Colloid systems and wetting agents; subcombinations thereof; pro – Continuous liquid or supercritical phase: colloid systems;... – Aqueous continuous liquid phase and discontinuous phase...
Reexamination Certificate
2000-12-13
2003-11-25
Bos, Steven (Department: 1754)
Colloid systems and wetting agents; subcombinations thereof; pro
Continuous liquid or supercritical phase: colloid systems;...
Aqueous continuous liquid phase and discontinuous phase...
C423S610000, C423S615000, C423S616000
Reexamination Certificate
active
06653356
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally concerns photocatalytic particles and aggregates and coatings, especially as may incorporate nanoparticulate titanium dioxide, and to processes for the production and the use thereof.
The present invention further generally concerns photocatalytic materials as are effective for, inter alia, killing microorganisms, including algae and bacteria, on contact in the presence of light in the visible or ultraviolet wavelengths. More particularly as regards these photocatalytic materials, the present invention concerns (1) composite photocatalytic materials in the form of particles and other bodies with surfaces which particles and bodies have (1
a
) cores nondeleterious to photocatalytic action coupled with (1
b
) photocatalytic surfaces; and (2) liquids, aggregates and solids incorporating such (1) photocatalytic materials.
2. Description of the Prior Art
2.1 Photocatalytic Coatings, Especially as May Incorporate Nanoparticulate Titanium Dioxide
A first aspect of the present invention will be seen to concern the production, and use, of photocatalytic coatings, especially as may incorporate nanoparticulate titanium dioxide.
For the purposes of the present invention, nanoparticulate titanium dioxide coating (“nano-coating”) is taken to be surface coatings of rutiles, anatases and amorphous titanium dioxide having a particle size of 1 to 100 nm, preferably of 1 to 50 nm, and more preferably of 1 to 10 nm, or titanium dioxide having the above-stated particle size dispersed and adhering on a surface.
2.1.1 Applications for Titanium Dioxide Nano-coatings
Applications for such titanium dioxide nano-coatings include the following. Pigmentary particles may be coated with titanium dioxide to impart improved U.V. absorption or opalescent effects. In this application the light transparency of the titanium dioxide due to the small particle size is a particularly desirable characteristic of the nano-coating.
Titanium dioxide nano-coatings may be applied to building materials as a photocatalytic coating providing anti-fouling benefits. Photocatalytic surfaces so created are particularly useful in public areas such as rest rooms and hospitals to reduce bacterial contamination.
A titanium dioxide nano-coating may be applied as a photocatalytic coating to a waste water treatment apparatus.
A titanium dioxide nano-coating may be applied to both powders and continuous surfaces as a coating for absorption of U.V. radiation,
A titanium dioxide nano-coating may be applied to a surface as a flame retardant surface.
A titanium dioxide nano-coating may be applied to a surface as a support layer in a dye solar cell.
The use of titanium dioxide nano-coatings is, however, currently still restricted by the fact no economic process is known which is capable of producing nano-coatings comprised of the stated particle size on an industrial scale. The present invention deals with this issue.
2.1.2 Sol/gel Coatings of Nano-particulate TiO
2
The most important previous methods for the formulation of nano-particulate TiO
2
coatings—also known as titanium dioxide nano-coatings—may be grouped together under the superordinate term of “sol/gel coatings”. Sol/gel coatings have been described in many journal articles and patents. Nano-particles of TiO
2
in the sol/gel form are attracted to surfaces by van der Waals' forces and may be further anchored to surfaces by stronger chemical bonds 1% such as fusion bonds.
Sol/gel materials are desirable because, when applied as a film to surfaces, these nano-particulate suspensions create the thinnest surface coatings, disperse evenly, and have good adhesion properties.
