DESC:Field of the Invention
The present invention relates to improved rutile titanium dioxide (TiO2) pigments and a process for their preparation thereof.

Background of the Invention
Surface treatments of TiO2 pigments to improve the quality of the pigment for a variety of end use applications is known. Such methods involve precipitation or adsorption of organic and/or inorganic materials on the surface of the TiO2 particles. Desired quality of the pigment such as dispersion, durability and optical properties would depend on the surface agents used, levels of coating and the methods of their incorporation. Except for durability, other properties are typically achieved through minimal surface treatment of the pigment. Improved durability and decreased photo activity are typically obtained by coating the pigment with metal oxides. Metal oxides such as silicon dioxide, zirconium dioxide, aluminium oxide is known for coating the pigment. The metal oxides behave as a barrier by enhancing the band gap or promoting the recombination of photo-generated charges. Different coating types produce different levels of durability in the final product. These inorganic coatings also enhance the dispersion and optical properties of TiO2 pigment.
Several coating types and processes are known in the art. US Patent Nos. 2,885,366 and 3,437,502 disclose a process for making a durable titanium dioxide pigment having an amorphous silica coating followed by a coating of crystalline alumina.
US 4,052,224 discloses a process for treating a TiO2 pigment using first a mixed solution of water-soluble compounds of aluminium, zirconium and titanium and then providing a final inorganic surface treatment with an aluminium phosphate. Resulting pigments are useful in the manufacture of paints having reduced photochemical activity. The lower TiO2 content in the pigment may adversely affect the optical properties of the pigment.
US Patent No. 4,405,376 discloses a titanium dioxide pigment along with a process for producing the pigment, wherein the pigment displays improved durability and dispersibility and comprises a pigmentary titanium dioxide core particle, an inner coating of hydrous oxides of tin and zirconium, and an outer coating of a hydrous oxide of aluminium. However, there is reduction in the whiteness of the pigment due to the use of tin compounds.
US Patent No. 4,447,271 discloses a highly durable and weather-resistant pigment made by treating TiO2 with a dense amorphous silica and then with a hydrous oxide of zirconia. Optionally, an outer coating of a hydrous oxide of alumina is provided. The silica precipitation is done at alkaline pH (pH 7.5 to 9.4). The alumina layer is precipitated at pH 10.0 to 10.5. Apart from the possible adverse impact on optical properties due to high surface coating levels, the contribution to durability from alumina coating will also be lower.
US Patent No. 5,203,916 discloses a pigmentary titanium composite possessing good durability and excellent optical properties, consisting essentially of a particulate titanium dioxide base, a hydrous zirconium oxide layer deposited on the titanium dioxide base at pH 8, and a hydrous alumina layer deposited on the hydrous zirconium oxide layer. A process for producing such a pigment is also described. Higher zirconia levels escalate pigment cost due the high cost of zirconium chemicals. Also, contribution to durability from alumina coating will be least.
US Patent No. 7,135,065 discloses a post treatment of TiO2 to obtain weather resistant pigment with good optical properties. The pigment is coated sequentially with hydrous tin and zirconium. At least one other component from silicon and titanium is additionally precipitated on the pigment particle surface. Then, final layer of alumina is precipitated. The post treatment components are added to the aqueous TiO2 suspension either in an acidic pH range or in an alkaline pH range. The possible reduction in whiteness due to tin compounds and higher zirconia levels are matters of concern.
US Patent No. 8,105,432 discloses a method for making high durability and easily dispersed pigment by adding citric acid to stabilize alumina. The combination of silica and citric acid-stabilized alumina are described as the cause of improved dispersion and durability. The precipitation of silica and alumina is done at various controlled pH levels by using either NaOH or HCl at 95°C. The patent offers no teaching on the deposition of zirconia layer and on the durability contribution from dense alumina coating.
US Patent No. 9,505,022 discloses a method for the use of wet surface treatment of TiO2 in order to produce durable universal grade TiO2 with superior optical properties. The pigment is coated with a hydrous silica and zirconia composite layer at acidic pH followed by a layer of alumina. The pigment exhibited improved durability with superior optical performance. Higher zirconia levels escalate pigment cost due the high cost of zirconium chemicals. Also, contribution to durability from alumina coating will be least.
Apparently, the product durability improves with the increase in coating level but at the risk of optical properties being affected adversely. Furthermore, apart from the nature of oxide used for coating and the coating level, the process of coating also influences the coating formed which in turn affect the durability and optical properties of the pigment.
Despite extensive prior art targeting the improvement in durability and optical properties of titanium dioxide, further improvements are continually being sought. Therefore, there is a need in the art for a process for preparing a universal type pigment having low surface coatings that provides a pigment with the desired durability and improved optical properties. The advantages of imparting co-precipitated composite layer of hydrous oxide of zirconium and silicon on TiO2 in the presence of a carboxylate additive, the dosing of aluminum citrate as a precursor for alumina coating, deposition of alumina coating both as dense and porous crystalline boehmite layers, and the benefits of lower treatment levels of zirconia especially in view of escalating costs of zirconium chemicals are disclosed herein.

