluorescent marker. It is advantageously chosen from inorganic fluorophores and organo-lanthanide complexes. According to another alternative, the fluorescent marker may also be chosen from organic fluorophores. Preferably, the luminescent element is chosen from pyrenic derivatives and thiophene derivatives. According to another embodiment, several luminescent elements are used to dope the particles, so as to form a more complex luminescence signature that is more difficult to copy. .

The amount of fluorescent molecules is selected to be sufficient to impregnate the particles and provide them with detectable fluorescent properties. The particles thus obtained are luminescent. The luminescent molecules are concentrated in the defects and / or pores of the black particles, and their confinement is ensured by transport using the supercritical solvent (002). The luminescent element is not covalently bonded to the carbon-based particle.

The particles are then used to form a luminescent black ink. The ink comprises a solvent in which the particles are dispersed. The solvent may be an aqueous solvent or an organic solvent. The solvent may also be a mixture of water and at least one organic solvent.

According to a particular embodiment, the ink further comprises a binder. The binder may be a polymer, natural or synthetic. Preferably, the binder is chosen from polyvinyl alcohol (PVA) and polyvinyl butyral (PVB).

The binder advantageously makes it possible to improve the adhesion of the particles to the substrate. Those skilled in the art will advantageously choose a solvent that can dissolve the binder in the ink so as to obtain a homogeneous composition. The ink is then deposited on a substrate by any technique suitable for forming a black and luminescent pattern. It may be an inkjet printing technique. After deposition of the ink, a substrate, provided with a black and luminescent pattern, is obtained. The substrate may optionally be annealed to evaporate the residual solvent. Subject to appropriate light excitation, the fluorescence of the particles is detectable by any suitable device. It may be, for example, a portable spectrometer. The presence of luminescence can, in particular, be detected with low cost commercially available detectors. The marking is, advantageously, difficult to identify by a person unfamiliar with the specificity of the marking.

The method for identifying an object, provided with the marking as described above, comprises the following successive steps: subjecting the object to ultraviolet radiation, detecting the fluorescence radiation emitted by said marking, identifying the object in question function of fluorescent radiation detected. For more accurate detection, the fluorescence spectrum of the labeling can be measured by a spectrophotometer to determine the exact position of the fluorescence band.

In the case where the ink used contains several fluorescent elements, several fluorescence peaks will be visible on the fluorescence spectrum. This type of marking, by these specificities, is highly secure and particularly difficult to copy. The detection of marking is easy and it is thus easy to spot imitations.

The production process will now be described by means of the following examples given for illustrative and not limiting. The synthesis of Cu / 8-cyclodextrin particles was carried out.

The precursors of these particles are copper nitrate Cu (NO3) 2.5H20 and p-cyclodextrin. The copper nitrate (1.198 g) is added in deionized water (5 mL) to form a first solution at 5.16 mmol. A second aqueous solution containing p-cyclodextrin (1171 mg or 1.03 mmol) is added to the first with vigorous stirring at room temperature. The molar ratio Cu / (3CD is about 5. The reaction mixture is sonicated, and stirred at room temperature for 2 h, then oven dried at 60 ° C for 48h.

The powder obtained is then calcined in the oven at 300 ° C. for 2 hours under a stream of nitrogen (100 ml / min) to form black nanoparticles. The same protocol was followed using a molar ratio Cu / (3CD of 1.

The particles obtained were observed by dynamic light scattering (or DLS for "Dynamic Light Scattering"). As shown in FIG. 1, the particles have a monodisperse size distribution. The particle size is centered around 20 nm, if the molar ratio Cu / f3CD is about 5, and the particle size centered around 200 nm if the molar ratio Cu / f3CD is 1. The particles were then doped with a luminescent element. The first doping was carried out with a pyrene derivative on the nanoparticles.

The nanoparticles 20 nm in diameter (100 mg) were introduced into a tubular reactor in the presence of 1-pyrenebutanol (50 mg). The tubular reactor is closed and then charged with liquid CO2 until a pressure of 80 bar is obtained. By means of a heating cord, the reactor is heated to 110 ° C., to reach a pressure of 300 bar. The pressure is maintained for 2 hours. After this treatment, the temperature is lowered to room temperature, then the pressure is broken to return to ambient pressure. The material obtained is recovered, washed in a mixture of ethanol and water, centrifuged and dialyzed. The resulting material is oven dried at 60 ° C overnight. The powder is then dispersed in 2-butanone (10 mL), and the fluorescence properties are analyzed.

