MXPA98008827A - Inorganic pigment coated with inorganic oxides - Google Patents

Abstract

Improved coated inorganic pigments having a first coating of Al2O3, SiO2, or optionally other oxide such as a source of P2O5, or mixtures thereof, an optional coating of ZrO2, SnO2, TiO2, SiO2, CeO2 or mixtures thereof, and a final coating of Al2O3 which exhibit good durability, dispersibility, and optical properties in plastic or paint compositions are disclosed. Also disclosed is a process whereby an aqueous slurry of an inorganic pigment is milled to achieve a particular particle size distribution, a first coating of Al2O3, SiO2, or optionally other oxide such as a source of P2O5, or mixtures thereof is precipitated onto the pigment material wihin a first pH range, an optional amount of ZrO2, SnO2, SiO2, TiO2, CeO2 or mixtures thereof is added to the aqueous slurry within a second pH range to obtain an optional coating over the first coating, and a final coating of Al2O3 is added over the prior coating(s).

Description

AN IMPROVED INORGANIC PIGMENT AND IMPROVED PROCESS FOR YOUR PREPARATION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improved inorganic pigments, coated with Zr02, Si02, Al203, Ti02, Sn02 or Ce02 or mixtures thereof, and with a method for multi-coating inorganic pigments for provide the latter with good resistance, low photochemical and chemical reactivity and good optical and dispersibility properties in plastics and paint applications. More specifically, the present invention relates to an improved titanium dioxide pigment having a first coating of A1203, Si02 or optionally of another inorganic coating material, or mixtures thereof, an optional coating of Zr02, Si02, Ti02, Sn02, Ce02 or mixtures thereof on the first coating, and a final coating of A1203. 2. Description of the State of the Art The surface treatment and coating of inorganic pigments, such as titanium dioxide for example, to achieve improved performance characteristics in plastics compositions is already known from the state of the art. The characteristics of dispersibility, processability and good optical properties are usually achieved through the minimal surface treatment of the pigment, while the characteristics of resistance and low chemical and photochemical reactivity are usually achieved by coating the pigment with silica, alumina and others. coatings of metal oxides. When precipitated, using state-of-the-art practices, these metal oxide coatings tend to make the pigments more difficult to disperse in plastics or paints. Normally, these problems have been solved with a single pigment. In this way, the technicians dedicated to the combination of plastics have had to choose generally between the characteristic of resistance, which can be achieved with coated pigments, and the characteristics of dispersibility, processability and good optical properties, which can be achieved with both pigments treated as untreated. Similarly, the performance of paints can be improved through conventional surface treatments, but by adding the first treatment agent before going through the isoelectric point that maximizes both resistance and dispersibility. For example, US Patent No. 3,523,810 describes a process for coating titania pigments with 0.5 to 10% boehmite alumina at a pH of 7 to 9 and at temperatures between 70 and 100 ° C. A pigment of this type alone with boehmite and without amorphous alumina may have good optical and dispersibility properties, but does not achieve good strength. Similarly, US Patent No. 4,022,636 describes the addition of two alumina coatings to titanium dioxide pigments: an amorphous coating at a pH of about 2 followed by a boehmite coating at a pH of about 8, get a pigment with a good behavior in plastics. Likewise, US Patent No. 4,416,699 describes a method for coating a pigment with 2 to 6% alumina, to achieve the characteristics of resistance to collapse, gloss and dispersibility required for the paint qualities of titanium dioxide. However, this process contains higher than permissible alumina levels to achieve optimal performance of titanium dioxide pigments in plastics applications. Examples of coating processes of inorganic pigments to which the