Summary of the Invention
In one aspect, the present invention relates to a titanium dioxide pigment. The titanium dioxide pigment comprises a composite first coating of 0.25% to 0.50% by weight of hydrous oxide of zirconium and 0.25% to 0.50% by weight of hydrous oxide of silicon based on total pigment weight; and a second coating of 1.5% to 4% by weight of alumina based on total pigment weight on said first coating surface.
The composite first coating of hydrous oxide of zirconium comprises 0.25% to 0.50% by weight of zirconia based on total pigment weight and calculated as ZrO2 and the first coating of hydrous oxide of silicon comprises 0.25% to 0.50% by weight of silica based on total pigment weight and calculated as SiO2. The second coating comprises 1.5% to 4.0% by weight of alumina based on total pigment weight and calculated as Al2O3. In an embodiment, the source of alumina is a sodium aluminate solution containing about 28% to 34% wt. of Al2O3. The titanium dioxide pigment comprises 93.0% to 96.0% by weight of titanium dioxide based on total pigment weight and calculated as TiO2.
In another aspect, the present invention relates to a process for preparing a titanium dioxide pigment wherein the process comprising the steps of:
a. preparing an aqueous slurry of base TiO2 particles at a pH range of 7.0 to 8.0;
b. heating said slurry at a temperature of 55°C to 60°C followed by contacting said slurry with a carboxylate additive compound;
c. adding a zirconium orthosulphate solution and a sodium silicate solution simultaneously to said slurry by maintaining temperature of 55°C to 60°C thereby forming a first coprecipitated coating of hydrous oxide of zirconium and hydrous oxide of silicon at a pH range of 6.8 to 7.5;
d. adding balance zirconium orthosulphate solution to the above slurry maintaining the pH of 6.8 to 7.5 with the help of NaOH solution and digesting for 10 to 30 minutes thereby maintaining temperature of 55°C to 60°C;
e. lowering the pH of the above slurry to a range of 3.0 to 4.5 and adding sodium aluminate solution in presence of a carboxylic acid compound at a temperature of 55°C to 75°C and digesting for 10 to 15 minutes, thereby forming a second coating of amorphous dense alumina; and
f. raising the pH of above dense alumina coated slurry to 7.0 to 8.0 by maintaining the temperature at 55°C to 75°C and digesting for 10 to 35 minutes to effect partial conversion of dense alumina to porous boehmite alumina coating.

In the context of the present invention, the carboxylate additive compound which is added prior to addition of zirconium orthosulphate solution and sodium silicate solution is a polyacrylate polymer of lower molecular weight selected from acrylic monomers, oligomers or polyacrylate polymers of lower molecular weight wherein the carboxylate additive compound is 0.10% to 0.50% by weight based on total pigment weight. The carboxylic acid compound added along with sodium aluminate is citric acid and its sodium or ammonium salt selected from hydroxypolycarboxylic acids thereof wherein the carboxylic acid is 0.10% to 0.50% by weight based on total pigment weight.

The aqueous slurry comprises base TiO2 particles in a range of 35% to 50% weight. The first coating comprises coprecipitation of 0.25% to 0.50% weight of hydrous oxide of zirconium and 0.25% to 0.50% weight of hydrous oxide of silicon based on total pigment weight. The second coating comprises 1.5% to 4.0% of alumina based on total pigment weight.

The alumina coating is imparted from aluminum citrate solution comprising citric acid or its sodium salt or ammonium salt and a sodium aluminate wherein the aluminium citrate is prepared by adding citric acid and sodium aluminate under agitation in a molar ratio of 0.008:1.0.

