U.S. patent application number 10/724451 was filed with the patent office on 2004-06-24 for nanoparticulate titanium dioxide coatings, and processes for the production and use thereof.
Invention is credited to Sherman, Jonathan.
Application Number | 20040120884 10/724451 |
Document ID | / |
Family ID | 27496913 |
Filed Date | 2004-06-24 |
United States Patent
Application |
20040120884 |
Kind Code |
A1 |
Sherman, Jonathan |
June 24, 2004 |
Nanoparticulate titanium dioxide coatings, and processes for the
production and use thereof
Abstract
Nanoparticulate titanium dioxide coating produced by educing
flocculates of titanium dioxide nanoparticles from a titanyl
sulfate solution and dispersing the nanoparticles in a polar
sol-forming medium to make a sol suitable as a coating usable to
impart photocatalytic activity, U.V. screening properties, and fire
retardency to particles and to surfaces. The photocatalytic
material and activity is preferably localized in dispersed
concentrated nanoparticles, spots or islands both to save costs and
leverage anti-microbial effects.
Inventors: |
Sherman, Jonathan;
(Brentwood, TN) |
Correspondence
Address: |
William C. Fuess
FUESS & DAVIDENAS
Suite II-G
10951 Sorrento Valley Road
San Diego
CA
92121
US
|
Family ID: |
27496913 |
Appl. No.: |
10/724451 |
Filed: |
November 28, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10724451 |
Nov 28, 2003 |
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09736738 |
Dec 13, 2000 |
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6653356 |
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60216937 |
Jul 10, 2000 |
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60202033 |
May 5, 2000 |
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60188761 |
Mar 13, 2000 |
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60170509 |
Dec 13, 1999 |
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Current U.S.
Class: |
423/608 ;
423/615 |
Current CPC
Class: |
B01J 35/0013 20130101;
C01P 2006/10 20130101; C09C 1/3607 20130101; B01J 35/004 20130101;
C01P 2004/04 20130101; C01P 2004/03 20130101; B01J 21/063 20130101;
C01P 2006/22 20130101; C01P 2006/12 20130101; C01P 2006/80
20130101; C01P 2004/84 20130101; C01P 2006/60 20130101; B01J 35/002
20130101; C01P 2004/32 20130101; C01P 2006/19 20130101; B01J 21/06
20130101; B82Y 30/00 20130101; C01G 23/0532 20130101; C01P 2004/50
20130101; C01P 2002/84 20130101; C01P 2006/33 20130101; B01J
13/0047 20130101; C01P 2004/64 20130101 |
Class at
Publication: |
423/608 ;
423/615 |
International
Class: |
C01G 023/047 |
Claims
What is claimed is:
1. A process for producing particulate titanium dioxide comprising:
a) mixing an alkaline-reacting liquid with an aqueous solution of
titanyl sulfate at elevated temperature until the resultant mixture
reacts acidically and is neutralized to a pH between 5 and 9,
forming flocculates of titanium dioxide nanoparticles; b)
first-isolating the formed titanium dioxide nanoparticle
flocculate; c) first-washing in water the isolated titanium dioxide
nanoparticle flocculate; d) second-washing in an acid or an alkali
the isolated and first-washed nanoparticle flocculate, e)
second-isolating as a product of the second-washing an acidic or an
alkaline titania concentrate of particulate titanium dioxide.
2. The process for the producing particulate titanium dioxide
according to claim 1 extended and enlarged to a process of
producing a sol of particulate titanium dioxide comprising as a
step after the e) second-isolating the further step of: f)
dispersing the second-isolated titania concentrate in a polar
sol-forming medium to make a sol suitable to serve as a coating in
which is present particulate titanium dioxide.
3. The extended and enlarged process of producing both (i)
particulate titanium dioxide and (ii) a sol of particulate titanium
dioxide according to claim 2 wherein the a) mixing through f)
dispersing makes a sol that is transparent.
4. The extended and enlarged process of producing both (i)
particulate titanium dioxide and (ii) a sol of particulate titanium
dioxide according to claim 2 still further extended and enlarged to
use the sol as a coating, the method comprising as a step after the
f) dispersing the further step of: g) applying a film of the
titania sol to a surface.
5. The process according to claim 4 further comprising as a step
after the g) applying, h) neutralizing the surface with a suitable
acidic- or alkaline-reacting compound; and i) washing the surface
with water.
6. The process according to claim 4 wherein the surface is prepared
after the g) applying of the film of the titania sol by: coating
said surface with 0.1 to 1,000 wt., relative to TiO.sub.2 that is
within the titania sol in the g) applying step, of at least one
oxide, hydroxide or hydrous oxide compound dawn from the group
consisting of aluminum, silicon, zirconium, tin, magnesium, zinc,
cerium and phosphorus.
