ALUMINA COATED TIO2

Abstract: A chalk-fade resistant TiO₂ pigment exhibiting excellent gloss and dispersion that is comprised of TiO₂ particles with a composite coating of dense alumina and boehmite alumina totaling 2-6% by weight alumina based on the TiO₂ and a pro­ cess for the preparation of such TiO₂ pigment.

Description: Alumina Coated TiO2 The present invention relates to titanium dioxide pigment coated with a composite of dense alumina and boehmite alumina and the process for their preparation. More specifically, the present invention relates to a titanium dioxide pigment with 2-6% by weight alumina composed of a composite of dense/boehmite alumina exhibiting good resistance to chalk-fade reaction, high gloss and excellent dispersibility. The treatment of TiO2 to improve durability by applying a protective coating is well-known in the art. U.S. Patent 3,437,502 describes an improved TiO2 pigment with high opacity and outstanding ease of dispersibility in liquid coating obtained by applying a dense silica coating to TiO2 followed by a coating of alumina. U.S. Patent 3,928,057 discloses a TiO2 pigment having a first coating of porous silica or alumina and a dense coating ofsilica over the porous coating to achieve a pigment with a high degree of hiding power and durability. U.S. Patent 3,523,810 discloses a TiO2 pigment coated with boehmite alumina having a crystal size of at 0 least 50 A in diameter to provide the TiO2 which will give the latex paint composition excellent viscosity stability. U.S. Patent 4,022,636 discloses a method for preparing a TiO2 pigment that has improved material handling characteristics for plastic systems by coating the TIROS first with amorphous alumina and then boehmite alumina. However, the total level of alumina coated is insufficient to provide a satisfactory product for paint systems where both gloss, dispersibility and chalkfade resistance are desired. Such a pigment would not exhibit the chalk-fade resistance, gloss and dispersibility of the pigment of the present invention. British Patent 1,368,601 discloses coating TiO2 with alumina and then dissolving off a portion of the alumina coating and applying a second coating of hydrous titania and more alumina in order to prepare a TiO2 pigment with improved gloss retention and resistance to chalking. Durability for rutile TiO2 pigment has been achieved in the past by depositing a dense silica coating on the pigment. An alumina coating has been applied over the silica coating to enhance pigment processing, as well as gloss and dispersibility. The gloss and dispersibility of a rutile TiO2 pigment having only a coating of silica is inferior to a rutile TiO2 pigment with an alumina coating. However, alumina coatings that are all boehmite alumina do not provide chalkfade resistance. A durable TiO2 pigment has now been found comprising TiO2 particles having a composite coating of dense alumina and boehmite alumina totaling 2-6% by weight alumina based on the Ti02, said composite dense and toehinite alunina being present in a weight ratio of 8:1-1:2 of dense to boehmite alumina,tile dense alumina defined as alumina leaving incorporated therein sulfate ion in a weight ratio of aluminum to sulfate ion of lO:l-1.3:1. The present invention provides a TiO2 rutile pigment with an alumina coating that exhibits excellent gloss and dispersibility along with lightfastness (resistance to chalk fade). The present Ti02 pigment has an alumina coating that is characterized by a p6rtion being dense (containing sulfate) to achieve the chalk-fade resistance and a portion being boehmite, a form of hydrated alumina, to achieve both acceptable gloss and dispersibility. The composite coating of the invention is applied to TiO2 particles that have been prepared either by the vapor phase oxidation. of titanium tetrachloride or with hydrolysis of titanium sulfate. One process for preparing the durable TiO2 pigment having 2-6% by weight of alumina based on the TiO2, said alumina having a 8:1-1:2 weight ratio of dense to boehmite alumina and a weight ratio of aluminum to sulfate ion of 10:1-1.3: :1 comprises (a) adding TiO2 rutile pigment to water with agitation to form a slurry; -(b) heating the thus prepared slurry to 40-900C; (c) adding a soluble basic aluminum com pound and/or an inorganic base to the slurry to attain a pH of 7.0 9.5; (d) adding a sufficient amount of a soluble aluminum compound to de posit a total of 0.5-2% by weight alumina based on the Ti02 while maintaining the pH at 7.0-9.5, (e) lowering the pH to 5.2-6.5 with an acid; (f) adding a sufficient amount of soluble aluminum compound to de posit in the presence of the re quired amount of sulfate ions 2-6% by weight of total alumina based on the Tio2 while maintaining -the pH at 5.2-6.5 at a tempera ture of 40-650C; ; (g) optionally agitating the slurry at 40-650C for 1-180 minutes; and (h) separating and washing the thus treated TiO2 free of soluble salts. The above process involves depositing boehmite alumina on the TiO2 and then depositing dense alumina under conditions whereby a composite coating of dense and boehmite alumina is formed. Preferably, the soluble aluminum compound is sodium aluminate and the acid used to reach the pH of steps (d), (e) and (f) is sulfuric acid. Another process for preparing the composite coating involves first depositing dense alumina on the TiO2 and then subjecting the dense alumina to conditions whereby some of the dense alumina is converted to boehmite alumina. Accordingly, another process for preparing the durable TiO2 pigment having 2-6% by weight of alumina based on the Tri02, said alumina having an 8:1-1:2 weight ratio of dense to boehmite alumina and a weight ratio of aluminum to sulfate ion of 10:1-1.3: :1 comprises (a) adding TiO2 rutile pigment to water with agitation to form a slurry; (b) heating the thus prepared slurry to 30-650C; (c) adjusting the photo 5.2-6.5; (d) adding a sufficient amount of the soluble aluminum compound to de posit in the presence of the re quired amount of sulfate ions 2-68 by weight of total alumina based on the Ti02 while maintaining the pH at 5.2-6.5 at a temperature of 30-650C; (e) agitating the slurry at 30-650C for 5-180 minutes; (f) adjusting the pH to 7.0-9.5; (g) agitating the slurry for 5-60 minutes at 40-700C while main taining the pH at 7.0-9.5; and (h) separating and washing the thus treated TiO2 free of soluble salts. The composite- dense and boehmite alumina structure of the present invention provides the TiO2 with improved durability, gloss and dispersibility. Composite structures outside the range of the invention do not provide the desired chalk resistance, gloss and dispersibility. For example, higher levels of dense relative to boehmite alumina gives-higher chalk-fade resistance but marginal dispersibility and gloss. Dense alumina is alumina having sulfate ions strongly adsorbed therein at a weight ratio of 10:11.3:1 of aluminum to sulfate ion. Alumina is densified by curing the alumina with the sulfate ions for a period of time at a temperature of 30-650C and a pH of 5.2-6.5. Higher temperatures during precipitation or curing, e.g., 900C would result in less than the required sulfate ion retention. Likewise, precipitation and curing outside the pH range will affect the amount of sulfate ion retention. Boehmite- is a crystalline form of alumina that contains sorbed water. The water is commonly intercalated between A1(OH)6 octahedral layers in both random and regular arrangements. The dense alumina provides the TiO2 pigments witch durability, e.g., lightfastness as measured by chalk-fade resistance. The boehmite alumina enhances gloss and dispersibility of the TiO2 pigments. The TiO2 used to form the pigment with the improved durability, gloss and dispersibility of this invention is rutile TiO2. The sulfate ion may come from a variety of sources. It may come from the acid used to precipitate the alumina, e.g., sulfuric acid. It may come from the same source as the alumina, e.g., Al2 (SO4)3. It may also be separately added into the slurry when the slurry does not contain sufficient sulfate. For example, if sodium aluminate is used as the source of alumina and HC1 a-s the acid, one could add a soluble sulfate such