TITANIUM DIOXIDE COATED MICACEOUS IRON OXIDE PIGMENTS AND METHOD FOR PRODUCING THE SAME

Abstract: The weatherability and lightfastness of TiO₂ coated micaceous iron oxide (MIO) pigment may be improved by transforming the TiO₂ present in the coating layer to rutile TiO₂. Further improvement in the weatherability and lightfastness may be achieved by coating the pigment with an oxide of Cr, V, Mo, Al or a mixture thereof and/or a hydrous oxide of Cr or V, or a combination of two or more hydrous oxides of Cr, V, Al and Si.

Description: BACKGROUND OF THE INVENTION This invention relates to titanium dioxide coated micaceous iron oxide pigments having improved weather-ability and lightfastness. It also relates to the production of such pigments. Japanese Patent Publication No. 7176/1976 assigned to the assignee of this application discloses a pigment composed of flaky micaceous iron oxide (MIO) having a thin coating layer of a metal oxide such as titanium dioxide. This coating layer is transparent and has a high refractive index so that the pigment exhibits a unique metallic glamor owing to the interference of light. It has been found, however, that the above pigment tends to lose its unique glamor upon exposing to light for a long period of time particularly under outdoor conditions. This is because the TiO₂ crystals forming the coating layer are of anatase type. Attempts have been made to produce a similar pigment coated with rutile type TiO₂ which is known to have a greater weatherability than anatase type TiO₂. Unfortunately, since the transition of TiO₂ to rutile requires a higher temperature and/or a longer period of time than the transition to anatase, the above attempts failed to produce a pigment having the same or similar optical properties as the prior art pigment. This is because the MIO substrate is not stable and/or reacts with TiO₂ under such conditions. DE-A-2255464 describes titanium dioxide-coated MIO pigments which are further coated with aluminium oxide. Chemical Abstracts, volume 102, 115231t describes anatase-coated mica pigments further coated with a Group II metal halide. SUMMARY OF THE INVENTION It is an object of the present invention to provide an MIO pigment having a coating of rutile TiO₂ with improved weatherability while retaining a unique light interference glamour. Other objects and advantages of the invention will become apparent as the description proceeds. In one aspect of the present invention, there is provided a pigment composed of flaky micaceous iron oxide particles having a thin coating of TiO₂ characterised in that said TiO₂ in the coating is rutile TiO₂; that said pigment has on said TiO₂ coating a plurality of protective coatings of an oxide of Cr, V, Mo, Al or a combination thereof; and that the primary protective coating has been calcined at an elevated temperature at which a major proportion of said TiO₂ in the coating layer is transformed to rutile TiO₂. According to another aspect of the present invention, there is provided a method for producing a pigment of the invention 1 to 4 which comprises the steps of (a) adding a water-soluble titanium compound and a water-soluble salt of Zn, Sn, Sb, Li or a mixture thereof to an aqueous suspension of flaky micaceous iron oxide particles, (b) hydrolysing said titanium compound and said water-soluble metal salt and depositing the resulting hydrous TiO₂ and the hydrous oxide of said metal on the surface of said micaceous iron oxide particles, (c) resuspending the hydrous TiO₂ coated particles in an aqueous medium, (d) adding a water-soluble metallic compound of Cr, V, Mo, Al or a mixture thereof, (e) hydrolysing the water-soluble metallic compound and depositing the resulting hydrous metal oxide on said hydrous TiO₂ coating, (f) calcining the coated particles at a temperature from 500 °C to 1000 °C, (g) suspending the calcined particles in an aqueous medium, (h) repeating said step (d) and step (e), (i) recovering and drying the resulting coated particles. DETAILED DESCRIPTION OF THE INVENTION The process for producing an MIO pigment having a coating of rutile type TiO₂ according to the present invention may be as disclosed in the above-cited Japanese Patent Publication No. 7176/1976 except that a metal salt capable of promoting the transformation of TiO₂ to rutile type is added to an aqueous suspension of MIO prior to the calcination step of TiO₂ coated MIO particles. The starting MIO particles may be naturally occurring or synthetic. They are flaky in shape and preferably have an average diameter of 2 to 200 microns and an average thickness of 0.3 to 5 microns. In a first embodiment of the invention, the MIO particles are suspended in water to make a slurry and