advantages of improved performance have been ascribed include the coating process described in US Patent No. 2,671,031. This patent describes the precipitation of a coating of hydrous aluminum oxide on titanium dioxide followed by thermal treatment at temperatures between 500 and 800 ° C to decrease the collapse of the pigment. US Patent No. 2,780,558 describes a process for coating pigment materials with incorporated alumina as an alcoholate compound. In US Patent No. 3,146,119 the claimed process requires the precipitation of a layer of titanium dioxide on the pigment prior to the precipitation of an alumina layer. US Patent No. 3,409,454 describes a pigment of titanium dioxide coated with alumina and a process for obtaining it, wherein the alumina is precipitated on the pigment under alkaline conditions (pH greater than 10) to achieve improved dispersibility of the pigment in plastics. US Patent No. 3,529,985 discloses a doubly coated aluminum dioxide pigment wherein the first metal oxide coating is calcined at a temperature of at least 450 ° C prior to the addition of the second coating to thereby achieve improved characteristics of resistance and optical properties. US Patent No. 3,853,575 discloses the production of titanium dioxide pigments coated with alumina wherein the alumina coating is delivered through the addition of an alkali salt of alumina to an aqueous solution of titanium dioxide. US Patent No. 4,239,548 discloses a titanium dioxide pigment with improved photochemical stability achieved through a first coating of cerium and phosphate radicals on the pigment and of a second coating of aluminum and phosphate radicals on the first coating. US Patent No. 5,114,486 describes a process for coating pigments of titanium dioxide with zinc, phosphate and aluminum ions to achieve an improved processing capacity. None of the aforementioned patents suggest or describe the coating of inorganic pigments with a first layer of A1203, Si02 or optionally another inorganic material or mixtures thereof, and an optional layer of Zr02, SiO2, TiO2, SnO2, CeO2 or mixtures thereof. the same, and a final coating of A1203, to produce the coated pigments that constitute the object of this invention. SUMMARY OF THE INVENTION It is an object of the present invention to provide coated, resistant inorganic pigments with good optical properties and with good dispersibility and chemical stability characteristics when applied to both paints and plastics. Another object is to provide improved inorganic pigments, preferably titanium dioxide pigments, which have deposited, thereon, a first coating of Si02, Al203 or optionally other oxides or mixtures thereof before passing through the isoelectric point, and an optional coating of Zr02, Si02, Sn02, Ce02 or mixtures thereof and a final coating of A1203. Another object of the present invention is to provide a process by means of which such coated inorganic pigments are obtained. DESCRIPTION OF THE PREFERRED MODALITIES The present invention relates to an inorganic pigment having a coating of Al203 or Zr02 or Si02 or Ce02 or Sn02 or Ti02 or mixtures thereof and comprising an inorganic pigment material wherein the coating is at least about 0.25% by weight based on the weight of said pigment material, and having a first coating of A1203, SiO2 or optionally of another oxide or mixture thereof, deposited on said inorganic pigment beforehand that said pigment flocculates or passes through its isoelectric point, and an optional coating of Zr02, Ce02, Ti02, SiO2, SnOz or mixtures thereof, deposited on said first coating once said pigment has passed through its isoelectric point; said first coating being achieved in a discontinuous range and said optional coating being achieved in a discontinuous or continuous range. Preferably, the other optional oxide is a source of P205. Conveniently, the inorganic pigment has a coating of about 0.25 to 5% by weight, based on the weight of said pigment. Preferably, the inorganic pigment has a coating of from 0.5 to 3% by weight based on the weight of said pigment material and, more preferably, the inorganic pigment has a coating of approximately 1.3% by weight based on the weight of said pigment material.

