Processes for particle size reduction of graphitic carbon nitride particles

Abstract

The present invention mainly relates to a process for preparing sub-micrometer-sized graphitic carbon nitrides, comprising a step of milling or sonicating graphitic carbon nitrides. The present invention also relates to a sub-micrometer-sized graphitic carbon nitride having a volume size distribution in which the particle size of less than 1 μm dominates more than 50% of a volume of total particles in the suspension.

Description

[0001] DESCRIPTION

[0002] TITLE OF INVENTION

[0003] PROCESSES FOR PARTICLE SIZE REDUCTION OF GRAPHITIC CARBON NITRIDE PARTICLES

[0004] TECHNICAL FIELD

[0005] The present invention mainly relates to a process for particle size reduction of graphitic carbon nitride particles. Also, the present invention relates to graphitic carbon nitride particles with reduced particle size.

[0006] BACKGROUND ART

[0007] A UV protecting effect is one of key factors for cosmetic products. In order to filter radiation in UV region (lower than 400 nm in wavelength), organic UV filtering compounds and inorganic UV filtering compounds, such as TiO2 and ZnO, are widely used. Graphitic carbon nitride, which is an inorganic compound, is known to exhibit UV absorbing property. Some documents relating to graphitic carbon nitride have previously been reported.

[0008] For example, WO2020 / 246715 discloses an ultraviolet absorber which comprises polymeric carbon nitrides having a heterocyclic structure. However, this document is silent about a particle size of graphitic carbon nitrides.

[0009] Also, regarding techniques for decreasing particle size of carbon materials, some documents have previously been published so far.

[0010] For example, US7300958B2 discloses a method of manufacturing an ultra-dispersion of primary particles of nanometer-sized carbon, comprising: applying a wet milling method to an aggregate structure of said primary particles of nanometer-sized carbon to overcome van der Waals forces, by which forces said primary particles of nanometer-sized carbon are held together to form said aggregate structure, wherein said wet milling method is carried out with balls as a breaking medium, each of the balls having a diameter less than or equal to 0.1 mm.

[0011] Also, EP3096866A1 discloses a method of mercury removal, comprising: injecting activated carbon into flue gas generated from coal combustion, wherein the activated carbon has a d95 particle size distribution ranging from 1 pm to 28 pm and a d95 / d50 ratio ranging from 1 .5 to 3, wherein the d95 particle size distribution is obtainable by milling and air classification.

[0012] Also, CN103975468A discloses a composite particle comprising: one or more kinds of carbon materials selected from the group consisting of (i) fibrous carbon material, (ii) chain carbon material, and (iii) carbon material in which fibrous carbon material and chain carbon material are bonded to each other; with a phosphate containing lithium; and a manufacturing method thereof.

[0013] However, these documents do not mention graphitic carbon nitrides at all.

[0014] Make-up cosmetic compositions are used in order to provide keratinous substances, such as skin, in particular facial skin, with desired color appearance. A development for a new manufacturing process for graphitic carbon nitrides as eco-friendly, UV absorbing materials is demanded.

[0015] DISCLOSURE OF INVENTION

[0016] As objective of the present invention is to provide a process for reducing particle size of graphitic carbon nitrides efficiently to obtain sub-micrometer-scaled graphitic carbon nitrides.

[0017] Another objective of the present invention is to provide graphitic carbon nitrides with reduced particle size, which can exhibit improved UV protection property and transparent appearance.

[0018] The above objective of the present invention can be achieved by a process for preparing sub- micrometer-scaled graphitic carbon nitride particles, wherein the graphitic carbon nitride is subject to a milling process or a sonication process.

[0019] The milling step may be a wet milling step.

[0020] The wet milling step may be a wet jet milling step or a wet beads milling step.

[0021] The sonication may be carried out with a homogenizer.

[0022] The process may comprise the step of milling or sonicating graphitic carbon nitrides two or more times.

[0023] The process may comprise an additional step of preparing the graphitic carbon nitrides by heating at least one precursor compound at 450 °C or more for at least 1 minute, prior to the step of milling or sonicating graphitic carbon nitrides.

[0024] The heating may be carried out in a presence of oxygen-containing species, such as O2 (especially with oxygen flux) and / or humidity.

[0025] The present invention also relates to a sub-micrometer-sized graphitic carbon nitride having a volume size distribution in which the particle size of less than 1 pm dominates more than 50 % of a volume of total particles.

[0026] A suspensions of the sub-micrometer-sized graphitic carbon nitride at 0.01 % by weight concentration in water may have a turbidity of 300 NTU or less, preferably 200 NTU or less.

[0027] The present invention also relates to a use of the sub-micrometer-sized graphitic carbon nitride according to the present invention as a paint active, as a pigment, as a filler especially of plastics or as a cosmetic active, and in particular as a UV absorber.

