TITANIUM DIOXIDE PIGMENT COATED WITH BORIA-MODIFIED SILICA

Abstract: Pigments are disclosed which consist essentially of rutile TiO₂ particles bearing densified coatings comprising SiO₂ and B₂O₃.

Description: BACKGROUND OF THE INVENTION The present invention relates generally to coated TiO₂ particles for use as pigments, and particularly to TiO₂ particles having densified silica coatings. 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 plant, resulting in coating degradation or failure. Therefore, minimizing free radical migration is desireable in order to provide lightfast and stable pigments for coating use. A widely-used approach to the problem of providing lightfastness involves coating TiO₂ particles with a layer of densified silica. The following patent references disclose conventional silica coating densification techniques. Werner, U. S. Patent, 3,437,502, discloses TiO₂ pigment having high opacity and dispersibility, which is obtained by applying a dense silica coating to TiO₂ followed by topcoating with alumina. The silica coating is applied by increasing the pH of a slurry of TiO₂ particles to above 7. A predetermined quantity of sodium silicate is then added to the slurry, resulting in a rise in pH of the slurry to above 8, generally above 11. The pH is then gradually reduced over a period of several hours by addditon of dilute acid, and the resulting silica-coated particles are cured at pH 6.0 to 7.5, at 60° to 100°C, for 30 minutes to several hours. The resulting product is then coated with alumina. West, U. S. Patent 4,125,412, discloses preparation of durable TiO₂ pigments by slurrying silica-coated TiO₂ particles at pH 9 to 10.5 while maintaining slurry temperature at 80° to 100°C. European Patent Application 73,340 describes a similar method for coating TiO₂ particles with amorphous dense silica prior to topcoating with alumina. Silica is applied by adding a soluble silicate to a TiO₂ slurry at a temperature of at least 85°C and at a pH of from 9.8 to 10.1. The slurry is then neutralized in at least three neutralization steps by addition of acid. Common to the teachings of the foregoing references is a requirement for relatively high temperatures and high alkalinity during the silica densification step. An extremely lightfast white pigment for use in paint systems can be made by depositing 4-6 weight percent amorphous dense silica on rutile TiO₂ pigment. To coat the base TiO₂ pigment with this quantity of dense silica requires several hours at 85°-90°C, during which mineral acids are used to precipitate silica from Na₂SiO₃. This high temperature coating operation requires use of steel tanks, since less costly fiberglass tanks are not rated for use in the 85°-90°C temperature range. Methods for providing dense coatings of more than 4 weight percent silica at lower temperatures e.g., 65°-80°C, to allow use of fiberglass tanks, would be of significant interest to the TiO₂ pigment industry. It has now been found that codeposition of B₂O₃ with SiO₂ provides dense silica coatings at process temperatures which permit use of fiberglass slurry tanks. The boria-containing pigments resulting from this process are highly lightfast, and exhibit excellent gloss and dispersibility. SUMMARY OF THE INVENTION The present invention provides a pigment consisting essentially of TiO₂ particles bearing coatings comprising SiO₂ and B₂O₃. In a process aspect, the present invention provides a process for preparing TiO₂ particles bearing coatings of boria-modified silica, comprising: (a) forming an aqueous slurry of rutile TiO₂ at a temperature from about 65°C to about 90°C; (b) adjusting pH of the slurry to the range 7-10.5; (c) adding a predetermined quantity of a solution comprising Na₂SiO₃ and B₂O₃, under conditions which maintain silicate and boria ions in solution; (d) gradually lowering pH of the slurry to about 7.5-8.5 by addition of acid, thereby depositing a coating of silica and boria; and (e) curing the resulting coated pigment at a temperature from about 65°C to about 90°C for a period of at least 15 minutes. DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, B₂O₃ is codeposited with SiO₂ on TiO₂ particles, providing lightfast, durable pigments. The TiO₂ particles making up the pigments bear dense silica coatings, yet are produced at temperatures significantly lower than those required by other silica densification processes capable of providing dense silica coatings at greater than 4 weight percent. In practice, silica/boria layers are codeposited from a master solution of Na₂SiO₃ and Na₂B₂O₄. A standard acid solubility test can be to measure effectiveness of coating densification. In addition to enhanced densification of SiO₂ in the presence of B₂O₃, an improvement in pigment brightness is provided. 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. The amount of silica or silica topcoated with alumina which the particles bear as coatings can be varied depending primarily upon the pigment's intended use. Generally, the silica coatings will constitute from 2-10% of the total pigment weight, and an alumina topcoat, if present, will constitute 0.5-5% of the total weight of the particles. Preferably, the silica coating constitutes 4-8% of the total weight of the pigment particles. Preferably, the alumina topcoat, if present, constitutes 1-4% of the total weight. The amount of alumina the particles bear as coatings, expressed as percent by weight, is calculated by first determining the alumina content of the coated pigment by ion plasma spectroscopy. 