a solution of a water-soluble titanium compound is added with stirring. In order to hydrolyze the titanium compound to hydrous TiO₂, the slurry is gradually heated to a temperature between 80°C and the boiling point. Alternatively, the slurry is gradually neutralized by adding a base portionwise with stirring. The resulting hydrous TiO₂ particles will deposit on the surfaces of MIO particles. Usable titanium compounds include titanyl sulfate, titanium tetrachloride and a mixture thereof. Titanium tetrachloride is preferable. The rutile-transforming promoter may be added to the aqueous slurry of MIO prior to, simultaneously with or subsequent to the addition of the solution of said titanium compound. Usable promoters include water-soluble salts of Zn, Sn, Sb or Li such as chlorides, nitrates, sulfates and mixtures of these salts. Zn and Sn salts are preferred. Preferably the aqueous slurry is constantly stirred during the coating operation. After the coating operation, the slurry may be filtered. Then the wet cake is resuspended in water to make an aqueous slurry and a solution of a water-soluble salt of Cr, V, Mo or Al such as chloride, nitrate or sulfate, or a mixture of these salts is added. The slurry is then gradually heated to a temperature between 80°C and the boiling point or gradually neutralized with the addition of a base portionwise to hydrolyze the water-soluble metal salt to the corresponding hydrous metal oxide. Preferbly the slurry is constantly stirred during this operation. The resulting hydrous metal oxide will deposit on the previously formed hydrous TiO₂ coating layer. The resulting coated MIO particles are recovered by filtration, washed with water, dried and calcined at a temperature ranging from 500°C to 1000°C,preferably from 600°C to 900°C. Among metal species tested for this purpose including Cr, V, Al, Mn, Mo, Zr, Fe, Co, Cu, rare earth metals and their combinations. Cr, V, Mo and Al have been proven satisfactory when used alone or in combination. When a plurality of oxides of these metals are used, they may be deposited on the substrate particles either simultaneously or successively. The amount of this (primary) protective coating prior to the calcination step preferably ranges between 0.001 and 1.0% calculated as metal oxide by weight of the TiO₂ coated MIO substrate. A secondary protective coating layer is then provided in addition to the primary protective coating on the TiO₂ coated MIO pigment. This may be performed by adding a solution of a water-soluble salt of Cr, V, Mo or Al to an aqueous slurry of the pre-coated MIO pigment, hydrolysing the metal salt and depositing the resulting hydrous metal oxide, recovering the coated particles, washing and drying in the same manner as the previous embodiments except for the omission of the calcination step. This post coating also has an effect of added improvement in weatherability of the finished pigment. When a plurality of hydrous hydroxides of these metals are to be coated, they may be deposited either simultaneously or successively. The amount of this post-coating preferably ranges between 0.1 and 10% calculated as hydrous metal oxide by weight of the TiO₂ coated MIO substrate. The invention is further illustrated by the following examples in which all percents are by weight unless otherwise indicated. Examples 10-13 illustrate the invention, and the other examples are included for reference. EXAMPLE 1 To a suspension of 200 g of MIO in 1.4 liter of water were added with stirring 146 g of an aqueous solution of titanium tetrachloride (containing 27.4% of TiO₂ and 32.3% of HCl) and 14 g of SnCl₄·5H₂O. The mixture was heated gradually at a rate of 1°C/minute to 98°C and maintained at the same temperature for 3 hours with stirring. Then the suspension was filtered and the resulting cake was washed, dried and calcined at 700°C for 1 hour to give TiO₂ coated MIO pigment. X-ray diffraction analysis revealed that the TiO₂ coating layer consisted of rutile TiO₂. EXAMPLE 2 To a suspension of 200 g of MIO in 1.4 liter of water was added with stirring 146 g of anaqueous solution of titanium tetrachloride (containing 27.4% of TiO₂ and 32.3% of HCl). The mixture was heated gradually at a rate of 1°C/minutes to 95°C and maintained at the same temperature for 3 hours. The suspension was filtered and the resulting cake was washed and resuspended in 1 liter of water. After 15 g of ZnCl₂ dissolved in a small amount of water was added, the suspension was neutralized gradually to pH 6.5 with aqueous ammonia, stirred for 30 minutes and then filtered. The resulting cake was washed, dried and calcined at 730°C for 1 hour to give TiO₂ coated MIO pigment. X-ray diffraction analysis revealed