[0028] The present invention also relates to a composition comprising the sub-micrometer-sized graphitic carbon nitride according to the present invention, and water and / or at least one organic media.

[0029] The present invention also relates to a composition, preferably a cosmetic composition for keratinous substances, such as skin, in particular a sunscreen composition, comprising the sub-micrometer-sized graphitic carbon nitride according to the present invention. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 shows particle the size distribution of as-synthesized graphitic carbon nitride powder in the suspension (A) and the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 1 (B), measured by the dynamic light scattering.

[0031] Figure 2 shows UV-vis absorption spectra of the suspensions with 0.01 % by weight of as- synthesized graphitic carbon nitride powder and that according to Example 1 in water / isopropanol (1 % by weight isopropanol).

[0032] Figure 3 shows the particle size distribution of as-synthesized graphitic carbon nitride powder in the suspension (A), and the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 2 (B), measure by the laser diffraction / scattering method.

[0033] Figure 4 shows a SEM image of the as-synthesized graphitic carbon nitride (A), and an ADF- STEM image of the graphitic carbon nitride according to Example 2 with reduced particle size (B).

[0034] Figure 5 shows a UV-vis absorption spectrum of the suspension of the graphitic carbon nitride according to Example 2 in water at the concentration of 0.1 % by weight compared to water.

[0035] Figure 6 shows the particle size distribution of as-synthesized graphitic carbon nitride powder in the suspension (A) and the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 3 (B), measured by the dynamic light scattering.

[0036] Figure 7 shows the particle size distribution of as-synthesized graphitic carbon nitride powder in the suspension (A) and the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 4 (B), measured by the dynamic light scattering.

[0037] Figure 8 shows the absorption coefficient of the suspension of the graphitic carbon nitride according to Example 4 compared with the suspension of the as-synthesized graphitic carbon nitride, TiCh (average primary particle size: 15 nm), and ZnO (average particle size: max. 200 nm) (A), and the retrodiffusion coefficient in the UV-Vis wavelength region of the suspension of the graphitic carbon nitride according to Example 4 compared with the suspension of the as-synthesized graphitic carbon nitride, TiOz (average primary particle size: 15 nm), and ZnO (average particle size: max. 200 nm) (B).

[0038] BEST MODE FOR CARRYING OUT THE INVENTION

[0039] After diligent research, the inventors have surprisingly found that particle size of graphitic carbon nitrides can be reduced by a milling or sonication process, and obtained smaller graphitic carbon nitrides with improved UV protection property and transparency, and thus completed the invention.

[0040] Thus, the present invention mainly relates to a process for preparing sub-micrometer-scaled graphitic carbon nitrides, comprising a step of milling or sonicating the graphitic carbon nitrides.

[0041] The inventors of the present invention surprisingly discovered that making graphitic carbon nitrides in sub-micrometer-scale provides the graphitic carbon nitrides with enhanced UV protection property and improved transparency.

[0042] Thus, the process according to the present invention can produce sub-micrometer-sized graphitic carbon nitrides with enhanced UV protection property and improved transparency.

[0043] Hereafter, the present invention will be described in a detailed manner.

[0044] [Process]

[0045] The present invention relates to a process for preparing sub-micrometer-sized graphitic carbon nitrides, comprising a step of milling or sonicating graphitic carbon nitrides.

[0046] The term "graphitic" in graphitic carbon nitride here means that the carbon nitride has a sheet graphite-like structure. Thus, the graphitic carbon nitride of the present invention may have a layered or a sheet structure. The graphitic carbon nitride is generally solid at room temperature, and in a powder form.

[0047] The term “sub-micrometer-sized” here can mean that particles have a particle size of less than 1 pm. Thus, the process according to the present invention can produce increased concentration of graphitic carbon nitride particles having a particle size of less than 1 pm in the suspension. In the present specification, the particle size in the suspension can be measured by, for example, using dynamic light scattering (DLS; Nanotrac Wave EX, MicrotracBEL Corp.) or laser diffraction / scattering method (LS 13 320 XR, Beckman Coulter Inc.).

[0048] The sub-micrometer-sized carbon nitrides of the present invention may have a volume size distribution in which the particle size of less than 1 pm dominates more than 50%, preferably more than 60% of a volume of total particles. Also, in one embodiment, the sub- micrometer-sized carbon nitrides of the present invention may have an average particle size of greater than 0.01 pm, preferably 0.05 pm. In another embodiment, the sub-micrometer-sized carbon nitrides of the present invention may have an average particle size of greater than 0.1 pm.

[0049] The term “average particle size” used herein can represent a volume-average size mean diameter which is given by the statistical particle size distribution to half of the population, referred to as d50. The volume-average size mean diameter and the volume size distribution of sub-micrometer-sized carbon nitrides can be measured by, for example, a dynamic light scattering particle size distribution analyzer.

[0050] The graphitic carbon nitride will be described in more detailed manner in the item “graphitic carbon nitride” later.