Brief Description of Drawings
Figure 1 shows Transmission electron microscopy (TEM) images of the titanium dioxide (TiO2) pigment prepared in accordance with the present invention.
Detailed Description of Invention
The present invention is related to an improved process for preparing rutile titanium dioxide (TiO2) pigment. In particular, the present invention relates to a rutile titanium dioxide (TiO2) pigment comprising a pigmentary core of titanium dioxide (TiO2), a first coating of hydrous oxide of zirconium and hydrous oxide of silicon layer on TiO2 and a second outer coating of alumina from aluminum citrate as a precursor thereon and a process for preparing said titanium dioxide pigment. The zirconia-silica composite layers and alumina layers precipitated during the coating process substantially improve the quality of the coating layer formed over the TiO2 base thereby improving the properties of the pigments such as durability, dispersion and optical properties.
In an embodiment, the present invention provides a titanium dioxide pigment. The titanium dioxide pigment comprises a composite first coating of 0.25% to 0.50% by weight of hydrous oxide of zirconium and 0.25% to 0.50% by weight of hydrous oxide of silicon based on total pigment weight; and a second coating of 1.5% to 4.0% by weight of alumina based on total pigment weight on said first coating surface. The first coating of hydrous oxide of zirconium comprises 0.25% to 0.50% by weight of zirconia based on total pigment weight and calculated as ZrO2 and hydrous oxide of silicon comprises 0.25% to 0.50% by weight of silica based on total pigment weight and calculated as SiO2. The second coating comprises 1.5% to 4.0% by weight of alumina based on total pigment weight and calculated as Al2O3. The source of alumina is a sodium aluminate solution containing about 28% to 34% wt. of Al2O3.
In an embodiment, the pigment comprises 93.0% to 96.0% by weight of titanium dioxide based on total pigment weight and calculated as TiO2.
In another embodiment, the present invention provides a process for preparing a titanium dioxide pigment. The process comprising
a. preparing an aqueous slurry of base TiO2 particles at a pH range of 7.0 to 8.0;
b. heating said slurry at a temperature of 55°C to 60°C followed by contacting with a carboxylate additive compound;
c. adding a zirconium orthosulphate solution and a sodium silicate solution simultaneously to said slurry by maintaining temperature of 55°C to 60°C thereby forming a first coprecipitated coating of hydrous oxide of zirconium and hydrous oxide of silicon at a pH of 6.8 to 7.5;
d. adding balance zirconium orthosulphate solution to the above slurry by maintaining the pH of 6.8 to 7.5 with the help of NaOH solution and digesting for 10 to 30 minutes a temperature thereby maintaining of 55°C to 60°C;
e. lowering the pH of the above slurry to a range of 3.0 to 4.5 and adding sodium aluminate solution in presence of a carboxylic acid compound at a temperature of 55°C to 75°C and digesting for 10 to 15 minutes, thereby forming a second coating of dense alumina; and
f. raising the pH of above dense alumina coated slurry to 7.0 to 8.0 by maintaining the temperature of 55°C to 75°C and digesting for 10 to 35 minutes to effect partial conversion of dense alumina to porous boehmite alumina coating.
In this embodiment, the carboxylate additive compound can be selected from a polyacrylate polymer of lower molecular weight selected from acrylic monomers, oligomers or polyacrylate polymers of lower molecular weight. The carboxylate additive compound can be about 0.10% to 0.50% by weight based on total pigment weight. The carboxylic acid compound used during the alumina coating can be selected from citric acid and its sodium or ammonium salt selected from hydroxypolycarboxylic acids thereof. The carboxylic acid can be about 0.10% to 0.50% by weight based on total pigment weight. The composite first coating comprises coprecipitation of 0.25% to 0.50% weight of hydrous oxide of zirconium and 0.25% to 0.50% weight of hydrous oxide of silicon based on total pigment weight. The addition of zirconioum orthosulfate to the TiO2 suspension results in its hydrolysis and as a result zirconia will be precipitated which anchors as hydrous oxide of zirconium coating layer on TiO2 surfaces via Zr-O-Ti bonds. Similarly, hydrolysis of sodium silicate precipitates silica, which combines onto the surface of TiO2 surface as hydrous oxide of silicon through Si-O-Ti bond.
In an embodiment, the second coating comprises 1.5% to 4.0% by weight of alumina based on total pigment weight.
In an embodiment, said aqueous slurry comprises base TiO2 particles in a range of 35% to 50% weight.
In an embodiment, an aqueous slurry of titanium dioxide pigment particles can be prepared, and sand milled before depositing a composite layer. An aqueous suspension of base titanium dioxide material can be used. The suspension can be at acidic pH, for example, at a pH of from 3.0 to 5.0 or an alkaline pH, for example, at a pH of 9.0 or higher or about 10.0 or higher. Preferably, an alkaline TiO2 suspension can be used. To this end, the suspension pH value is set to at least about 10, preferably at least 10.5. The aqueous slurry is preferably a high intensity zircon sand milled in the presence of a dispersing agent to cause at least about 70%, more preferably at least about 80%, of the base pigment particles in the slurry to have a particle size of less than 0.5 microns as measured by a particle size analyser (Horiba LA960). The aqueous slurry thus formed can contain TiO2 from about 350 gdm-1 to about 500 gdm-1 by weight, based on the total volume of the slurry. The viscosity of the slurry can be less than 50 centipoises. The dispersing agent can be selected from the group consisting of phosphates, acrylates, polyols and/or amines. Preferably, the dispersing agent is sodium hexametaphosphate. The dispersing agent can be in an amount in the range of from about 0.05% to about 0.50% by weight based upon the weight of said pigment material. Preferably the amount of said dispersing agent is about 0.10% to 0.30% by weight based upon the weight of said pigment material. Preferably the milling media can be zircon sand. The milling media is then removed from the aqueous slurry of the pigment and the slurry is heated to a temperature of 55°C to 60°C and contacted with carboxylate additive. The carboxylate additive compound can be selected from acrylic monomers, oligomers or polyacrylate polymer of lower molecular weight. The carboxylate additive compound aids to form a stabilized hydrous zirconium oxide coating layer on the pigment surface which will give additional durability even in lower amounts compared to traditionally zirconia coated TiO2. The process of depositing a composite layer of co-precipitated mixture oxides of hydrous zirconium and silicon on a slurry of titanium dioxide pigment includes forming a co-precipitated coating on the base particles in situ in the aqueous slurry. The co-precipitated metal oxides essentially consist of metal oxides of zirconium or silicon or both. Preferably, the co-precipitated metal oxides are inorganic coating consisting of silica and zirconia. Preferably, silica, based on the weight of TiO2, can be added to the slurry simultaneously with zirconia, sufficient to maintain the pH of the suspension within 6.8 to 7.5. In another aspect, zirconia and silica can be added sequentially.
The co-precipitated coating of zirconia and silica can be formed on the pigment at a pH in the range of from about 6 to about 9, preferably from about 6 to about 8, more preferably from about 6.8 to about 7.5. The amount in which the composite zirconia and silica coating can be formed on the base pigment can be in equal quantities preferably each in the range of from about 0.10% to about 0.75% by weight, even more preferably from about 0.25% to about 0.50% by weight, based on the weight of the base pigment.
In an embodiment, the zirconia compound, can comprise water soluble zirconium salts. The acid soluble zirconium salts are useful as sources of zirconium. Preferred are salts of sulphuric or hydrochloric acids and most preferably, zirconium orthosulphate containing about 18.0 weight % ZrO2 with a specific gravity of 1.40. The source is added in the range of about 0.10% to 0.75%, preferably in the range of about 0.25% to 0.50% wt. hydrous zirconia expressed as ZrO2.
In an embodiment, the silica compound can comprise a water-soluble alkaline sodium and potassium silicates. Most preferably, sodium silicate solution containing about 25% weight SiO2 with specific gravity of 1.25 preferred as source of SiO2. Preferably, the sodium silicate is added in amounts between about 0.10% and about 0.75%, most preferably about 0.25% to 0.50% wt. of silica, expressed as SiO2.
In an embodiment, the aqueous slurry and the co-precipitate component zirconia and the silica precursor compounds are both added to the suspension simultaneously at a pH maintaining at about 6.8 to about 7.5. The balance zirconia precursor compound is then added to the suspension maintaining the pH of about 6.8 to about 7.5 with the help of NaOH solution. The person skilled in the art is familiar with the quantity of NaOH required for pH control. In order to ensure completion of precipitation and to avoid particle aggregation, the temperature of titanium dioxide suspension can be maintained high prior to addition of coating agents. The temperature can range from about 40°C to about 90°C, more preferably in the range of about 40°C to about 80°C and still more typically from about 55°C to about 60°C, although low temperatures might also be effective. A retention time of 5 to 45 minutes, preferably 10 to 30 minutes, or 10 to 15 minutes can be used until precipitation is substantially or fully completed.
In an embodiment, hydrous aluminium oxide can be continuously coated on the surface of ZrO2/SiO2 coated TiO2 with a compact layer at an elevated temperature. The amorphous hydrous alumina can be formed on the base pigment at a pH in the range of from about 3 to about 6, preferably from about 3 to about 5, more preferably from about 3 to about 4.5. The temperature can range from about 40°C to about 90°C, more preferably in the range of about 40°C to about 80°C and still more typically from about 55°C to about 75°C. The alumina coating formed on the base pigment can be in the range of from about 1.5% to about 9%, more preferably from about 1.5% to about 6%, even more preferably in the range of from about 1.5% to about 4.0%, calculated as Al2O3 and referred to the total TiO2 pigment weight.