7. The process according to claim 6 wherein the coating with at
least one oxide, hydroxide or hydrous oxide compound is 5 to 200
wt. %, relative to the TiO.sub.2.
8. The process according to claim 5 wherein after the coating the
surface is dried.
9. The process according to claim 5 wherein after the coating the
surface is annealed.
10. The process according to claim 1 wherein the mixing is until
the resultant mixture reacting acidically is neutralized to a pH
between 6.5 and 7.5.
11. The process according to claim 1 wherein the mixing is with an
alkaline-reacting liquid drawn from the group consisting
essentially of aqueous solutions of ammonium hydroxide, sodium
hydroxide, and potassium hydroxide.
12. The process according to claim 1 wherein the mixing is with an
alkaline-reacting liquid drawn from the group consisting
essentially of carbonates of sodium, potassium and ammonium.
13. The process according to claim 1 wherein the mixing is with an
ammonium hydroxide solution having a concentration from 1 to 8
molar NH.sub.4OH.
14. The process according to claim 1 wherein the mixing proceeds at
a temperature within the range of 60.degree. C. to 100.degree.
C.
15. The process according to claim 1 wherein, between the a) and
the b) first-isolating transpires the further step of a1) letting
cool a mixture created by the a) mixing.
16. The process according to claim 15 wherein the al) letting cool
the mixture comprising: quenching to a temperature below
600.degree. C, for greater than {fraction (1/4)} hour.
17. The process according to claim 1 wherein the b) first-isolating
comprises: separating, by filtering or other method conventionally
recognized in the art.
18. The process according to claim 17 wherein the d) second-washing
in the acid or the alkali is with monobasic acid or alkali so as to
both (i) remove contaminants from the isolated and first-washed
nanoparticle flocculate and (ii) introduce ions necessary for sol
formation.
19. The process according to claim 18 wherein the d) second-washing
is in monobasic acid or alkali 1 to 6 times the weight of the
titanium flocculate precipitate.
20. The process according to claim 18 wherein the d) second-washing
in the acid or the alkali is with hydrochloric acid.
21. The process according to claim 20 wherein the d) second-washing
in the acid or the alkali is with 3 to 6 molar hydrochloric
acid.
22. The process according to claim 1 wherein the e) second-isolated
acid or alkaline titania concentrate contains 4 to 40 wt. % of
TiO.sub.2, the remainder being any of (i) wash acid or wash alkali,
(ii) water moisture and (iii) small quantities of contaminants.
23. The process according to claim 2 wherein the f) dispersing of
the second-isolated titania concentrate in a polar sol-forming
medium is so to make a transparent sol in which TiO.sub.2 is
present exclusively as nano-particles having a diameter of between
1 and 100 nm.
24. The process according to claim 2 wherein the f) dispersing of
the second-isolated titania concentrate is in a polar sol-forming
medium consisting essentially of water, or an alcohol containing 1
to 10 carbon atoms and at least one hydroxide group per molecule,
or mixtures thereof.
25. A sol suitable as a coating consisting essentially of titanium
dioxide nanoparticles educed from an aqueous titanyl sulfate
solution neutralized with an alkali to precipitate titania
floculates that are water-washed and then acid-washed; dispersed in
a polar sol-forming medium.
26. The sol according to claim 25 wherein the sol is
transparent.
27. The sol according to claim 25 wherein the aqueous titanyl
sulfate solution from which the-titanium dioxide nanoparticles are
educed contains sulfuric acid.
28. The sol according to claim 25 wherein the titanyl sulfate
solution is obtained by digesting with sulfuric acid material drawn
from the group consisting of ilmenite and titanium slag; dissolving
a digestion cake resultant from the digesting in water; and
performing clarification to derive the aqueous titanyl sulfate
solution suitable as an educt.
29. The sol according to claim 25 wherein the titanyl sulfate
undergoing dissolution in water is commercial grade.
30. The sol according to claim 25 wherein the titanyl sulfate
solution is. obtained by dissolution of titanium dioxide and
TiO.sub.2 hydrates, including orthotitanic acid and metatitanic
acid, in sulfuric acid (H.sub.2SO.sub.4).
31. The sol according to claim 25 wherein the titanyl sulfate
solution is obtained by dissolution in H.sub.2SO.sub.4 of alkali
metal and magnesium titanates in hydrous form.
32. The sol according to claim 25 wherein the titanyl sulfate
solution is obtained by reaction of TiCl.sub.4 with H.sub.2SO.sub.4
to form TiOSO.sub.4 and HCl.
