Titanium dioxide-based chromatic pigments

Abstract: TiO₂-based chromatic pigments are provided by coating core particles of TiO₂ having a mean particle size from 0.01 to 0.1 µm with a layer of metal oxide of Ni, Co, Ce, Cu, Cr, Mn, V, W or a mixtrure thereof.

Description: This invention relates to titanium dioxide-based chromatic pigments useful as a colorant of paints, printing inks, plastics, cosmetics and the like. Titanium dioxide particles are well-known as a white pigment and have been used in the paint, plastics, cosmetics and many other industries. The pigment grade TiO₂ particles generally have a mean particle size of about 0.2 - 0.3 µm. US-A-3 324 071 discloses compositions useful as delustring agents comprising pigment grade (0.2-0.3 µm particle size) titanium oxide particles coated with manganese oxide. US-A-2219129 discloses cobalt oxide coated pigment grade (0.2-0.3 µm particle size) titanium oxide particles as pigments for paint compositions. Finer particles of TiO₂ having a mean particle size less than 0.1 µm exhibit different optical properties from the TiO₂ pigments. They are almost colorless or transparent to visible light but selectively block the passage of UV light therethrough. Accordingly, they find uses as UV blocking agents rather than chromatic pigments in the paint, plastics, cosmetics and other industries. In addition, U.S. Patent No. 4,753,829 discloses the incorporation of TiO₂ microparticles into the base coat of multi-layer metallic coatings to promote "down-flop" effect. TiO₂ based chromatic pigments have been known including those disclosed in U.S. Patent No. 2,257,278 and Japanese Patent publication No. 25685/72. They are generally solid solutions of coloring oxides of metals such as Sb, Ba, Ni and Co in a TiO₂ lattice. These pigments are produced by intimately mixing TiO₂ and the coloring metal oxides or oxide precursors, calcining the mixture at a temperature of 1000 - 1300°C, cooling and grinding the resulting products. Pigments thus produced are very hard and it is difficult to grind them to a mean particle size of less than 0.1 µm. Accordingly, needs exist for chromatic pigments useful as colorants of paints, plastics, cosmetics and the like while retaining at the same time most of beneficial properties of TiO₂ microparticles such as UV blocking and down flop promoting properties. In one aspect of the present invention, there is provided a chromatic pigment comprising core particles of titanium dioxide having a mean particle size from 0.01 to 0.1 µm, said particles being coated with a layer of 1 to 30%, by weight of titanium oxide particles, of an oxide of Ni, Co, Ce, Cu, Cr, Mn, V, W or a mixture of these oxides, so that the coated core particles substantially retain the core particles transparency to visible light. In another aspect of the present invention, there is provided a method for producing such a chromatic pigment comprising the steps of suspending titanium dioxide microparticles having a mean particle size of 0.01 to 0.1 µm in an aqueous medium, adding 1 to 30%, calculated as NiO, CoO, CeO₂, CuO, Cr₂O₃, MnO₂, V₂O₅ or WO₂ by weight of said titanium dioxide particles, of a water-soluble compound of Ni, Co, Ce, Cu, Cr, Mn, V, W or a mixture of said compounds, hydrolyzing said compound and depositing the resulting metal hydroxide on said titanium dioxide particles, and recovering and calcining the resulting product at a temperature of 300°C to 800°C, wherein the coated titanium dioxide microparticles in the resulting product substantially retain the transparency of the core particles to visible light. Fig. 1 shows X-ray diffraction patterns of Ti0₂ microparticles before and after coating with nickel oxide; Fig. 2 shows light transmission curves of the pigment of Example 1 calcined at varying temperatures; and Figs. 3A and 3B are chromaticity diagrams of multilayer metallic coatings incorporating the pigment of the present invention and conventional TiO₂ microparticles, respectively. The chromatic pigment of the present invention comprises core particles TiO₂ having a mean particle size of 0.01 to 0.1 µm and a colored metal oxide layer covering the core particles. The starting TiO₂ microparticles may be produced by the well-known colloid chemical process or the chemical concentration process. Their crystallographic structure may be anatase, rutile or a mixture thereof. Commercially available TiO₂ microparticles having a protective coating of