Suitable aluminium components for the surface treatment method according to the present invention can be alkaline or acid-reacting, water-soluble salts, e.g. sodium aluminate, aluminium sulphate, aluminium nitrate, aluminium chloride, aluminium acetate, etc. Most preferably, the aluminium containing material is sodium aluminate solution containing about 28% to 34% wt. of Al2O3 with specific gravity of 1.50 as source of Al2O3. This selection is not to be interpreted as a restriction. All the amounts of zirconia, silica and alumina are percentages expressed as weight percentage to the untreated TiO2 weight. The layer of aluminium oxide can be applied to the ZrO2/SiO2 composite layer in such a way that the pH value is preferably maintained in the range from about 3 to about 4.5, e.g. by parallel addition of an alkaline aluminium component, such as sodium aluminate, and an acid, e.g. sulphuric acid or hydrochloric acid, or together with a base, e.g. NaOH.
In this context, either the components can be added in such a way that the pH value remains constant at a value in the range from about 3 to about 4.5, or the components can be added in such a combination that the pH value varies within the pH value range from about 3 to about 4.5 during addition. The person skilled in the art would be familiar with these procedures. Suitable for setting the pH value are, for example, base or acids (e.g., NaOH/H2SO4).
In an embodiment, coating of amorphous dense alumina is carried out at an acidic pH of about 3.0 to about 6.0. During this step, the pH of the suspension is preferably maintained at a pH of from about 3.0 to about 4.5 range throughout with the help of strong mineral acid. Those persons skilled in the art are familiar with the quantity of acid required for pH control. A retention time of 5 to 45 minutes, 10 to 30 minutes, preferably 10 to 15 minutes can be used to ensure the completion of precipitation of dense alumina on the titanium dioxide particles to which a first zirconia-silica composite coating is already deposited.
In another embodiment, the coating involves first depositing dense alumina at acidic pH and then raising pH of the dense alumina coated titanium dioxide comprising a zirconia-silica composite coating to a pH of 7.0 to 8.0 thereby converting majority portion of dense amorphous alumina coating to porous and crystalline boehmite alumina coating.
The coating film of amorphous hydrous alumina is dense and compact at acid conditions, while at a pH value in the alkaline range the coating film was transferred into a continuous and porous boehmite structure. The pH of the suspension is adjusted to about 7.0 to about 8.0 and maintained the temperature of about 55°C to about 75°C with the help of NaOH solution. A retention time of 15 to 60 minutes, 20 to 45 minutes, or 25 to 30 minutes or preferably 10 to 35 minutes can be used to ensure the partial conversion of amorphous dense alumina to crystalline and porous boehmite alumina. The invention involves alumina coating in both dense phase for improved durability as well as boehmite phase for enhanced optical properties and hence only partial conversion. In the present invention majority, (= 60%) i.e. about 60% to 70%, is required as boehmite phase.
The pigment requires the advantages accruing from the coatings of both dense and boehmite alumina. The pigment with dense alumina will have high durability characteristics but at the cost of reduced optical properties and dispersion. The durability of the zirconia coated TiO2 pigment can be enhanced further by the presence of dense alumina. Increasing the pH of the dense alumina suspension to 7.0 to 8.0 will convert (significant or partial) portion of dense alumina to porous crystalline boehmite alumina. Porous boehmite alumina in the presence of citric acid will improve the optical properties and dispersion of the pigment. The advantages of both dense and porous coatings are taken use of. The highly porous and continuous boehmite alumina film contributes to the increase of steric hindrance and electrostatic repulsion between the TiO2 pigment particles, and thus prevents the particles from agglomerate and promoted the dispersion stability.
The coating step of alumina includes additives while coating amorphous dense alumina. The additive is a carboxylic acid and is generally hydrocarbon comprising compounds of aliphatic type having the length of the main hydrocarbon comprising chain preferably does not exceed 10 carbon atoms, for example, 6 carbon atoms. Examples of carboxylic acid compound for coating of alumina can be selected from hydroxypolycarboxylic acids and more particularly the hydroxytricarboxylic acid is citric acid. Citric acid or its corresponding sodium or ammonium salts are preferred. The specific additives are believed to result in compact coating of alumina for enhanced durability.
In an embodiment, citric acid or its sodium salt can be incorporated in the coating composition by using it to produce aluminium citrate or by adding it separate. The alumina coating is imparted from the aluminium citrate solution comprising citric acid or its sodium salt or ammonium salt and a sodium aluminate. Aluminium citrate is therefore prepared in such a way as to generate corresponding citrate as citric acid in the range of about 0.10% to about 2%, preferably about 0.10% to about 0.50% and most preferably in the range of from about 0.15% to about 0.25%, by weight of TiO2. The aluminium citrate is prepared by adding citric acid and sodium aluminate under agitation in a molar ratio of 0.008:1.0.
The said alumina coatings contiguous to zirconia-silica composite skin will combine maximum durability, dispersibility and optical properties. This combination of sodium aluminate and citric acid presumably eliminates the sticky and thixotropic behaviour of filter cake during pressure filtration and spin flash drying.
Citric acid can be added to sodium aluminate solutions under agitation in the preferred molar ratio of citric acid to aluminium. Suitable amount of citric acid can be in the range of about 0.10% to about 2%, preferably about 0.10% to about 0.50 % and most preferably in the range of from about 0.15% to about 0.25%, by weight of TiO2. The carboxylic acid used is 0.10% to 0.25% by weight, on total pigment weight.
Synergistic benefits have been found when both sulphate and citrate ions are present as the preferred additives because of dispersion enhancing properties of citrate ions and synergistic effect of SO4- ions to result in compact coating of alumina for enhanced durability. The SO4- anions encourage the formation of insoluble dense alumina on the TiO2 surface in the presence of citrate ions. Sodium aluminate based aluminium citrate will provide citrate ions while SO4-will be available from H2SO4 being used for pH adjustments. Addition of sodium aluminate solution in the presence of agents such as citrate and sulphate ions forms compact and dense hydrous amorphous alumina at a pH of from about 3.0 to about 6.0. The coating involves first coating dense alumina at an acidic pH preferable at 3.0 to 4.5 and then raising pH of the dense alumina coated to 7.0 to 8.0 whereby majority i.e. about 60% to 70%, of the dense alumina is converted to crystalline and porous boehmite alumina. The highly porous and continuous boehmite alumina film contributes to the increase of steric hindrance and electrostatic repulsion between the TiO2 pigment particles, and thus prevent the particles from agglomeration and promote the dispersion stability. The present invention involves two-phase coating of Al2O3, wherein the first phase is of dense amorphous alumina done in acidic pH to promote durability and second phase is of crystalline and porous boehmite alumina that promotes the dispersion characteristics and further enhances optical properties of the TiO2 pigment. The composite dense and porous boehmite structure of the present invention provides the TiO2 with improved durability, gloss and dispersibility. Presence of dense alumina promotes the chalk-free resistance. The hydrous alumina when applied to the pigment in the needle-like boehmite form allows for the pigment to be more easily dispersed. The ratio of the dense alumina to the boehmite alumina is preferable in the range of about 1:1 and most preferably in the range of about 0.7:1.3.
Referring to Figure 1, transmission electron microscopy (TEM) images of the TiO2 pigment prepared in accordance with the present invention are shown. The TEM images show that the surface-coating comprising hydrous oxides of zirconia, silica and alumina on the surface of titanium dioxide pigment is uniform and continuous. This uniform and continuous coating contributes to higher durability and dispersibility of the TiO2 pigment.
The surface treated TiO2 pigment can be separated from the suspension by filtration methods known to the person skilled in the art, and the resultant filter cake is washed in order to remove the soluble salts. During subsequent drying and milling, e.g. in a steam mill, an organic compound can be added to the pigment, taken from the range customarily used in the manufacture of TiO2 pigments and familiar to the person skilled in the art, such as polyalcohols (trimethylol propane or trimethylol ethane). The surface treatment method according to the invention is customarily performed in batch mode. It is, however, also possible to perform treatment continuously, in which case suitable mixing equipment, such as is familiar to the person skilled in the art, must guarantee sufficiently thorough mixing.
In embodiment, the alumina coating on the surface of rutile TiO2 pigment takes place through both chemical interaction/bonding and physical adsorption. The composite coating consisting of hydrous zirconia and silica and boehmite alumina displays improved durability and dispersion stability in the water-based dispersions. The boehmite content in the coating film improves with the elevation of aging temperature and pH.
The layers of zirconia-silica composite and alumina precipitated under the controlled conditions as described herein can be useful in avoiding any chance for dissolution or damage of these layers. The order of addition of zirconia-silica and alumina can be important as the zirconia-silica composite layer can be used directly on the TiO2 base to induce photo-durability. Zirconia-silica composite layers and alumina layers thus precipitated improve the quality of the layer formed over the TiO2 base and thereby improve the properties of the pigments such as durability, optical properties and dispersion. The average particle size of the TiO2 pigment is 0.30 microns.
The titanium dioxide pigment produced in accordance with the present invention can be used with advantage as a multipurpose pigment having improved durability with good optical performance both in interior and exterior coating applications, including but not limited to powder, coil, can, and /or automotive applications.

