FR3161579A3 - METHODS FOR REDUCING THE PARTICLE SIZE OF GRAPHIC CARBON NITRIDE - Google Patents

Abstract

PROCESSES FOR REDUCING THE PARTICLE SIZE OF GRAPHITICAL CARBON NITRIDE The present invention relates primarily to a process for preparing submicrometer-sized graphitic carbon nitrides, comprising a step of grinding or sonicating the graphitic carbon nitrides. The present invention further relates to a submicrometer-sized graphitic carbon nitride exhibiting a volume size distribution in which particles smaller than 1 μm dominate more than 50% of the total particle volume in the suspension. Figure for abstract: Figure 4

Description

METHODS FOR REDUCING THE PARTICLE SIZE OF GRAPHIC CARBON NITRIDE

The present invention relates primarily to a method for reducing the particle size of graphitic carbon nitride. Furthermore, the present invention relates to reduced-size graphitic carbon nitride particles.

STATE OF THE ART

UV protection is a key factor in cosmetic products. To filter radiation in the UV region (wavelengths below 400 nm), organic and inorganic UV-filtering compounds, such as _TiO₂ and ZnO, are widely used. Graphitic carbon nitride, an inorganic compound, is known to exhibit UV absorption properties. Some documents relating to graphitic carbon nitride have already been published.

For example, patent WO2020/246715 discloses an ultraviolet absorber comprising polymeric carbon nitrides with a heterocyclic structure. However, the document makes no mention of the particle size of the graphitic carbon nitrides.

Furthermore, regarding techniques for reducing the particle size of carbon materials, some documents have already been published.

For example, US patent 7300958B2 discloses a method for manufacturing an ultra-dispersion of nanometer-sized primary carbon particles, comprising: applying a wet milling process to an aggregated structure of said nanometer-sized primary carbon particles to overcome van der Waals forces, by which forces said nanometer-sized primary carbon particles are held together to form said aggregated structure, wherein said wet milling process is performed with beads as a breaking medium, each bead having a diameter less than or equal to 0.1 mm.

Furthermore, patent EP3096866A1 discloses a method for removing mercury, comprising: the injection of activated carbon into combustion gases generated by the combustion of coal, in which the activated carbon has a particle size distribution d95 of between 1 μm and 28 μm and a ratio d95/d50 of between 1.5 and 3, the particle size distribution d95 being obtained by pneumatic grinding and separation.

In addition, patent CN103975468A discloses a composite particle comprising: one or more types of carbon materials selected from the group consisting of (i) a fibrous carbon material, (ii) a chain carbon material, and (iii) a carbon material in which the fibrous carbon material and the chain carbon material are bonded together; with a lithium-containing phosphate; and a method for manufacturing it.

However, the aforementioned documents do not mention graphitic carbon nitrides at all.

Cosmetic makeup compositions are used to give keratinous substances, such as skin, and in particular facial skin, a desired color appearance. The development of a new manufacturing process for graphitic carbon nitrides as environmentally friendly, UV-absorbing materials is needed.

DISCLOSURE OF THE INVENTION

The objective of the present invention is to propose a process for efficiently reducing the particle size of graphitic carbon nitrides in order to obtain submicrometer-sized graphitic carbon nitrides.

Another objective of the present invention is to provide graphitic carbon nitrides with a reduced particle size, which may exhibit improved UV protection properties and a transparent appearance.

The above objective of the present invention can be achieved by a process for preparing submicrometer-sized graphitic carbon nitride particles, in which the graphitic carbon nitride is subjected to a grinding or sonication process.

The grinding stage can be a wet grinding stage.

The wet grinding stage can be a wet jet grinding stage or a wet ball grinding stage.

Sonification can be carried out using a homogenizer.

The process may include the step of grinding or sonicating the graphitic carbon nitrides two or more times.

The process may include an additional step of preparing the graphitic carbon nitrides by heating at least one precursor compound to 450 ℃ or more for at least 1 minute, before the step of grinding or sonicating the graphitic carbon nitrides.

Heating can be carried out in the presence of species containing oxygen, such as _O2 (especially with oxygen flux) and/or humidity.

The present invention further relates to a submicrometer-sized graphitic carbon nitride which has a volume particle size distribution in which the particle size less than 1 μm dominates more than 50% of a total particle volume.

A submicrometer-sized graphitic carbon nitride suspension at a concentration of 0.01 wt% in water may exhibit a turbidity of 300 NTU or less, preferably 200 NTU or less.

The present invention further relates to the use of submicrometer-sized graphitic carbon nitride according to the present invention as an active ingredient in paint, as a pigment, as a filler, in particular for plastics, or as a cosmetic active ingredient, and in particular as a UV absorber.

The present invention further relates to a composition comprising submicrometer-sized graphitic carbon nitride according to the present invention, and water and/or at least one organic medium.