The material has been characterized by fluorescence spectroscopy. The emission and fluorescence spectra are shown in FIG. 2. The second doping was carried out with a thiophene derivative on the micro particles.

The microparticles of 200 nm in diameter (100 mg) were introduced into a tubular reactor in the presence of 2.2 ': 5'2 "-terthiophene (50 mg) .The tubular reactor is closed and then charged with liquid CO2 until a 70 bar pressure The reactor is heated to 130 ° C. by means of a heating cord to reach a pressure of 340 bar, the pressure is maintained for 4 hours and after this treatment the temperature is lowered to at room temperature, then the pressure is broken in order to return to ambient pressure The material is recovered, washed in EtOH + H20, centrifuged and then dialyzed.

The resulting material is oven dried at 60 ° C overnight. The powder is then dispersed in 2-butanone (10mL), and the fluorescence properties are analyzed.

The material has been characterized by fluorescence spectroscopy. The emission and fluorescence spectra are shown in FIG. 3. Fluorescence tests were also conducted on a solution of 2-butanone containing undoped particles. No fluorescence emission was observed after light excitation of the particles.

The production process uses mild operating conditions. It is simple to implement and inexpensive. The doping of black particles of the ink with a luminescent element according to the method previously described does not influence the color of the final product.

The resulting particles are both black and fluorescent and effectively mark different types of substrates. The particles may be nanoparticles or microparticles.

Claims (22)

REVENDICATIONS1. A method of producing a black luminescent ink comprising the following successive steps: - providing porous particles based on carbon, the pore diameter being less than 2 nm, - impregnating the carbon-based particles with a supercritical fluid, in the presence a luminescent element. 2. Method according to claim 1, characterized in that the carbon-based particles are synthesized from a metal and a precursor of carbon. 3. Method according to claim 2, characterized in that the synthesis of the carbon-based particles comprises the steps of: providing a first aqueous solution containing metal ions, adding to the first solution a second solution containing the carbon precursor, to form aggregates based on carbon, - subjecting the aggregates to a heat treatment, the heat treatment being carried out at a temperature greater than or equal to 100 ° C and less than or equal to 300 ° C, so as to form the particles to carbon base. 4. Method according to one of claims 2 and 3, characterized in that the carbon precursor is a pore-forming agent. 5. Process according to claim 4, characterized in that the blowing agent is chosen from cyclodextrins and their derivatives. 6. Process according to claim 5, characterized in that the pore-forming agent is p-cyclodextrin. 7. Process according to any of the preceding claims, characterized in that the pore diameter of the particles is less than 1 nm. 8. Method according to one of claims 2 to 7, characterized in that the metal is selected from copper, cobalt, zinc, manganese, nickel, palladium, vanadium, tungsten, platinum, molybdenum, iron and chromium. 9. Method according to any one of claims 2 to 8, characterized in that the molar ratio between the metal ions and the carbon precursor is between 1 and 10, and preferably of the order of 5. 15 10. Process according to any one of claims 2 to 9, characterized in that the carbon-based particles are Cu / C particles. 11. Process according to any one of claims 1 to 10, characterized in that the luminescent element is chosen from pyrenic derivatives and thiophene derivatives. 12. Method according to any one of claims 1 to 11, characterized in that the supercritical fluid is carbon dioxide. 25 13. Porous carbon-based particle impregnated with a luminescent element, the pore diameter of the particle being less than 2 nm. 14. Particle according to claim 13, characterized in that the particle 30 is formed of a carbon element and a metal, the metal being selected from copper, cobalt, zinc, manganese, nickel, palladium, vanadium, tungsten, platinum, molybdenum, iron and chromium. 15. Particle according to one of claims 13 and 14, characterized in that the luminescent element is selected from pyrenic derivatives and thiophene derivatives. 16. Particle according to any one of claims 13 to 15, characterized in that the pore diameter of the particle is less than or equal to 1 nm. 17. Particle according to any one of claims 13 to 16, whose diameter is less than or equal to 100nm. 15 18. Particle according to any one of claims 13 to 17, characterized in that cyclodextrin molecules are disposed in the pores of the particle. 19. A black luminescent ink comprising a solvent and particles as claimed in any one of claims 13 to 18. 20. Ink according to claim 19, characterized in that it further comprises a binder. 25 21. A method of marking a substrate with a black luminescent ink according to one of claims 19 and 20, said ink being deposited on the substrate, by an ink jet printing technique, to form a black pattern. and luminescent. 30 22. A black and luminescent patterned substrate obtained by the process of claim 21.