Conveniently, the inorganic pigment used is titanium dioxide. The process for the preparation of the inorganic pigment of the present invention coated with at least 0.25% by weight, based on the weight of the pigment, of Zr02, Ce02, A1203, Ti02, Si02, Sn02 or mixtures thereof, comprises the steps of: (a) grinding an aqueous slurry containing the inorganic pigment material, in the presence of a dispersant agent, to achieve a size distribution of particle of said material wherein not more than about 5% of said pigment particles are greater than 0.49 microns, as measured by a Microtrac 9200 FRA; (b) separating the grinding media from said aqueous slurry; (c) depositing a first coating of A1203; Si02 or optionally other oxides or mixtures thereof on the inorganic pigment material prior to said pigment flocculating or passing through its isoelectric point; (d) add an optional coating of Zr02, Ce02, Si02, Sn02, Ti02 or mixtures thereof to said slurry once the inorganic pigment material has passed through its isoelectric point and adjust the pH of the slurry to form a second coating on said pigment; (e) digest the slurry at a temperature of about 50 to 80 ° C for a time of about 5 to 60 minutes; (f) adding a final coating of A1203; (g) adjusting the pH of the slurry to a level within the range of about 6 to 9 and digesting said slurry at a temperature of about 50 to 70 ° C for a time of about 5 to 60 minutes, to complete the formation of said second coating deposited in step (f) on said previous coating; and (h) recovering said coated pigment. Preferably, the other optional oxide of step (c) above is a source of P205. Conveniently, the pigment material is coated with a total amount of coating or coatings of about 1% to 3% of Zr02, Ti02, Sn02, Si02, A1203, Ce02 or mixtures thereof by weight, based on the weight of said pigment material. Preferably, the pigment material is coated with about 1.3 to 2% of Zr02, Ti02. Sn02, Si02, A1203, Ce02 or mixtures thereof, by weight based on the weight of said pigment material. Suitably, and for pigments to be used in plastics, the first coating is from about 0.1 to 0.3% by weight. For pigments to be used in paints, the first coating is conveniently from 0.3 to 1% by weight approximately. Suitably, the final layer of Al203 in pigments used in plastics will be from about 0.1 to 0.3% by weight. For pigments to be used in paints, the final coating is conveniently 1 to 3% by weight approximately. The process of the present invention may optionally include a second amount of a dispersing agent added in step (d). Conveniently, the dispersing agent of steps (a) and (d) is selected from the group consisting of phosphates, polyols and amines. Preferably, the dispersing agent is sodium hexametaphosphate. Suitably, the dispersing agent of step (a) is present in an amount of about 0.05 to 0.50% by weight, based on the weight of said pigment material. Preferably, the dispersing agent of step (a) is present in an amount of about 0.15% by weight, based on the weight of said pigment material. Suitably, the dispersing agent of step (d) is present in an amount of about 0.05 to 0.50% by weight, based on the weight of said pigment material. Preferably, the dispersing agent of step (d) is present in an amount of approximately 0.30% by weight, based on the weight of said pigment material. According to the present invention, from about 0.1 to 10% of said pigment particles are 0.49 microns or more, measured through a Micro-trac 9200 FRA. Adequately, about 0.1 to 5% of said pigment particles are 0.49 microns or more and preferably less than about 2% of said pigment particles are 0.49 microns or more. Conveniently, the grinding medium of step (a) is sand. Preferably, the temperature during steps (d) to (g) is maintained within the range of about 50 to 80 ° C and more preferably the temperature is maintained at about 60 ° C. Conveniently, the coating provided is a salt acid or alkaline of Zr02, Ti02, Si02, Sn02, Ce02 or mixtures thereof Preferably, said Zr02 coating agent is zirconium orthosulfate In the process of the present invention, the amount of Al203, SiO2 or optionally of other oxides or mixtures thereof, in the coating incorporated in step (c), is sufficient to provide said first coating in an amount of the order of 0.1 to 1% by weight based on the weight of said material Pigment Suitably, the amount of A1203, SiO2 or mixtures thereof, added in step (c), is sufficient to provide said first coating in an amount of the order of 0.3 to 0.7% by weight approximately, based on the weight of said pigment material for applications in paints and from about 0.1 to 0.3% for plastics applications. Preferably, the amount of A1203, Si02 or mixtures thereof, added