LT7181B - METHOD FOR MANUFACTURING CERIC PHOSPHATE CEPO4 PARTICLES FOR PROTECTION OF SKIN AGAINST UV RADIATION AND FOR USE AS WHITE PIGMENTS IN COLORED COSMETICS - Google Patents

Abstract

The invention is directed to a method for the production of optically active submicron or micron-sized cerium phosphate CePO4 particles by synthesizing them. According to this method, cerium oxalate is first produced, it is dry homogeneously mixed with ammonium phosphate and heated at a temperature of 1000 oC. The obtained pure monazite phase cerium phosphate is ground in a ball mill to obtain cerium phosphate particles of the appropriate size. Later, the surface of the cerium phosphate particles is additionally modified with stearic or other fatty acid to prevent them from agglomerating with each other, to become lipophilic, so that they can be incorporated into the oil phase of emulsions. The particles are non-toxic, biologically inert, and therefore can be used in such areas as cosmetics, medicine, etc. Such particles have the potential to be used as novel inorganic filters in ultraviolet (UV) protection products and/or as fillers or white pigments that further enhance sun protection in color cosmetics.

Description

FIELD OF THE INVENTION

The invention is directed to a method for producing optically active submicron or micron-sized cerium phosphate CePO4 particles. The particles are non-toxic, biologically inert, and therefore can be used in such areas as cosmetics, medicine, etc. Such particles have the potential to be used as new inorganic filters in ultraviolet (UV) protection products and/or as fillers or white pigments that additionally enhance sun protection in colored cosmetics. The synthesis method of the created particles is environmentally friendly and does not produce any harmful by-products.

TECHNICAL LEVEL

UV rays not only damage organic materials such as plastics, but also have a negative impact on humans. UV-B radiation (290-320 nm) causes sunburn and skin inflammation, while UV-A radiation (320-400 nm) causes tanning. In order to protect the skin from UV rays, various organic and inorganic sunscreen products have been developed.

Currently popular inorganic particles titanium dioxide and zinc oxide (TiO2, ZnO), used for UV protection in cosmetic products, are criticized for their small size, measured in nanometers, and are associated with negative effects on both consumers and the environment, manifested in clogged skin pores, damaged coral reefs, etc. In addition, TiO2 is well known for its photocatalytic activity, which often contributes to faster product degradation. Due to the negative effects of inorganic UV filters containing TiO2, ZnO, a more environmentally friendly alternative is needed.

It is also important to know that the EU has initiated regulations on the currently popular sunscreen ingredient TiO2, which is now considered unsafe. For example, according to EU Regulation (EC) No. 1272/2008, titanium dioxide is recognized as carcinogenic by inhalation, and from 2021, titanium dioxide (E171) is no longer considered safe when used as a food additive.

Japanese patent JP3819575B2 describes a cerium phosphate-based UV absorber, described by the general formula

CeMx(PO4)yAz ₂ nH ₂ O, with an average particle size of 0.01 to 1 μm. The method for producing a cerium phosphate-based ultraviolet absorber comprises mixing a basic aqueous solution containing phosphate ions and an aqueous solution containing cerium ions so that the pH of the suspension obtained after mixing is from 1 to 11, and hydrothermally treating the resulting amorphous precipitate suspension at a temperature of 120-200 °C. The adsorber produced by the method of the present invention using cerium phosphate does not have the disadvantages inherent in TiO2 and ZnO-based UV filters, and in addition, the present invention has confirmed that the use of cerium phosphate significantly reduces the photocatalytic activity, which was the main problem of conventional inorganic UV absorbers.

ESSENCE OF THE INVENTION

This invention is intended for a new method of obtaining cerium phosphate particles by synthesizing them by producing cerium oxalate, mixing it dry homogeneously with ammonium phosphate and heating it at a temperature of 1000 ^o C in air atmosphere conditions. The obtained pure monazite phase cerium phosphate was crushed using a ball-mill apparatus, obtaining cerium phosphate particles of the appropriate size. Later, the surface of the cerium phosphate particles was additionally modified with stearic acid so that they would not agglomerate with each other, become lipophilic, so that they could be incorporated into the oil phase of emulsions. Such emulsions are more aesthetic, and the cerium phosphate particles themselves are distributed and remain on the skin surface for a longer and more reliable time (the particles are lipophilic, human skin is also lipophilic). The method is ecological, there are no extraneous toxic products.