33. The sol according to claim 25 wherein the titanyl sulfate
solution contains 100 to 300 g of titanium/l, calculated as
TiO.sub.2.
34. The sol according to claim 25 wherein the titanyl sulfate
solution contains 170 to 230 g of titanium/l, calculated as
TiO.sub.2.
35. The sol according to claim 25 having less than 0.1 wt. % of
carbon.
36. A composite body exhibiting a photocatalytic effect consisting
essentially of a core particle consisting essentially of a material
without deleterious effect on a photocatalytic reaction; and a
multiplicity of nanoparticles, each less than 33% the diameter of
the core particles, of photocatalytic material upon the surface of
the core particle, the photocatalytic material being. less than 20%
by weight of (i) the combined multiplicity of photocatalytic
material nanoparticles and (ii) the core particle.
37. The composite body according to claim 36 wherein the core
particle is less than 1 centimeter in diameter; and wherein each of
the multiplicity of nanoparticles is of diameter less than 100
nanometers.
38. The composite body according to claim 36 wherein the core
particle's material without deleterious effect on a photocatalytic
reaction consists essentially of a material drawn from the group
consisting essentially of silicates and carbonates including
silicate and carbonate powders, mineral and mineral composites
including calcined clay and wollastonite, metal oxides including
zinc oxide, inorganic pigments, and construction aggregates
including roofing granules.
39. The composite body according to claim 36 wherein the core
particle consists essentially of a polymer.
40. The composite body according to claim 39 wherein the core
particle's polymer consists essentially of polymer drawn from the
group consisting essentially of acrylics, acrylonitriles,
acrylamides, butenes, epoxies, fluoropolymers, melamines,
methacrylates, nylons, phenolics, polyamids, polyamines,
polyesters, polyethylenes, polypropylenes, polysulfides,
polyurethanes, silicones, styrenes, terephthalates, vinyls.
41. The composite body according to claim 39 wherein the polymer
core particle is less than 1 centimeter in diameter.
42. The composite body according to claim 36 wherein the
photocatalytic material of the multiplicity of nanoparticles is
drawn from the group of metal compound semiconductors consisting
essentially of titanium, zinc, tungsten and iron, and oxides of
titanium, zinc, tungsten and iron, and strontium titanates.
43. The composite body according to claim 42 wherein the metal
compound semiconductor photocatalytic material is combined with a
metal or metal compound drawn from the group consisting of
vanadium, iron, cobalt, nickel, copper, zinc, ruthenium, rhodium,
silicon, tin, palladium, gold, platinum, and silver.
44. The composite body according to claim 36 wherein the
photocatalytic material is drawn from the group of metal compound
semiconductors consisting essentially of anatase titanium dioxide
and zinc oxide.
45. The composite body according to claim 36 wherein the
photocatalytic material consists of particles of a diameter from 1
nanometer to 100 nanometers.
46. The composite body according to claim 36 wherein the
photocatalytic material consists of particles of diameter from 1
nanometer to 50 nanometers.
47. The composite body according to claim 36 wherein the
photocatalytic material consists of particles of diameter from 1
nanometer to 10 nanometers.
48. The composite body according to claim 36 wherein the core
particles consist of particles of diameter from 100 nanometers to 1
centimeter.
49. The composite body according to claim 36 wherein weight of the
photocatalytic material of the combined multiplicity of
nanoparticles is less than 10% of weight of the core particle.
50. A great multiplicity of composite bodies in accordance with
claim 36 incorporated in amount from 0.001% to 85% by volume within
a composition suitable for use as an additive or a coating.
51. The great multiplicity of composite bodies in accordance with
claim 50 incorporated in a composition that further includes one or
more materials from the group of building materials consisting of
concrete, cement, ceramic, stucco, hard flooring, masonry, roofing
shingles, wall shingles, building siding and swimming pool
surfaces.
52. The great multiplicity of composite bodies in accordance with
claim 50 incorporated in a composition that is effective as an
anti-fouling coating.
53. The composite body according to claim 36 effective in killing
by contact any of algae, bacteria, mold or fungus.
54. The composite body according to claim 36 wherein, at a
proportion by weight of the photocatalytic material in the
composite particle of less than 10%, the efficacy of the
photocatalytic material within the composite particle to kill by
contact algae, bacteria, mold, and fungus upon the composite
particle's surface is at least one-half (0.5) as good as is the
efficacy of this same photocatalytic material to kill in purest
form, making that at least equal killing effect is realized with a
five to one (5:1) reduction in the amount of photocatalytic
material when this photocatalytic material is upon the surface of
the composite particle.
