PROCESS FOR PREPARING COATED CADMIUN BASED PIGMENTS

Abstract: A heat-resistant pigment can be produced by forming a dense and uniform coating on the surface of the pigment by contacting a hydrophilic pigment with a metal alkoxide or a boron or silicon alkoxide in at least partly water-miscible solvent.

Description: This invention relates to a process for preparing cadmium based pigments coated with one or more species of metal oxide and having excellent heat resistance, weatherability, light fastness and chemical stability. Pigments which are inferior in heat resistance, weatherability, light fastness and chemical stability (resistance to acids, alkalis and other chemicals) can be improved by coating the surface thereof with a physically and chemically stable metal oxide or treating the surface thereof with a coupling agent. For instance, cadmium pigments undergo discoloration when subjected to a temperature 700°C or higher and their brilliant color is lost, because cadmium sulfide, their principle, is converted to cadmium sulfate or cadmium oxide. Therefore, cadmium pigments cannot be used for products such as ceramic tiles which are subjected to high temperatures. Cadmium yellow (pale light), for example is inferior in acid resistance. As methods for surface treatment of pigment for the purpose of improving heat resistance, weatherability, light fastness, acid resistance, alkali resistance, etc., the following are known; (a) To form a coating of silica on the surface of pigments by addition of a silicate salt when the pigments are formed by precipitation. (b) To form a coating by adding a metallic salt which forms a water-insoluble metal hydroxide or oxide upon neutralization or a metal salt pair which forms a water-insoluble salt by double decomposition reaction in a dispersion of a pigment. (c) To modify the surface of pigments using a titanium coupler or silane coupler (as described for example in DE-A-3529145). In addition to the above, there is known a method for preventing oxidation decomposition of pigments for ceramic tiles comprising mixing a stable material such as zirconium silicate which is not attacked by glaze into a pigment and firing the mixture. In the methods (a) and (b), precipitate of silica or a water-insoluble metal salt is formed on the surface of pigments. Formation of precipitate is usually influenced by temperature, pH, etc. and adjustment of many factors are required, and yet it is very difficult to form dense and homogeneous coating. The method (c) is effective for improving weatherability, light fastness and dispersibility of pigments. However, the couplers are mere surface-modifiers for pigments and do not form a dense coating on the surface of pigments, and therefore, they are of little use for improving heat resistance, acid resistance, alkali resistance, etc. When a coating for improving heat resistance, light fastness, acid resistance and alkali resistance of pigment is formed, formation of the coating is usually carried out under the heated condition, since the reactivity is low at room temperature or lower temperatures. In GB-A 803454 for example, TiO₂ pigments coated with alumina are prepared by hydrolysis of aluminium alkoxide after the coating with overheated steam. By the reaction at an elevated temperature, no dense coating film is formed. The reason is surmised to be that metal hydroxide molecules formed by rapid hydrolysis attach to the already formed suspended metal hydroxide or oxide particles in preference to the surface of pigments, meaning that the percentage of the molecules depositing on the pigment surface is low. We have found that if a hydrophilic cadmium based pigment is contacted with an alkoxide such as an alkoxide titanium, silicon zirconium, zinc and boron in an at least partially water-miscible organic solvent, a dense and uniform coating is formed at relatively lower temperatures. Further, we found that if a secondary coating is formed on the thus formed primary coating by repeating the above procedure or by the known precipitation process, a firm coating is formed and heat-resistance, weatherability, light fastness, chemical stability such as acid resistance and alkali resistance, etc. of the pigment is further improved. This invention provides a process for preparing coated cadmium based pigments comprising contacting a hydrophilic cadmium based pigment with an alkoxide selected from the group consisting of an alkoxide of