Pigment for shielding of ultraviolet radiation

Abstract: [Object] To provide an ultraviolet radiation shielding pigment having a high power of shielding of ultraviolet radiation, particularly UV-A, a low cohesive force, and excellent extensibility and adhesiveness as a pigment for cosmetics. [Solution] A pigment for shielding of ultraviolet radiation which comprises a flaky powder having particle surfaces coated with particles of barium sulfate having an average diameter of 0.1 to 2.0 microns, and with needle crystal particles of zinc oxide having an average major-axis diameter of 0.05 to 1.5 microns, and a process for manufacturing it.

Description: [Detailed Description of the Invention] [Field of the Invention] This invention relates to a pigment for shielding of ultraviolet radiation, and more particularly, to a pigment obtained by coating the surfaces of particles of a flaky powder with particles of zinc oxide and of barium sulfate for shielding of, among others, UV-A, a process for manufacturing the same, and the use thereof. [Background Art] The amount of ultraviolet radiation of a wavelength range of 290 to 400 nm on the ground of the earth occupies about 6% of the light of the sun, of which about 0.5% is occupied a relatively short wavelength in the range of 29C to 320 nm (hereinafter referred to as UV-B), while about 5.5%, or the greater part thereof is occupied a relatively long wavelength in the range of 320 to 400 nm (UV-A). UV-A passes through clouds and window glass more easily because of its long wavelength than ultraviolet radiation having a short wavelength, and our human skin is exposed to more UV-A in our daily life, and UV-A penetrates into the skin tissue. While UV-B is scattered or absorbed on the surface of the skin and causes sunburn, or like inflammation to the skin, UV-A reaches the inner layer of the skin, and is said to produce in the skin tissue a radical causing aging of the skin by light, such as wrinkling, loosening, and a reduction of elasticity, while exerting an adverse effect on the cell membranes, and genes. When intercepting ultraviolet radiation, therefore, it is not sufficient to think merely of shielding of the whole range of ultraviolet radiation, but it is of greater importance and interest to shield UV-A (Journal of Cosmetic Technology, 31, No. 1, pp. 14-30, 1997). As materials having the power of shielding of ultraviolet radiation, there are, for example, known and commercially available metal oxides such as fine titanium oxide (see e.g. Japanese Patent Publication No. Sho 47-42502), fine iron oxide (see e.g. Japanese Patent Laid-open No. Hei 5-279235), fine zinc oxide (see e.g. Japanese Patent Laid-open No. Sho 60-231607) and fine cerium oxide (see e.g. Japanese Patent Laid-open No. Hei 2-823312), and benzotriazole-based, or other organic agents for absorbing ultraviolet radiation. These commercially available agents for shielding of or absorbing ultraviolet radiation have, however, a number of problems including an insufficient power of shielding of ultraviolet radiation and a limited scope of use, as will be pointed out below. Fine titanium oxide, for example, has a low power of absorbing UV-A, and is required to have a larger particle diameter to exhibit a power of shielding UV-A by scattering effect of it, but its larger particles lose transparency and have a higher degree of whiteness, thereby limiting the scope of its use. Fine iron oxide has a limited scope of use, since it is inferior in its power of shielding of ultraviolet radiation to any other metal oxide, and has a brown color. Fine zinc oxide has a high power of shielding of ultraviolet radiation, but its high agglomeration property presents an obstacle to its use. It is necessary to use a special dispersing apparatus having a strong stirring force to disperse zinc oxide into primary particles having a diameter which enables it to shield ultraviolet radiation most effectively, but even if it may be divided into primary particles, it easily agglomerates again. Fine cerium oxide is so expensive that it can hardly be used except for special purposes. Benzotriazole-based, and other organic ultraviolet radiation absorbing agents have a high power of absorbing ultraviolet radiation, but as they are organic compounds, they are basically lacking in stability, and cannot be expected to durable effect for a long time. Moreover, the use of any organic ultraviolet radiation absorbing agent as an additive to cosmetics is very strictly restricted from the standpoint of safety. EP-A-0 142 695 discloses platelet-shaped pigments coated with barium sulfate and if