TITANIUM DIOXIDE PIGMENT COATED WITH CERIUM CATIONS, SELECTED ACID ANIONS, AND ALUMINA

Abstract: A pigment consisting essentially of rutile TiO₂ particles coated with alumina or alumina-silica, and having particle surfaces associated with cerium cations and borate anions or water-soluble polyfunctional organic acid anions, is useful in coating compositions.

Description: BACKGROUND OF THE INVENTION The present invention relates generally to pigments composed of coated TiO₂ particles, and particularly to TiO₂ pigments having coatings comprising cerium cations, selected acid anions, and an outer coating of alumina. TiO₂ has a high refractive index for its density, which renders it a superior pigment for use in coatings, e.g., paints. However, TiO₂ is photoactive; exposure to ultraviolet radiation results in generation of free radicals on the surface of TiO₂ particles. Thus, where a TiO₂ pigment is employed in a paint exposed to sunlight, free radicals are generated which can migrate to the film-forming component of the paint, resulting in coating degradation or failure. Therefore, minimizing free radical migration is desirable in order to provide lightfast and stable pigments for coating use. Heretofore, the majority of commercial TiO₂ pigments have been coated with dense silica or densified alumina to provide lightfastness. However, some patent references have disclosed use of cerium in association with TiO₂ pigments to promote lightfastness. Lederer, U. S. Patent 3,513,007, discloses TiO₂ particles having coatings formed from dispersions containing water-soluble hydrolyzable compounds of silicon, titanium, zirconium, or phosphorus, to which water-soluble hydrolyzable compounds of aluminum, cerium, and or calcium and alkali are added. Barnard, U.S. Patent 4,239,548, discloses pigments having inner coatings comprising cerium and phosphate radicals and outer coatings comprising aluminum and phosphate radicals and optionally hydrous alumina. Jacobson, U.S. Patent 4,460,655, describes pigments of TiO₂ particles associated with compounds selected from the group consisting of Na₇Ce₆F₃₁, K₇Ce₆F₃₁, Li₇Ce₆F₃₁, and(NH₄)₇Ce₆F₃₁. Jacobson, U.S. Patent 4,461,810, discloses pigments of TiO₂ particles, the surfaces of which are associated with cerium cations and sulfate, phosphate, or silicate anions. The particles are also provided with an outer coating of alumina. Newland et al., U.S. Patent 4,022,632, describes TiO₂ pigments treated with carboxylate salts of cobalt, cerium or manganese. Bramekamp, U.S. Patent 3,506,466, discloses TiO₂ pigment particles coated with inorganic oxides, where the inorganic oxide bears on its surface an amine salt of an oxycarboxylic acid selected from the group consisting of lactic, citric, malic, tartaric and glycolic acids. GB-A-1580882 describes a method of making a coated inorganic pigment which performs adequately in terms of gloss and dispersibility both organic and aqueous-based paint media. The inorganic pigment particles, which are preferably TiO₂ are coated with alumina and then contacted with one or more organic hydroxy acids or salts or with a water-soluble reaction product of one or more di- or polyhydric alcohols and one or more di- or poly-basic organic hydroxy acids. It has now been found that pigments of TiO₂ particles treated with cerium cations in association with certain polyfunctional water-soluble acid anions or borate ions, and then topcoated with alumina, exhibit excellent lightfastness, gloss and dispersion characteristics. SUMMARY OF THE INVENTION The present invention provides a pigment consisting essentially of rutile TiO₂ particles bearing coatings of alumina or alumina-silica, the particle surfaces having associated therewith 0.05-2%, by weight of the TiO₂, of cerium cations and an associated quantity of borate anions or polyfunctional organic acid anions having a solubility in water of at least 10 grams per liter at 25°C. DETAILED DESCRIPTION OF THE INVENTION The pigments of the present invention are composed of rutile TiO₂ particles treated with cerium cations in association with selected polyfunctional organic acid anions such as citrate anions, and then topcoated with alumina or alumina-silica. Optionally, the acid anions can be borate. In extreme lightfastness exposures, the pigments of this invention provide performance equivalent to TiO₂ coated with 6% by weight silica and 2% by weight alumina. It is believed that cerium citrate in association with the TiO₂ particle surfaces provides recombination centers for electron/hole pairs produced when pigments containing such particles are exposed to actinic radiation. Quenching of electron/hole pairs eliminates free radical production which might otherwise degrade a paint film or other organic materials in proximity to pigment particles. The TiO₂ used to prepare the pigment of the invention can be of the conventional rutile variety, prepared by either the chloride or sulfate process. Associated with the particle surfaces of the TiO₂ are cerium cations and selected acid anions. "Associated with the particle surfaces" means that the ions are bound to the TiO₂ particles by a chemical or physical attraction. The acid anions associated with the cerium cations are borate anions or polyfunctional organic acid anions having water solubilities greater than 10 grams per liter at 25°C. Exemplary of such organic acid anions are citrate, succinate, fumarate, adipate, tartrate, and maleate ions. Of the foregoing, citrate ions are preferred. The cerium cations can be supplied by any water-soluble cerium salt, e.g., ceric sulfate or ceric nitrate, which are added to rutile slurries at pH 2-7. Polyfunctional organic acid anions are supplied by addition of the appropriate acid. Borate anions are supplied by addition of Na₂B₂O₄. The cerium ion content of the pigment can be determined by ion plasma chromatography. The polyfunctional organic acid anion content can be determined using a Leco or other total carbon analyzer. The amount of alumina or alumina-silica which the particles bear as coatings can be varied depending primarily upon the pigment's intended use. Ordinarily, the alumina coating will constitute 0.5-10% of the total weight of the particles. Preferably, the alumina coating constitutes 1-6%, and most preferably, 2-4%, of the total weight of the pigment particles. The amount of alumina the particles bear as coatings, expressed as percent by weight, is calculated by first determining, by ion plasma spectroscopy, the alumina content of the coated pigment. The alumina content of the uncoated rutile TiO₂ is similarly determined, and the alumina content attributable to the coating is determined by computing the difference between coated and uncoated alumina contents. The pigment of the present invention can be prepared from an aqueous slurry containing 200-450 grams per liter TiO₂. This slurry is brought to 45°-70°C and is held at that temperature throughout the preparation procedure. The slurry is adjusted to a pH at which added cerium ions will remain in solution by addition of mineral acid, e.g., HCl, and then sufficient cerium salt is added to provide a cerium ion concentration in the slurry of 0.05-2%, by weight of the TiO₂. For example, if cerium ions are supplied by addition of ceric nitrate, pH should be adjusted to 2-4; if cerium ions are added in the form of ceric sulfate, pH should be adjusted to the range 2-7. At this point, a sufficient quantity of a selected anion species (i.e, organic acid or borate) is added to the slurry to provide a concentration of added anion ranging from 100% to 300% of the cerium ion concentration. As a result, cerium will precipitate upon the surfaces of the TiO₂ particles in association with the added anion. Alumina is then precipitated on the TiO₂ particles by slowly adding enough sodium aluminate to the slurry to provide a concentration of about 2-8% by weight of the TiO₂. Acid, normally sulfuric or hydrochloric, is added at the same time to maintain the pH of the slurry within the range 6-9 during the precipitation step. After addition of aluminate is complete, the slurry is allowed to cure, with continuous stirring, for 15-30 minutes. The resulting pigment is then separated from the liquid by filtration or centrifugation, washed with water, and dried. Coating compositions can be prepared with the pigments of the invention in conventional ways, e.g., by blending the pigment with a film-forming component and a liquid carrier. The following examples illustrate particular aspects of the present invention. In the examples, all parts and percentages are by weight and all degrees are Celsius unless otherwise indicated. Example 1 In a large plastic vessel equipped with stirring apparatus and a pH probe, 11,000 parts water are mixed with 5000 parts rutile TiO₂. The resulting slurry is heated to 60°, and the pH is adjusted to about 3.0 by addition of HCl. 42 parts of a 1.5 M Ce(NO₃)₄ solution are then added, followed by 40 parts citric acid. The resulting mixture is then stirred for a period of about 5 minutes, and then the pH is increased to about 7.5 by addition of NaOH. A coating of 2% hydrous alumina is then deposited on the pigment by simultaneously adding 250 mL NaAlO₂ solution (containing the equivalent of 400 g A1₂O₃ per liter solution) and HCl, such that the pH of the slurry is continuously maintained from about 7.0 to about 7.5. The resulting coated pigment is cured by holding for about 30 minutes at 60° and pH 7.5. The pigment is then filtered from the slurry, washed free of salts with deionized water, and air-dried. The pigment is then micronized, using 3 lbs. superheated steam per lb. pigment. The resulting pigment, containing about 0.2% CeO₂ as Ce₃(C₆H₅O₇)₄, and a 2.0% alumina coating, is suitable for use in paint systems. Example 2 A mill base is made by mixing This mixture is sand ground, and then the sand is filtered from the mill base. A paint is made by mixing 100 parts of the mill base with the following:

Claims: A pigment consisting essentially of rutile TiO₂ particles bearing a coating of alumina or alumina-silica, the TiO₂ particle surfaces having bound therewith 0.05-2%, by weight of the TiO₂, of cerium cations and an associated quantity of borate anions or polyfunctional organic acid anions having a solubility in water of at least 10 grams per liter at 25°C, said coating of alumina or aluminasilica forming a top coat on the TiO₂ particles. A pigment according to claim 1, wherein the alumina or alumina-silica coating constitutes 0.5-10% of the total weight of the TiO₂ particles. A pigment according to claim 2, wherein the alumina or alumina-silica coating constitutes 1-6% of the total weight of the TiO₂ particles. A pigment according to claim 3, wherein the alumina or alumina-silica coating constitutes 2-4% of the total weight of the TiO₂ particles. A pigment according to any one of the preceding claims wherein the associated anions are borate ions. A pigment according to claim 1, wherein the associated anions are polyfunctional organic acid anions having a solubility in water of at least 10 grams per liter at 25°C. A pigment according to any one of claims 1 to 4, wherein the associated anions are citrate, succinate, fumarate, adipate, tartrate or maleate ions. A coating composition comprising a pigment according to any one of claims 1 to 7, a film-forming material, and a liquid carrier. A method of producing coated TiO₂ pigment particles which comprises (a) forming an aqueous slurry containing 200 to 450 grams per litre of TiO₂ and having a pH at which cerium ions will remain in solution, (b) adding sufficient cerium salt to provide a cerium ion concentration in the slurry of 0.05 to 2% by weight of TiO₂, (c) adding a sufficient quantity of a source of borate or polyfunctional organic acid anions to provide a concentration of said anions of from 100 to 300% of the cerium ion concentration thereby causing the cerium ions to precipitate onto the surfaces of the TiO₂ particles in association with the said borate or polyfunctional organic acid anions and (d) depositing an alumina or silicaalumina coating on the treated TiO₂ particles. A method according to claim 9 wherein in step (d) alumina is precipitated onto the treated TiO₂ particles by adding enough sodium aluminate to the slurry to provide a concentration of abut 2 to 8% by weight of the TiO₂ and adjusting the pH to within the range of 6 to 9 during the precipitation step.

Inventors: JACOBSON. HOWARD WAYNE
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1987
Main classification: C09C1
IPC codes: C09C1, C09D7, C09C3