The cerium phosphate particles of the monazite crystalline phase with a modified surface became completely hydrophobic, when dropped into water they simply bounced off the water surface and stayed on it, did not wet at all and were perfectly distributed in the oil phase.

Studies have shown that the obtained pure monazite crystalline phase cerium phosphate particles are perfectly incorporated into the composition of cosmetic products, have good UV absorption, and are not phototoxic. The particles are suitable both as a UV filter for cosmetics and provide effective protection from sunlight, achieved through UV absorption rather than reflection, and as white pigments for makeup products (powders, etc.). In addition, the particles are biologically inert, produced ecologically and would contribute to a more sustainable environment and healthier skin for consumers in the long run.

DESCRIPTION OF DRAWINGS

The invention will be described below with reference to the accompanying drawings, in which:

Fig. 1 shows an X-ray diffraction (XRD) analysis diagram of cerium phosphate CePO4 particles obtained according to the present invention;

Fig. 1 shows images of the morphology of CePO4 particles obtained according to the present invention, determined by a scanning electron microscope (SEM);

Fig. 1 shows a diagram of the size distribution by number and volume of CePO4 particles obtained according to the present invention in an aqueous medium;

Fig. 1 shows the reflection spectrum (left) and excitation and emission spectra (right) of the cerium phosphate CePO4 particles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following materials were used to obtain the cerium phosphate particles of this invention:

- oxalic acid dihydrate, C2H2O4^2H2O;

- cerium (III) nitrate hexahydrate, Ce(NO3)3^6H2O;

- ammonium dihydrogen phosphate, NH4H2PO4.

First, cerium oxalate Ce2(C2O4)3 was prepared by precipitation. Stoichiometric amounts of Ce(NO3^6H2O and C2H2O4^2H2O were dissolved in separate beakers with deionized (DI) water. After mixing the resulting solutions, insoluble cerium oxalate Ce2(C2O4)3 precipitated. The resulting white solid was washed with deionized (DI) water using a vacuum filtration system. The material was washed until the pH of the filtrate became neutral. The cerium oxalate was dried in a drying oven for 12 hours at 60 °C.

Subsequently, white monazite-phase cerium phosphate CePO4 particles were prepared by a conventional solid-phase reaction. Stoichiometric amounts of cerium oxalate Ce2(C2O4)3 and ammonium dihydrogen NH4H2PO4 were homogeneously mixed, transferred to crucibles and annealed at 1000 °C for 4 hours in air.

After annealing, the cerium phosphate CePO4 particles obtained were ground in a Retsch E-Max ball mill. Deionized water and CePO4 were mixed in a 1:1 ratio and ground in a ball mill using 5 mm diameter ZrO2 balls. The milling speed and duration were set to 300 rpm and 20 min, respectively. The milled mixture was then sieved (sieve mesh diameter - 200 μm) and dried at 80 °C for 12 hours. The resulting dry particles were re-ground in an agate mortar.

Additionally, the surface of the cerium phosphate particles was later modified with stearic acid so that they:

1) do not agglomerate with each other. Particles tend to agglomerate, especially at certain pH values, because particles on an unmodified surface are stabilized only by surface charge, i.e. electrostatic stabilization, which is highly dependent on ionic strength, pH, etc. factors. Such stabilization is not reliable;

2) become lipophilic so that they can be incorporated into the oil phase of emulsions. Such emulsions are, first of all, more aesthetic, and by modifying the surface of the particles with stearic acid, they become hydrophobic and more easily distributed in oil emulsions. This is very important in cosmetic products, as it improves the stability of the particles, their distribution on the skin and the aesthetics of the final product.

