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.degree. 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.
Of the chromium yellow pigments, the principle of which is lead chromate, and chromium orange are inferior in heat resistance and acid resistance and lemon yellow is inferior in heat resistance and resistance to alkali and they all suffer discoloration. Molybdenum red, which essentially consists of lead chromate and lead molybdate, is inferior in heat resistance, acid resistance, alkali resistance, weatherability and light fastness, and suffers discoloration.
Cadmium yellow (pale light), ultramarine, chromium green, lithopone, Bordeaux-10B, etc. are inferior in acid resistance, and prussian blue, chromium green, brilliant carmine 6B, lake red C, Bordeaux-10B, rhodamine lake Y, etc. are inferior in alkali 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.
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. 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 pigment is contacted with a metal alkoxide such as alkyl silicate 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.