as sodium sulfate to provide sulfate ions. Any inorganic sulfate that will not interfere with the process can be used as a source of soluble sulfate. The soluble aluminum compound of this invention provides the alumina needed. The soluble aluminum compound may be -a basic or acidic aluminum salt. Representative examples of such compounds include sodium aluminate, potassium aluminate, aluminum sulfate, aluminum chloride, etc. Thus in some instances it may be convenient to use a basic aluminum salt to provide alumina as well to adjust the pH. The preferred ratio of dense to boehmite alumina is :3:lA:2 especially 3: 2. 'lone preferred ratio of aluminum to sulfate in the dense alumina is 5:1-1.3:1. The preferred pH for the slurry- - prior to addition of the alumina source is 1.5-3. The preferred alumina sources are sodium aluminate and aluminum'sulfate, The most preferred alumina source is sodium aluminate. The preferred acid for pH adjustment is sulfuric acid. The preferred base for pH adjustment is sodium hydroxide. The preferred process for forming the composite coating on TiO2 involves subjecting the TiO2 to conditions for boehmite formation followed by depositing alumina under conditions that will produce or form dense alumina. It has surprisingly also been found that the addition of tetrapotassium pyrophosphate to the slurry of TiO2 after or with the addition of the soluble aluminum compound results in a pigment of further improved durability over the durability achieved with only the composite alumina of the invention. Generally, 0.1-0.4% by weight of tetrapotassium pyrophosphate based on the TiO2 is added to achieve improved results. The tetrapotassium pyrophosphate may be added simultaneously with the soluble aluminum compound or after the addition of the soluble aluminum compound but before the curing is completed by the agitation of the slurry for the time of up to 180 minutes. The process of the invention wherein a soluble aluminum compound is added to the TiO2 slurry at a pH of 7.0-9.5 and 40-90dC forms boehmite alumina. Preferred conditions for forming the boehmite are 45600C at a pH of 8-9. The subsequent deposition of alumina at a pH of 5.2-6.5 results in the formation of dense alumina during the treatment at 40-650C. The preferred conditions for depositing dense. alumina are 5.7-6.5 pH and a temperature of 45-600C. The coated product may be optionally cured for 1-180 minutes while maintaining the pH at 5.2-6.5 and 40-650C. It is preferable to cure the coated product for 1-180 minutes at 5.2-6.5 pH and 40-65"C. This process is preferred since the conversion of the dense alumina to boehmite alumina is more sensitive to the conditions and therefore more difficult to control in commercial operations. Boehmite alumina is stable at the low pH after being formed at the high pH of the process. Dense alumina is formed at the low pH but not stable at the high pH of the process. The process of the invention wherein the soluble aluminum compound is added to the slurry of TiO2, after adjusting the pH to 5.2-6.5, involves formation of dense alumina when cured at 30-650C for 5-180 minutes. The preferred initial pH of the slurry is 1.0-3.5 when the soluble aluminum compound is a sodium or potassium aluminate. The preferred subsequent treatment of the dense alumina at 45-600C and pH of 8-9 for 5-40 minutes is to form boehmite alumina. Thus the alumina deposited on the TiO2 initially that is dense alumina is partially converted to boehmite alumina to form the composite dense and boehmite structure of the inveniton. Any inorganic base may be used for pH adjustment. The preferred base is sodium hydroxide. Representative examples include sodium hydroxide, potassium hydroxide and ammonium hydroxide. Any acid may be used in the process of this invention that has sufficient acid strength to reach the desired pH under the conditions involved. Representative examples include HCl, H2SO4, HNO3, acetic, H3PO4, etc. EXAMPLES The following examples further illustrate the invention. Example 1 - (Process for Preparing the Product of the Invention by First Forming Boehmite Alumina