The present invention further relates to a composition, preferably a cosmetic composition for keratinous substances, such as skin, in particular a sunscreen composition, comprising submicrometer-sized graphitic carbon nitride according to the present invention.

FIG. 1 There FIG. 1 illustrates the particle size distribution of graphitic carbon nitride powder in the synthesized state in suspension (A) and the particle size distribution of graphitic carbon nitride powder in suspension according to example 1 (B), measured by dynamic light scattering.

FIG. 2 There FIG. 2 illustrates the UV-visible absorption spectra of suspensions with 0.01 wt% of graphitic carbon nitride powder in the synthesized state and those according to example 1 in water/isopropanol (1 wt% isopropanol).

FIG. 3 There FIG. 3 illustrates the particle size distribution of graphitic carbon nitride powder in the synthesized state in the suspension (A), and the particle size distribution of graphitic carbon nitride powder in the suspension according to example 2 (B), measured by the laser diffraction/scattering method.

FIG. 4 There FIG. 4 illustrates an SEM image of graphitic carbon nitride in the synthesized state (A) and an ADF-STEM image of graphitic carbon nitride according to example 2 with a reduced particle size (B).

FIG. 5 There FIG. 5 illustrates a UV-Visible absorption spectrum of the suspension of graphitic carbon nitride according to example 2 in water at a concentration of 0.1 wt% relative to water.

FIG. 6 There FIG. 6 illustrates the particle size distribution of graphitic carbon nitride powder in the synthesized state in suspension (A) and the particle size distribution of graphitic carbon nitride powder in suspension according to example 3 (B), measured by dynamic light scattering.

FIG. 7 There FIG. 7 illustrates the particle size distribution of graphitic carbon nitride powder in the synthesized state in suspension (A) and the particle size distribution of graphitic carbon nitride powder in suspension according to example 4 (B), measured by dynamic light scattering.

FIG. 8 There FIG. 8 illustrates the absorption coefficient of the graphitic carbon nitride suspension according to example 4 with respect to the synthesized state of graphitic carbon nitride suspension, _TiO2 (average primary particle size: 15 nm) and ZnO (average particle size: 200 nm max.) (A), and the backscattering coefficient over the UV-visible wavelength range of the graphitic carbon nitride suspension according to example 4 with respect to the synthesized state of graphitic carbon nitride suspension, _TiO2 (average primary particle size: 15 nm) and ZnO (average particle size: 200 nm max.) (B).

After extensive research, the inventors surprisingly discovered that the particle size of graphitic carbon nitrides could be reduced by a grinding or sonication process, and obtained smaller graphitic carbon nitrides with improved UV protection and transparency, and thus realized the invention.

Thus, the present invention relates mainly to a process for preparing submicrometer-sized graphitic carbon nitrides, comprising a step of grinding or sonicating the graphitic carbon nitrides.

The inventors of the present invention have surprisingly discovered that the manufacture of submicrometer-sized graphitic carbon nitrides conferred upon the graphitic carbon nitrides improved UV protection and transparency.

Thus, the process according to the present invention can produce submicrometer-sized graphitic carbon nitrides with improved UV protection and transparency.

The present invention will be described in detail below.

[Process]

The present invention relates to a process for preparing submicrometer-sized graphitic carbon nitrides, comprising a step of grinding or sonicating the graphitic carbon nitrides.

The term "graphitic" in the expression "graphitic carbon nitride" here means that the carbon nitride has a structure similar to that of a sheet of graphite. Thus, the graphitic carbon nitride of the present invention may have a layered or laminated structure. Graphitic carbon nitride is generally solid at room temperature and in powder form.

The term "submicrometer size" can be used here to mean that the particles are smaller than 1 μm. Thus, the process according to the present invention can produce an increased concentration of graphitic carbon nitride particles with a particle size of less than 1 μm in the suspension. In this patent application, the particle size in the suspension can be measured, for example, using dynamic light scattering (DLS; Nanotrac Wave EX, MicrotracBEL Corp.) or the laser diffraction/scattering method (LS 13 320 XR, Beckman Coulter Inc.).

The submicrometer-sized carbon nitrides of the present invention may have a volume particle size distribution in which particles smaller than 1 μm dominate by more than 50%, preferably more than 60%, of the total particle volume. Furthermore, in one embodiment, the submicrometer-sized carbon nitrides of the present invention may have an average particle size greater than 0.01 μm, preferably 0.05 μm. In another embodiment, the submicrometer-sized carbon nitrides of the present invention may have an average particle size greater than 0.1 μm.

The expression "average particle size" used here can represent an average diameter in volume-average size which is given by the statistical particle size distribution at half the population, designated by d50. The average diameter in volume-average size and the volume-size distribution of submicrometer-sized carbon nitrides can be measured, for example, by a dynamic light scattering particle size distribution analyzer.

Graphitic carbon nitride will be described in more detail in the section "graphitic carbon nitride" later.