SiO₂, Al₂O₃ or ZrO₂ on the TiO₂ core particles may also be used. Particles greater than 0.1 µm have a greater opacity than desirable to use as the starting material of the pigment of the present invention. The starting TiO₂ microparticles are suspended in water preferably at a concentration less the 300 g/L, typically at a concentration of 200 g/L. To the suspension is added a water-soluble metal compound with stirring. The color characteristic of final pigment is determined by the particular metal species employed. To this end, Ni, Ce, Cr, V and W give yellow, Co gives green, Cu and Mn give black. Mixtures of two metal species give an intermediate color. Examples of water-soluble compounds include chlorides, sulfates and nitrates of Ni, Ce, Co, Cu, Cr and V, and ammonium tungstate. The ratio of the water-soluble metal compound to TiO₂ microparticles is such that the final pigment particles are coated with 1 to 30 % as NiO, CeO₂, CoO, CuO, Cr₂O₃, MnO₂, V₂O₅ or WO₂ by weight of the starting TiO₂ microparticles. The color intensity of the final pigment is directly proportional to the amount of coating. Therefore, the exact amount of water-soluble metal compounds to be added will depend upon the color intensity as desired. After the addition of water-soluble metal compounds, the suspension is heated to a temperature from 40 °C to the boiling point thereof for at least 30 minutes with stirring. Then the suspension is gradually neutralized to a pH 8-9 with a suitable base or acid such as aqueous ammonia, sodium hydroxide, hydrochloric acid, sulfuric acid or nitric acid to hydrolyze the metal salt and deposit the resulting metal hydroxide on the TiO₂ microparticles. After neutralization, the suspension is allowed to stand for at least 30 minutes and then filtered. The resulting filter cake is washed with water and dried at 100-150 °C to obtain TiO₂ microparticles coated with a metal hydroxide layer. Color development takes place when calcining the above product at a temperature above 300 °C for 0.5 to 4 hours. Care should be taken in this step to prevent the pigment particles from sintering to large hard aggregates. Experiments have shown that about 800 °C is the upper limit to maintain the desired particle size and transparency in the final pigment. Experiments have also shown that the addition of a small amount of calcium or aluminum salts, silicates, phosphates, antimony salts, zirconium salts, tin compounds, zinc compounds or niobium compounds to the suspension prior to the neutralization step is effective for the prevention of sintering into large aggregates, color development, particle size-regulating and other purposes. This calcining step may be carried out in any coventional furnace of tunnel, electrical and othe types. The pigment thus produced may be further coated with one or more hydrated oxides of Si, Al, Zn, Sn and like metals to enhance weatherability, dispersibility and other properties. As can be easily appreciated, the pigment of the present invention has unique optical properties compared with prior art TiO₂-based colored pigments such as titanium yellow. The pigment has a sufficiently fine particle size to selectively transmit visible light while blocking the passage of UV light as do TiO₂ microparticles. This is because the pigment is calcined at an elevated temperature at which the pigment particles are not sintered to large hard aggregates. The color characteristics of the pigment of the present invention vary depending upon the metal species and the amount of colored metal oxide layer from pastel to deep tone and find a variety of uses in the paint, plastics, cosmetics and other industries. The pigment of the present invention is particularly useful to impart multilayer metallic coatings comprising a metallic base coat and a clear top coat with down-flop effect by incorporating the pigment of this invention into the base coat as taught by U.S. Patent No. 4,753,829. Such multilayer metallic coatings are well-known in the art to finish, for exmaple, automobile bodies. The base coat composition may comprise (a) 20 to 40 % by weight of a film-forming synthetic resin; (b) 1 to 10 % by weight of a metallic or metallic-like pigment; (c) 1 to 15 % by weight of the colored TiO₂ microparticle pigment of the present invention; and (d) the balance of an organic volatile solvent for said film-forming synthetic resin. The film-forming synthetic resin (a) may be any of the polymers known to be