[0051] The process according to the present invention comprises at least one step of milling or sonicating graphitic carbon nitrides. Thus, the process according to the present invention comprises at least one step in which graphitic carbon nitrides are subject to a milling or a sonication step to produce the sub-micrometer-sized graphitic carbon nitride particles.

[0052] Any methods including media-less process, such as wet jet milling, thin-film spin mixer, and high speed laminar-flow-type mixer, and media-type processes, such as wet and dry beads milling can be used for milling the graphitic carbon nitrides.

[0053] As the milling method, mention can be particularly made of wet milling methods and dry milling methods. As the wet milling methods, mention can be particularly made of wet jet milling methods and wet beads milling methods.

[0054] 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. The examples of organic solvents include linear or branched alcohols, such as ethanol, propanol, butanol, isopropanol, and isobutanoL

[0055] The wet jet milling methods are generally categorized to two methods; one method is to collide the suspension including the materials to be grinded; another method is to pass the suspension through the nozzle having a narrow gap to cause turbulence.

[0056] In wet jet milling methods, the suspensions of the graphitic carbon nitrides are discharged under high pressure. The method for applying the pressure is not particularly limited. The pressure applied to the suspension may range from 50 MPa to 300 MPa and the number of times passing the nozzle may be 1 time to 400 times.

[0057] For the wet beads milling methods, peripheral velocity may range from 6 m / s to 18 m / s, preferably from 10 m / s to 14 m / s, considering the practical production. The constituent materials of the beads are not limited, but for examples, mention can be made of metal oxides, such as SiCh, glass, ZrCh, and Y2O3-stabilized ZrC ; metal nitrides, such as SisN4; metal carbides such as WC; and metals and metal alloys such as Steel. The diameter size of the beads may range from 0.01 mm to 2 mm, preferably from 0.05 mm to 1 mm, considering the practical production. The milling apparatus include any type of those applicable to the beads with abovementioned diameter, such as annular-type and disc-type apparatus, and their derivatives, where the apparatus is not limited to the apparatus specified.

[0058] As the dry milling methods, mention can be made of dry jet milling methods having high energy, such as steam jet milling methods.

[0059] As the sonication method, any devices for irradiation of sonication can be used. In one embodiment, the sonication is carried out with a homogenizer. The output of the sonication is not particularly limited, but may range from 5 to 5,000 W, preferably 10 to 1,000 W. The period for sonication is not particularly limited, but may be up to 200 hours. The temperature for sonication is not particularly limited, and in general sonication is carried out at the temperature ranging from 25 °C to 80°C.

[0060] The process according to the present invention may comprise the step of milling or sonicating graphitic carbon nitrides once or two or more times.

[0061] For example, for the beads milling methods may include single step or multiple steps, while the multiple-step beads milling such as two-step millings can efficiently produce submicrometer-sized graphitic carbon nitride. The example for the two-step milling is to mill using beads with the diameter in a range of 0.3 to 2 mm, such as 0.3 mm or 0.5 mm diameter, for the first step and then to mill using beads with the diameter smaller than 0.3 mm, such as 0.1 mm diameter, for the second step. In one embodiment of the present invention, the process according to the present invention may include additional step of preparing graphitic carbon nitrides to be milled of sonicated, as a starting material for the process, prior to the step of milling or sonicating graphitic carbon nitrides. The preparation of the graphitic carbon nitrides can be synthesized by heating at least one precursor compound.

[0062] Thus, the process according to the present invention may be a process for preparing the submicrometer-sized graphitic carbon nitrides, comprising: i) preparing the graphitic carbon nitrides by heating at least one precursor compound at 450°C or more for at least 1 minute; and ii) milling or sonicating the graphitic carbon nitrides to produce the sub-micrometer-sized graphitic carbon nitrides.

[0063] In the preparation of the as-synthesized graphitic carbon nitrides, one precursor compound may be used, or two or more precursor compounds may be are used in combination.

[0064] The precursor compound may be selected from the precursor known by one skilled in the art, for example, urea, thiourea, melamine, guanidine, arginine, cyanamide, dicyandiamide, and a salt thereof, and combinations thereof (Chem. Rev. 2016, 116, 7159-7329, Ong, W.J.; Tan, L.L.; Ng, Y.H.; Yong, S.T.; Chai, S.P., Catalysts 2019, 9(10), 805, Seong Jun Mun and Soo-Jin Park; https: / / doi.org / 10.3390 / catal9100805). Preferably, the precursor compound is selected from urea, melamine, guanidine, arginine, and a salt thereof and the combination thereof.

[0065] The salt of the precursor compound is not particularly limited, but mention can be made of salts with inorganic acids, such as carbonic acid and HalH wherein Hal represents halogen atom such as chloride (hydrochloric acid).

[0066] In one preferred embodiment of the present invention, only one precursor compound is used for the preparation of the graphitic carbon nitride.

