Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2001-02-05
2003-02-11
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S442000, C106S445000, C106S447000, C423S610000, C423S611000, C423S612000
Reexamination Certificate
active
06517629
ABSTRACT:
FIELD OF INVENTION
The present invention relates to the field of titanium dioxide pigments. More specifically, the present invention relates the production of low volatile titanium dioxide pigments.
BACKGROUND OF THE INVENTION
Titanium dioxide is widely used as a pigmenting agent in a vast number of applications. Due to its unique combination of high refractive index and chemical inertness it has become the white pigment of choice. One of the main industrial applications of TiO
2
is pigmenting of plastics articles.
Polymeric compounds (e.g., polyolefins) that are used to make plastics articles are required to be processed at high temperatures for a number of industrial processes. At elevated temperatures, inorganic compounds such as titania pigments that have been incorporated in the polymeric compounds can release volatile species such as water molecules, which can cause defects in polymers such as bubbles or holes. Holes formed in thin films may be referred to as “lacing,” which is a phenomenon that limits the operating conditions of the film manufacturer. Consequently, film manufacturers often desire titanium dioxide pigments that have a reduced tendency to cause lacing.
Titanium dioxide is widely manufactured by either of two established processes, the sulfate process or the chloride process, both of which are well known to persons skilled in the art. The “sulfate” process is the older of the commercial processes. In the sulfate process, titanium bearing ores are dissolved in sulfuric acid, and the extracted titanium is purified by selective precipitation and washing, followed by calcination to produce raw crystalline TiO
2
, of either the anatase or the rutile form.
In the “chloride” process, titanium bearing ores are reacted with Cl
2
gas and a reducing agent such as coke (the “chlorination” reaction) to produce titanium tetrachloride. The titanium tetrachloride is purified by distillation and reacted with oxygen to form TiO
2
, usually of the rutile form. Aluminum chloride is usually reacted along with the TiCl
4
to enhance the properties of the base titanium dioxide such as improved resistance to chalking and to act as a rutilizing agent. Alumina levels can range from 0.1-10% by weight with respect to TiO
2
, as suggested in U.S. Pat. Nos. 3,501,262 and 3,505,091, both of which are incorporated by reference. The alumina level is usually less than 10%, typically around 0.5-1.5%. However, alumina levels greater than 2% and preferably between 3% and 5% have been suggested, as in U.S. Pat. No. 5,824,146, which is incorporated by reference. The final product of the oxidation reaction is a substantially rutile particle that contains some aluminum in the rutile crystal and a surface that is enriched in aluminum species.
After the oxidation reaction, the raw base is collected on a filter to allow the liberated chlorine to be recycled to chlorination, and the collected base is mixed with water to form a slurry. This slurry can then be further processed, for example by milling in a media mill and/or treating with inorganic and/or organic compounds to enhance the properties of the pigment.
Historically, the raw base was “treated” after oxidation with an inorganic compound such as alumina to improve properties such as dispersion and material handling such as filtration and conveying. The term “treatment” refers to the precipitation or addition of a species, for example by addition of aluminum sulfate to the slurry followed by neutralization with caustic soda. This treatment occurs after oxidation, while the previously mentioned addition of aluminium chloride occurs prior to oxidation. Common inorganic treatments include alumina, phosphate, silica and zirconia.
In response to the demands of customers for grades of titania that can be processed in polymers at increasing temperatures and pigment loadings, manufacturers have been reducing the amount of inorganic surface treatment. The reason is that inorganic species are usually hydrated, either as part of the chemical structure or water adsorbed from the atmosphere onto the surface of the pigment. Upon heating, this water is driven off and is one of the key contributors to volatiles and subsequent defects such as lacing. Reducing the inorganic surface treatment reduces the total moisture content and reduces the volume of volatile species driven off of the pigment at elevated temperatures. The minimum moisture content on the base is obtained by adding no further inorganic compound after oxidation, that is by taking the raw oxidizer discharge, optionally wet milling, then neutralizing, washing, drying and micronizing the base.
An organic compound is commonly added to the pigmentary base in order to enhance further the dispersability and other properties such as rheology in the polymeric article. Typical organic compounds include polyols, alkanolamines, silanes, organophosphorous compounds, fatty acids and esters. A key benefit of organic compounds is that they usually further lower the moisture content of the pigment either by reacting with surface hydroxyl groups or by adsorbing to polar sites and blocking moisture pickup. The final product is usually milled in a device such as a fluid energy mill, referred to as a micronizer.
A limitation of the current technology is that the moisture content of the final micronized pigment is largely predetermined by the amount of moisture inherent to the oxidizer discharge and the ability of the organic to reduce further the moisture content. A further limitation is that reliance on organic additives to reduce the moisture content is heavily constrained by factors such as FDA approval for food contact applications. Not all organics that are effective at blocking moisture are approved for food contact applications. A base with a lower inherent moisture content will allow manufacture of products with equivalent moisture content using lower organic treatment levels and reduced manufacturing cost.
Thus, there presently exists a need for a titanium dioxide pigment with a reduced volatile content to allow higher processing temperatures and increased TiO
2
loadings in plastics applications without significant lacing. A benefit of the present invention is that it allows the manufacture of pigments that have a much lower level of volatiles associated with them than can be obtained with the current practice.
SUMMARY OF THE INVENTION
The present invention provides a novel method for making a titanium dioxide pigmentary base of reduced moisture content by dissolving readily soluble alumina species present in oxidizer discharge and removing the soluble species by filtration or some similar process such as settling and decantation. Such a base exhibits substantially lower moisture content when the base is dried and may be used to prepare low volatile pigments that are used to produce polymeric matrices in high temperature applications such as extrusion coating that have minimal or no lacing defects.
In one embodiment, the present invention provides a method for producing a low volatile pigmentary base that may be further processed to form a finished titanium dioxide pigment, comprising:
a. oxidizing titanium tetrachloride and aluminum chloride to form an oxidizer discharge comprising co-oxidized alumina and titanium dioxide; and
b. separating and removing a portion of the co-oxidized alumina, said separating and removing causing a reduction in the ratio of co-oxidized alumina to titanium dioxide, causing a titanium dioxide slurry to be formed, said titanium dioxide slurry comprising said low volatile pigmentary base.
In another embodiment, the present invention provides a method for producing a low volatile pigment comprising:
a. obtaining an oxidizer discharge, the oxidizer discharge comprising titanium dioxide and co-oxidized alumina;
b. combining the oxidizer discharge with a solvent;
c. dissolving a portion of the co-oxidized alumina to form dissolved alumina;
d. removing the dissolved alumina from the titanium dioxide, causing a titanium dioxide slurry to be formed, said titanium dioxide slurry compris
Bell Mark L.
Hailey Patricia L.
Kalow David A.
Kalow & Springut LLP
Locke Scott D.
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