The surface of cerium phosphate particles can be modified in two ways:

1) in dispersion: during the procedure of surface modification of cerium phosphate particles with stearic acid in dispersion, dry cerium phosphate particle powder and stearic acid in a mass ratio of 10:1 were dispersed in an ethanol solution with constant stirring (1 hour, at 75 °C), then the dispersion was left to settle for 3 hours. Then the dispersion of cerium phosphate particles modified with stearic acid was filtered, the powder was additionally washed several times with warm ethanol solution to remove free stearic acid adsorbed on the surface, and dried at 80 °C for 24 hours. 1-propanol or isopropyl alcohol, or any mixtures of these solvents, can be used instead of ethanol;

2) dry one-step method, more suitable for commercial quantities. Dry cerium phosphate particle powder was ground in a mill together with stearic acid powder in a ratio of 1:10. Since stearic acid has a relatively low melting point of 69.5 °C, the heat generated during grinding due to high shear becomes sufficient to melt the stearic acid and coat the surface of the cerium phosphate particles.

Although stearic acid was used to modify the surface of the cerium particles in the present invention, any other fatty acid may be used as well.

X-ray diffraction (XRD) analysis of the obtained cerium phosphate particles was performed using a Rigaku MiniFlexII diffractometer operating in Bragg-Brentan geometry in the range of 10° < 2Θ < 80° using Ni-filtered Cu Kα radiation (scanning step width was 0.02° and scanning speed was 5°/min). X-ray diffraction (XRD) analysis showed (Fig. 1) that the obtained cerium phosphate CePO4 particles are a single-phase material with a monoclinic (monazite) crystal structure. The XRD pattern of the synthesized CePO4 powder matches well with the reference pattern #96-403-1390 and no impurity peaks were observed. This indicates that the obtained phase-pure material with a monoclinic (monazite) crystal structure.

To determine the morphology of the resulting cerium phosphate CePO4 particles, field emission scanning electron microscope (SEM) images (electron acceleration voltage - 10 kV) were taken (Fig. 2). The figure shows that the ball-milled CePO4 powder consists of irregularly round submicron or micron-sized particles with an average diameter of 1 μm.

The surface area of the resulting cerium phosphate particles was investigated by nitrogen gas adsorption by the Brunauer-Emmett-Teller (BET) method using a surface area and porosity analyzer “TriStar II 3020”. The BET surface area of the resulting CePO4 particles was 1.37 ^m2 /g, and after grinding in a ball mill, the surface area of the particles increased to 2.0 m2/g.

Particle size distribution measurements of cerium phosphate CePO4 in aqueous media (c = 1 mg/ml, pH = 6.2) were performed using a dynamic light scattering apparatus “Zeta Sizer Nano ZS” (Malvern, UK) with a He-Ne 632.8 nm laser operating in the range of 10-10000 nm at a temperature of 25 °C. The results of 100 individual particle size distribution measurements by number and volume are presented in Figure 3. A very small (0.1%) fraction of nanoparticles (in the range from 85 nm to 100 nm) was detected (see Figure 3a). The majority of CePO4 particles in aqueous dispersions are submicron or micron in size, with diameters ranging from 0.1 to 3 μm and 0.2 to 8 μm, respectively, by number and volume.

The reflection, excitation and emission spectra of CePO4 particles were recorded on an Edinburgh Instruments FLS980 spectrometer. The particle reflection spectrum was recorded using an integrating sphere coated with Teflon. Barium sulfate BaSO4 was used as a reflection standard (excitation and emission slits were set to 5.0 nm and 0.15 nm, respectively). When measuring the excitation spectrum (λβιυ = 340 nm), the excitation and emission slits were set to 0.5 nm and 1.0 nm, respectively. When measuring the emission spectrum (λβχ = 280 nm), the excitation and emission slits were set to 1.0 nm and 0.5 nm, respectively. Each spectrum was recorded with a 0.5 nm step width and a 0.2 s integration time. The emission spectra were corrected for the instrument response using a correction file provided by Edinburgh Instruments. The excitation spectra were corrected for the reference detector.

The reflectance spectrum of cerium phosphate CePO4 is shown in Fig. 4 on the left and shows that the particles absorb almost all radiation in the UV-C region (up to 280 nm). As the wavelength of the radiation increases from 280 to 315 nm (UV-B region), the absorption of the particles decreases from about 95% to about 35%, respectively. In the case of UV-A radiation (315 nm < λ < 400 nm), the absorption of CePO4 decreases to about 15%, reaching the limit of the UV-A region. Finally, the particles reflect almost all electromagnetic radiation in the visible and near infrared regions.