titanium, silicon, zirconium, zinc and boron in an at least partially water-miscible organic solvent at a temperature of -10°C to +10°C and warming the reaction mixture to a temperature up to 80°C to form a coating on the surface of said pigment, and drying and/or firing the thus coated pigment. The hydrophilic cadmium pigments treatable by the process of the present invention are tinting pigments. The alkoxides usable in the process of the present invention are methoxide, ethoxide, n-propoxide, isopropoxide, butoxide of titanium, silicon, zirconium, zinc or boron. Specific examples are silicon alkoxide (alkyl silicate) such as silicon tetramethoxide (methyl silicate), silicon tetraethoxide (ethyl silicate), etc.; zirconium tetraethoxide, zirconium tetrabutoxide, titanium tetra-n-propoxide, titanium tetraisopropoxide, boron triethoxide, etc. Metal alkoxides condensation-polymerized up to hexamer or so can be used. Two or more alkoxides can be used in combination. By combining alkoxides of different properties, pigments can be provided with enhanced heat an alkoxide of titanium, silicon, zirconium, zinc and boron. resistance, weatherability, light fastness, chemical stability and other characteristics by virtue of composite or synergistic effect. The solvents usable in the present invention are those which are miscible with the alkoxides and at least partially miscible with water. Specific examples thereof are methyl alcohol, ethyl alcohol, propyl alcohol, acetone, acetylacetone, ethylene glycol, butyl alcohol, methyl acetate, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone, etc. The mixing ratio of the alkoxide and the solvent is not specifically restricted. The amount of the hydrated metal oxide formed on the surface of pigments should be preferably at least 0.1 wt%, more preferably not less than 1.0 wt% as the metal oxide. When the amount of the metal oxide coating is less than 0.1 wt%, the surface of the pigment may not be well covered, and heat resistance, weatherability, light fastness and chemical stability of the pigment is apt to be insufficient. When the amount of the metal oxide coating is in excess of 10 wt%, dispersibility of the pigment may be poor, and use of such pigments in paints and plastics will be restricted. Therefore, a metal alkoxide should be present in the treating solution in an amount of not less than 0.1 wt% and not more than 10 wt% as long as paints and plastics are concerned. Although hydrated metal oxide is formed on the surface of the pigment, the hydrated metal oxide is converted to metal oxide upon drying and/or firing. In the process of the present invention, it is preferred that the pigment to be treated be soaked in one of the usable solvents for a long period of time (2∼20 hours) prior to being contacted with the alkoxide. This increases the affinity of the pigment to alkoxide. The contacting of pigment and alkoxide at a lower temperature (-10°C∼10°C) gives a dense coating. A small amount of suitable dispersant or wetting agent (surfactant) can be used for the purpose of promoting dispersion of pigment. It is preferable to add water to the reaction mixture in the later stage of the contacting of pigment and metal alkoxide in order to promote decomposition of the metal alkoxide. The amount of the added water should not exceed 50 wt% of the amount of the metal alkoxide. It is also preferable to warm the reaction mixture at the stage of addition of water. If a small amount of a mineral acid, preferably hydrochloric acid, is added to the reaction mixture, hydrolysis of metal alkoxide is further promoted. In accordance with the present invention, the hydrolysis of metal alkoxide proceeds very slowly, and therefore a dense and uniform layer of hydrated oxide of the corresponding metal is deposited. In the above mentioned conventional methods, the hydrated metal oxide or metal salt depositing on the surface of pigments tends to partially coagulate to form chains because of rapid reaction and thus the pigment surface is not well coated. In contrast, according to the present invention, the pigment surface is coated densely and uniformly with hydrated metal oxide by slow hydrolysis of metal alkoxide. If two or more alkoxides of titanium, silicon, zirconium, zinc or boron, of different properties are used in combination, pigments can be provided with improved heat resistance, weatherability, light fastness, chemical stability, etc. in combination. It is noteworthy that cadmium