The process according to the present invention comprises at least one step of grinding or sonicating graphitic carbon nitrides. Thus, the process according to the present invention comprises at least one step in which the graphitic carbon nitrides are subjected to a grinding or sonication step in order to produce submicrometer-sized graphitic carbon nitride particles.

For the grinding of graphitic carbon nitrides, it is possible to use all processes, including media-free processes such as wet jet milling, thin film spin mixer and high-speed laminar flow mixer, and media-based processes such as wet or dry ball milling.

Examples of grinding processes include wet grinding and dry grinding. Examples of wet grinding processes include wet jet grinding and wet ball grinding.

The media used in the process according to the present invention may include water and/or organic solvents. The organic solvents may be hydrophilic or hydrophobic. Examples of organic solvents include linear or branched alcohols, such as ethanol, propanol, butanol, isopropanol, and isobutanol.

Wet jet grinding processes are generally classified into two processes: one process involves impacting the suspension including the materials to be ground; another process involves passing the suspension through the nozzle which has a narrow space in order to cause turbulence.

With wet jet milling processes, graphitic carbon nitride suspensions are discharged under high pressure. The pressure application method is not particularly limited. The pressure applied to the suspension can range from 50 MPa to 300 MPa, and the number of passes through the nozzle can range from 1 to 400.

For wet ball milling processes, the peripheral speed can range from 6 m/s to 18 m/s, preferably from 10 m/s to 14 m/s, taking into account practical production requirements. The materials used for the balls are not limited, but examples include metal oxides such as _SiO₂ , glass, _ZrO₂ , and _{Y₂O₃- stabilized ZrO₂} ; metal nitrides such as _Si₃N₄ ; metal carbides such as _WC ; and metals and metal alloys such as steel. The ball diameter can range from 0.01 mm to 2 mm, preferably from 0.05 mm to 1 mm, again taking into account practical production requirements. The grinding apparatus includes any type of apparatus applicable to balls having the aforementioned diameter, such as an annular and disc-shaped apparatus, and their derivatives, and is not limited to the apparatus specified.

Examples of dry grinding processes include high-energy dry jet grinding processes, such as steam jet grinding processes.

Any irradiation or sonication device can be used for the sonication process. In one embodiment, sonication is performed with a homogenizer. The sonication power is not particularly limited but can range from 5 to 5,000 W, preferably from 10 to 1,000 W. The sonication time is not particularly limited but can be up to 200 hours. The sonication temperature is not particularly limited, and sonication is generally carried out at a temperature ranging from 25°C to 80°C.

The process according to the present invention may include the step of grinding or sonicating the graphitic carbon nitrides once or twice, or more.

For example, in ball milling, processes can include one or more steps, while multi-step ball milling, such as two-step milling, can efficiently produce submicrometer-sized graphitic carbon nitride. Examples of two-step milling include milling with balls ranging from 0.3 to 2 mm in diameter, such as 0.3 mm or 0.5 mm, in the first step, followed by milling with balls smaller than 0.3 mm in diameter, such as 0.1 mm, in the second step.

In one embodiment of the present invention, the process according to the present invention may include an additional step of preparing graphitic carbon nitrides for grinding or sonication, as a raw material for the process, prior to the grinding or sonication step of the graphitic carbon nitrides. The preparation of the graphitic carbon nitrides may be synthesized by heating at least one precursor compound.

Thus, the process according to the present invention can be a process for preparing submicrometer-sized graphitic carbon nitrides, comprising:

(i) the preparation of graphitic carbon nitrides by heating at least one precursor compound to 450°C or more for at least 1 minute; and

ii) the grinding or sonication of graphitic carbon nitrides in order to produce submicrometer-sized graphitic carbon nitrides.

In the preparation of graphitic carbon nitrides in the synthesized state, one precursor compound may be used, or two or more precursor compounds may be used in combination.

The precursor compound may be chosen from precursors known to the person skilled in the art, for example, urea, thiourea, melamine, guanidine, arginine, cyanamide, dicyandiamide and one of their salts, and combinations thereof (Chem. Rev. 2016, 116, 7159–7329, Ong, WJ; Tan, LL; Ng, YH; Yong, ST; Chai, SP, Catalysts 2019, 9(10), 805, Seong Jun Mun and Soo-Jin Park; https://doi.org/10,3390/catal9100805). Preferably, the precursor compound is chosen from urea, melamine, guanidine, arginine and one of their salts, and combinations thereof.

The salt of the precursor compound is not particularly limited, but we can mention salts with inorganic acids, such as carbonic acid and HalH, in which Hal represents a halogen atom such as chloride (hydrochloric acid).

In a preferred embodiment of the present invention, a single precursor compound is used for the preparation of graphitic carbon nitride.

The heating temperature of at least one precursor compound is at least 450 ℃. Preferably, heating is carried out at 500 C or more, and more preferably at 525 C or more.