and Then Dense Alumina) In a stirred vessel, 6209 g rutile TiO2 pigment in 15.5 liters of water were heated to 600C and had a pH of 3.85. Sixty-three ml of concentrated H2S04 were added to lower the pH of the pigment slurry to 1.5. The pH was adjusted to 8.5 with 34 ml of 508 NaOH and 196 ml of a sodium aluminate solution (315 g A1203/liter) over a 15 minute period, this formed 1% boehmite alumina based on the TiO2. The pH of the system was then lowered to pH=5.5 with 6 ml of concentrated H2S04. During a 30 minute period, a total of 393 my of sodium aluminate solution (315 g A1203/ liter) were added while holding the pH between 5.5 and 6.0 with concentrated H2SO4 (125 ml). After the alumina addition, the slurry was given a 15 minute cure at pH=5.7 and temperature at 600C. 12.4 Grams of tetrapotassium pyrophosphate (TKPP) in 75 ml of water were added along with 2 ml concentrated H2SO4 to hold the pH at 5.7. The treated pigment slurry was then given a 30 minute cure at pH=5.7 and 60"C. The pigment was then filtered, washed free of salts and dried at 1200C. The pigment was then micronized in an 8" steam micronizer using 3 lbs steam/lb pigment and evaluated for lightfastness, gloss and dispersion. The data is summarized in the table below. Example 2 The procedure of Example 1 was followed except the TKPP treatment step was omitted. The data is summarized in the table below. Example 3 - (Coating Dense Alumina First Followed by Increasing pH to Form Boehmite Alumina) In a stirred vessel 5000 g rutile TiO2 pigment in 12.5 liters of water were heated to 600C and had a pH=3.65. Thirty-two ml of concentrated H2S04 were added to lower the pH to 1.5. During a 30 minute period, 417 ml of sodium aluminate solution (360 g Al203/liter) were added at pH 6.2-6.4 controlling the pH with 93 ml of concentrated H2SO4. The slurry was cured for 30 minutes at pH=6.2, temperature 60 C. Ten grams of tetrapotassium pyrophosphate (TKPP) dissolved in 50 ml of water were added along with 2 ml concentrated H2 SO4 to maintain the pH at 6.2. The slurry was then cured for 15 minutes. The pH of the slurry was increased to 7.5 with 50% NaOH solution for boehmite formation. After 30 minutes, the pigment was filtered, washed and dried at I200C. The pigment was then micronized in an 8" steam micronizer using 2.8 lbs steam/lb of pigment and evaluated for lightfastness, gloss and dispersion. The data is summarized in the table below. Example 4 The procedure of Example 3 was followed except the TKPP treatment was omitted. The pigment was evaluated. The data is summarized in the table below. Example 5 - (High Ratio of Dense Alumina to Boehmite) In a stirred vessel 6200 g rutile TiO2 pigment in 15.5 liters of water were heated to 60 C and had a pH of 3.43. Fifty ml of concentrated H2SO4 were added to lower the pH to 1.5. During a 30 minute period, 517 ml of sodium aluminate solution (360 g A1203/liter) were added at a pH=6.2 with pH control maintained with Ill ml of concentrated H2SO4. The pigment slurry was cured for 30 minutes. 12.4 Grams of tetrapotassium pyrophosphate (TKPP) were added along with 1 ml concentrated H2S04 to hold pH at 6.2. The slurry was cured for 15 minutes at pH=6.2 and 600C. The pigment was then filtered, washed free of soluble salts and dried at 1200C. The dried pigment was steam micronized in an 8" micronizer using 3 lbs steam/lb pigment and evaluated for lightfastness, gloss and dispersion. The data is summarized in the table below. Example 6 The same procedure as in Example 5 was followed except without the TKPP treatment. The data is summarized in the table below. Example A - (All Boehmite Alumina Coating) In a stirred vessel 6200 g rutile TiO2 pigment i-n 15.5 liters of water were heated to 600C and had a pH=3.35. Twenty-eight ml of concentrated H2S04 were added to lower the pH to 1.5. During a 30 minute period, 517 ml of sodium aluminate solution (360 g Al203/liter) were added at pH=8.5 controlling the pH with 102 ml of concentrated H2SO4. The alumina coated pigment slurry was cured for 30 minutes at pH=8.5 and 600C. The pigment was then filtered, washed free of soluble salts and