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 brough to 65°-90°C and is held at that temperature throughout the preparation procedure. The slurry is adjusted to pH 7-10.5, and a sufficient quantity of solution containing Na₂SiO₃ and Na₂B₂O₄, or separate solutions of Na₂SiO₃ and Na₂B₂O₄, is added to provide a coating of the desired composition and weight. Upon addition of the Na₂SiO₃ and Na₂B₂O₄ solution(s), pH of the slurry increases to about 11. Generally, slurry pH must be maintained above about 10 to maintain silicate and boria ions in solution. Generally, the weight and composition of the coatings to be deposited will be determined by the composition of the master Na₂SiO₃/Na₂B₂O₄ solution, the quantity added to the slurry containing TiO₂, and quantity of TiO₂ in the slurry. For TiO₂ slurries containing 400 g/L TiO₂, about 40-50 mL of a master aqueous solution, containing 400 g SiO₂ equivalent and 40 g B₂O₃ equivalent per liter, should be added per liter TiO₂ slurry to provide coatings approximately 5% of pigment weight. The compositions of the coating solutions can be varied as required to provide variations in coating weight and composition. The coatings applied to TiO₂ particles in accordance with the present invention can contain from 60-95 percent by weight SiO₂ and 0.5-30% by weight B₂O₃, based upon coating weight. Preferably, coatings will contain 80-90 percent by weight SiO₂ and 2-20 percent by weight B₂O₃. Upon addition of the coating solutions, the pH of the TiO₂ slurry will rise to the range 10.5-11.5. Over a period of 2-4 hours, 15% aqueous HCl or other mineral acid is added to gradually decrease slurry pH to about 8.0. As acid is added, SiO₂/B₂O₃ will be deposited and densified on the surfaces of the TiO₂ particles. After all acid is added, the pigment should be cured at 65°-90°C for 30 minutes. If desired, alumina can be precipitated on silica/boria coated TiO₂ particles by adding enough sodium aluminate to the slurry, at a temperature from about 50°-90°C, 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 as the alumina is precipitated. 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, 9000 parts water are mixed with 3000 parts rutile TiO₂. The resulting slurry is heated to 75°, and the pH is adjusted to about 7.5 by addition of NaOH. 400 parts of an aqueous solution containing 400 parts SiO₂ equivalent (as Na₂SiO₃) and 40 parts B₂O₃ equivalent (as Na₂B₂O₄) per 1000 parts water are added. The pH of the resulting mixture is then determined, and sufficient 15% aqueous HCl is added, over a period of about 3 hours, to reduce the slurry pH to about 8.0. After all HCl has been added, the slurry is cured for 30 minutes at pH 8.0 and 75°. The resulting coating, which constitutes about 5.5% by weight of the total pigment, contains SiO₂ and B₂O₃ in a ratio by weight of about 90:10. A coating of 2.5% hydrous alumina is then deposited on the pigment by simultaneous gradual addition of 200 mL NaAlO₂ solution (containing NaAlO₂ equivalent to 400 g Al₂O₃ 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 ambient temperature and pH 7.5. The pigment is then filtered from the slurry, washed free of salts with water, and air-dried. The pigment is then micronized, using 3 lbs. superheated steam per lb. pigment. The resulting pigment is suitable for use in paint systems where lightfastness is required. Example 2 In this series of experiments, several batches of pigment were prepared to evaluate the effect of B₂O₃ codeposition and various processing temperatures upon acid solubility. Generally, acid solubility is inversely related to chalk-fade resistance for TiO₂ pigments. The pigments described below were prepared by procedures substantially similar to that described in Example 1, above, except that processing temperatures were varied as indicated in Table 1, below. Acid solubilities were determined as follows. 10 mL 66% sulfuric acid is added to a test tube containing a magnetic stirring bar, and the tube is placed in an aluminum heating block and heated to 175°. A 0.2000 g pigment sample is added to the tube, and digested for 1 hour with stirring. At the conclusion of the digestion period, the tube containing the pigment sample is cooled by pouring the acid mixture into a beaker of ice, and the residue in the tube and beaker is washed with distilled water. The pigment residue in the tube and beaker is collected in a 100 mL volumetric flask, the volume made up to 100 mL by addition of distilled water, and the contents mixed thoroughly. The contents of the volumetric flask are then filtered, and 10 mL of the resulting filtrate are added to a 25 mL volumetric flask, to which 2 mL 20% hydrogen peroxide, and sufficient 10% sulfuric acid to make 25 mL, are added. The resulting solution is allowed to stand one hour. Absorbance of the solution is then read at 400 nm using a 10 mm cell path. Soluble TiO₂ is determined by reference to a previously prepared spectrophotometric curve obtained by measurement of samples containing known quantities of dissolved TiO₂. Example 3 A mill base is made by mixing Pigment of Example 1 387 parts alkyd resin (Syntex ® 3145; Celanese Coatings Co.) 149.6 parts n-butanol 9.4 parts xylol 79 parts. This mixture is sand ground, and then the sand is filtered from the mil base. A paint is made by mixing 100 parts of the mill base with the following: xylol 1.4 parts alkyd resin (Syntex ® 3145; Celanese Coatings Co.) 71.8 parts melamine resin solution (Cymel ® 248-8, 55% solids, American Cyanamid Co.) 38.5 parts.