The heating time of at least one precursor compound is at least 1 minute. Preferably, the heating time is at least 10 minutes, more preferably at least 20 minutes, and/or a maximum of 30 hours, and more preferably a maximum of 25 hours.

The heating of the precursor compound can be carried out in air, in a noble gas such as argon or helium, or in an inert gas such as nitrogen. In preferred embodiments of the present invention, the heating of the precursor compound is carried out in air or in argon.

In a preferred embodiment, the heating process can be carried out in the presence of oxygen-containing species, such as _O₂ , moisture, _O₃ , atomic O₂, and/or ionic oxygen, as an oxidizing agent. Without being limited to theory, it is estimated that more porous graphitic carbon nitride can be obtained when heating is carried out in the presence of oxygen-containing species. In the preferred embodiment, the heating is carried out in air, or in a noble gas or an inert gas, including oxygen-containing species.

In a preferred embodiment, in addition to oxygen in the air, the heating process is carried out in the presence of oxygen-containing species, such as _O2 , moisture, ozone _O3 , atomic O and/or ionic oxygen, as an oxidizing agent.

In a preferred embodiment, the heating process is carried out in the presence of _O2- containing species, in particular an oxygen flow and/or moisture. The term "oxygen flow" may mean an oxygen stream in this patent specification.

Preferably, the oxidizing agent used during the heating step is in gaseous form.

According to one embodiment, the oxygen source is neither permanganate salt nor hydrogen peroxide.

In one embodiment of the present invention, the heating process includes at least two heating steps at the same or different temperatures. In other words, the heating process may include a post-heating step. Thus, in one embodiment, the heating process may comprise a first heating step of at least one precursor compound to 450 °C or higher for at least 1 minute, followed by a second heating step of at least one precursor compound to 450 °C or higher for at least 1 minute. The temperature for the first heating step and the temperature for the second heating step may be the same or different, but, in general, the temperature for the second heating step is equal to or higher than the temperature for the first heating step. The temperature and duration of the first and second heating steps are identical to those described above.

In one embodiment of the present invention, a cooling step is included between the heating steps. Thus, in one embodiment, the cooling step is located between the first and second heating steps. The temperature for the cooling step is not specifically limited, but, for example, is reduced to ambient temperature (approximately 25°C). The duration of the cooling step is also not specifically limited, but is, for example, from approximately 1 minute to 24 hours.

In another embodiment of the present invention, the process according to the present invention may include at least one pre-grinding process prior to the grinding or sonication step of the graphitic carbon nitride. The pre-grinding processes may include not only wet grinding processes, but also dry grinding processes using dry ball milling, dry ball milling, and dry jet milling.

In cases where a suspension of submicrometer-sized graphitic carbon nitrides is obtained by the process according to the present invention, the process may include an additional step of drying the suspension to obtain dried submicrometer-sized graphitic carbon nitrides. The drying methods are not particularly limited and may include freeze-drying, spray drying, and fluidized bed sludge drying. The dried submicrometer-sized particles can be resuspended in liquids.

[Graphitic carbon nitride]

The present invention further relates to the graphitic carbon nitride particle, in particular the submicrometer-sized graphitic carbon nitride particle. The submicrometer-sized graphitic carbon nitride particle can be prepared by the process according to the present invention explained above.

The term "submicrometer size" can be used here to mean that the particles have an average particle size of less than 1 μm. Thus, the submicrometer-sized graphitic carbon nitride according to the present invention has an average particle size of less than 1 μm. In this patent application, the particle size can be measured, for example, using dynamic light scattering (DLS; Nanotrac Wave EX, MicrotracBEL Corp.) or the laser diffraction/scattering method (LS 13 320 XR, Beckman Coulter Inc.).

The submicrometer-sized graphitic carbon nitride of the present invention may include at least one heptazine motif in its structure. In this patent application, _the term "heptazine motif" means a heterofused ring consisting of three hetero-rings composed of carbon and nitrogen atoms, represented by _C6N7 . Thus, the graphitic carbon nitride of the present invention may have a heptazine-based monolayer structure. The graphitic carbon nitride of the present invention may include at least one heptazine motif, at least one triazine motif, and combinations thereof. The presence of the heptazine motif can be determined by X-ray diffraction (XRD) analysis, Fourier transform infrared (FT-IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy.

The submicrometer-sized graphitic carbon nitride of the present invention can have a porous structure. More specifically, the graphitic carbon nitride of the present invention can have a nanoporous structure. The pores can exist on the layer of the graphitic carbon nitride between the heptazine and triazine motifs.

