Chromatic Pigments Comprising a Coloring Metal Oxide Coating on Transparent Titanium Oxide Core Particles of 0.01 to 0.1 Micron Size

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 TiO.sub.2 particles generally have a mean particle size of about 0.2-0.3 .mu.m. Finer particles of TiO.sub.2 having a mean particle size less than 0.1 .mu.m exhibit different optical properties from the TiO.sub.2 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. Pat. No. 4,753,829 discloses the incorporation of TiO.sub.2 microparticles into the base coat of multi-layer metallic coatings to promote "down-flop" effect.

TiO.sub.2 based chromatic pigments have been known including those disclosed in U.S. Pat. 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 TiO.sub.2 lattice. These pigments are produced by intimately mixing TiO.sub.2 and the coloring metal oxides or oxide precursors, calcining the mixture at a temperature of 1000.degree.-1300.degree. 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 .mu.m.

Accordingly, needs exist for chromatic pigments useful as a colorants of paints, plastics, cosmetics and the like while retaining at the same time most of beneficial properties of TiO.sub.2 microparticles such and UV blocking and down flop promoting properties.

In one aspect of the present invention, there is provided a chromatic pigment comprising core particles of TiO.sub.2 having a mean particle size of 0.01 to 0.1 .mu.m coated thereon with a layer of an oxide of Ni, Co, Ce, Cu, Cr, Mn, V, W or a mixtue of these oxides.

In another aspect of the present invention, there is provided a method for producing a chromatic pigment comprising the steps of suspending TiO.sub.2 microparticles having a mean particle size of 0.01 to 0.1 .mu.m in an aqueous medium, adding a water-soluble compound of Ni, Co, Ce, Cu, Cr, Mn, V, W or a mixture of said compound, hydrolyzing said metal compound and depositing the resulting metal hydroxide on said TiO.sub.2 particles, recovering and calcining the resulting product at a temperature of 300.degree. C. to 800.degree. C.

FIG. 1 shows X-ray diffraction patterns of TiO.sub.2 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 TiO.sub.2 microparticles, respectively.

The chromatic pigment of the present invention comprises core particles TiO.sub.2 having a mean particle size of 0.01 to 0.1 .mu.m and a colored metal oxide layer covering the core particles.

The starting TiO.sub.2 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 avilable TiO.sub.2 microparticles having a protective coating of SiO.sub.2, Al.sub.2 O.sub.3 or ZrO.sub.2 on the TiO.sub.2 core particles may also be used. Particles greater than 0.1 .mu.m have a greater opacity than desirable to use as the starting material of the pigment of the present invention.

The starting TiO.sub.2 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 TiO.sub.2 microparticles is such that the final pigment particles are coated with 1 to 30% as NiO, CeO.sub.2, CoO, CuO, Cr.sub.2 O.sub.3, MnO.sub.2, V.sub.2 O.sub.5 or WO.sub.2 by weight of the starting TiO.sub.2 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.degree. 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 TiO.sub.2 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.degree.-150.degree. C. to obtain TiO.sub.2 microparticles coated with a metal hydroxide layer. Color development takes place when calcining the above product at a temperature above 300.degree. 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.degree. 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 conventional 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 TiO.sub.2 -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 TiO.sub.2 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. Pat. 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 TiO.sub.2 microparticle pigment of the present invention; and (d) the balance of an organic volatile solvent for said film-forming synthetic resin.

The film-forming systhetic 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, TiO.sub.2 -coated mica flake, TiO.sub.2 -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 solvnets 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 TiO.sub.2 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 TiO.sub.2 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 TiO.sub.2 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.

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 NiSO.sub.4 .multidot.6H.sub.2 O with stirring. After heating the suspension to 80.degree. 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.degree. C., calcined in an electrical furnace at 600.degree. 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.

The process of Example 1 was repeated except that the pigment was calcined at 700.degree. C.

The process of Example 1 was repeated except that amount of nickel sulfate was decreased to 250 g.

The process of Example 1 was repeated except that 672 g of cerium sulfate as Ce.sub.2 (SO.sub.4).sub.3 .multidot.8H.sub.2 O was replaced for 496 g of NiSO.sub.4 .multidot.6H.sub.2 O.

The process of Example 1 was repeated except that 529 g of cobalt sulfate as CoSO.sub.4 .multidot.7H.sub.2 O was replaced for 496 g of NiSO.sub.4 .multidot.6H.sub.2 O.

The process of Example 1 was repeated except that 472 g of copper sulfate as CuSO.sub.4 .multidot.5H.sub.2 O was replaced for 496 g of NiSO.sub.4 .multidot.6H.sub.2 O.

The process of Example 1 was repeated except that 200 g of chromium chloride as CrCl.sub.3 .multidot.6H.sub.2 O was replaced for 496 g of NiSO.sub.4 .multidot.6H.sub.2 O.

The process of Example 1 was repeated except that 319 g of manganese sulfate was MnSO.sub.4 .multidot.H.sub.2 O was replaced for 496 g of NiSO.sub.4 .multidot.6H.sub.2 O.

The process of Example 1 was repeated except that 515 g of vanadium sulfate as VSO.sub.4 .multidot.7H.sub.2 O was replaced for 496 g of NiSO.sub.4 .multidot.6H.sub.2 O.

The process of Example 1 was repeated except that 535 g of ammonium tungstate as (NH.sub.4).sub.2 WO.sub.4 was replaced for 496 g of NiSO.sub.4 .multidot.6H.sub.2 O.

The process of Example 1 was repeated except that 132 g of antimong chloride as SbCl.sub.3 was used in combination with 496 g of nickel sulfate as NiSO.sub.4 .multidot.6H.sub.2 O.

The TiO.sub.2 microparticles used as starting material in Example 1 were calcined in an electrical furnace at 600.degree. C. for 2 hours and then pulverized in an atomizer to give white TiO.sub.2 microparticles.

Amount of Coating: Chemical analysis.

Specific surface area: N.sub.2 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.

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 phathalate 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.

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.

a) The following ingredients were charged in a 400 ml plastic bottle and thoroughly dispersed using a paint shaker.

b) The resulting mill base obtained in step a) was mixed with the following ingredients.

c) A metallic coating composition was prepared accordance to the following formulation.

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.

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 costs simultaneously at 140.degree. C. for 30 minutes.

g) FIGS. 3A and 3B are chromaticity diagrams of the multilayer metallic coating films incorporating the colored TiO.sub.2 microparticles produced in Example 11 and the colorless TiO.sub.2 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.degree. 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 TiO.sub.2 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 ring 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 ALUMINTUM K.K.

Note 7. Product of DAINIPPON INK AND CHEMICALS, INC.