that the TiO₂ coating layer consisted of rutile TiO₂. EXAMPLE 3 To a suspension of 200 g of MIO in 1.4 liter of water were added with stirring 146 g of an aqueous solution of titanium tetrachloride (containing 27.4% of TiO₂ and 32.3% of HCl) and 14 g of SnCl₄·5H₂O. The mixture was heated gradually at a rate of 1°C/minutes to 98°C and stirred for 3 hours at the same temperature. The suspension was filtered and the resulting cake was washed to obtain wet cake of hydrous TiO₂ coated MIO. This cake was resuspended in 1 liter of water and heated to 50°C. To this were added 8 ml of an aqueous solution of aluminum sulfate having a concentration of 50 g/liter as Al₂O₃ and an amount of aqueous solution of NaOH having a concentration of 5 g/liter over 60 minutes while maintaining the pH of the suspension at 5-7. The suspension was neutralized to pH 7.0, stirred for 30 minutes and filtered. The resulting cake was washed, dried and calcined at 800°C for 1 hour to obtain TiO₂ coated MIO pigment. EXAMPLE 4 To the aqueous suspension of hydrous TiO₂ coated MIO prepared in Example 3 was added a solution of 0.075 g of (NH₄)₆Mo₇O₂₄·4H₂O in 50 ml of water. The mixture was neutralized with a 5 g/liter solution of NaOH to pH 7.0 over 30 minutes. Then 8 ml of an aqueous solution of aluminum sulfate (50 g/liter as Al₂O₃) and an amount of the above NaOH solution were added over 60 minutes while maintaining the pH of the suspension at 5-7. The suspension was neutralized to pH 7.0, stirred for 30 minutes and filtered. The resulting cake was washed, dried and calcined at 800°C for 1 hour to obtain TiO₂ coated MIO pigment. EXAMPLE 5 Example 4 was followed except that 0.075 g of (NH₄)₆Mo₇O₂₄ was replaced by 0.21 g of CrCl₃·6H₂O. EXAMPLE 6 Example 4 was followed except that 0.075 g of (NH₄)₆Mo₇O₂₄ was replaced by 0.077 g of NH₄VO₃. EXAMPLE 7 100 g of TiO₂ coated MIO obtained in Example 1 was suspended in 600 ml of water and heated to 50°C. To this was added 3.5 g of CrCl₃·6H₂O in 100 ml of water. The mixture was neutralized with a 5 g/liter solution of NaOH to pH 7.0 over 30 minutes. Then 40 ml of an aqueous solution of aluminum sulfate (50 g/liter as Al₂O₃) was added. The mixture was neutralized with the above NaOH solution to pH 7.0 over 30 minutes, stirred for additional 30 minutes and filtered. The resulting cake was washed and dried at 120°C for 8 hours to obtain TiO₂ coated MIO pigment. EXAMPLE 8 Example 7 was followed except that 3.5 g of CrCl₃·6H₂O was replaced by 1.3 g of NH₄VO₃. EXAMPLE 9 100 g of TiO₂ coated MIO obtained in Example 1 was suspended in 600 ml of water. The suspension was heated to 90°C and adjusted to pH 9,0 with NaOH. To this were added 100 ml of an aqueous solution of sodium silicate (containing 20 g/liter of SiO₂ and 6 g/liter of Na₂O) and an amount of dilute sulfuric acid sufficient to maintain pH 9.0 over 180 minutes. After the addition of sodium silicate solution, the mixture was neutralized to pH 7.0 with dilute sulfuric acid and allowed to stand for 10 minutes. Then 40 ml of an aqueous solution of aluminum sulfate (50 g/liter as Al₂O₃) was added. The mixture was neutralized to pH 7.0 with a 5 g/liter solution of NaOH over 30 minutes, stirred for additional 30 minute and filtered. The resulting cake was washed and then dried at 120°C for 8 hours to obtain TiO₂ coated MIO pigment. EXAMPLE 10 Example 7 was followed starting from TiO₂ coated MIO obtained in Example 3. EXAMPLE 11 Example 7 was followed starting from TiO₂ coated MIO obtained in Example 4. EXAMPLE 12 Example 7 was followed starting from TiO₂ coated MIO obtained in Example 5. EXAMPLE 13 Example 7 was followed starting from TiO₂ coated MIO obtained in Example 6. COMPARATIVE EXAMPLE To a suspension of 200 g of MIO in 1.4 liter of water was added 200 ml of a solution of titanyl sulfate (200 g/liter of TiO₂ and 340 g/liter of H₂SO₄). The slurry was gradually heated to 98°C at a rate of 1°C/minutes with stirring, stirred for additional 3 hours at the same temperature and filtered. The resulting cake was washed, dried and calcined at 700°C for 1 hour to obtain TiO₂ coated MIO pigment. X-ray diffraction analysis revealed that the coating layer consisted of anatase TiO₂. WEATHERING TEST TiO₂ coated MIO pigments prepared in the above examples were subjected to the following weathering test. The above formulation was dispersed in a mixer at 1800 rpm for 5 minutes. A clear coating composition was prepared from the above formulation as the base enamel. Test Panel A carbon black enamel was applied on a bonderized mild steel plate and baked. The coated plate was polished with No. 500 polishing paper, washed with water and dried. Then the polished surface was sprayed with the above base enamel to a dry film thickness of 25 microns, set for 10 minutes, sprayed with the above clear coating composition, and set for 30 minutes. The coating film was finally baked at 140°C for 25 minutes. Weathering test was carried out by exposing the panel to light in Weather-O-Meter. Lightfastness Lightfastness with time was evaluated visually in terms of the total score made by five panelers according to the following scale. 