The heptazine motif is preferably represented by formula (I), its salts and its solvates such as hydrates:
(I)

1. un atome d'hydrogène,
2. un atome d'halogène,
3. un groupe contenant de l'oxygène, comme un groupe carboxy, nitro ou nitroso,
4. une chaîne hydrocarbonée saturée ou insaturée, acyclique linéaire ou ramifiée et/ou cyclique, aromatique ou non aromatique, contenant de 1 à 10 carbones, ladite chaîne hydrocarbonée étant potentiellement interrompue par un ou plusieurs hétéroatomes tels que O, S, N ou N(O) ;
5. un hydroxy,
6. un amino R⁴R⁵N-, dans lequel R⁴et R⁵, identiques ou différents, représentent un atome d'hydrogène, un groupe alkyle (C₁-C₆) ou un autre groupe heptazine monovalent, de préférence un groupe heptazine monovalent (II)dans lequel R¹et R²sont tels que définis ici précédemment ;
7. R⁴R⁵N(O)-, dans lequel R⁴et R⁵, identiques ou différents, sont tels que définis ici précédemment ; et
8. R⁴-N(O)- ou ; dans lequel R⁴est tel que défini ici précédemment.

Formula (I) in which ^R1 , ^R2 , and ^R3 , identical or different, represent:

1. a hydrogen atom,
2. a halogen atom,
3. an oxygen-containing group, such as a carboxy, nitro or nitroso group,
4. a saturated or unsaturated, linear or branched acyclic hydrocarbon chain and/or cyclic, aromatic or non-aromatic, containing from 1 to 10 carbons, said hydrocarbon chain being potentially interrupted by one or more heteroatoms such as O, S, N or N(O);
5. a hydroxy,
6. an amino ^{R4 R5} N-, in which ^R4 and ^R5 , identical or different, represent a hydrogen atom, an alkyl group ( _C1 - _C6 ) or another monovalent heptazine group, preferably a monovalent heptazine (II) group in which ^R1 and ^R2 are as defined here previously;
7. ^{R4 R5} N(O)-, in which ^R4 and ^R5 , whether identical or different, are as defined here previously; and
8. R ⁴ -N(O)- or ; in which R ⁴ is such as defined here previously.

it being understood that:

- at least one of the radical ^R1 , ^R2 or ^R3 represents v) a hydroxy group, more preferably R1 represents v) a hydroxy group and ^R2 and ^R3 , identical or different, preferably identical, represent iii) a nitroso group chosen from vi) to viii), more preferably viii), and

- one or more nitrogens in the rings can be oxidized (N-oxide or N-OH).

Halogen ii) can be chosen from Cl and Br.

The hydrocarbon chain iv) may be a saturated or unsaturated hydrocarbon chain, preferably saturated, linear acyclic or branched, preferably linear acyclic. The hydrocarbon chain iv) may contain from 1 to 6, preferably from 1 to 4, carbon atoms. Thus, the hydrocarbon chain iv) may be a saturated, linear, acyclic hydrocarbon chain containing from 1 to 6, preferably from 1 to 4, carbon atoms, which may be interrupted by one or more heteroatoms such as O, S, N, or N(O).

Submicrometer-sized graphitic carbon nitride can be suspended in water and in organic media, such as polar oils, including diisopropyl sebacate. Without being limited to theory, it is estimated that the reason why the graphitic carbon nitride of the present invention can be dispersible in both water and oils is that it contains a relatively large number of functional groups produced by discretization and/or fission within its structure, particularly heptazine motifs, which can contribute to a change in surface polarity, such as the hydrophilic and hydrophobic nature of the graphitic carbon nitride surface. It is further estimated that the quantity of functional groups is increased by reducing the size of the graphitic carbon nitride, as this can induce discretization and/or fission within its structure.

The submicrometer-sized graphitic carbon nitride of the present invention exhibits improved UV absorption properties. Preferably, the graphitic carbon nitride exhibits absorption in both the UV-B and UV-A regions. UV-B rays here refer to UV rays with wavelengths between 280 and 320 nm. UV-A rays here refer to UV rays with wavelengths between 320 and 400 nm. An absorption curve over a range of ultraviolet and visible light can be measured, for example, by ultraviolet-visible (UV-Vis) absorption spectroscopy and diffuse reflectance spectroscopy.

The inventors of the present invention have made a surprising discovery: transforming graphitic carbon nitrides into submicrometer-sized particles can improve the UV absorption properties of the suspension due to increased dispersibility. Furthermore, the inventors of the present invention have made a surprising discovery: transforming graphitic carbon nitrides into submicrometer-sized particles can eliminate the opacity caused by the presence of larger particles in the graphitic carbon nitrides.

Thus, the submicrometer-sized graphitic carbon nitride suspension of the present invention exhibits low turbidity. For example, suspensions at a concentration of 0.01 wt% in water may exhibit a turbidity of 300 NTU or less, preferably 200 NTU or less; suspensions at a concentration of 0.005 wt% in water may exhibit a turbidity of 200 NTU or less, preferably 150 NTU or less; and suspensions at a concentration of 0.001 wt% in water may exhibit a turbidity of 50 NTU or less, preferably 30 NTU or less, respectively. The turbidity can be measured, for example, using a portable 2100Q turbidimeter (HACH).