in step (c), is sufficient to provide said first coating in an amount of about 0.5% by weight, based on the weight of said pigment material. In the process of the present invention, the amount of Zr02, Si02, Sn02, Ti02, Ce02 or mixtures thereof, optionally added in step (d), is sufficient to provide said optional coating in an amount of the order of 0 , 2 to 1.2% by weight approximately, based on the weight of said pigment material. Conveniently, the amount of Zr02, Si02, Sn02, Ti02 Ce02 or mixtures thereof, added in step (d), is sufficient to provide said optional coating in an amount of the order of 0.5 to 1% by weight approximately , based on the weight of said pigment material. Preferably, the amount of Zr02, Si02, Sn02, Ti02, Ce02 or mixtures thereof, added in step (d), is sufficient to provide said optional coating in an amount of about 0.60% by weight, based on in the weight of said pigment material. Conveniently, the pH of said slurry during step (a) is about 10. The pH of said slurry in step (d) is suitably adjusted to a level within the range of about 3 to about 7 and preferably the pH of said slurry is adjusted to about 5. Conveniently, the pH of said slurry in step (f) is adjusted to about 5. Suitably, the pH of said slurry in step (g) is adjusted to a level within the range of about 7 to 9 and preferably the pH of said slurry is adjusted to about 7.8. Conveniently, the agent for adjusting the pH in step (d) is a mineral acid, preferably sulfuric acid. Preferably, the agent for adjusting the pH in steps (f) and (g) is an alkali metal hydroxide base. Preferably, said agent for adjusting the pH is sodium hydroxide. Conveniently, said slurry is digested in step (e) for a time of about 10 to 45 minutes and preferably said slurry is digested for about 15 minutes. Suitably, said slurry is digested in step (g) for a time of about 10 to 45 minutes and preferably said slurry is digested for about 30 minutes. Suitably, the coated pigment is recovered by filtration, drying and milling. EXAMPLE 1 An aqueous slurry comprising 1800 g of Ti02 was dispersed with 0.15% sodium hexametaphosphate and then grinded with sand to obtain a particle size distribution such that 7% of the particles have a larger size to 049 microns After grinding, the sand was separated. The slurry was then diluted to a volume of 4.5 liters, 2.54 g of calgon (a source of P205) was added and the temperature was adjusted to 60 ° C. 0.4899 g / ml of A1203 was added as sodium aluminate. The pH was adjusted to 3.5 with 5 ml of sulfuric acid and the solution was digested for 15 minutes. The pH of the solution was then adjusted to 5.75 with 5 ml of sodium hydroxide. Then 0.9802 g / ml of alumina was added as sodium aluminate, the pH was maintained in less than 6 and the solution was digested for 30 minutes. The pH was then adjusted to 7.8 with 4.3 ml of sodium hydroxide and the solution was digested for a further 15 minutes. The samples were then filtered, dried and micronized with 0.35% TMP. The results are shown in Table 1. EXAMPLE 2 The treatment of Example 1 was repeated. The pigment was micronized with 0.40% TMP / 0.3% TEA. Reference 1 consisted of a pigment for plastics, strongly treated and "resistant". Reference 2 consisted of a pigment for untreated plastics. Reference 3 consisted of a pigment for plastics of standard quality. TABLE 1 * Lower numbers indicate less reactivity in these trials and, therefore, are preferred. EXAMPLE 3 An aqueous slurry containing 872 g of Ti02 was dispersed with 0.15% sodium hexametaphosphate and then proceeded to the sand milling to obtain a particle size distribution such that 10% of the particles were larger than 0, 49 microwaves The sand was removed after grinding. The temperature of the slurry was adjusted to 70 ° C. 0.8% A1203 (23.2 ml of 301 g / liter of A1203 as sodium aluminate was added. 6.8 ml of sulfuric acid to precipitate the alumina and reduce the slurry pH was added. The slurry was digested for 15 minutes at 70 ° C. to the slurry was added 2% A1203 or 57.9 ml of sodium aluminate solution and the slurry was digested for 30 minutes. 13.5 ml of sulfuric acid to the slurry to precipitate were added alumina and reduce the pH of the slurry to 7.3 After 15 minutes more digestion, the pigment was filtered, washed and dried.The pigment was ground with fluidic energy with 0.35% of an organic milling aid. The optical behavior of the material in this example was compared to Reference 4 which consisted of a pigment for standard quality paints.

TABLE 2 Average results The data indicates that the maximum benefits of grinding with sand are not achieved without using the process of the invention, which initiates the coating process before flocculation is induced.