dried at 1200C. The dried pigment was steam micronized in an 8" micronizer using 3 Ibs steam/lb pigment and then evaluated for lightfastness, gloss and dispersion. The data is summarized in the table below. Example B - (Coating of 1.5% SiO2/3.0% A1203) In a stirred vessel 6800 g rutile TiO2 pigment in 17 liters of water were added to 900C and had a pH= 2.90. Added 27.2 ml of 50% NaOH to pH=9.0. Added 255 ml-sodium silicate solution (400 g/liter SiO2) controlling the pH at 9.4 with 21 ml of concentrated H2SO4. The silica coated TiO2 slurry was cured for 30 minutes at pH=8.5 and 900C. During a 30 minute period, 570 ml of sodium aluminate solution (360 g/A1203 per liter) were added while controlling the pH at 8.5 and temperature at 600C. The silica/alumina coated TiO2 slurry was cured 15 minutes at pH=8.5 and 600C. The coated pigment was filtered, washed free of soluble salts and dried at 1200C. The dried pigment was steam micronized in an 8" micronizer using 3 lbs steam/lb pigment and then evaluated for lightfastness, gloss and dispersion. A scale of 0-10 was used to rate the pigments after exposure to ultraviolet light. A pigment with a value of 10 would show no UV activity whereas an extremely photoactive pigment would get a value of zero. Titanium dioxide pigment with lightfastness values in excess of 5 can be used in applications where UV durability is desired. Any lightfastness test normally used for paint systems can be used to obtain lightfastness values. The data is summarized in the table below. The following tables summarize the pertinent data for the above examples: tightfastness 30J TFW-124 Boehmite* Value Gloss Dispersion Alumina % Example 1 6.25 72 7·-6¸(3) 1.0 Example 2 5.25 73 7-6(7) 1.1 Example 3 6.0 71 7-6(7) 1.2 Example 4 5.25 72 7-6(6) 0.9 Example 5 7.5 67 7-5(22)-- 0.4 Example 6 6.5 67 7-5(20) 0.4 Controls Example A 3.75 73 7i-6h(5) 2.8 All Boehmite Coating Example B 6.25 63 7-3(22) 1.1 1.5% SiO2/ 3.0% Al 23 *Estimated from infrared absorption curves and thermal gravimetric analysis based on the weight of coated Tio2 product. Ratio Sulfate* Dense/Boehmite * Content % Al2O3 Si02% Example 1 0.36 2 Example 2 0.38 1.8 Example 3 0.34 1.5 Example 4 0.40 2.3 Example 5 0.67 6.5 Example 6 0.62 6.5 Controls Example A 0.02 0.06 All Boehmite Coating Example B 0.02 1.5 1.5% SiO2/ 3.0% A1203 *Based on the weight of TiO2 coated product. When preparing pigments in accordance with the invention by first precipitating dense alumina at an acid pli, it is not always necessary to raise the pH to a value between 7 and 9.5 in order to convert a proportion of dense alumina to boehmite. As illustrated in Examples 5 and 6, a small proportion of boehmite alumina is formed, along with dense alumina, if the alumina is precipitated at an acid pH close to neutrality, e.g. at pH 6.2 or higher. Further quantities of boehmite may be formed by digesting the aluminacoated pigment under similar pH conditions, although the rate of conversion to boehmite proceeds more readily at a pH between 7 and 9.5 and at elevated temperature. It will be noted from the Tables of Results above that the light-fastness values of pigments prepared in accordance with the invention are comparable with, and in some cases superior to, the Example B control in which separate after treatments with both silica and alumina are employed. Also, in general, the dispersibility of the pigments of the invention are superior. This illustrates a significant advantage arising from this invention i.e. that it is possible to achieve comparable pigment properties without requiring to precipitate another metal oxide, such as silica, onto the pigment particles. It is to be understood that any of the components and conditions mentioned as suitable herein can be substituted for its counterpart in the foregoing examples and that although the invention has been described in considerable detail in the foregoing, such detail is solely for the purpose of illustration. Variations can be made in the invention by those skilled in the art without departing from the spirit and scope of the invention except as set forth in the claims.