necessary with zinc oxide, whereby the ratio of amounts in parts by weight is not defined. Said pigments are mainly used in cosmetic applications but have no practical impact in the use for shielding of ultraviolet radiation. EP 0 414 049 describes pigments having an effect of preventing ultraviolet rays, comprising of platelet-shaped pigments coated with barium sulfate and titanium oxide or zirconium oxide. A method of coating flaky grains with ultra-fine barium sulfate grains discloses EP 0.565 043. Said barium sulfate grains have a mean grain size of 0.1 µm or less and are produced wet-chemically using complexing agents. Under these circumstances, we, the inventors of this invention, have found as a result of our earnest study for developing a pigment for shielding of ultraviolet radiation that a pigment obtained by coating a flaky powder with particles of zinc oxide and barium sulfate as a high power of shielding of ultraviolet radiation, particularly UV-A, while zinc oxide becomes less agglomerate, and we have succeeded in developing an ultraviolet radiation shielding pigment which, when added to cosmetics, exhibits high extensibility and adhesiveness as the basic properties required of cosmetics. These results form a basis for this invention. [Disclosure of the Invention] This invention provides a pigment for shielding of ultraviolet radiation which comprises a flaky powder having particle surfaces coated with particles of barium sulfate having an average diameter of 0.1 to 2.0 µm (microns), and with needle crystal particles of zinc oxide having an average major-axis diameter of 0.05 to 1.5 µm (microns), wherein the amount in parts by weight of said barium sulfate is smaller than that of said zinc oxide, relative to the amount of said flaky powder. This invention also provides a process for manufacturing a pigment for shielding of ultraviolet radiation, which comprises suspending a flaky powder in water to form its suspension; dropping (a) a water-soluble barium compound, and (b) a solution containing a member of the group consisting of sulfuric acid, sodium sulfate and potassium sulfate and containing a higher chemical stoichiometric equivalent ratio of sulfate ions than barium ions in (a), into said suspension in such a way as either dropping (b) after adding an appropriate amount of (a), or dropping (a) and (b) simultaneously, whereby the particles of said flaky powder in said suspension are coated with particles of barium sulfate; dropping (c) a water-soluble zinc compound of the group consisting of zinc sulfate and zinc acetate, and (d) a basic solution into said suspension in such a way as either dropping (d) after adding an appropriate amount of (c), or dropping (c) and (d) simultaneously, whereby said particles of said flaky powder are coated with the hydroxide or carbonate of zinc; collecting said coated particles by filtration; washing them; drying them; and calcining them. This invention further provides a cosmetic, paint, or plastic resin material containing an appropriate amount of a pigment for shielding of ultraviolet radiation as defined above. The ultraviolet radiation shielding pigment of this invention which comprises a flaky powder having particle surfaces coated with particles of barium sulfate having a specific diameter and needle crystal particles of zinc oxide has a high power of shielding of ultraviolet radiation, particularly UV-A, and if it is used in a cosmetic, it is excellent not only in extensibility and adhesiveness, but also in resistance to color dulling, as the properties required of the cosmetics. The following is a detailed description of this invention. The flaky powder which is used for the purpose of this invention may, for example, be mica, sericite, talc or kaolin having a particle diameter of 0.5 to 100 µm (microns), and is usually mica (muscovite). A suspension is prepared by suspending 5 to 20 parts by weight of such flaky powder in 100 parts by weight of water. The suspension is heated to at least 50°C, preferably about 60-80°C, under stirring, and is, then, employed for A: Step of coating with particles of barium sulfate, and 3: Step of coating with zinc oxide, as will now be described. A: Step of coating with particles of barium sulfate Either of two methods as described at (1) and (2) below can be employed for coating the flaky powder in the suspension with particles of barium sulfate. (1) An appropriate amount of a water-soluble barium compound (a) is added to the suspension heated to about 60-80°C, and dissolved therein under stirring, and a separately prepared solution containing sulfate ions (b) is thereafter