Examples
The following examples are given to describe further the details of the instant invention. TiO2 pigment by chloride process manufactured by KMML is used in all examples. Properties of the pigments prepared in the examples are shown in Table 1 and Table 2 below. All percentages, based on weight basis, were analysed by XRF (X-ray fluorescence) as their oxides and refer to the TiO2 base material.
Example 1
A wet milled slurry was formed of TiO2 base material having 0.7% by weight to 1% by weight alumina within the lattice, manufactured by the chloride process. The slurry had a TiO2 concentration of 400 g/l and was adjusted to a pH value of 7.0 with H2SO4 at 58°C. Carboxylate solution (0.15% by weight, based on weight of TiO2) is dosed to this slurry in a span of 5 minutes. While stirring, 0.30% by weight of SiO2, based on the weight of TiO2, was added to the suspension in the form of sodium silicate solution simultaneously with ZrO2 in the form of zirconium orthosulphate solution (18% by weight ZrO2) sufficient to maintain the pH of the suspension within 6.8 to 7.5. Immediately afterwards, balance zirconium orthosulphate (0.30 % total ZrO2 based on the weight of TiO2 particles in the slurry) was added to the solution simultaneously with NaOH to maintain pH within 6.8 to 7.5. The resulting slurry was digested for 15 minutes at the same temperature. The pH of the slurry was lowered to 3.0 to 4.5 by adding H2SO4 and the temperature was adjusted to about 70°C. Subsequently, citric acid (0.20% by weight, based on weight of TiO2) mixed with sodium aluminate solution equivalent to 2.2% by weight of Al2O3, based on the weight of the TiO2, was added to the slurry in the form of sodium aluminate, and the pH was maintained at 3.0 to 3.5 by adding H2SO4. The resulting slurry was digested for another 15 minutes at the same temperature. The pH of the slurry was then adjusted to 7.0 to 8.0 with NaOH solution in a span of 15 minutes. The slurry was then cured for another 25 minutes and the final pH value was adjusted to 7.0 to 8.0. Water soluble salts were then washed off and the resulting product was dried at 120°C and milled.
The chemical composition of the product was analysed by XRF and the durability was estimated by UV reactivity rating, whereby, the higher the rating, the better the durability in the scale of 0 to 10. Dispersibility is determined by methods known in the art. For example, the coated titanium dioxide pigments of the present invention can be mixed in a plastic or paint and the distribution of the pigment particles measured. Uniform distribution of the pigment throughout the paint or plastic indicates good dispersibility, while agglomerate formation would indicate poor dispersibility of the pigment. Some methods of determining dispersibility known in the art include tinting strength, Q test, and the like.
Gloss is determined by incorporating the pigments made by the methods of the present invention into acrylic based paints at 13.2% PVC (pigment volume concentration) and the 20° gloss measured.
Example 2
Example 2 was similar to Example 1 except that the SiO2 and ZrO2 were added sequentially.