useful in coating compositions such as acrylic polymers, alkyd resins and polyester resins having cross-linkable functional groups such as hydroxy and/or carboxyl groups. Cross-linkers such as melamine resins and polyisocyanate compounds, and catalysts for the reaction of the cross-linkers with the film-forming polymers such as sulfonates and tin compounds, where present, should be considered as being part of said film-forming synthetic resins. The metallic or metallic-like pigment may be any of conventional pigments such as aluminum flake, bronze flake, TiO₂-coated mica flake, TiO₂-coated micaceous iron oxide flake and the like. They preferably have an aspect ratio from 10 to 100. These metallic or metallic-like pigments may be used in combination with conventional non-metallic pigments such as carbon black, iron oxide, perylene pigments and the like. The organic volatile solvents may be any of liquids which are conventionally used as polymer solvents in coating compositions such as aliphatic or aromatic hydrocarbons, ester, ketones, alcohols and the like. The clear topcoat composition to be applied on the base coat may be generally the same as the base coat composition except that it does not contain pigments and thus transparent. In use, the base coat composition is applied on a suitable substrate such as antomobile bodies to a film thickness of, for example, 10 to 50 microns and then the clear topcoat compositions is applied thereon preferably wet-on -wet followed by baking both coats simultaneously. The multilayer metallic coatings incorporating the pigment of this invention exhibit higher down-flop effect and chromaticity than the corresponding coatings incorporating conventional colorless or white TiO₂ microparticles. Particularly, the pigment of this invention having yellowish colors gives the multilayer coatings which have warmer flop (color viewed on the same side as the incident light at a near angle) and more intense golden face (color viewed on the opposite side to the incident light) than the conventional colorless or white TiO₂ microparticles. Furthermore, the pigment of this invention have a number of unique optical properties including adequate degrees of light scattering, refracting and transmitting properties owing to the TiO₂ core microparticles. These unique properties are not found in conventional color pigment microparticles such as iron oxide microparticles. The following examples are offered for illustrative purposes only. Example 1 1400 g of rutile type titanium dioxide microparticles having a mean particle size of 15 nm was dispersed in water at a concentration of 200 g/L. To the suspension was added 496 g of nickel sulfate as NiSO₄·6H₂O with stirring. After heating the suspension to 80 °C, an amount of 50 g/L aqueous solution of sodium hydroxide was added over 60 minutes until the pH of the suspension became 9. Then the suspension was allowed to stand for 30 minutes and filtered. The resulting filter cake was washed with 7 L of water, centrifuged, dried at 120 °C, calcined in an electrical furnace at 600 °C for 2 hours, and pulverized in an atomizer. Properties of the resulting pigment are shown in Table 1. X-ray diffraction patterns of the titanium dioxide microparticles before and after coating with nickel oxide are shown in Fig. 1. Example 2 The process of Example 1 was repeated except that the pigment was calcined at 700 °C. Example 3 The process of Example 1 was repeated except that amount of nickel sulfate was decreased to 250g. Example 4 The process of Example 1 was repeated except that 672 g of cerium sulfate as Ce₂(SO₄)₃·8H₂O was replaced for 496 g of NiSO₄·6H₂O. Example 5 The process of Example 1 was repeated except that 529 g of cobalt sulfate as CoSO₄·7H₂O was replaced for 496 g of NiSO₄·6H₂O. Example 6 The process of Example 1 was repeated except that 472 g of copper sulfate as CuSO₄·5H₂O was replaced for 496 g of NiSO₄·6H₂O. Example 7 The process of Example 1 was repeated except that 200 g of chromium chloride as CrCl₃·6H₂O was replaced for 496 g of NiSO₄·6H₂O. Example 8 The process of Example 1 was repeated except that 319 g of manganese sulfate as MnSO₄·H₂O was replaced for 496 g of NiSO₄·6H₂O. Example 9 The process of Example 1 was repeated except that 515 g of vanadium sulfate as VSO₄·7H₂O was replaced for 496 g of NiSO₄·6H₂O. Example 10 The process of Example 1 was repeated except that 535 g of ammonium tungstate as (NH₄)₂WO₄ was replaced for 496 g of