dropped into the suspension; (2) An aqueous solution (a-1) is prepared from an appropriate amount of water-soluble barium compound (a), and the solution (a-1) and the above solution (b) are dropped simultaneously into the suspension heated to about 60-80°C, while it is under stirring. Examples of the water-soluble barium compound (a) or (a-1) are its chloride, nitrate and hydroxide. Examples of the source of sulfate ions (b) are sulfuric acid, sodium sulfate and potassium sulfate, and it contains a higher chemical stoichiometric equivalent ratio of sulfate ions than the barium ions in (a). B: Step of coating with particles of zinc oxide Either of two methods as described at (1) and (2) can be employed for the step of coating with particles of zinc oxide. (1) An appropriate amount of a water-soluble zinc compound (c) is added to, and dissolved in the suspension as obtained by Step A above, and a separately prepared basic solution (d) is thereafter dropped in the suspension, so that it may have a pH of 7 or above, and preferably 8 or above. (2) An aqueous solution (c-1) is prepared from an appropriate amount of a water-soluble zinc compound (c), and the solutions (c-1) and (d) are dropped simultaneously into the suspension as obtained by Step A above, while it is kept at a pH of 7 or above. Customary methods are employed for separating solid matter by filtration from the suspension obtained by either method (1) or (2) and containing the flaky powder coated with particles of barium sulfate and the hydroxide or basic carbonate of zinc, and for washing, drying and calcining it to obtain a pigment coated with particles of barium sulfate and of zinc oxide. Its calcining is performed at a temperature of 500-900°C, and preferably of 600-800°C. At any temperature below 500°C, zinc fails to be completely oxidized, or its oxidation takes place at an undesirably low speed. At any temperature above 900°C, the particles coating the flaky particle surfaces undesirably undergo sintering to become larger in diameter than expected, or eventually agglomerate into larger masses. Examples of the water-soluble zinc-compound (c) or (c-1) are its sulfate and acetate. Examples of the basic substance in (d) are sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogen carbonate and potassium hydrogen carbonate. The selection of the method (1) or (2) in each of Steps A and B depends on the size of the coating particles as intended. If large coating particles are desired, for example, the method (2) (hereinafter referred to as the simultaneous dropping method) is suitable, while the method (1) is suitable if fine particle coating is desired. Apart from the coating method as discussed, the factors which determine the size of the particles of barium sulfate and zinc oxide include the temperature of the suspension, the shape of the reaction vessel employed, the shape of the stirring blade employed, and the stirring speed. The particles tend to have a smaller diameter if, for example, a lower temperature is employed, or if a higher stirring speed is employed, or if the reaction vessel is provided with a baffle, or if a higher dropping speed is employed. It is, therefore, advisable to select these conditions so that they may be suitable for the intended particle diameter. It is, of course, also possible to employ a combination of the method (1) in Step A and the method (2) in Step B, or of the method (2) in Step A and the method (1) in Step B. The barium sulfate particles prevent the agglomeration of the zinc oxide particles, and also exhibit their advantages in giving a cosmetic adhesiveness and extensibility on the skin as its important properties. The zinc oxide particles are a substance having a high power of shielding of ultraviolet radiation, and more particularly, UV-A, and in order that their excellent power may further be raised, or may not be lowered, the barium sulfate particles have their average diameter controlled within the range of 0.1 to 2.0 µm (microns). If their average diameter is smaller than 0.1 µm (micron), the pigment has a lower degree of extensibility on the skin, and if it is larger than 2.0 µm (microns), the pigment has a lower degree of transparency. The zinc oxide particles are of needle crystals, and preferably have an average major-axis diameter of 0.05 to 1.5 µm (microns). If it is smaller than 0 .05 µm (micron), the pigment has a lower degree of extensibility on the skin, and if it is larger than 1.5 µm (microns), the pigment has a lower power of shielding of ultraviolet radiation. The coating amount of the barium sulfate particles is the minimum as