[0067] The temperature for heating the at least one precursor compound is at least 450°C.

[0068] Preferably, the heating is carried out at 500°C or more, and more preferably at 525°C or more.

[0069] The period or the heating of the at least one precursor compound is at least 1 minute. Preferably, the period of the heating is at least 10 minutes, more preferably at least 20 minutes, and / or within 30 hours, and more preferably within 25 hours.

[0070] The heating of the precursor compound can be carried out in air, in noble gas, such as argon or helium, or in 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.

[0071] In one preferred embodiment, the heating process may be carried out in a presence of oxygencontaining species, such as O2, humidity, O3, atomic O, and / or ionic oxygen, as an oxidizing agent. While not wishing to be bound by theory, it is believed that the more porous graphitic carbon nitride can be obtained when the heating is carried out in the presence of oxygencontaining species. In the preferred embodiment, the heating is carried out in air, or in noble gas or inert gas including oxygen-containing species.

[0072] 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, humidity, ozone O3, O atomic and / or ionic oxygen, as an oxidizing agent.

[0073] In a preferred embodiment, the heating process is carried out in a presence of oxygencontaining species of O2, in particular oxygen flux, and / or humidity. The term "oxygen flux" can mean an oxygen flow in the present specification.

[0074] Preferably the oxidizing agent used during the heating step is in a gas form.

[0075] According one embodiment the oxygen source is neither from permanganate salt nor from hydrogen peroxide.

[0076] 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 the at least one precursor compound at 450°C or more for at least 1 minute, and then a second heating step of the at least one precursor compound at 450°C or more 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 greater than the temperature for the first heating step. The temperature and period for the first and second heating steps are as explained above.

[0077] In one embodiment of the present invention, a cooling step is present between heating steps. Thus, in one embodiment, the cooling step is included between the first heating step and the second heating step. The temperature for the cooling step is not particularly limited, but for example, the temperature is cooled to a room temperature (about 25°C). The period of the cooling step is not particularly limited, but for example is about from 1 minute to 24 hours.

[0078] In another embodiment of the present invention, the process according to the present invention may comprise at least one pre-grinding processes prior to the step milling or sonicating the graphitic carbon nitride. The pre-grinding processes may include not only wet milling processes but also dry grinding processes using dry beads milling, dry ball milling, and dry jet milling.

[0079] In the case that suspension of the sub-micrometer-sized graphitic carbon nitrides is obtained by the process according to the present invention, the process may comprise additional step for drying the suspension to obtain dried sub-micrometer-sized graphitic carbon nitrides.

[0080] The drying methods are not particularly limited, and may include freeze-drying, spray-drying and media slurry drying. The dried sub-micrometer-sized particles can be re-suspended to liquids.

[0081] [Graphitic Carbon Nitride]

[0082] The present invention also relates to the graphitic carbon nitride particle, in particular the sub- micrometer-sized graphitic carbon nitride particle. The sub-micrometer-sized graphitic carbon nitride particle can be prepared by the process according to the present invention as explained above.

[0083] The term “sub-micrometer-sized” here can mean that particles have an average particle size of less than 1 pm. Thus, the sub-micrometer-sized graphitic carbon nitride according to the present invention has an average particle size of less than 1 pm. In the present specification, the particle size can be measured by, for example, using dynamic light scattering (DLS; Nanotrac Wave EX, MicrotracBEL Corp.) or laser diffraction / scattering method (LS 13 320 XR, Beckman Coulter Inc.).

[0084] The sub-micrometer-sized graphitic carbon nitride of the present invention may comprise at least one heptazine unit in the structure. In the present specification, the heptazine unit means a hetero-fused ring consisting of three hetero rings consisting of C atoms and N atoms, represented with CeN?. 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 comprise at least one heptazine unit, at least one triazine unit, and a combination thereof. The presence of the heptazine unit can be determined by X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FT-IR) analysis, and nuclear magnetic resonance spectroscopy (NMR) analysis.

[0085] The sub-micrometer-sized graphitic carbon nitride of the present invention may have a porous structure. More specifically, the graphitic carbon nitride of the present invention may have a nano-porous structure. The pores may exist on the layer of the graphitic carbon nitride between heptazine units and triazine units.

[0086] The heptazine unit is preferably represented by formula (I), its salts and its solvates such as hydrates:

[0087] Formula (I) wherein, R1, R2, and R3, identical or different, represent: i) a hydrogen atom, ii) an halogen atom, iii) an oxygen-containing group as carboxy, nitro, or nitroso group, iv) a saturated or unsaturated, acyclic linear or branched, and / or cyclic, aromatic or nonaromatic, hydrocarbon chain containing from 1 to 10 carbon carbons, the said hydrocarbon chain being potentially interrupted by one or more heteroatom such as O, S, N or N(O); v) hydroxy, vi) amino R4R3N-, wherein R4and R5, identical or different, represent a hydrogen atom, (Ci-Ce)alkyl group or another monovalent heptazine group, preferably a monovalent wherein R1and R2are as defined herein before; vii) R4RSN(O)-, wherein R4and R3, identical or different, are as defined herein before; and viii)R4-N(O)- or ; wherein R4is as defined herein before. it being understood that: at least one of radical R1, R2or R3represents v) a hydroxy group, more preferably R1 represents v) hydroxy group and R2and R3, identical or different, preferably identical, represent iii) nitroso group selected from vi) to viii), more preferably viii), and one or more nitrogen into the cycles can be oxidized (N-oxide, or N-OH).