Claims (45)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following claims is claimed as property: 1. An inorganic pigment having a coating of Zr02, SiOz, Sn02, Ti02, Al203, Ce02 or mixtures thereof and comprising an inorganic pigment material wherein the coating constitutes at least about 0.25% by weight based on the weight of said pigment material, characterized in that it has a first coating of Al203, Si02 or mixture thereof, deposited on said inorganic pigment prior to said pigment flocculating or passing through its isoelectric point, an optional coating selected from the group consisting of Zr02, Sn02, Si02, Ti02, Ce02 or mixtures thereof. same, deposited on said first coating once said pigment has passed through its isoelectric point; said first coating being achieved in a discontinuous range and said second coating being achieved in a discontinuous or continuous range, and a final coating of A1203 deposited on the first coating and on the optional coating when the latter is present.
2. 2. A coated inorganic pigment according to claim 1, characterized in that said coating is from 0.25 to 5% by weight based on the weight of said pigment.
3. 3. - A coated inorganic pigment according to claim 2, characterized in that said coating is from 1 to 3% by weight based on the weight of said pigment.
4. 4. A coated inorganic pigment according to claim 3, characterized in that said coating is from 1.3 to 2% by weight approximately based on the weight of said pigment.
5. 5. - An inorganic pigment according to claim 1, characterized in that the first coating includes a source of P2Os.
6. 6. - A coated inorganic pigment according to claim 1, characterized in that said pigment material is titanium dioxide.
7. 7. A process for the preparation of an inorganic pigment coated with at least about 0.25% by weight, based on the weight of the pigment, of Zr02, Sn02, SiO2, TiO2, A1203, CeO2 or mixtures thereof, characterized in that it comprises the steps of: (a) grinding an aqueous slurry containing the inorganic pigment material, in the presence of a dispersing agent, to achieve a particle size distribution of said material wherein not more than about 5% of said pigment particles are greater than 0.49 microns, as measured by a Microtrac 9200 FRA; (b) separating the grinding media from said aqueous slurry; (c) depositing a first coating of A1203, Si02 or mixtures thereof on the inorganic pigment material prior to said pigment flocculating or passing through its isoelectric point; (d) digest the slurry at a temperature of about 50 to 80 ° C for a time of about 5 to 60 minutes; (e) coating the pigment with a final coating of A1203; (f) adjusting the pH of the slurry to a level within the range of about 6 to 9 and digesting said slurry at a temperature of about 50 to 80 ° C for a time of about 5 to 60 minutes, to complete the formation of said second coating on said first coating; and (g) recovering said coated pigment.
8. 8. - A process according to claim 7, characterized in that it comprises the step of adding a coating of Zr02, Sn02, Si02, Ti02, Ce02 or mixtures thereof to said slurry once the inorganic pigment material has passed through its isoelectric point and adjust the pH of the slurry to a level of at least about 3 to form a second coating on said pigment.
9. 9. - A process according to claim 8, characterized in that, in said additional coating step, the pigment material is coated with approximately 1 to 5% of Zr02., Si02, Sn02, m Ti02, Ce02 or mixtures thereof, by weight based on the weight of said pigment material.
10. 10. A process according to claim 9, characterized in that, in said additional coating step, the pigment material is coated with about 1.3 to 2% of Zr02 or Ce02 or mixtures thereof, by weight based on the weight of said pigment material.
11. 11. A process according to claim 7, characterized in that step (c) comprises depositing a coating containing a source of P205.
12. 12. A process according to claim 7, characterized in that said pigment material is titanium dioxide.
13. 13. A process according to claim 7, characterized in that in step (d) a second amount of a dispersing agent is added.
14. 14. A process according to claim 13, characterized in that said dispersing agent in steps (a) and (d) is chosen from the group consisting of phosphates, polyols and amines.
15. 15. -A process according to claim 14, characterized in that said dispersing agent is sodium hexametaphosphate.
16. 16. A process according to claim 7, characterized in that said dispersing agent in step (a) is present in an amount of about 0.05 to 0.50% by weight, based on the weight of said pigment material. .
17. 17. A process according to claim 13, characterized in that said dispersing agent in step (d) is present in an amount of approximately 0.05 to 0.50% by weight, based on the weight of said pigment material.