1: No fading 2: Slight fading 3: Fading The results obtained are shown in Table 1. Crosscut Tape Test The coated film was crosscut into 100 square sections of 1 × 1 mm. A cellophan adhesive tape was applied on the cut surface and peeled off rapidly. The number of remaining sections was counted. The results are shown in Table 2. EXAMPLE 14 The wet cake of hydrous TiO₂ coated MIO prepared in Comparative Example was resuspended in 1 liter of water and heated to 50°C. To this suspension was added a solution of 3.5 g of CrCl₃·6H₂O in 100 ml of water. The mixture was neutralized with a 5 g/liter solution of NaOH to pH 7.0 over 30 minutes. Then 8 ml of an aqueous solution of aluminum sulfate having a concentration of 50 g/liter as Al₂O₃ was added over 60 minutes while maintaining the pH of the mixture at 5-7. The mixture was adjusted at pH 7.0 stirred for additional 30 minutes and filtered. The resulting cake was washed, dried and calcined at 700°C for 1 hour to give TiO₂ coated MIO pigment. EXAMPLE 15 100 g of TiO₂ coated pigment produced in Example 14 was suspended in 600 ml of water and heated to 50°C. To this was added 3.5 g of CrCl₃·6H₂O in 100 ml of water. The mixture was neutralized to pH 7.0 with a 5g/liter solution of NaOH over 30 minutes. Then 40 ml of an aqueous solution of aluminum sulfate (50 g/liter concentration as Al₂O₃) was added. The mixture was neutralized with the above NaOH solution to pH 7.0 over 30 minutes, stirred for additional 30 minutes and filtered. The resulting cake was washed and dried at 120°C for 8 hours to give TiO₂ coated MIO pigment. Weathering Test The products of Examples 14 and 15 were tested for their weatherability in the same manner as described hereinbefore. The results are shown in Tables 3 and 4 below.

Claims: 1. A pigment composed of flaky micaceous iron oxide particles having a thin coating of TiO₂ characterised in that said TiO₂ in the coating is rutile TiO₂; that said pigment has on said TiO₂ coating a plurality of protective coatings of an oxide of Cr, V, Mo, Al or a combination thereof; and the primary protective coating has been calcined at an elevated temperature at which a major proportion of said TiO₂ in the coating layer is transformed to rutile TiO₂. 2. The pigment according to claim 1 characterised in that said micaceous iron oxide particles have an average diameter from 2 to 200 microns and an average thickness of from 0.3 to 5 microns. 3. The pigment according to claim 1 or 2 characterised in that the amount of TiO₂ coating is 0.1 to 30 % by weight of said micaceous iron oxide. 4. The pigment according to any of claims 1 to 3 characterised in that the amount of the primary protective coating is 0.001 to 1 % calculated as metal oxide by weight of the TiO₂ coated micaceous iron oxide substrate. 5. A method for producing the pigment as claimed by any one of claims 1 to 4 which comprises the steps of (a) adding a water-soluble titanium compound and a water-soluble salt of Zn, Sn, Sb, Li or a mixture thereof to an aqueous suspension of flaky micaceous iron oxide particles, (b) hydrolysing said titanium compound and said water-soluble metal salt and depositing the resulting hydrous TiO₂ and the hydrous oxide of said metal on the surface of said micaceous iron oxide particles, (c) resuspending the hydrous TiO₂ coated particles in an aqueous medium, (d) adding a water-soluble metallic compound of Cr, V, Mo, Al or a mixture thereof, (e) hydrolysing the water-soluble metallic compound and depositing the resulting hydrous metal oxide on said hydrous TiO₂ coating, (f) calcining the coated particles at a temperature from 500 °C to 1000 °C, (g) suspending the calcined particles in an aqueous medium, (h) repeating said step (d) and step (e), (i) recovering and drying the resulting coated particles. 6. The method according to claim 5 characterised in that the amount of said water-soluble titanium compound is 0.1 to 30 % as TiO₂ by weight of said micaceous iron oxide. 7.The method according to claim 6 characterised in that said water-soluble salt of Zn, Sn, Sb, Li or a mixture thereof is less than 50 % calculated as metal oxide by weight of said TiO₂.

Inventors: MITSUO, HARADA, NOBUMOTO, HIRAOKA, SANJI, FUJIWARA
Applicants: TAYCA CORPORATION
Application year: 1988
Main classification: C09C1
IPC codes: C09C1