Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-09-15
2002-01-29
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S438000, C106S442000, C106S450000, C428S403000
Reexamination Certificate
active
06342099
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to titanium dioxide pigments coated with a first layer of a phosphate compound, a layer of silica or a zirconia compound, a second layer of a phosphate compound and a layer of alumina. The titanium dioxide pigments of the present invention are easily dispersible and retain gloss and durability.
BACKGROUND
Titanium dioxides, in either rutile or anatase forms, are well known as opacifying pigments due to their high refractive index: they are among the compounds which when prepared in a size range of from about 0.2 to about 0.4 microns diffuse light as well as or better than any other known compound. However, titanium dioxide (TiO
2
) is a semiconductor and will convert UV radiation (for example, from UV light) into low energy &bgr;-particles (electrons) which can ultimately initiate the degradation of many organic-based materials in which the pigment is used, such as paints, plastics and other organic-based compositions, through the production of various radical species.
One method to limit the effects of light on titanium dioxide pigment is to coat the pigment with inorganic compounds such as oxides and hydroxides of alumina, silica, zirconium or phosphate. Such coatings can result in a more durable product that is resistant to U.V. light induced degradation. However, some coatings cause the formation of agglomerates leading to difficulties in dispersing pigments into various organic-based compositions thereby reducing opacity and gloss.
In general, phosphate coatings are desirable to use because they exhibit good cost effectiveness. However, conventional prior art teaches that phosphate coatings greater than 0.5% tend to cause pigments to agglomerate causing reduction in dispersibility and thus opacity of the end product, as well as gloss. Accordingly, prior art phosphate coatings on titanium dioxide particles have typically been limited to phosphate contents ranging from 0.1% to 0.5% by weight based on the weight of titanium dioxide base.
Based on the foregoing, there is still a need for phosphate coated titanium dioxide pigments that are easily dispersible and exhibit excellent gloss and durability. Such coated pigments are useful in the manufacture of paints, and other coating systems.
SUMMARY OF THE INVENTION
The present invention provides titanium dioxide pigments coated with at least two layers of a phosphate compound. Preferably, a layer of silica or zirconia separates the two phosphate layers. The pigments of the present invention are easily dispersible and maintain gloss and durability comparable to the so-called “superdurable” pigments (e.g. pigments having up to 6% silica).
In one embodiment, the present invention provides a coated titanium dioxide pigment, which includes a base titanium dioxide particle; a first deposit of a phosphate compound contiguous with the base TiO
2
particle; a deposit of a dense silica compound contiguous with the phosphate deposit; a second deposit of a phosphate compound contiguous with the dense silica compound; and a deposit of an alumina compound contiguous with the second phosphate deposit.
In second embodiment, the present invention provides a process for preparing coated titanium dioxide pigment, comprising the steps of: a) preparing an aqueous slurry of base titanium dioxide, b) adding a first phosphate compound to the slurry to form a first phosphate compound coating, c) adding a silica compound to the slurry to form a silica compound coating, d) adding a second phosphate compound to the slurry e) adding an alumina compound to the slurry to form an alumina compound coating, thereby forming the coated titanium dioxide pigment.
In another embodiment, the present invention provides a process for preparing coated titanium dioxide pigment comprising the steps of: a) preparing an aqueous slurry of titanium dioxide base, b) adding a first phosphate compound to the slurry to form a first phosphate deposit, c) increasing the pH of the slurry to about 7.0, d) heating the slurry to a temperature of about 80° C. to about 95° C., e) adding a silica compound to the slurry over a period of about 30 minutes, f) aging the slurry for about 20 minutes, g) adjusting the pH of the slurry to about 6.5 over a period of about 60 minutes, h) adding a second phosphate compound to the slurry, i) adding an alumina compound to the slurry under conditions such that the pH of the slurry is maintained at about 6.5, j) aging the slurry for about 30 minutes, k) adjusting the pH of the slurry to about 5.3 and, l) discharging and filtering the coated titanium dioxide pigment.
In yet another embodiment, the present invention provides a process for preparing coated titanium dioxide pigment, comprising the steps of: a) preparing an aqueous slurry of titanium dioxide base, b) adding a first phosphate compound to the slurry to form a first phosphate compound coating, c) adding a silica or zirconia compound or combination thereof to the slurry to form a coating comprising silica or zirconia or combination thereof, d) adding a second phosphate compound to the slurry to form a second phosphate compound coating, e) adding an alumina compound to the slurry to form an alumina compound coating, thereby forming the coated titanium dioxide pigment.
For a better understanding of the present invention together with other and further embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
In preparing the preferred embodiments of the present invention, various alternatives may be used to facilitate the objectives of the invention. These embodiments are presented to aid in an understanding of the invention and are not intended to, and should not be construed to, limit the invention in any way. All alternatives, modifications and equivalents which may becomes obvious to those of ordinary skill upon a reading of the present disclosure are included within the spirit and scope of the present invention.
This disclosure is not a primer on the manufacture of titanium dioxide pigments, basic concepts known to those skilled in the art have not been set forth in detail.
Coated Titanium Dioxide Pigments
The coated titanium dioxide pigments of the present invention include base titanium dioxide particles, either rutile or anatase. Titanium dioxide base particle suitable for use in the present invention have a particle size range of less than about 1.0 microns, and more preferably from about 0.1 microns to about 0.5 microns, and most preferably from about 0.1 microns to about 0.35 microns.
The titanium dioxide base particle is coated with a first layer of a phosphate compound by depositing the phosphate compound onto the TiO
2
base particle. Preferably, the phosphate compound is a water soluble phosphate compound, such as for example, tetrapotassium pyrophosphate, sodium polyphosphate, tetrasodium pyrophosphate (Tetron™), sodium tripolyphosphate, potassium tripolyphosphate, sodium hexametaphosphate (Calgon™), phosphoric acid, and the like. Most preferably, the water soluble phosphate compound is sodium hexametaphosphate. The weight percent of the phosphate compound can vary depending on the layer deposited on the titanium dioxide base. It is not necessary that the phosphate compound coat each titanium dioxide particle, but only that some phosphate is deposited on the particle. Preferably, the phosphate compound in the first layer is deposited in an amount of from about 0.05% to about 1.0%, more preferably from about 0.05% to about 0.75% and most preferably, from about 0.05% to about 0.5% based upon the weight of titanium dioxide base.
The titanium dioxide particles of the present invention, optionally, are coated with a silica compound. Preferably, the silica coating is dense and contiguous with the first phosphate compound. The weight percent silica content can vary depending on the layer deposited on the first phosphate layer. Silica compounds suitable for use in the present invention include water soluble alka
Chegwidden Philip
Hiew Michael
Bell Mark L.
Hailey Patricia L.
Kalow & Springut LLP
Kalow, Esq. David A.
Millennium Inorganic Chemicals, Inc.
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