NiSO₄·6H₂O. Example 11 The process of Example 1 was repeated except that 132 g of antimon chloride as SbCl₃ was used in combination with 496 g of nickel sulfate as NiSO₄·6H₂O. Comparative Example The TiO₂ microparticles used as starting material in Example 1 were calcined in an electrical furnace at 600 °C for 2 hours and then pulverized in an atomizer to give white TiO₂ microparticles. Amount of coating : Chemical analysis. Specific surface area : N₂ gas adsorption method using Model SA-1000 sold by Shibata Rikagaku Kikai Kabushiki Kaisha. Munsell value : Measurement was made on sample pigments tapped on a glass cell using Minolta chroma meter Model CR-200. Measurement of light transmission The pigments of Example 1 calcined at varying temperatures were tested for light transmission properties. 1.57 g of sample pigment was intimately dispersed in a mixture of 21.8 g of nitrocellulose lacquer (note 1), 10.4 g of solvent mixture (note 2) and 2.1 g of dibutyl phthalate in a paint conditioner for 1 hour. This composition was applied on a polypropylene film using a 1.5 mil applicator and dried at room temperature overnight. Note 1. Nitrocellulose lacquer. Nitrocellulose 10 parts by weight Butyl acetate 9 parts by weight Ethyl acetate 6 parts by weight Ethylcellosolve 3 parts by weight Toluene 8 parts by weight Note 2. Solvent mixture. Butyl acetate 9 parts by weight Ethyl acetate 6 parts by weight Ethylcellosolve 3 parts by weight Toluene 9 parts by weight Percents transmission over a wavelength range between 300 nm and 700 nm were determined using specimen films thus prepared. The results are shown in Fig. 2. Multilayer metallic coatings incorporating the pigment of this invention a) The following ingredients were charged in a 400 ml plastic bottle and thoroughly dispered using a paint shaker. TiO₂ microparticles produced in Example 11 or Comparative Example 20.0 g Acrylic varnish ACRYDIC 47-712 (note 3), 50 % nonvolatile 40.0 g Solvent mixture (note 4) 40.0 g Zirconia beads of 0.8 mm diameter 500.0 g b) The resulting mill base obtained in step a) was mixed with the following ingredients. Acrylic varnish ACRYDIC 47-712 40.0 g Melamin resin SUPER BEKKAMIN L-117 (note 5), 60 % nonvolatile 16.7 g Solvent mixture (note 4) 16.0 g c) A metallic coating composition was prepared according to the following formulation. Aluminum flake paste ALUMIPASTE 7130 N (note 6), 64 % nonvolatile 40.0 g Acrylic varnish ACRYDIC 47-712 160.0 g Melamine resin SUPER BEKKAMIN L-117 33.3 g Solvent mixture (note 4) 135.0 g d) 120 g of the composition in step b) and 35 g of the composition in step c) were intimately mixed in a paint mixer for 30 minutes to obtain a base coat composition. e) A clear top coat composition was prepared according to the following formulation. Acrylic varnish ACRYDIC 44-179 (note 7) 100.0 g Melamin resin SUPER BEKKAMIN L-117 35.0 g f) The base coat composition was sprayed on a steel plate to a dry film thickness of 25 microns and allowed to set at room temperature for 5 minutes. Then the clear top composition was sprayed thereon wet-on-wet, allowed to set for 8 minutes and baked both coats simultaneously at 140 °C for 30 minutes. g) Figs. 3A and 3B are chromaticity diagrams of the multilayer metallic coating films incorporating the colored TiO₂ microparticles produced in Example 11 and the colorless TiO₂ microiparticles produced in Comparative Example, respectively. The measurement was made using a gonio-photometric color measurement system GCMS-3 sold by Murakami Color Research Laboratories Co. Ltd., at an constant incident angle of 70 °C. The color of reflected light was measured at varying angles. The results show that the coating film incorporating the pigment of Example 11 exhibit a variety of colors compared with the corresponding coating film incorporating the colorless TiO₂ microparticles of Comparative Example. This means that the coating film incorporating the pigment of the present invention will change its color delicately with the change in viewing angle. Furthermore, the results show that a bluish color has been diminished greatly on the flop side of the coating film incorporating the pigment of the present invention. Note 3. Product of DAINIPPON INK AND CHEMICALS, INC. Note 4. Toluene/xylene/ethyl acetate/butyl cellosolve = 5/2/2/1 by volume. Note 5. Product of DAINIPPON INK AND CHEMICALS, INC. Note 6. Product of TOYO ALMINIUM K.K. Note 7. Product of DAINIPPON INK AND CHEMICALS, INC.