[Use]

The present invention relates to the use of submicrometer-sized graphitic carbon nitride as a UV absorber, particularly a UV-A and/or UV-B absorber, to protect products from UV damage. For example, the UV absorber of the present invention can be used in paints, coatings, and cosmetics.

Since the submicrometer-sized graphitic carbon nitride of the present invention exhibits enhanced UV protection, its use can provide improved protection with the products used. Furthermore, because the graphitic carbon nitride of the present invention is transparent in liquids, it can be used in a wide variety of applications.

[Composition]

The present invention further relates to a composition comprising the submicrometer-sized graphitic carbon nitride of the present invention. Preferably, the composition according to the present invention is a cosmetic composition, and in particular a cosmetic composition intended for a keratinous substance such as skin. In a preferred embodiment, the composition according to the present invention is a sunscreen composition.

Furthermore, the composition according to the present invention can be used as an active ingredient in paint, as a pigment, as a filler in plastics, as a cosmetic active ingredient and/or as a sunscreen, such as a UVA and/or UVB absorber.

The composition according to the present invention preferably does not comprise _TiO₂ or ZnO. In another embodiment, the composition according to the present invention comprises _TiO₂ and/or ZnO in an amount of 5% by weight or less, more preferably 1% by weight or less, relative to the total weight of the composition. The graphitic carbon nitride of the present invention can be used in the composition in place of _TiO₂ and ZnO, which are known as traditional inorganic UV filters.

Since the submicrometer-sized graphitic carbon nitride of the present invention may exhibit an improved UV filtering property, the composition of the present invention may exhibit an improved UV protection effect.

EXAMPLES

The present invention will be described in more detail by means of examples. However, these examples should not be interpreted as limiting the scope of the present invention.

[Assessment]

The following evaluations were performed on samples of graphitic carbon nitride powder.

(Morphological analysis)

The morphology of suspended graphitic carbon nitride was observed by ring dark-field scanning transmission electron microscopy (ADF-STEM) using a liquid cell.

The IR spectrum of the graphitic carbon nitride powder was obtained using the attenuated total reflection (ATR) method. The suspension sample was lyophilized for analysis. The peak attributed to the heptazine motifs appears at 804 ^cm⁻¹ , which is a higher wavenumber than that of the triazine motifs (814 ^cm⁻¹ for melamine, 808 ^cm⁻¹ for melam), according to Nan Liu, et al. (ACS Omega, 2020, volume 5 12557−12567).

(Particle size analysis)

The particle size distribution in the graphitic carbon nitride suspension was evaluated using dynamic light scattering (DLS; Nanotrac Wave EX, MicrotracBEL Corp.) or the laser diffraction/scattering process (LS 13 320 XR, Beckman Coulter Inc.).

(UV absorption property)

The UV-visible absorption spectra of graphitic carbon nitride suspended in liquids in a fine quartz cell (two transparent sides, 2 mm (optical path length) × 10 mm × 45 mm (H), Tokyo Garasu Kikai Co., Ltd.) were collected using a UV-Visible spectrophotometer (V750, Jasco Inc.) coupled to an integrating sphere. The suspension samples were sonicated using an ultrasonic cleaner (ASU-3M, AS ONE Corporation) prior to measurement.

(Absorption and backscattering coefficients)

The transmission and reflection spectra of graphitic carbon nitride suspended in liquids at three different concentrations were measured over the wavelength range of 250 nm to 780 nm using a spectroscopic curve and converted into absorption coefficient (µa) and backscattering coefficient (µs'). The backscattering coefficient over the visible wavelength range, which is greater than 400 nm, represents the degree of opacity.

(Turbidity)

Turbidity in nephelometric turbidity units (NTU) was evaluated using a 2100Q portable turbidimeter (HACH).

(Color appearance)

The colour appearance of each of the graphitic carbon nitrides was evaluated by observing the sample with the naked eye.

[Preparation]

Graphitic carbon nitride according to the present invention was prepared in the following examples 1 to 4, and was evaluated as follows.

Example 1

5 g of melamine powder as a precursor compound were heated to 550 ^° C in air for 2 hours to prepare the graphitic carbon nitride in the synthesized state.

The synthesized graphitic carbon nitride was suspended at a concentration of 0.1 wt% in a water/isopropanol mixture (volume ratio: 99:1). The suspension was then jet-milled once using a jet mill at 70 MPa. Next, the suspension was diluted to 0.01 wt% graphitic carbon nitride by adding the water/isopropanol mixture (volume ratio: 99:1). After dilution, the suspension was jet-milled again at 70 MPa under the same conditions to produce the graphitic carbon nitride suspension as described in Example 1.

There FIG. 1 illustrates the particle size distribution of the graphitic carbon nitride powder in its synthesized state in the suspension, while the FIG. 1 (B) illustrates the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 1, measured by dynamic light scattering. As shown in the FIG. 1 (B), the peak, on which d50 is at 0.24 μm and d90 is at 0.41 μm, appeared in the range of 0.07 μm to 0.82 μm with a frequency of 100%. Thus, submicrometer-sized graphitic carbon nitride with a volume particle size distribution in which the particle size less than 1 μm dominates approximately 100% of the total particle volume was obtained in Example 1.