[0088] The ii) halogen may be selected from Cl and Br.

[0089] The iv) hydrocarbon chain may be a saturated or unsaturated, preferably saturated, acyclic linear or branched, preferably acyclic linear, hydrocarbon chain. The iv) hydrocarbon chain may contain from 1 to 6, preferably 1 to 4 carbon carbons. Thus, the iv) hydrocarbon chain may be a saturated and acyclic linear hydrocarbon chain containing from 1 to 6, preferably 1 to 4 carbon carbons, which can be interrupted by one or more heteroatom such as O, S, N or N(O).

[0090] The sub-micrometer-sized graphitic carbon nitride can be suspensible in water and in organic media, such as polar oils including diisopropyl sebacate. While not wishing to be bound by theory, it is believed that the reason why the graphitic carbon nitride of the present invention can be dispersible in both of water and oils is because the graphitic carbon nitride includes relatively large amount of functional groups produced by a discretization and / or a crack occurred in the structure, in particular heptazine units, which may contribute a change of surface polarity, such as hydrophilicity and hydrophobicity of the surface of the graphitic carbon nitride. It is also believed that the amount of the functional groups is increased by reducing the size of the graphitic carbon nitride because it may cause a discretization and / or a crack in the structure of the of the graphitic carbon nitride.

[0091] The sub-micrometer-sized graphitic carbon nitride of the present invention can exhibit an improved UV absorption property. Preferably, the graphitic carbon nitride has the absorption effect against both regions of UV-B and UV-A rays. UV-B rays here means UV rays having a wavelength between 280 to 320 nm. UV-A rays here means UV rays having a wavelength between 320 to 400 nm. An absorption curve in a range of ultraviolet light and visible light can be measured by, for example, ultraviolet-visible (UV-vis) absorption spectroscopy and diffuse reflectance spectroscopy.

[0092] The inventors of the present invention surprisingly discovered that making the graphitic carbon nitrides into sub-micrometer-sized particles could enhance the UV absorption property of suspension due to the improved dispersibility. Also, the inventors of the present invention surprisingly discovered that making the graphitic carbon nitrides into sub-micrometer-sized particles could suppress opacity caused the presence of large-sized particles of the graphitic carbon nitrides.

[0093] Thus, the suspension of the sub-micrometer-sized graphitic carbon nitride of the present invention exhibit low turbidity. For example, the suspensions at 0.01 % by weight concentration in water may be 300 NTU or less, preferably 200 NTU or less; the suspensions at 0.005 % by weight concentration in water may be 200 NTU or less, preferably 150 NTU or less: and the suspensions at 0.001 % by weight concentration in water may be 50 NTU or less, preferably 30 NTU or less, respectively. The turbidity can be measured by, for example, using 2100Q Portable Turbidimeter (HACH).

[0094] [Use]

[0095] The present invention may relate to a use of the sub-micrometer-sized graphitic carbon nitride of the present invention as UV absorbers, in particular UV A and / or B absorbers, in order to protect products from damages caused from UV radiation. For example, the UV absorber of the present invention can be used in paints, coatings, and cosmetics.

[0096] Because the sub-micrometer-sized graphitic carbon nitride of the present invention can show improved UV protection property, the use of the present invention can provide improved protection effect with products to be used. In addition, because the graphitic carbon nitride of the present invention can exhibit a transparent appearance in liquids, it can be used in a wide range of applications.

[0097] [Composition]

[0098] The present invention also relates to a composition including the sub-micrometer-sized graphitic carbon nitride of the present invention. Preferably, the composition according to the present invention is a cosmetic composition, in particular a cosmetic composition for keratinous substances, such as skin. In one preferred embodiment, the composition according to the present invention is a sunscreen composition.

[0099] Also, the composition according to the present invention can be used as a paint active, a pigment, a filler of plastics, a cosmetic active, and / or a sunscreen, such as a UVA and / or B absorber.

[0100] The composition according to the present invention preferably does not comprise TiO? or ZnO. In another embodiment, the composition according to the present invention comprises TiOz 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 instead of TiO2 and ZnO, which are known as traditional inorganic UV filters.

[0101] Because the sub-micrometer-sized graphitic carbon nitride of the present invention can show improved UV filtering property, the composition of the present invention can exhibit improved UV protection effect.