Example 3
Example 3 was similar to Example 1 except that the citric acid was dosed to the suspension immediately prior to sodium aluminate addition.
Example 4
Example 4 was similar to Example 1 except that the aluminate addition was performed at pH of 7.0 to 8.0 for precipitation of boehmite alumina and later reduced to 3.0 to 4.5 for partial conversion to dense alumina.
Test Methods
Samples from above Examples and Comparative Examples were tested for UV reactivity rating as per the procedure described below.
1. Ultraviolet Reactivity Rating (UVRR)
The durability of a pigment is usually measured as resistance to chalking over a long term, for example, about two years of outdoor exposure. Tests are typically carried out on paint containing the pigment. However, chalk/fade/degradation of exterior paints containing TiO2 pigment can also be attributed to the photooxidation of organic binder catalysed by the titanium dioxide under ultraviolet radiation in the presence of oxygen and water vapor, as reported in H. B Clark, “Titanium Dioxide Pigments,” Treatise on Coatings, Vol. 3, 5 Pigments, Marcel Dekker, 1975. So, the Ultraviolet Reactivity Rating (UVRR) of the pigments produced in the present Examples was measured as described in US Patent Nos. 5,554,216 and 5,824,145. The test used was based on the TiO2-catalyzed reduction of lead carbonate to lead metal under UV radiation. An air sealed dispersion of non-durable pigment and lead carbonate in an organic medium was used and turns from white to almost black by exposure to ultraviolet light. With a durable pigment, however, the paste turns to light grey. The exposure used was for 7 hours and the relative ultraviolet reactivity rating was calculated against a reference sample of the same composition. The value was then converted to a durability rating. Higher rating values represent higher durability.
The test results from testing the Examples are illustrated in Table 1 and Table 2 below. Reproducibility and repeatability of the experiments were confirmed, and all the values given in the Table are representative of at least 3 duplicate experiments.
TABLE- 1
Example ZrO2 (%) SiO2 (%) Al2O3 (%) Durability Rating*
1 0.33 0.35 3.0 7.6
2 0.33 0.34 3.0 7.4
3 0.32 0.33 3.0 7.5
4 0.34 0.34 3.0 7.0
* The higher the rating values, the higher the durability