Claims: CLAIMS: 1. TiO2 particles comprising a composite coating of dense alumina and boehmite alumina totalling 2-6% by weight alumina based on the TiO2, said composite dense and boehmite alumina being present in a weight ratio of 8:1-1:2 of dense to boehmite alumina, the dense alumina defined as alumina having incorporated therein sulfate ion in a weight ratio of aluminum to sulfate of 10:1-1.3:1. .TiOZ particles according to Claim 1 wherein the weight ratio of dense to boehmite alumina is 5:1-1:2. 3. TiO2 particles according to Claim 1 or Claim 2 having incorporated with the alumina 0.1-0.4% -by weight of tetrapotassium pyrophosphate based on the Tit2. 4. A process for preparing a chalk-fade resistant TiO2 pigment having 2-6% by weight of alumina based on the TiO2, said alumina having a 8:1-1:2 weight ratio of dense to boehmite alumina and a weight ratio of aluminum to sulfate of 10:1-1.3:1 comprising (a) adding TiO2 rutile pigment to water with agitation to form a slurry; (b) heating the thus prepared slurry to 40-90 C; (c) adding a soluble basic aluminum com pound and/or an inorganic base to the slurry to raise the pH to 7.0-9.5; - (d) adding a sufficient amount of the soluble aluminum compound to deposit a total of 0.5-2% by weight alumina based on the TiO2 while maintaining the pH at 7.0-9.5; (e) lowering the pH to 5.2-6.5 with an acid; ; (f) adding a sufficient amount of the soluble aluminum compound to deposit in the presence of the required amount of sulfate ions 2-68 by weight of total alumina based on the TiO2 while maintain ing the pH at 5.2-6.5 at a temper ature of 40-650C; (g) optionally agitating the slurry at 40-650C for 1-180 minutes; and (h) separating and washing the thus treated TiO2 free of soluble salts. 5. A process according to Claim 4 wherein 0.1-0-4% by weight of tetrapotassium pyrophosphate based on the TiOS is added after (f) but before (g). 6. A process for preparing a durable TiO2 pigment having 2-6% by weight of alumina based on the TiO2, said alumina having a 8:1-1:2 weight ratio of dense to boehmite alumina and a weight ratio of aluminum to sulfate of 10:1-1.3:1 comprising (a) adding TiO2 rutile pigment to water with agitation to form a slurry; (b) heating the thus prepared slurry to 30-650C; (c) adjusting the pH to 5.2-6.5; (d) adding a sufficient amount of the soluble aluminum compound-to deposit in the presence of the required amount of sulfate ions 2-6% by weight of total alumina based on the TiO2 while maintain ing the pH at 5.2-6.5 at a temper ature of 30-650; (e) agitating the slurry at 30-650C for 5-180 minutes; (f) adjusting the pH to 7 to 9.5; ; (g) agitating the slurry for 5-60 minutes at 40-700C while maintaining the pH of 7 to 9.5; and (h) separating and washing the thus treated TiO2 free of soluble salts. 7. A process according to Claim 6 wherein the pH of step (c) is 1.5-3. 8. A process according to Claim 6 or 7 wherein 0.1-0.4 by weight of tetrapotassium pyrophosphate based on the TiO2 is added aftcr (d) but before (e). 9. A process according to Claim 6 or 7 wherein 0.1-0.4% by weight of tetrapotassium pyrophosphate based on the TiO2 is added simultaneously with the aluminum compound. 10. A paint composition pigmented with TiO2 particles having a composite coating of dense alumina and boehmite alumina totalling 2-6% by weight alumina based on the TiO2, said composite dense and boehnite alumina present in a weight ratio of 8:1-1: of dense to boehinite, the dense alumina having incorporated therein sulfate in a weight ratio of aluminum to sulfate of 10:1-1.3:1.

Inventors: JACOBSON, HOWARD WAYNE
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1983
Main classification: C09C1
IPC codes: C09C1