required for lowering the agglomeration of the zinc oxide particles and, when used in a cosmetic, for giving it adhesiveness and extensibility on the skin, and is preferably 10 to 50 parts by weight relative to 100 parts by weight of flaky powder. If their amount is less than 10 parts by weight, they do not exhibit extensibility on the skin, and even if it may be more than 50 parts by weight, they do not exhibit any greatly improved extensibility. On the other hand, the zinc oxide particles are employed for shielding of UV-A, and for that purpose, it is preferable to employ the maximum possible amount thereof. Accordingly, the coating amount of the zinc oxide particles is at least larger than that of the barium sulfate particles, and is preferably 50 to 200 parts by weight relative to 100 parts by weight of flaky powder. If it is less than 50 parts by weight, they do not effectively shield UV-A, and if it is more than 200 parts by weight, the coating particles agglomerate on the particle surfaces of the flaky powder and thereby fail to be of the desired size, or cause the particles of the flaky powder to agglomerate, or even fail to coat the particles of the flaky powder, depending on the chemical nature of the surfaces of the latter. If the total amount of the barium sulfate and zinc oxide relative to the flaky powder exceeds the range as stated above, the particles undergo agglomeration, or solidification to the extent that no primary particles may be obtained even by crushing, or otherwise. There is, thus, obtained a pigment for shielding of ultraviolet radiation which comprises a flaky powder having particle surfaces coated with particles of barium sulfate having an average diameter of 0.1 to 2.0 µm (microns) and needle crystal particles of zinc oxide having an average major-axis diameter of 0.05 to 1.5 µm (microns), wherein the amount in parts by weight of said barium sulfate is smaller than that of said zinc oxide, relative to the amount of said flaky powder. The ultraviolet radiation shielding pigment of this invention exhibits an outstanding power of shielding of ultraviolet radiation when mixed in a cosmetic, paint, or plastic material, as stated above. It has particularly high power of shielding of UV-A as required of a cosmetic, exhibits good extensibility and adhesiveness on the skin, hardly has its color dulled when mixed with a body pigment, such as talc or mica, or oil, and is suitable for use in a foundation. The invention will now be described in further detail by way of examples and comparative examples, though these examples are not intended for limiting the scope of this invention. [Examples] Example 1: A suspension was prepared by suspending 150 g of fine muscovite particles having a diameter of 1 to 15 µm (microns) in 1.5 liters of water, and heated to about 80°C, and 50.7 g of barium hydroxide were added to it under stirring. Then, an aqueous solution of sulfuric acid having a concentration of 10% by weight was dropped at a rate of 2 ml/min. into the suspension under stirring until it had a pH of 3. Thereafter, its stirring was continued for about 10 minutes, and 662.5 g of zinc sulfate were added, and after about 10 minutes of stirring, a 32% by weight aqueous solution of sodium hydroxide was dropped into the suspension at a rate of 5 ml/min. until it had a pH of 8.5. Solid matter was separated from the suspension by filtration, washed, dried at about 105°C for 15 hours, and calcined at 700°C. As a result, there was obtained an ultraviolet radiation shielding pigment in which 100 parts by weight of fine muscovite particles were coated with 25 parts by weight of barium sulfate particles and 125 parts by weight of zinc oxide particles. Its examination by a SEM revealed that the barium sulfate particles had an average diameter of about 0.3 µm (micron), while the zinc oxide particles in needle crystal form had an average major-axis diameter of about 0.2 µm (micron). The pigment as obtained showed good extensibility and adhesiveness on the skin, and did not show any appreciable dulling in color when mixed with oil. Example 2: A suspension was prepared by suspending 150 g of fine muscovite particles having a diameter of 1 to 15 µm (microns) in 1.5 liters of water, and heated to about 80°C, and 507 g of a 10% by weight aqueous solution of barium hydroxide were added to it under stirring, so that the suspension might have a pH of 8.5, and an aqueous solution of sulfuric acid having a concentration of 30% by weight was dropped therein at a rate of 7.5 ml/min. Then, after 30 minutes of stirring, 2208 g of a 30% by weight aqueous solution of zinc