pigments coated with silica in accordance with the process of the present invention are neither discolored nor decolored at a high temperature of 1150°C or higher. When cadmium yellow (pale and light) pigments are coated in accordance with the process of the present invention, their resistance to visible light and ultra violet rays is enhanced, and their light fastness and weatherability are also improved. The coated pigments in accordance with the present invention have excellent heat-resistance, weatherability and chemical stability, and can be suitably used as ceramic pigments, which require heat resistance and chemical stability, and as tinting agents for engineering plastics. The process of the invention is described in more details in the following examples. Example 1 In a solution of zirconium tetra butoxide and a solvent as indicated in Table 1 in the indicated ratio, a cadmium yellow pigment in the amount indicated in the table was immersed and the mixture was stirred at 0°C for 10 hours, and thereafter further stirred at room temperature (20°C) for 20 hours. The thus treated pigment was collected by filtration, and the collected pigment was air-dried at room temperature for 24 hours, further dried at 100°C for 10 hours and at 200°C for 1 hour, and thereafter pulverized. To 100 parts by weight of soft poly(vinyl chloride), 1 part by weight of each of the thus obtained coated pigments and 2 parts by weight of titanium dioxide were added. The mixture was mixed well by means of heated rolls (120°C), shaped into 1 mm thick plates at 170°C by means of a press, and cut into 30 × 100 mm test pieces. Time course change in color of the test pieces was checked by immersing some test pieces in water and exposing them outdoors for 3 months and by soaking some test pieces in a 1N HCl solution or 1N NaOH solution for 1 month. The change in color was determined by measuring tristimulus values X, Y and Z in accordance with the method of JIS Z8722, the L,a,b values in accordance with the method of JIS Z8730 and calculating the color difference from the value of the untested control test piece. Example 2 and 3 To a solution of a metal alkoxide in a solvent as indicated in Table 1 in the indicated ratio, the pigment indicated in the table in the amount indicated in the table was added and the mixture was mixed for 2 hours and then slowly stirred at 0°C for 5 hours. The used pigment had been soaked in ethanol for more than 2 hours beforehand. The mixture was then stirred at room temperature (20°C) for 20 hours. Then the pigment was collected by filtration, air-dried at room temperature for 24 hours, and further dried at 100°C for 10 hours and at 200°C for 3 hours. The thus obtained coated pigment was added to a low temperature frit glaze (SK 010∼01) and a medium temperature frit glaze (SK 4∼6) in an amount of 10 wt%. The mixture was made into a slurry by addition of a suitable amount of water, and the slurry was ball-milled for 2 hours. The milled slurry was applied on ceramic tiles. After drying, the tiles were kept at 300°C for 3 hours in an electric oven. Thereafter the tiles were heated to 1100°C and 1200°C in 4∼5 hours. The tiles were fired for 15∼20 minutes at the respective temperatures, and change in color was examined. Example 4 To a solution of zirconium tetraethoxide in methyl alcohol in the ratio as indicated in Table 1, cadmium red pigment in the indicated amount was added. The mixture was stirred at room temperature for 20 hours. The pigment was collected by filtration, air-dried at room temperature, further dried at 100°C for 10 hours and at 200°C for 1 hour and pulverized. The thus obtained coated pigment was tested in the same manner as in Example 2 and 3 and change in color was examined. The results are indicated in the table. Example 5 A cadmium yellow (light) pigment indicated in Table 1 in the amount indicated in the table was soaked in the indicated amount of the indicated solvent for 2 hours. Thereafter, the mixture was chilled to 0°C, and the indicated metal alkoxide in the indicated amount was added to the mixture. The mixture was stirred at 0°C for 20 hours. Thereafter water and 1N HCl in the indicated amounts were added dropwise. The water and the HCl were mixed with the used solvent in an amount equal to the amount of the water beforehand in order that the water and hydrochloric acid would be easily mixed with the alkoxide solution. The mixture was further stirred well at 0°C for 