The UV-visible absorption spectra of the suspensions with 0.01 wt% of synthesized graphitic carbon nitride powder and those according to Example 1 in water/isopropanol (1 wt% isopropanol) are illustrated on the FIG. 2 The suspensions of both samples exhibited absorption in the UV wavelength region below 400 nm, while the sample according to example 1 exhibited higher absorption due to improved dispersibility.

Example 2

The synthesized graphitic carbon nitride was suspended at a concentration of 0.1 wt% in a water/isopropanol mixture (weight ratio: 99:1). The suspension was then wet-jet milled using a 200 MPa jet mill with a circulation time equivalent to 300 passes to obtain ground graphitic carbon nitride as described in Example 2. The suspension was then lyophilized to remove the suspension medium and obtain dried ground graphitic carbon nitride. The dried ground graphitic carbon nitride was then resuspended in water at 0.1 wt% by sonication for 20 minutes.

There FIG. 3 (A) illustrates the particle size distribution of the graphitic carbon nitride powder in the synthesized state in the suspension, while the FIG. 3 (B) illustrates the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 2, measured by laser diffraction/scattering. 67% of the detected size of the graphitic carbon nitride powder according to Example 2 in the suspension was in the range of 0.13 μm to 0.17 μm for the differential volume. Thus, the submicrometer-sized graphitic carbon nitride with a particle size distribution in which the particle size less than 1 μm dominates approximately 67% of the total particle volume was obtained in Example 2.

The turbidity of each suspension, comprising 0.1 wt% of the synthesized graphitic carbon nitride and of the graphitic carbon nitride according to Example 2, was measured. The suspension samples were diluted prior to measurement by adding water to the aforementioned suspensions to obtain concentrations of 0.01 wt%, 0.005 wt%, and 0.001 wt%. The turbidity of the suspensions at concentrations of 0.01 wt%, 0.005 wt% and 0.001 wt% of graphitic carbon nitride in the synthesized state was 549 NTU, 299 NTU and 57 NTU, respectively, while the turbidity of the suspensions at concentrations of 0.01 wt%, 0.005 wt% and 0.001 wt% of graphitic carbon nitride according to Example 2 was 176 NTU, 103 NTU and 26 NTU, respectively.

Based on the morphology of the suspended graphitic carbon nitride shown in Example 2 via ADF-STEM, it was confirmed that the graphitic carbon nitride had submicrometer-sized pores and chain-like fibrous structures. FIG. 4 (A) illustrates a SEM image of graphitic carbon nitride in the synthesized state, while the FIG. 4 (B) illustrates an ADF-STEM image of graphitic carbon nitride according to Example 2 with a reduced particle size in a water/isopropanol mixture (weight ratio: 99/1). It is confirmed that the process according to the present invention can transform large particles of agglomerated graphitic carbon nitrides in the synthesized state into submicrometer-sized particles.

In the IR analysis of graphitic carbon nitride according to example 2, a peak appeared at 806 ^cm⁻¹ , located midway between the peak of heptazine motifs at 804 cm⁻¹ and that of triazine motifs at 808 ^cm⁻¹ for melamine. This result indicates that graphitic carbon nitride according to example 2 exhibits both heptazine and triazine motifs.

The UV-visible absorption spectrum of the graphitic carbon nitride suspension according to Example 2 in water at a concentration of 0.1 wt% is illustrated on the FIG. 5 , compared to water. The suspension exhibited absorption in the UV wavelength region below 400 nm.

Example 3

The same synthesized graphitic carbon nitride as in Example 1 was used in Example 3. The synthesized graphitic carbon nitride was suspended at a concentration of 1 wt% in water. The suspension was then subjected to a two-stage ball mill: stage 1 (balls: 0.3 mm diameter, peripheral speed: 10 m/s then 12 m/s after 10 minutes, duration: 60 minutes) and stage 2 (balls: 0.1 mm diameter, peripheral speed: 14 m/s, duration: 120 minutes), to obtain milled graphitic carbon nitride as described in Example 3.

There FIG. 6 (A) illustrates the particle size distribution of the graphitic carbon nitride powder in the synthesized state in the suspension, while the FIG. 6 (B) illustrates the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 3, measured by dynamic light scattering. Micrometer-sized particles decreased after wet ball milling, and submicrometer-sized particles increased.

Example 4

The same synthesized graphitic carbon nitride powder was suspended at a concentration of 0.1 wt% in a water/propylene glycol mixture (50/50 wt ratio). The suspension was then wet-jet milled three times using a 70 MPa jet mill to obtain ground graphitic carbon nitride as described in Example 4. Finally, polyoxyethylene sorbitan monolaurate (Tween20) was added at a concentration of 0.3 wt%.