sulfate, and a 32% by weight aqueous solution of sodium hydroxide were dropped at a rate of 7.5 ml/min. into the suspension under stirring, while it was maintained at a pH of 8.5. Then, after 10 minutes of stirring, solid matter was separated from the suspension by filtration, washed, dried at about 105°C for 15 hours, and calcined at 700°C. There was, thus, obtained an ultraviolet radiation shielding pigment in which 100 parts by weight of muscovite were coated with 25 parts by weight of barium sulfate particles and 125 parts by weight of zinc oxide particles. Its examination by a SEM revealed that the barium sulfate particles had an average diameter of about 1.5 µm (microns), while the zinc oxide particles in needle crystal form had an average major-axis diameter of about 0.8 µm (micron). The pigment as obtained showed good extensibility and adhesiveness on the skin, and did not show any appreciable dulling in color when mixed with oil. [Comparative Examples] Comparative Example 1: A suspension was prepared by suspending 150 g of fine muscovite particles having a diameter of 1 to 15 µm (microns) in 1.5 liters of water, and heated to about 80° C, and 530 g of zinc sulfate were added to it under stirring. A 32% by weight aqueous solution of sodium hydroxide was dropped into the suspension at a rate of 5 ml/min. until it had a pH of 8.5. Then, after one hour of stirring, solid matter was separated from the suspension by filtration, washed, dried at about 105°C for 15 hours, and calcined at 700°C. As a result, there was obtained a pigment in which 100 parts by weight of muscovite were coated with 100 parts by weight of zinc oxide particles. Its examination by a SEM revealed that the zinc oxide particles in needle crystal form had an average major-axis diameter of about 0.2 µm (micron). The pigment as obtained was inferior to the pigment of Example 1 in extensibility and adhesiveness on the skin, and was inferior to those of Examples 1 and 2 in its power of shielding of ultraviolet radiation. Comparative Example 2: A pigment in which 100 parts by weight of muscovite were coated with 40.6 parts by weight of barium sulfate and 15 parts by weight of zinc oxide (Example 6 in Japanese Patent Publication No. Hei 2-42388). This pigment was inferior to the pigments of Examples 1 and 2 in its power of shielding of ultraviolet radiation. It was also inferior to Example 1 in its resistance to color dulling. [Testing of the power of shielding of ultraviolet radiation] Preparation of test samples: A test sample was prepared by admixing 0.3 g of each of the pigments according to Examples 1 and 2 and Comparative Examples 1 and 2 in 9.7 g of a medium ink based on a vinyl chloride resin to form a dispersion, applying the dispersion onto a glass plate by an applicator having a thickness of 120 µm (microns), and drying it. Method of measuring ultraviolet radiation transmittance: A spectrophotometer (Model 228 of Hitachi) was used for examining each test sample for its transmittance of radiation having a wavelength of 200 to 700 nm (see Figure 1). The ordinate axis represents the transmittance (%), while the abscissa axis represents the wavelength (nm). The powders as obtained in Examples 1 and 2 showed a low transmittance of radiation in they ultraviolet wavelength range (up to 400 nm) and the UV-A range, as compared with the products of Comparative Examples 1 and 2, and moreover, showed a minimum point of transmittance considered as absorption in the UV-A range. These results confirmed that the pigments of this invention had a high power of shielding of ultraviolet radiation. [Testing on color dullness] The pigment as prepared in each of the above Examples and Comparative Examples was mixed with liquid paraffin, and the hue which it presented in the mixture was compared with its original hue by a colorimeter (CR-300 of Minolta) as a measure of its color dullness having an important factor in the preparation of a cosmetic containing it. The results are shown in Figure 2. In Figure 2, the ordinate axis, ΔE*, represents the color difference as calculated by the following equation from the L value (lightness), the a value (red or green index) and the b value (yellow or blue index), while the abscissa axis represents the amount (%) of liquid paraffin in the mixture. [Equation 1] ΔE* = ΔL 2 +Δa 2 +Δb 2 As a result, it was confirmed that the pigment of Example 1 was more resistant to color dulling than that of Comparative Example 2 and a micaceous body pigment, and was suitable as a pigment for cosmetics. Each of the pigments as prepared in the above Examples and Comparative Examples and