Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2002-10-29
2004-08-31
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S404000, C106S436000, C106S447000, C106S454000, C427S214000, C427S215000, C427S219000, C427S376200, C427S419300
Reexamination Certificate
active
06783586
ABSTRACT:
BACKGROUND OF THE INVENTION
Addition, during TiO
2
surface treatment, of certain chemicals under specific conditions significantly enhance a number of pigment performance properties in paints. These properties include durability and ease-of dispersion. The chemicals added during surface treatment that improve pigment end-use performance include metal oxides, particularly aluminum oxides and silicon dioxide. The exact nature of these oxides may in turn be influenced by the conditions under which they are formed and the presence of other reagents during the precipitation process. The present invention provides an easy dispersing, high durability TiO
2
pigment and method of making that pigment. Such super-durable TiO
2
pigment with improved gloss retention (durability) and dispersibility (easy dispersing) is useful in a variety of exterior applications, including but not limited to powder, coil, automotive.
Some multi-purpose TiO
2
pigment grades that are commercially available are treated with a coating of amorphous alumina. In these products there is no silica present as a surface treatment. Although these products have durability that is sufficient for architectural exterior coatings, they are not super-durable. In these products materials such as zirconia and tin are used to stabilize the amorphous alumina. The use of zirconia and tin makes these grades more expensive to manufacture and to use. The present invention uses citric acid to stabilize the amorphous alumina. The combination of silica and citric acid stabilized amorphous alumina on a pigment seems to be a new approach that provides performance and cost efficiencies.
U.S. Pat. No. 3,825,438 teaches a process to make a coated titanium dioxide pigment with one or more hydrous oxides in the presence of a polyhydric alcohol and/or a carboxylic acid. In this process, the alcohol or acid may be added at any time in the process prior to the completion of the deposition of the coating. The teachings in U.S. Pat. No. 3,825,438 solve the problem of pigment agglomeration on storage. A broad series of organic additives are taught in this patent to accomplish this desired result. A critical teaching is that the point at which the alcohol and/or acid are added in the process has no influence on the results achieved in the pigment product.
U.S. Pat. No. 5,340,393 teaches a method of making a non-agglomerated water insoluble inorganic particle having a dense, amorphous silica coating. Citric acid is identified as a possible optional charge stabilization dispersing acid that may be used in the process to prevent agglomeration of particles in the process. This patent offers no teaching as to the deposit of amorphous alumina or special properties resulting from the use of citric acid over other dispersants taught.
U.S. Pat. No. 5,730,796 teaches a process to make a durable pigment having a coating of cerium oxide, a coating of dense, amorphous silica and preferably an outer coating of crystalline (hydrous) alumina. It makes no teaching or suggestion with respect to the use of citric acid in deposition of silica or aluminia to enhance pigment properties.
U.S. Pat. No. 5,700,318 teaches an alumina coated inorganic pigment having a first coating of boehmite alumina, a second coating of amorphous alumina and a third coating of boehmite alumina. Adjustment of precipitation pH is taught as a means to produce the crystalline and amorphous alumina coatings. No teaching or suggestion is made with respect to the use of citric acid in this process.
U.S. Pat. No. 3,506,466 teaches the use of a salt of a water soluble alcohol amine and an easily water soluble oxycarboxylic acid to enhance dispersibility. The salt is added after wet treatment and before grinding the pigment.
U.S. Pat. No. 5,824,146 teaches a titanium dioxide pigment with amorphous alumina product in vapor phase oxidation of titanium tetrachloride in the presence of aluminum trichloride. The pigment is slurried using a citric acid dispersant.
U.S. Pat. No. 5,824,145 teaches mixtures of titanium dioxide pigment slurry and slurry of amorphous alumina hydroxide formed from mixing the two component slurries and using citric acid as a dispersing agent foe the mixed slurries. There is no wet treatment process disclosed in this patent.
U.S. Pat. Nos. 2,885,366 and 3,437,502 provide the basic teachings for making a durable titanium dioxide pigment having an amorphous silica coating followed by a coating of mixed crystalline alumina. Neither of these patents teach the use of citric acid during the wet treatment process used to deposit these treatments on the pigment surface.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a method for making an easy-dispersing, high durability titanium dioxide pigment comprising the steps in sequence:
A. (1) heating a slurry of titanium dioxide particles to a temperature of from 85 to 100° C.,
(2) adding citric acid as a solution in water to the slurry to form a mixture,
(3) adjusting the pH of the mixture to 10 or more,
(4) adding sufficient sodium silicate as a water solution to the mixture to deposit silica on the surface of the particles of from 1 to 3% based on the weight of the titanium dioxide particles in the slurry,
(5) neutralizing the slurry by addition of a mineral acid over the course of one hour, thereby forming a slurry of silica coated titanium dioxide particles; and
B. (1) adjusting the temperature of the slurry of silica coated titanium dioxide particles to a temperature of from 55 to 90° C.,
(2) adding sufficient sodium aluminate as a water solution to the slurry in step B(1) and adjusting the pH of the mixture formed to from 5 to 9 by addition of a strong mineral acid to deposit aluminia as Al
2
O
3
of from 1 to 4% by weight based on the weight of titanium dioxide particles present in the slurry of step A(1) on the surface of the silica coated particles, and digesting the resulting mixture for from 15 to 30 minutes.
The method of the present invention may be used as a batch or a continuous process. In practicing the method it is preferred that in step A(1) the slurry is heated to a temperature of from 90 to 95° C. It is also preferred that citric acid be added to the slurry to a concentration based on the weight of the titanium dioxide particles of from 0.1 to 2%, and even more preferred that citric acid is added to a concentration of 0.3 to 0.5%.
The strong mineral acid used in the present invention is selected from the group consisting of nitric, hydrochloric and sulfuric acids. The most preferred acid to use when treating pigment made by the chloride process is hydrochloric acid, and the most preferred acid to use when treating pigment made by the sulfate process is sulfuric acid.
In the method of the present invention it is preferred in step B(2) that the addition of sodium aluminate solution and the pH adjustment by addition of mineral acid is done simultaneously.
The present process offers an improved method of making a titanium dioxide particles wherein the particles in a water slurry are wet treated with water solution of sodium silicate and sodium aluminate to form titanium dioxide having a first coating of silica followed by a second coating of aluminia wherein the improvement comprises adding citric acid to the titanium dioxide slurry before the addition of sodium silicate solution.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a product and a process for making that product in which TiO
2
is coated sequentially in a wet treatment process with amorphous silica and amorphous alumina following procedures similar to those currently employed except that citric acid is added during the deposition process.
In a typical application of this invention, slurry of TiO
2
is heated to a predetermined temperature and citric acid is added. The slurry is then pH adjusted to a target and sodium silicate is added. The pH is slowly decreased to neutral by addition of acid, after which the slurry is allowed to digest for a period of time, at the end of which the pigment particles are well coated with a
Bettler Charles Robert
Deibold Michael Patrick
E. I. du Pont de Nemours and Company
Koslow C. Melissa
Manlove Shalie
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