[0102] EXAMPLES The present invention will be described in a more detailed manner by way of examples. However, these examples should not be construed as limiting the scope of the present invention.

[0103] [Evaluation]

[0104] The following evaluations were conducted on graphitic carbon nitride powder samples.

[0105] (Morphology Analysis)

[0106] The morphology of the graphitic carbon nitride in suspension was observed via annular darkfield scanning transmission electron microscopy (ADF-STEM) using a liquid cell.

[0107] The IR spectrum of the graphitic carbon nitride powder was obtained using attenuated total reflection (ATR) method. The suspension sample was freeze dried for the analysis. The peak assigned to heptazine units appears at 804 cm'1, which is higher wavenumber than those of triazine units (814 cm'1for melamine, 808 cm'1for melam), according to Nan Liu, et al. (ACS Omega, 2020, volume 5,12557-12567).

[0108] (Particle Size Analysis)

[0109] The particle size distribution in the suspension of the graphitic carbon nitride was evaluated using dynamic light scattering (DLS; Nanotrac Wave EX, MicrotracBEL Corp.) or laser diffraction / scattering method (LS 13 320 XR, Beckman Coulter Inc.).

[0110] (UV Absorption Property)

[0111] The UV-vis absorption spectra of the graphitic carbon nitride suspended in liquids in Fine quartz cell (two transparent Sides, 2 mm (optical path length) x 10 mm x H45 mm, Tokyo Garasu Kikai Co., Ltd.) were collected using UV-Visible spectrophotometer (V750, Jasco Inc.) coupled with an integrating sphere. The suspension samples were sonicated using Ultrasonic Cleaner (ASU-3M, AS ONE Corporation) before the measurement.

[0112] (Absorption and Retrodiffusion Coefficients)

[0113] The transmission and reflectance spectra of the graphitic carbon nitride suspended in liquids at 3 different concentrations were measured for the wavelength range from 250 nm to 780 nm using a spectroscopic curve and converted to absorption coefficient (pa) and retrodiffusion coefficient (ps’). The retrodiffusion coefficient in the visible wavelength range, which is larger than 400 nm, represents the degree of opacity.

[0114] (Turbidity)

[0115] The turbidity in Nephelometric Turbidity Unit (NTU) was evaluated using 2100Q Portable Turbidimeter (HACH).

[0116] (Color Appearance)

[0117] Color appearance of each of the graphitic carbon nitride was evaluated by observing the sample with naked eyes. [Preparation]

[0118] The graphitic carbon nitride according to the present invention was prepared in the following Examples 1 to 4, and was evaluated as below.

[0119] Example 1

[0120] 5 g of melamine powder as the precursor compound was heated at 550°C in air for 2 hours to prepare graphitic carbon nitride as synthesized.

[0121] The as-synthesized graphitic carbon nitride was suspended at the concentration of 0.1% by weight in a mixture of water / isopropanol (volume ratio: 99 / 1). Then, the suspension was subject to jet milling using a jet mill at 70 MPa for one time. Then, the suspension was diluted to 0.01 % by weight of graphitic carbon nitride by addition of the mixture of water / isopropanol (volume ratio: 99 / 1). After dilution, the jet milling at 70 MPa in the same condition above was carried out again on the suspension to produce suspension of the graphitic carbon nitride according to Example 1 .

[0122] Figure 1 (A) shows the particle size distribution of as-synthesized graphitic carbon nitride powder in the suspension, while Figure 1 (B) shows the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 1 , measured by the dynamic light scattering. As can be seen from Figure 1(B), the peak, in which d50 is 0.24 pm and d90 is 0.41 pm, appeared in the range from 0.07 pm to 0.82 pm with the frequency of 100%. Thus, the sub-micrometer-sized graphitic carbon nitride with a volume size distribution in which the particle size of less than 1 pm dominates about 100% of a volume of total particles was obtained in Example 1.

[0123] The UV-vis absorption spectra of the suspensions with 0.01 % by weight of as-synthesized graphitic carbon nitride powder and that according to Example 1 in water / isopropanol (1 % by weight isopropanol) are shown in Figure 2. The suspensions of both samples exhibited absorption in the UV wavelength region lower than 400 nm, while the sample according to Example 1 exhibited higher absorption due to the improvement of dispersibility.

[0124] Example 2

[0125] The as-synthesized graphitic carbon nitride was suspended at the concentration of 0.1% by weight in a mixture of water / isopropanol (weight ratio: 99 / 1 ). Then, the suspension was subject to wet jet milling using a jet mill at 200 MPa via circulation for the duration equivalent to 300 times of passes to obtain milled graphitic carbon nitride according to Example 2. Then the suspension was subjected to freeze-drying to remove the suspension media and obtain dried milled graphitic carbon nitride. Then the dried milled graphitic carbon nitride was re-suspended in water at 0.1 % by weight by sonication for 20 minutes.