2 hours, kept at 0°C for 30 hours and finally was left to stand at room temperature for 20 hours. The thus treated pigment was collected by filtration, air-dried at room temperature for 24 hours, further dried at 100°C for 10 hours and at 200°C for 1 hour and finally pulverized. The thus obtained coated pigments were tested in the same manner as in Examples 1∼4 and change in color was examined. The results are indicated in table 1. Example 6 To a solution of ethyl silicate (40%) in ethyl acetate in the ratio as indicated in Table 1, the indicated amount of cadmium red pigment was added, and the mixture was stirred at 0°C for 20 hours. Water and 1N HCl in the indicated amounts were added to the mixture. The water and the hydrochloric acid were beforehand mixed with the ethyl acetate in an amount equal to the amount of the water so that they could easily mix with the alkoxide mixture. The mixture was stirred well at 0°C for 2 hours, kept at that temperature for 30 hours and finally, allowed to stand at room temperature for 20 hours. The thus treated pigment was collected by filtration, air-dried at room temperature for 24 hours, dried at 100°C for 10 hours and finally at 200°C for 3 hours. The thus obtained coated pigment was mixed into a low temperature frit glaze (SK 010∼01) and a medium temperature frit glaze (SK 4∼6), respectively in an amount of 10 wt%, and made into a slurry by addition of a suitable amount of water. After the mixture was ball-milled for 2 hours, it was applied on ceramic tiles. The tiles were dried well at room temperature and then dried at 300°C for 3 hours. Thereafter, they were heated to 1100°C and 1200°C in 4∼5 hours, and they were fired at the respective temperatures for 15∼20 minutes. Change in color was examined. The results are indicated in the table. Example 7 Cadmium red pigment in the amount indicated in Table 1 was soaked in the indicated amount of propyl alcohol at room temperature for more than 2 hours with stirring. Then the indicated amount of titanium tetraisopropoxide was added dropwise and the mixture was stirred for 20 hours. Water and 1N HCl which were mixed with propyl alcohol in an amount equal to the amount of the water were added dropwise to the reaction mixture. The reaction mixture was stirred well for 2 hours, and allowed to stand at room temperature for 20 hours and then kept at 60°C for 1 hour. After cooling to room temperature, the pigment was collected by filtration, air-dried at room temperature for more than 24 hours, and further dried at 100°C for 10 hours and at 200°C for 3 hours. The thus obtained coated pigment was tested in the same manner as in Example 15, and change in color was examined. The results are shown in the table. Example 8 Cadmium yellow pigment (light) in the amount indicated in Table 1 was added to a mixture of methyl silicate and ethyl alcohol in the indicated ratio at 0°C and the mixture was stirred for 1 hour. Thereafter, a mixture of the indicated amounts of water and 1N HCl and the solvent of the amount equal to the amount of the water was added to the pigment mixture, and the mixture was stirred at 0°C for 10 hours at room temperature for 20 hours and finally the mixture was kept at 60°C for 1 hour. After cooling to room temperature, the pigment was collected by filtration, air-dried at room temperature for 24 hours, dried at 100°C for 10 hours and at 200°C for 1 hour and pulverized. The thus obtained coated pigment was tested in the same manner as in Example 4 and change in color was examined. The results are indicated in the table. Examples 9 and 10 Cadmium pigments indicated in Table 1 were respectively soaked in the indicated solvents in the indicated amount with stirring for 2 hours. Then the mixture was chilled to 0°C. To the pigment mixture, the respective indicated metal alkoxide was added, and the resulting mixture was stirred well for 1 hour. Thereafter, the indicated amounts of water and 1N HCl were added and the mixture was stirred well for 2 hours. Then the mixture was kept in a chilling bath for 10 hours with stirring, and thereafter, the mixture was allowed to stand at room temperature for 30 hours. The thus treated pigment was collected by filtration, air-dried at room temperature for more than 24 hours, and dried at 100°C for 10 hours and at 200°C for 3 hours. The thus obtained coated pigment was mixed into a low temperature glaze (SK 010∼01) and a medium temperature glaze (SK 4∼6) in the amount of 10 