The excitation and emission spectra of cerium phosphate CePO4 are shown in Fig. 4 on the right. The crystal field splits the 5d orbital of the Ce ³⁺ ion into five different energy components, so the excitation spectrum (λβιυ = 340 nm) has a single broad band up to about 310 nm with a maximum at about 280 nm, which arises from the interconfigurational ⁷ F5/2 ^ 5d optical transition of the Ce ³⁺ ion. The broadband emission spectrum (λβχ = 280 nm) ranges from about 290 nm to about 380 nm, with a peak at about 320 nm. The emission spectrum is asymmetric due to emission from the lowest crystal field component of the 5d orbital to the momentum of the Ce ³⁺ ion (5d ^ ⁷ F52 and 5d ^ Ψ7/2 optical transitions).

Thus, cerium phosphate CePO4 particles obtained by the method of this invention can be effectively used in the production of cosmetic creams and mineral powders intended for skin protection against UV rays and provide effective protection against sunlight, achieved through UV absorption, rather than reflection. The particles can also be used as white pigments in colored cosmetics. The concentration of particles in the preparations can vary from 0.1% to 30.0%. The preparation in the form of a cream can be used daily, applying it to exposed areas of the face and body. The recommended dose is 2 mg/1 cm ² of skin.

Claims (5)

DEFINITION OF THE INVENTION 1. A method for producing cerium phosphate CePO4 particles for protecting the skin from UV radiation and for use as white pigments in colored cosmetics, characterized in that: a) dissolve stoichiometric amounts of cerium (III) nitrate hexahydrate Ce(NO3^6H2O) and oxalic acid dihydrate C2H2O4^2H2O in separate beakers in deionized (DI) water, mix the solution, obtaining precipitates of insoluble cerium oxalate Ce2(C2O4)3; (b) the precipitated cerium oxalate is washed with deionized water using a vacuum filtration system until the pH of the filtrate becomes neutral; c) the cerium oxalate obtained in the form of a filtrate is dried in a drying oven for 12 hours at a temperature of 60 °C; d) homogeneously mix stoichiometric amounts of cerium oxalate Ce2(C2O4)3 and ammonium dihydrogen NH4H2PO4, transfer to crucibles and anneal at 1000 °C for 4 hours in an air atmosphere, obtaining white monazite phase cerium phosphate CePO4 particles; e) the resulting cerium phosphate CePO4 particles are mixed with deionized water in a 1:1 ratio and ground in a ball mill using 5 mm diameter ZrO2 balls at a grinding speed and duration of 300 rpm and 20 min, respectively; f) after grinding, the resulting mixture is sieved through a sieve with a mesh diameter of 200 μm and dried at 80 °C for 12 hours; g) re-grinding the dry cerium phosphate CePO4 particles in an agate mortar to obtain irregularly round submicron or micron sized cerium phosphate CePO4 particles with an average diameter of 1 μm and a surface area of up to 2.0 m ² /g; h) the resulting cerium phosphate CePO4 particles are additionally modified with a fatty acid in dispersion to prevent them from agglomerating with each other, to make them lipophilic and to improve their dispersion in oil phases. 2. The method according to claim 2, characterized in that the fatty acid is stearic acid. 3. The method according to claim 1, characterized in that during step e): - dry cerium phosphate particles and stearic acid are dispersed in an ethanol solution in a mass ratio of 10:1 with continuous stirring for 1 hour at a temperature of 75 °C, - the dispersion is left to settle for 3 hours, - the dispersion of cerium phosphate particles modified with stearic acid is filtered and additionally washed several times with warm ethanol solution to remove free stearic acid adsorbed on the surface, - dried at 80 °C for 24 hours. 4. The method according to claim 3, characterized in that instead of ethanol, 1-propanol or isopropyl alcohol, or any mixtures of these solvents, is used. 5. The method according to claim 1 and 3, characterized in that step e) is carried out in a dry one-step process, when dry cerium phosphate particles are ground in a mill together with stearic acid powder in a ratio of 1:10; since stearic acid has a sufficiently low melting point of 69.5 °C, the heat generated during grinding due to high shear melts the stearic acid and coats the surface of the cerium phosphate particles.