As discussed in U.S. Pat. No. 5,840,111, the sol/gel coatings are generally formulated using the alkoxide method, i.e. the carefully controlled, frequently base- or acid-catalyzed hydrolysis of metal alkoxides and similar molecular precursors in mixtures of water and one or more organic solvents. The solvent used is generally the same alcohol as is the basis of the alkoxide. One disadvantage of this previous process is that costly educts and complicated processing are required. Moreover, the products have an undesirably elevated carbon content.
Originally developed for silicon compounds, the alkoxide method is increasingly also being used for the synthesis of nano-titanium dioxide in accordance with the equation
Ti(OR)
4
+2H
2
O→TiO
2
+4 ROH
See, for example, J. Livage,
Mat. Sci. Forum
152-153 (1994), 43-54; J. L. Look and C. F. Zukoski,
J. Am. Ceram. Soc.
75 (1992), 1587-1595; WO 93/05875.
It is frequently possible to produce monodisperse particles, i.e. particles having a very narrow particle size distribution, by appropriate selection of the reaction conditions, permitting production of homogeneous particles ranging in diameter from some micrometers down to a few nanometers. One example of such a special processing method is working in microemulsions, by which means it is possible to limit particle size. See, for example, D. Papoutsi et al.,
Langmuir
10 (1994), 1684-1689.
The educts for virtually all sol/gel reactions for the production of TiO
2
nano-coatings, whether by conventional or microemulsion methods, are titanium alkoxides (Ti(OR)
4
), the alkyl residues R of which conventionally contain 2 to 4 carbon atoms. However, due to the high price of these alkoxides and particular handling requirements (protective gas, strict exclusion of moisture in order to prevent premature hydrolysis), the stated reactions have not been considered for a large scale industrial process.
Still furthermore, working in microemulsions has the disadvantage that, due to the frequently low concentration of the reactants, (i) the space/time yield is low and (ii) large quantities of water/solvent/surfactant mixtures are produced which must be disposed of.
An alternative, a non-hydrolytic sol/gel manufacturing process has recently been proposed which involves reacting metal halides with oxygen donors such as ethers or alkoxides. See S. Acosta et al.,
Better Ceramics through Chemistry VI (
1994), 43-54.
2.1.3 Chemical Vapor Reaction Processes for the Production of TiO
2
as May be Used in Nano-Coatings
Yet another group of methods for the production of ultra-fine titanium dioxide particles comprises the so-called CVR (chemical vapor reaction) processes, which are based upon the reaction of vaporizable metal compounds (generally alkoxides) with oxygen (air) or steam in the gas phase. This process is described, for example, in U.S. Pat. No. 4,842,832 and Europe patent no. EP-A 214 308. While small quantities of powders produced using such processes are presently (circa 2000) commercially available, they are extremely expensive.
2.1.4 Industrial Processes Producing TiO
2
Undesirably Coarse for Use in Nano-Coatings
Of the hitherto known processes performed on a large industrial scale for the production of finely divided (sub-pigmentary) titanium dioxide, none yields a product comparable in terms of fineness and transparency with sol/gel materials. These industrial processes include hydrolysis of TiCl
4
as is shown in Great Britain patent no. GB-A 2 205 288; production of rutile nuclei in the sulfate process as is shown in Europe patents nos. EP-A 444 798 and EP-A 499 863; and peptisation with monobasic acids of titanium dioxide hydrate which has been washed free of sulfate as is shown in Europe patent no. EP-A 261 560 and also in U.S. Pat. No. 2,448,683.
It is also known from U.S. Pat. No. 5,840,111 to react a solution comprising sulfuric-acid and titanyl sulfate by adding an alkaline-reacting liquid such that the alkaline liquid is present in a stoichiometric deficit relative to the “free sulfuric acid” (which is the total sulfur content minus that proportion bound in the form of foreign metal sulfates). The resultant solution is then flocculated by adding a monobasic acid. This process is inefficient because a significant portion, approximately 50%, of the titanyl sulfate does not react acidically with the stoichiometricall
Bos Steven
Fuess & Davidenas
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