There FIG. 7 (A) illustrates the particle size distribution of the graphitic carbon nitride powder in the synthesized state in the suspension, while the FIG. 7 (B) illustrates the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 4, measured by dynamic light scattering. Micrometer-sized particles decreased after jet milling, and submicrometer-sized particles increased.

There FIG. 8 (A) illustrates the absorption coefficient of the graphitic carbon nitride suspension according to Example 4 compared to the synthesized graphitic carbon nitride suspension, _TiO₂ (average primary particle size: 15 nm), and ZnO (average particle size: 200 nm max.). The graphitic carbon nitride according to Example 4 exhibits the highest UV absorption property in the UV-B region and good UV-A absorption property in the UV-A region. Furthermore, the increase in UV absorption property upon grinding the graphitic carbon nitride can be confirmed.

There FIG. 8 (A) illustrates the UV-visible backscattering coefficient of the graphitic carbon nitride suspension according to Example 4 compared to the synthesized graphitic carbon nitride suspension, _TiO₂ (average primary particle size: 15 nm), and ZnO (average particle size: 200 nm max.). The graphitic carbon nitride according to Example 4 exhibits the highest scattering property in the UV region. Furthermore, the suppression of scattering property in the visible light region for the graphitic carbon nitride according to Example 4 is almost identical to that of _TiO₂ and ZnO. This means that the graphitic carbon nitride according to Example 4 exhibits better transparency. In contrast, the synthesized graphitic carbon nitride exhibited high scattering property in the visible light region, indicating that it has high opacity.

Example 5

The same synthesized graphitic carbon nitride as in Example 1 was used in Example 5. The synthesized graphitic carbon nitride was suspended at a concentration of 0.05 wt% in water. The suspension was sonicated using a homogenizer (VIOLAMO SONICSTAR 85, AS ONE Corporation) at 20 W for 7 hours to obtain sonicated graphitic carbon nitride as described in Example 5.

The synthesized graphite nitride suspension was yellow, while the graphite nitride suspension from Example 5 was white and exhibited better dispersibility. This indicates that the particle size of the graphite nitride was reduced by sonication. This result suggests that it is possible to obtain a suspension containing submicrometer-sized graphite nitride particles by sonication using a homogenizer.

As can be seen from the results of the examples, the process according to the present invention can efficiently produce submicrometer-sized graphitic carbon nitrides. Furthermore, the submicrometer-sized graphitic carbon nitride produced according to the examples exhibited improved UV filtering (absorption and scattering) and transparency.

Consequently, it can be concluded that the process according to the present invention is very useful for producing submicrometer-sized graphitic carbon nitride, which is a novel, environmentally friendly UV filtering material. Furthermore, the submicrometer-sized graphitic carbon nitride of the present invention is very useful as a UV absorber for various products, as it can provide these products with improved UV protection without coloring them. In particular, the submicrometer-sized graphitic carbon nitride of the present invention is very useful as a UV absorber for cosmetic products, since it can provide keratinous substances, such as skin, with better UV protection while remaining transparent in liquids.

Claims (10)

Process for preparing submicrometer-sized graphitic carbon nitrides, comprising a step of grinding or sonicating the graphitic carbon nitrides. A method according to claim 1, wherein the grinding step is a wet grinding step. A method according to claim 2, wherein the wet grinding step is a wet jet grinding step or a wet ball grinding step. A process according to any one of the preceding claims, comprising an additional step of preparing the graphitic carbon nitrides by heating at least one precursor compound to 450 °C or more for at least 1 minute, before the step of grinding or sonicating the graphitic carbon nitrides. A process according to claim 4, wherein the heating is carried out in the presence of oxygen-containing species, such as _O2 , moisture, _O3 , atomic O and/or ionic oxygen, as an oxidizing agent, the oxidizing agent used during the heating step preferably being in gaseous form; the oxygen-containing species no longer preferably being either permanganate salt or hydrogen peroxide. A process according to any one of the preceding claims, comprising an additional step of evaporating the suspension media to obtain the dried graphitic carbon nitrides, after the step of grinding or sonicating the graphitic carbon nitrides. Graphitic carbon nitride of submicrometer size which exhibits a volume particle size distribution in which particle sizes smaller than 1 μm dominate more than 50% of the total particle volume Submicrometer-sized graphitic carbon nitride according to claim 7, wherein a suspension of submicrometer-sized graphitic carbon nitride at a concentration of 0.01 wt% in water has a turbidity of 300 NTU or less, preferably 200 NTU or less. Use of submicrometer-sized graphitic carbon nitride according to claim 7 or 8, as an active ingredient in paint, as a pigment, as a filler, in particular for plastics, or as a cosmetic active ingredient. Composition comprising submicrometer-sized graphitic carbon nitride according to claim 7 or 8 and water and/or at least one organic medium.