Claims: A chromatic pigment comprising core particles of titanium dioxide having a mean particle size from 0.01 to 0.1 µm, said particles being coated with a layer of 1 to 30%, by weight of the titanium dioxide particles, of an oxide of Ni, Co, Ce, Cu, Cr, Mn, V, W or a mixture of these oxides, so that the coated core particles substantially retain the core particles transparency to visible light. A pigment as claimed in claim 1, wherein said oxide is nickel oxide. A pigment as claimed in claim 1, wherein said oxide is cobalt oxide. A pigment as claimed in claim 1, wherein said oxide is cerium oxide. A pigment as claimed in claim 1, wherein said oxide is copper oxide. A pigment as claimed in claim 1, wherein said oxide is chromium oxide. A pigment as claimed in claim 1, wherein said oxide is manganese oxide. A pigment as claimed in claim 1, wherein said oxide is vanadium oxide. A pigment as claimed in claim 1, wherein said oxide is tungsten oxide. A pigment as claimed in any one of the preceding claims, wherein said titanium dioxide particles are of rutile type. A method for producing a chromatic pigment as claimed in claim 1, comprising the steps of suspending titanium dioxide microparticles having a mean particle size of 0.01 to 0.1 µm in an aqueous medium, adding 1 to 30%, calculated as NiO, CoO, CeO₂, CuO, Cr₂O₃, MnO₂, V₂O₅ or WO₂ by weight of said titanium dioxide particles, of a water-soluble compound of Ni, Co, Ce, Cu, Cr, Mn, V, W or a mixture of said compounds, hydrolyzing said compound and depositing the resulting metal hydroxide on said titanium dioxide particles, and recovering and calcining the resulting product at a temperature of 300°C to 800°C, wherein the coated titanium dioxide microparticles in the resulting product substantially retain the transparency of the core particles to visible light. A method as claimed in claim 11, wherein said water-soluble metal compound is the chloride, sulfate or nitrate of Ni, Co, Ce, Cu, Cr, Mn or V, or ammonium tungstate. A method as claimed in claim 11 or claim 12, wherein said hydrolyzing step includes adjusting the pH of said suspension at 8 to 9 with an alkali or acid. A coating composition comprising: (a) 20 to 40% by weight of a film-forming synthetic resin; (b) 1 to 10% by weight of a metallic or metallic-like pigment; (c) 1 to 15% by weight of a coloured TiO₂ microparticle pigment as claimed in claim 1; and (d) the balance of an organic volatile solvent for said film-forming synthetic resin, said composition being for use in finishing a substrate with a multilayer metallic coating comprising a base coat of said composition and a clear top coat applied thereon.

Inventors: IDEI SHUNJI, YASUKI TAKASHI, IDEI, SHUNJI, YASUKI, TAKASHI
Applicants: TAYCA CORP, TAYCA CORPORATION
Application year: 1990
Main classification: C09C1
IPC codes: C09D7, C09D201, C09C1, C09C3, C08K3, C09D5