[0126] Figure 3(A) shows the particle size distribution of as-synthesized graphitic carbon nitride powder in the suspension, while Figure 3(B) shows the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 2, measure by the laser diffraction / scattering method. 67% of the detected size of graphitic carbon nitride powder according to Example 2 in the suspension was in the range from 0.13 pm to 0.17 pm for differential volume. Thus, the sub-micrometer-sized graphitic carbon nitride with the particle size with a volume size distribution in which the particle size of less than 1 pm dominates about 67% of a volume of total particles was obtained in Example 2.

[0127] The turbidity of each of the suspensions comprising 0.1 % by weight of the as-synthesized graphitic carbon nitride and the graphitic carbon nitride according to Example 2 was measured. The suspension samples were diluted before the measuring by addition of water to the aforementioned suspensions so that the suspensions at 0.01 % by weight, 0.005 % by weight and 0.001 % by weight concentrations were obtained. The turbidity of the suspensions at 0.01 % by weight, 0.005 % by weight, and 0.001 % by weight concentrations of the as-synthesized graphitic carbon nitride were 549 NTU, 299 NTU, and 57 NTU, respectively, while the turbidity of the suspensions at 0.01 % by weight, 0.005 % by weight, and 0.001 % by weight concentrations of the graphitic carbon nitride according to Example 2 were 176 NTU, 103 NTU, and 26 NTU, respectively.

[0128] According to the morphology of the graphitic carbon nitride in suspension according to Example 2 via ADF-STEM, it was confirmed that the graphitic carbon nitride had sub- micrometer-sized pores and fibrous chain-like structures. Figure 4(A) shows a SEM image of the as -synthesized graphitic carbon nitride, while Figure 4(B) shows an ADF-STEM image of the graphitic carbon nitride according to Example 2 with reduced particle size in a mixture of water / isopropanol (weight ratio: 99 / 1). It is confirmed that the process according to the present invention could make large-sized particles of agglomerates of as-synthesized graphitic carbon nitrides into sub-micrometer-sized particles.

[0129] In the IR analysis of the graphitic carbon nitride according to Example 2, a peak appeared at 806 cm'1, which is located in the middle between the peak of heptazine units at 804 cm'1and that of triazine units at 808 cm’1for melam. This result indicates that graphitic carbon nitride according to Example 2 has both of heptazine units and triazine units.

[0130] The UV-vis absorption spectrum of the suspension of the graphitic carbon nitride according to Example 2 in water at the concentration of 0.1 % by weight is shown in Figure 5, compared to water. The suspension exhibited absorption in the UV wavelength region lower than 400 nm.

[0131] Example 3

[0132] The same as-synthesized graphitic carbon nitride as Example 1 was used in Example 3. The as-synthesized graphitic carbon nitride was suspended at the concentration of 1 % by weight in water. Then, the suspension was subject to beads milling comprising two steps: step 1 (beads: G 0.3 mm, peripheral velocity: 10 m / s and then 12 m / s after 10 minutes, duration time: 60 minutes) and then step 2 (beads: 0 0.1 mm, peripheral velocity: 14 m / s, duration time: 120 minutes), to obtain milled graphitic carbon nitride according to Example 3.

[0133] Figure 6(A) shows the particle size distribution of as-synthesized graphitic carbon nitride powder in the suspension, while Figure 6(B) shows the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 3, measured by the dynamic light scattering. The micrometer-sized particles decreased after the wet beads milling and the sub-micrometer-sized particles increased.

[0134] Example 4 The same as-synthesized graphitic carbon nitride powder was suspended at the concentration of 0.1 % by weight in the mixture of water / Propylene Glycol (weight ratio: 50 / 50). Then, the suspension was subject to wet jet milling using a jet mill at 70 MPa for three time to obtain milled graphitic carbon nitride according to Example 4. Then polyoxyethylene sorbitan monolaurate (Tween20) was added at the concentration of 0.3% by weight.

[0135] Figure 7(A) shows the particle size distribution of as-synthesized graphitic carbon nitride powder in the suspension, while Figure 7(B) shows the particle size distribution of the graphitic carbon nitride powder in the suspension according to Example 4, measured by the dynamic light scattering. The micrometer-sized particles decreased after the jet milling and the sub-micrometer-sized particles increased.

[0136] Figure 8(A) shows the absorption coefficient of the suspension of the graphitic carbon nitride according to Example 4 compared with the suspension of the as-synthesized graphitic carbon nitride, TiCb (average primary particle size: 15 nm), and ZnO (average particle size: max. 200 nm). The graphitic carbon nitride according to Example 4 could show the highest UV absorption property in the UV-B region, and showed good UV-A absorption property in the UV-A region. Also, the increase of the UV absorption property by milling the graphitic car bon nitride could be confirmed.