TABLE- 2
Example ZrO2 (%) SiO2 (%) Al2O3 (%) TiO2 (%) Dispersion* Gloss Filter Cake Nature
1 0.33 0.35 3.0 95.30 7.0 62 Stable porous cake
2 0.33 0.34 3.0 95.30 6.75 60 Stable porous cake
3 0.32 0.33 3.0 95.35 6.75 62 Sticky with Thixotropic tendency
4 0.34 0.34 3.0 95.30 7.0 65 Stable porous cake
* The higher the values, the higher the dispersion
It is very obvious from the results that, the durability is higher in all of the Examples of the present invention except Example 4.
It can be seen that the durability is higher when SiO2 and ZrO2 are co-precipitated on the TiO2 particles.
The optical performance, particularly the gloss in Example 4, was highest where initially alumina was precipitated in boehmite phase and later converted partially to dense alumina.
The resultant filter cake of Example 3 exhibited thixotropic behaviour where citric acid and sodium aluminate were added separately.
2. The inferences to the comparative evaluation results of the Finished Pigment against competitive grades are provided in the Table 3 below.

Table 3 - Comparative Evaluation of Finished Pigment against Competitive Grades
Property Comparative Grade-1 Comparative Grade-2 Product of the present invention
Oil Absorption 18.3 20.0 17.3
High Shear Dispersion 7.0 6.75 7.0
Average Particle Size, ? 0.29 0.33 0.30
Performance in Paints
Solvent Based (13.2% PVC)
Vehicle Brightness, L 95.1 95.0 95.1
Acrylic Gloss @ 20o 62.0 52.0 62.0
Reducing Strength 100.0 98.5 102.1
Water Based (26.0% PVC)
Vehicle Brightness, L 96.73 96.30 96.72
Gloss @ 60o 34.8 13.5 35.0
Gloss @ 85o 72.8 58.0 73.0
Reducing Strength 100.0 96.2 102.4
Contrast ratio 0.96 0.94 0.96
Durability Index 6.7 7.5 7.6