mica were examined for their absorption of linseed oil. The results are shown in Table 1. The results confirmed that the pigments of Examples 1 and 2 absorbed a smaller amount of oil than the pigment of Comparative Example 1 not coated with barium sulfate, and were satisfactory in moldability when forming a cake in preparing a cosmetic, such as a compact powder. [Table 1] Absorption of oil by pigments Pigment Amount of oil absorbed (ml/100 g) Example 1 81 Example 2 55 Comparative Example 1 97 Comparative Example 2 65 Mica-M 53 The following is a description of a few examples of use of the ultraviolet radiation shielding pigment according to this invention. [Examples of Use] Example 1 of Use (Compact Powder): Composition Ultraviolet radiation shielding pigment according to Example 1 or 2 25 parts by weight Color pigment 5 parts by weight Lanolin 3 parts by weight Isopropyl myristate Balance Magnesium stearate 2 parts by weight Talc 50 parts by weight Example 2 of Use (Automobile Paint): Composition Composition A (acrylmelamine resin): Acrydic® 47-712 70 parts by weight Super Beccamine® G821-60 30 parts by weight Composition B: Ultraviolet radiation shielding pigment according to Example 1 or 2 10 parts by weight Pearl pigment 10 parts by weight Composition C (thinner for acrylmelamine resin): Ethyl acetate 50 parts by weight Toluene 30 parts by weight n-butanol 10 parts by weight Solvesso® #150 40 parts by weight A mixture of Compositions A and B is diluted with Composition C to form a dilution having a viscosity suitable for spray coating (12 to 15 seconds with a Ford Cup #4), and it is sprayed to form a base coat layer. Example 3 of Use (Plastic Composition): Composition High-density polyethylene resin (pellets) 100 parts by weight Ultraviolet radiation shielding pigment according to Example 1 or 2 1 part by weight Magnesium stearate 0.1 part by weight Zinc stearate 0.1 part by weight The pellets of the above composition are dry blended, and injection molded. [Brief Description of the Drawings] Figure 1 is a graph comparing the pigments of Examples 1 and 2 and Comparative Examples 1 and 2 with respect to their transmittance of ultraviolet radiation having a wavelength in the range of 200 to 700 nm; Figure 2 is a graph showing the results of the tests conducted to examine the dulling in color of each of the pigments according to Examples 1 and 2 and Comparative Examples 1 and 2.

Claims: A pigment for shielding of ultraviolet radiation which comprises a flaky powder having particle surfaces coated with particles of barium sulfate having an average diameter of 0.1 to 2.0 µm (microns), and with needle crystal particles of zinc oxide having an average major-axis diameter of 0.05 to 1.5 µm (microns), wherein the amount in parts by weight of said barium sulfate is smaller than that of said zinc oxide, relative to the amount of said flaky powder. A pigment as set forth in claim 1, wherein the amount of said barium sulfate is 10 to 50 parts by weight and that of said zinc oxide is 50 to 200 parts by weight, relative to 100 parts by weight of said flaky powder. A process for manufacturing a pigment as set forth in claim 1 or 2, which comprises suspending a flaky powder in water to form its suspension; dropping (a) a water soluble barium compound, and (b) a solution containing a member of the group consisting of sulfuric acid, sodium sulfate and potassium sulfate and containing a higher chemical stoichiometric equivalent ratio of sulfate ions than barium ions in (a), into said suspension in such way as either dropping (b) after adding an appropriate amount of (a), or dropping (a) and (b) simultaneously, whereby the particles of said flaky powder in said suspension are coated with particles of barium sulfate; dropping (c) a water soluble zinc compound of the group consisting of zinc sulfate and zinc acetate, and (d) a basic solution into said suspension in such a way as either dropping (d) after adding an appropriate amount of (c), or dropping (c) and (d) simultaneously, whereby said particles of said flaky powder are coated with the hydroxide or carbonate of zinc; collecting said coated particles by filtration; washing them; drying them; and calcining them. A cosmetic, paint, or plastic material containing a pigment as set forth in claim 1 or 2.

Inventors: NOGUCHI TAMIO, WATANABE YUKITAKA, NOGUCHI, TAMIO, WATANABE, YUKITAKA, Noguchi, Tamio, c/o Merck Japan K.K., Watanabe, Yukitaka, c/o Merck Japan K.K.
Applicants: MERCK PATENT GMBH
Application year: 1998
Main classification: C09C1
IPC codes: C09D7, A61K8, C09C1, C08K9, C08L101, A61Q17, C09C3, C09D5, C01G9, C09B67, A61Q1