Processes for preparing hydrophobic inorganic oxide pigments

Details Processes for preparing hydrophobic inorganic oxide pigments Processes for preparing hydrophobic inorganic oxide pigments

2000-11-10

2003-09-16

Brunsman, David (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

Pigment, filler, or aggregate compositions, e.g., stone,...

C106S446000

Reexamination Certificate

active

06620234

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to the preparation of hydrophobic inorganic oxide-based pigments, and more specifically to methods for treating inorganic oxide pigments in aqueous media with organosilicon compounds in order to introduce hydrophobic characteristics to the pigment.
BACKGROUND OF THE INVENTION
Titanium dioxide (TiO
2
) is the premier white pigment used for whitening, brightening and opacifying paper, paints and plastics. Like many other inorganic oxides, untreated TiO
2
pigment is hydrophilic in nature and, as such, is readily wetted by water but not wet by nonpolar substances, such as hydrocarbons, plastics, etc. Accordingly, in order to incorporate TiO
2
into plastics, oil-based paints and other nonpolar substances, the surface of the TiO
2
must often be made hydrophobic to improve the dispersibility of the pigment in the nonpolar substance and to improve other properties of the resulting pigmented substance.
Numerous agents have been developed for making TiO
2
hydrophobic, including phosphorylated fatty acid derivatives, dialkylsulfosuccinates and various silicone-based agents. Some examples of “silicone-based agent” include silanes, substituted silanes, hydrosylates and condensates of these silanes, siloxanes, polysiloxanes, and substituted siloxanes and polysiloxanes. The substituted groups are often organic and/or functional, i.e. chemically reactive, groups. For the purposes of this disclosure this class of substituted silanes, siloxanes and polysiloxanes agents will be referred to as “organosilicon” agents.
In one approach for preparing hydrophobic TiO
2
pigment, the TiO
2
particles are treated with chemically nonreactive organosilicon compounds, such as polydimethylsiloxanes or other such agents. Typically, such agents are applied to inorganic oxides, like pigments, by spraying the agents undiluted or in solution onto the particle surfaces. However, the resulting pigments typically exhibit poor physical properties for high temperature applications and the nonreactive agent can migrate from the surface of the pigment leading to potential problems with heat sealing, print clarity and other such related problems in the final pigmented product.
Another approach for preparing hydrophobic TiO
2
pigment involves treating the inorganic pigment with a reactive silane that will bond directly to the pigment surface. U. S. Pat. Nos. 5,607,994; 5,631,310; 5,889,090; and 5,959,004 (all assigned to E. I. du Pont de Nemours and Company; herein referred to collectively as the “DuPont Patents”) teach the coating of TiO
2
pigment with octyltriethoxysilane where it is intended that the silane directly bonds to the pigment surface. Typically, the reactive silane, for example an octyltriethoxysilane in the DuPont Patents, is sprayed directly onto the pigment when the pigment is dry or incorporated in a filter cake. Some prior art methods use mechanical mixers to facilitate the dispersing of the silane onto the pigment.
The disadvantages of these prior art techniques include incomplete coating of the pigment particles by the reactive silane, even with mechanical mixing, and less than optimal binding of the reactive silane to the pigment particle surface. Additionally, such techniques also produced processing problems with respect to the manufacture of the pigment and often introduce environmental problems by generating volatile organic compounds, such as ethanol.
Other prior art methods include applying reactive silanes in an organic slurry in order to get more complete and uniform coating of the pigment particles. The disadvantages of these techniques is that usually the organic solvent must be removed before processing of the pigment can proceed; thus adding an additional and expensive processing step to the production of the pigment.
The inventors in their earlier patent, U.S. Pat. No. 5,653,794, the disclosure of which is hereby incorporated by reference, describe a technique which overcomes the disadvantages of using organic solvents and the prior art environmental problems with respect to the generation of volatile organic compounds. This patent describes a treatment method for TiO
2
pigment wherein a halosilane is added to an aqueous slurry of TiO
2
pigment and water. The use of a halosilane, such as octyltrichlorosilane, avoids generation of volatile organic compounds, such as ethanol. In this process, hydrochloric acid is generated in solution as the by-product of the hydrolysis reaction of the silanes. Since the hydrochloric acid is in solution, it can easily be neutralized and disposed of as a salt; thus avoiding environmental problems associated with the release of volatile organic compounds. Moreover, because the reaction takes place in an aqueous slurry of pigment and water this treatment procedure does not require the removal of organic solvents from the pigment in order to continue processing.
It has been found, however, that the addition of the halosilane to an aqueous solution of pigment and water results in a high viscosity mixture which is often difficult to process. The present invention overcomes this difficulty while retaining the advantages of U.S. Pat. No. 5,653,794.
SUMMARY OF THE INVENTION
The present invention provides a treatment method for rendering inorganic oxide particles, such as TiO
2
pigment, hydrophobic which avoids the disadvantages of the prior art. It has been found that by predispersing a suitably reactive organohalosilane into an aqueous media and using intensive mixing means (such as a rotor stator emulsifier or inline static mixer), a reactive dispersion can be prepared. When this dispersion is combined with inorganic oxide particles, such as TiO
2
pigment, under sufficient agitation, the problems discussed above of the prior art are eliminated. The present inventive method provides unexpected processing advantages in pigment preparation and yields a more economical pigment product. Pigment preparation time and the processing equipment energy requirements are reduced. Additionally, the hydrophobic pigments prepared by using the present invention have good performance properties, such as dispersibility in nonpolar substances (i.e., plastics), and they do not degrade physical properties of the pigmented nonpolar substance, such as lacing resistance of thermoplastics.
DETAILED DESCRIPTION OF THE INVENTION
The present section details the preferred embodiments of the invention. These embodiments are set forth to illustrate the invention and aid in its understanding. Since this disclosure is not a primer on inorganic oxides or TiO
2
pigment production or their treatment or the design, manufacture or operation of treatment vessels, basic concepts known or readily determinable by those skilled in the field of TiO
2
production have not been set forth in detail. Concepts such as choosing appropriate manufacturing materials or solvents, or appropriate additives for the treatment process, or suitable conditions for operating the treatment process are known or readily determinable by those skilled in the industry. Attention is directed to the appropriate texts and references known to those in the art field for details regarding these and other concepts that may be necessary in the practice of the invention.
Methods for treating TiO
2
pigment with organosilicon compounds are well known in the prior art and are discussed in numerous references, including U. S. Pat. Nos. 2,488,440; 2,559,638; 4,061,503; and 4,151,154, the DuPont Patents and U.S. Pat. No. 5,653,794 by the present inventors. The teachings of these patents are incorporated into the present disclosure by reference.
The subject invention provides a method for preparing a hydrophobic inorganic oxide by (i) mixing a reactive silane with an aqueous media under conditions so that the reactive silane substantially condenses and forms an aqueous reactive dispersion and (ii) mixing the resulting aqueous dispersion with a slurry of pigment and water under conditions so that the reactive dispersion chemically reacts with the inorganic oxide directly or upon

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