From above Table 3, it was observed that in solvent based formulation, the new product of the present invention has shown comparable gloss and higher reducing strength against comparative grade-1 while outperforming comparative grade-2 in both the above parameters. Further, the trend was found to be similar in emulsion formulation also. The High Shear Dispersion value exhibited by the product of this invention indicated very high dispersion quality. This high HSD value can be attributed to the higher fineness of grind, and tactful treatment recipe.
It was further observed that durability of the product of new invention, was found to be superior to comparative grade-1 and comparable with comparative grade-2. Further, the lower oil absorption value of new product is superior to both the comparative grades. In short, the new product was found to combine the best attributes of Comp-1 and Comp-2.
3. The TEM images of the TiO2 pigment as illustrated by Figure 1 clearly showed that the surface coating of zirconia-silica and alumina was uniform and continuous which contributed to higher durability and dispersibility of the TiO2 pigment.
The foregoing description of specific embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching.
The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others, skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated.
It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.

,CLAIMS:
1. A titanium dioxide pigment comprising;
a first coating of 0.25% to 0.50% by weight of hydrous oxide of zirconium and 0.25% to 0.50% by weight of hydrous oxide of silicon based on total pigment weight; and
a second coating of 1.5% to 4.0% by weight of alumina based on total pigment weight on said first coating surface.

2. The titanium dioxide pigment as claimed in claim 1, wherein the first coating of hydrous oxide of zirconium comprises 0.25% to 0.50% by weight of zirconia based on total pigment weight and calculated as ZrO2.

3. The titanium dioxide pigment as claimed in claim 1, wherein the first coating of hydrous oxide of silicon comprises 0.25% to 0.50% by weight of silica based on total pigment weight and calculated as SiO2.

4. The titanium dioxide pigment as claimed in claim 1, wherein the second coating comprises 1.5% to 4.0% by weight of alumina based on total pigment weight and calculated as Al2O3.

5. The titanium dioxide pigment, as claimed in claim 1, wherein the source of alumina is a sodium aluminate solution containing 28% to 34% wt. of Al2O3.

6. The titanium dioxide pigment, as claimed in claim 1, wherein the pigment comprises 93.0% to 96.0% by weight of titanium dioxide based on total pigment weight and calculated as TiO2.

7. A process for preparing a titanium dioxide pigment comprising
a. preparing an aqueous slurry of base TiO2 particles at a pH range of 7.0 to 8.0;
b. heating said slurry at a temperature of 55°C to 60°C followed by contacting with a carboxylate additive compound;
c. adding a zirconium orthosulphate solution and a sodium silicate solution simultaneously to said slurry by maintaining temperature of 55°C to 60°C thereby forming a first coprecipitated coating of hydrous oxide of zirconium and hydrous oxide of silicon at a pH of 6.8 to 7.5;
d. adding balance zirconium orthosulphate solution to the above slurry by maintaining the pH of 6.8 to 7.5 with the help of NaOH solution and digesting for 10 to 30 minutes thereby maintaining temperature of 55°C to 60°C;
e. lowering the pH of the above slurry to a range of 3.0 to 4.5 and adding sodium aluminate solution in presence of a carboxylic acid compound at a temperature of 55°C to 75°C and digesting for 10 to 15 minutes, thereby forming a second coating of amorphous dense alumina; and
f. raising the pH of above dense alumina coated slurry to 7.0 to 8.0 by maintaining the temperature of 55°C to 75°C and digesting said slurry for 10 to 35 minutes to effect partial conversion of amorphous dense alumina to crystalline boehmite alumina coating.

8. The process as claimed in claim 7, wherein the carboxylate additive compound is a polyacrylate polymer of lower molecular weight selected from acrylic monomers, oligomers or polyacrylate polymers of lower molecular weight.

9. The process as claimed in claim 8, wherein the carboxylate additive compound is 0.10% to 0.50% by weight based on total pigment weight.

10. The process as claimed in claim 7, wherein the carboxylic acid compound is citric acid and its sodium or ammonium salt selected from hydroxypolycarboxylic acids thereof.

11. The process as claimed in claim 10, wherein the carboxylic acid is 0.10% to 0.50% by weight based on total pigment weight.

12. The process as claimed in claim 7, wherein said aqueous slurry comprises base TiO2 particles in a range of 35% to 50% by weight.

13. The process as claimed in claim 7, wherein said first coating comprises coprecipitation of 0.25% to 0.50% weight of hydrous oxide of zirconium and 0.25% to 0.50% weight of hydrous oxide of silicon based on total pigment weight.

14. The process as claimed in claim 7, wherein the second coating comprises 1.5% to 4.0% of alumina based on total pigment weight.

15. The process as claimed in claim 7, wherein the alumina coating is imparted from aluminum citrate solution comprising citric acid or its sodium salt or ammonium salt and a sodium aluminate.

16. The process as claimed in claim 15, wherein the aluminium citrate is prepared by adding citric acid and sodium aluminate under agitation in a molar ratio of 0.008:1.0.