wt%, the mixture was made into a slurry by addition of a suitable amount of water and the slurry was ball-milled for 2 hours. The slurry was applied on ceramic tiles, which were dried well at room temperature, and further dried at 300°C for 3 hours. Then the tiles were heated to 1100°C and 1200°C in 4∼5 hours and fired at those temperatures for 15∼20 minutes, whereafter change in color was examined. The results are indicated in the table. Comparative Example 1 One part by weight of sodium metasilicate was dissolved in 100 parts by weight of water. Ten (10) parts by weight of a cadmium yellow (light) as indicated in Table 2 was added to the solution, and the mixture was stirred at 40°C for 2 hours. Then the pH of the mixture was adjusted to 6.5∼7.5 by dropwise addition of 1N H₂SO₄. Thereafter the mixture was stirred well for 5 hours, and then stirred at room temperature for more than 20 hours. The thus treated pigment was collected by filtration, washed with water, and dried at 100°C for 3 hours and at 200°C for 1 hour. The pigment was tested in the same manner as in Example 1, and change in color was examined. The results are indicated in the table 2. Comparative Examples 2 - 4 Two (2) parts by weight of sodium metasilicate was dissolved in 100 parts by weight of water. Ten (10) parts by weight of the indicated cadmium pigment and 0.3 part by weight of 2-amino-2-methyl-1-propanol were added to the solution and the mixture was mixed well at 40°C for 2 hours. The pH of the mixture was adjusted to 6.5∼7.5 by dropwise addition of 1N H₂SO₄. The mixture was mixed well for 5 hours, and further stirred at room temperature for more than 20 hours. The pigment was collected by filtration, and dried at 100°C for 3 hours and at 200°C for 1 hour. The thus obtained pigment was mixed into frit glazes and made into a slurry by addition of a suitable amount of water, and the slurry was applied on ceramic tiles and fired in the same manner as in Examples 9 and 10. In the above-described Examples and Comparative Examples, color change at elevated temperatures of cadmium pigments was examined. With respect to the low temperature glazes (900∼1100°C), the pigments of Comparative Examples which were coated by the conventional precipitation method were partly discolored or decolored in the temperature range of up to 1000°C, but the pigments coated in accordance with the present invention did not undergo color change. With respect to the intermediate temperature glazes (1100∼1200°C), the coated pigments of Comparative Examples darkened and remarkably decolored. In contrast the coated pigments of the present invention underwent little discoloration and retained their brilliant tint. In the higher temperature range up to 1200°C, the difference between the coated pigments of the present invention and those of Comparative Examples was far more remarkable. It was confirmed that the coated pigments of the present invention have excellent heat resistance and chemical stability.

Claims: A process for preparing coated cadmium-based pigments comprising contacting a hydrophilic cadmium-based pigment with an alkoxide selected from the group consisting of an alkoxide of titanium, silicon, zirconium, zinc and boron in an at least partially water-miscible organic solvent at a temperature of -10°C to 10°C, and warming the reaction mixture to a temperature up to 80°C to form a coating on the surface of the pigment, and drying and/or firing the thus coated pigment. The process for preparing coated cadmium-based pigments as described in claim 1, wherein water is added to the mixture of the pigment, the alkoxide and the organic solvent at the later stage of the contacting. The process for preparing coated cadmium-based pigments as described in claim 2, wherein a mineral acid is added to the mixture in addition to water. The process for preparing coated cadmium-based pigments as described in claim 1, wherein the pigment is soaked in the solvent for not less than 2 hours prior to contacting with the alkoxide. The process for preparing coated cadmium-based pigment as described in claim 1, wherein the alkoxide is used in an amount of 0.1 wt% of the pigment to be treated as the metal oxide. A coated cadmium-based pigment produced by a process of any of the preceding claims.

Inventors: NISHIHARA, AKIRA, TSUNASHIMA, MAKOTO
Applicants: MITSUBISHI KINZOKU KABUSHIKI KAISHA
Application year: 1987
Main classification: C09C3
IPC codes: C09C1, C09C3, C09B67