[0137] Figure 8(B) shows the retrodiffusion coefficient in the UV-Vis wavelength region of the suspension of the graphitic carbon nitride according to Example 4 compared with the suspension of the as-synthesized graphitic carbon nitride, TiCh (average primary particle size: 15 nm), and ZnO (average particle size: max. 200 nm). The graphitic carbon nitride according to Example 4 could show the highest diffusion property in UV region. In addition, the graphitic carbon nitride according to Example 4 showed suppressed diffusion property in visible light region which was almost the same as TiO2 and ZnO. This means that the graphitic carbon nitride according to Example 4 exhibits improved transparency. On the other hand, the as-synthesized graphitic carbon nitride exhibited high diffusion property in visible light region, which indicates it exhibits high opacity.

[0138] Example 5

[0139] The same as-synthesized graphitic carbon nitride as Example 1 was used in Example 5. The as-synthesized graphitic carbon nitride was suspended at the concentration of 0.05% by weight in water. The suspension was subject to sonication using homogenizer (VIOLAMO SONICSTAR 85, AS ONE Corporation) at 20 W for 7 hours to obtain sonicated graphitic carbon nitride according to Example 5.

[0140] The color appearance of the suspension of the as-synthesized graphitic carbon nitride was yellow, while the suspension of the graphitic carbon nitride according to Example 5 exhibited white appearance and better dispersibility in the suspension. This indicates that the particle size of the graphitic carbon nitride was reduced by the sonication. This result indicates that it is possible to obtain the suspension comprising sub-micrometer-sized graphitic carbon nitride by a sonication using a homogenizer.

[0141] As can be understood from the results of the examples, the process according to the present invention could efficiently produce sub-micrometer-sized graphitic carbon nitrides. Also, the sub-micrometer-sized graphitic carbon nitride according to the examples exhibited improved UV filtering effect (absorption and diffusion) and exhibited improved transparency. Accordingly, it can be concluded that the process according to the present invention is very useful to produce the sub-micrometer-sized graphitic carbon nitride which is a new eco- friendly, UV filtering material. In addition, the sub-micrometer-sized graphitic carbon nitride of the present invention is very useful as UV absorbers for various products, since it can provide products with improved UV protecting effect while it does not color the products. In particular, the sub-micrometer-sized graphitic carbon nitride of the present invention is very useful as UV absorbers for cosmetic products, since it can provide the keratinous substances, such as skin, with improved UV protection and while it is transparent in liquids.

Claims

CLAIMS1 . A process for preparing sub-micrometer-sized graphitic carbon nitrides, comprising a step of milling or sonicating graphitic carbon nitrides.

2. The process according to Claim 1 , wherein the milling step is a wet milling step.

3. The process according to Claim 2, wherein the wet milling step is a wet jet milling step or a wet beads milling step.

4. The process according to Claim 1, wherein the sonication is carried out with a homogenizer.

5. The process according to any one of the preceding claims, comprising the step of milling or sonicating graphitic carbon nitrides two or more times.

6. The 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 at 450 °C or more for at least 1 minute, prior to the step of milling or sonicating graphitic carbon nitrides.

7. The process according to Claim 6, wherein the heating is carried out in a presence of oxygen-containing species, such as O2, humidity, O3, O atomic and / or ionic oxygen, as an oxidizing agent, preferably the oxidizing agent used during the heating step is in a gas form; more preferably the oxygen-containing species is neither from permanganate salt nor from hydrogen peroxide.

8. The process according to any one of the preceding claims, comprising an additional step of evaporating the suspending media from the suspension to obtain the dried graphitic carbon nitrides, after the step of milling or sonicating graphitic carbon nitrides.

9. The process according to Claim 8, wherein the evaporation is carried out by freeze- drying.

10. The process according to Claim 8 or 9, comprising an additional step of resuspending the dried graphitic carbon nitrides in the media, after the step of drying the media.

11. A sub-micrometer-sized graphitic carbon nitride having a volume size distribution in which the particle size of less than 1 pm dominates more than 50% of a volume of total particles.

12. The sub-micrometer-sized graphitic carbon nitride according to Claim 11, wherein a suspensions of the sub-micrometer-sized graphitic carbon nitride at 0.01 % by weight concentration in water has a turbidity of 300 NTU or less, preferably 200 NTU or less.

13. A use of the sub-micrometer-sized graphitic carbon nitride according to Claim 11 or 12, as a paint active, as a pigment, as a filler especially of plastics or as a cosmeticactive.

14. A use of the sub-micrometer-sized graphitic carbon nitride according to Claim 11 or 12 as a UV absorber.

15. A composition comprising the sub-micrometer-sized graphitic carbon nitride according to Claim 11 or 12 and water and / or at least one organic media.

16. A composition, preferably a cosmetic composition for keratinous substances, such as skin, comprising the sub-micrometer-sized graphitic carbon nitride according toClaim 11 or 12.

17. The composition according to Claim 16, which is a sunscreen composition.