18. 18. A process according to claim 7, characterized in that from about 0.1 to 5% of said pigment particles are 0.49 microns or greater, measured by a Microtrac 9200 FRA.
19. 19. A process according to claim 18, characterized in that less than about 2% of said pigment particles are 0.49 microns or greater.
20. 20. - A process according to claim 7, characterized in that the grinding medium of stage (a) is sand.
21. 21. A process according to claim 8, characterized in that the temperature after stages (c) to (f) is maintained within the range of about 50 to 80 ° C.
22. 22. A process according to claim 21, characterized in that the temperature is maintained at approximately 60 ° C.
23. 23. A process according to claim 8, characterized in that the coating of Zr02, Sn02, Ti02, Si02, Ce02 or mixtures thereof is provided by an acidic or alkaline inorganic salt of Zr02, Sn02, Ti02, Si02, Ce02.
24. 24. A process according to claim 23, characterized in that said coating of Zr02 is zirconium orthosulfate.
25. 25. A process according to claim 11, characterized in that the coating amount of A1203, Si02, source of P205 or mixtures thereof, added in step (c), is sufficient to provide said first coating in an amount of order of 0.1 to 1% by weight approximately, based on the weight of said pigment material.
26. 26. A process according to claim 25, characterized in that the amount of A1203, Si02, source of P205 or mixtures thereof, added in step (c), is sufficient to provide said first coating in an amount of the order of 0.2 to 0.5% by weight approximately, based on the weight of said pigment material.
27. 27. A process according to claim 26, characterized in that the added amount of A1203, Si02, source of P205 or mixtures thereof is sufficient to provide said first coating in an amount of 0.35% by weight approximately, based on the weight of said pigment material.
28. 28. A process according to claim 8, characterized in that the additional coating of Zr02, Sn02, Si02, Ti02, Ce02 or mixtures thereof is added in an amount sufficient to provide said coating in an amount of the order of 0. , 2 to 1% by weight approximately, based on the weight of said pigment material.
29. 29. A process according to claim 28, characterized in that the added amount of Zr02, Sn02, Si02, Ti02, Ce02 or mixtures thereof is sufficient to provide said additional coating in an amount of the order of 0.5 to 1% in weight approximately, based on the weight of said pigment material.
30. 30. A process according to claim 29, characterized in that the additional coating amount of Zr02, Ce02, Sn02, Si02, Ti02 or mixtures thereof is sufficient to provide said coating in a sanity of 0.60% by weight approximately , based on the weight of said pigment material.
31. 31. A process according to claim 7, characterized in that the pH of said slurry during step (a) is about 10.
32. 32. A process according to claim 8, characterized in that the pH of said slurry in the optional step is adjusted to a level within the range of about 3 to 7.
33. 33.- A process according to claim 32, characterized in that the pH of said slurry is adjusted to a level of about 5.
34. 34. - A process according to claim 7, characterized in that the pH of said slurry in step (e) is adjusted to about 5.
35. 35. A process according to claim 7, characterized in that the pH of said slurry in step (f) is adjusted to a level within the range of about 7 to 9.
36. 36.- A process according to claim 35, characterized in that the pH of said slurry is adjusted to approximately 8.
37. 37.- A process according to claim 8, characterized in that the agent for adjusting the pH is a mineral acid.
38. 38.- A process according to claim 37, characterized in that said agent for adjusting the pH is sulfuric acid.
39. 39.- A process according to claim 7, characterized in that the agent for adjusting the pH in steps (e) and (f) is an alkali metal hydroxide base.
40. 40.- A process according to claim 39, characterized in that said agent for adjusting the pH is sodium hydroxide.
41. 41. A process according to claim 7, characterized in that said slurry is digested in step (d) for a time of about 10 to 45 minutes.
42. 42. A process according to claim 41, characterized in that said slurry is digested for approximately 30 minutes.
43. 43.- A process according to claim 7, characterized in that said slurry is digested in step (f) for a time of about 10 to 45 minutes.
44. 44. A process according to claim 43, characterized in that said slurry is digested for approximately 15 minutes.
45. 45. A process according to claim 7, characterized in that said coated pigment is recovered by filtration, drying and milling.