Claims: 1. A pigment consisting essentially of rutile TiO₂ particles bearing coatings comprising SiO₂ and B₂O₃. 2. A pigment according to claim 1, wherein the TiO₂ particles have coatings consisting essentially of from 60-­ 98 percent by weight SiO₂ and 0.5-30 percent by weight B₂O₃, based on coating weight. 3. A pigment according to claim 2, wherein the TiO₂ particles have coatings consisting essentially of from 80-­ 90 percent by weight SiO₂ and 2-20 percent by weight B₂O₃, based on coating weight. 4. A pigment according to claim 3, wherein the TiO₂ particles have SiO₂/B₂O₃ coatings constituting 2-10% of the total pigment weight. 5. A pigment according to claim 4, wherein the TiO₂ particles have SiO₂/B₂O₃ coatings constituting 4-8% of the total pigment weight. 6. A pigment according to any one of the preceding claims, wherein the TiO₂ particles have outer coatings of alumina constituting 0.5-5% of the total weight of the particles. 7. A pigment according to claim 6, wherein the TiO₂ particles have outer coatings of alumina constituting 1-4% of the total weight of the particles. 8. A process for preparing TiO₂ particles bearing coatings of boria-modified silica, comprising: (a) forming an aqueous slurry of rutile TiO₂ at a temperature from about 65°C to about 90°C; (b) adjusting pH of the slurry to the range 7-­ 10.5; (c) adding a predetermined quantity of a solution comprising Na₂SiO₃ and B₂O₃, under conditions which maintain silicate and boria ions in solution; (d) gradually lowering pH of the slurry to about 7.5-8.5 by addition of acid, thereby depositing a coating of silica and boria; and (e) curing the resulting coated pigment at a temperature from about 65°C to about 90°C for a period of at least 15 minutes. 9. A process according to claim 8, wherein sufficient Na₂SiO₃ and B₂O₃ are added in step (c) to provide a coating consisting essentially of from 60-98 percent by weight SiO₂ and 0.5-30 percent by weight B₂O₃ based on coating weight. 10. A process according to claim 9, wherein sufficient Na₂SiO₃ and B₂O₃ are added in step (c) to provide a coating consisting essentially of from 80-90 percent by weight SiO₂ and 2-20 percent by weight B₂O₃, based on coating weight. 11. A process according to claim 10, wherein sufficient Na₂SiO₃ and B₂O₃ are added in step (c) to provide a SiO₂/B₂O₃ coating constituting 0.5-10% of the total pigment weight. 12. A process according to claim 11, wherein sufficient Na₂SiO₃ and B₂O₃ are added in step (c) to provide a SiO₂/B²O³ coating constituting 4-8% of the total pigment weight. 13. A process according to any one of claims 8 to 12 comprising the following additional step: (f) adding sufficient NaAlO² to the slurry, at a temperature from about 50°-90°C, to provide a concentration of about 2-8% by weight of the TiO², while adding mineral acid to maintain the pH of the slurry from 6-9. 14. A coating composition comprising a pigment according to any one of claims 1 to 7, a film-forming material, and a liquid carrier.

Inventors: JACOBSON, HOWARD WAYNE
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1987
Main classification: C09C1
IPC codes: C09D7, C09D1, C09C1