55. A method of making composite photocatalytic particles
comprising: preparing an aqueous slurry of first particles,
consisting essentially of a material without deleterious effect on
photocatalytic reaction, having an associated first particle size
in the range from 100 nanometers to 1 centimeter diameter; adding a
colloidal suspension of 0.1% to 60% by weight second particles,
which second particles consist essentially of photocatalytic
material having diameters in the range from 1 to 100 nanometers,
the combined weight of second particles in the colloidal suspension
being less than 20% of the combined weight of the. first particles
that are also within the aqueous slurry; mixing the aqueous slurry
and the colloidal suspension so that the photocatalytic material
second particles attach through van der Waals or fusion chemical
forces to the nondeleterious material first particles, forming a
slurry of composite particles wherein the relatively smaller
photocatalytic material second particles (i) are upon the surfaces
of the relatively larger nondeleterious material first particles,
and (ii) are in weight less than 20% of these first particles.
56. The method according to claim 55 wherein the colloidal
suspension added is from 0.1% to 60% by weight second
particles.
57. The method according to claim 56 wherein the colloidal
suspension added is of the highest solids concentration at which
the suspension is stable, being in the range from 14% to 50% by
weight.
58. The method according to claim 56 further comprising: adjusting
the pH of the mixing so as to move away from, in the same
direction, the respective isoelectric points of the photocatalytic
material second particles and the nondeleterious material first
particles, the isoelectric points being those points at which the
particles have a neutral net charge.
59. The method according to claim 56 further comprising: adjusting
the pH of the mixing so that either the photocatalytic material
second particles or the nondeleterious material first particles
approach their respective isoelectric points, but only when the
mixture of both particles have low ionic strength and the pH is
such that both particles are above or below their isoelectric
points.
60. The method according to claim 56 further comprising:
establishing an opposite electrical charge on the nondeleterious
material first particles and the photocatalytic material second
particles.
61. The method according to claim 56 wherein either the adding of
the colloidal suspension of second particles, or the mixing of the
aqueous slurry and the colloidal suspension, or both the adding and
the mixing, transpires in the presence of at least one
dispersant.
62. The method according to claim 56 further comprising one or more
finishing steps drawn from the group consisting of separating,
washing and drying the composite photocatalytic particles.
63. The method according to claim 56 further comprising drying the
slurry of composite photocatalytic particles; and annealing in a
kiln the dried composite photocatalytic particles.
64. The method according to claim 63 that, after the annealing,
further comprises: rapidly cooling the annealed composite
photocatalytic particles to ambient room temperature within a time
period, which time period is necessarily dependent upon the
temperature of the annealing and the amount of the composite
photocatalytic particles, that is less than six hours.
65. The method according to claim 64 wherein the rapid cooling of
the annealed composite photocatalytic is accomplished by rapid
removal of the material from the kiln to a room temperature
environment.
66. A photocatalytic aggregate particle. consisting essentially of
an extender particle of material both non-photocatalytic and
non-interfering with photocatalytic reaction; with discrete
photocatalytic titanium oxide particles exposed on the surface.
67. The photocatalytic aggregate particle according to claim 66
wherein the photocatalytic titanium oxide particles consists
essentially of titanium dioxide in the anatase crystalline
form.
68. The photocatalytic aggregate particle according to claim 66
wherein the photocatalytic titanium oxide particles are less than
about 20% by weight.
69. The photocatalytic aggregate particle according to claim 66
wherein the extender particle is a material drawn from the group
consisting essentially of silicates and carbonates including
silicate and carbonate powders, mineral and mineral composites
including calcined clay and wollastonite, metal oxides including
zinc oxide, inorganic pigments, and construction aggregates
including roofing granules.
70. A process of making photocatalytic aggregate particles
comprising: mixing an aqueous slurry of extender particles made
from material both non-photocatalytic and non-interfering with
photocatalytic reactions with a solution of titanyl sulfate; then
adding an acid or an alkaline reacting agent to cause discrete
microparticles of titanium dioxide to be deposited onto the
extender particles.
71. A process for making photocatalytic aggregate particles
comprising: mixing an aqueous slurry of extender particles made
from material both non-photocatalytic and non-interfering. with
photocatalytic reactions with an alkaline or acidic titania sol
containing particles of titanium dioxide.
72. The process for making photocatalytic aggregate particles
according to claim 71 wherein the titanium dioxide particles in the
titania sol have an average diameter size within the range of about
1 to about 100 nanometers.
73. The process for making photocatalytic aggregate particles
according to claim 71 wherein the titanium dioxide particles in the
titania sol and the extender particles are both above or below
their respective isoelectric points.
74. The process for making photocatalytic aggregate particles
according to claim 71 wherein discrete particles of the titanium
dioxide that is within the titania sol are dispersed onto the
surfaces of the extender particles in an amount less than 20 weight
% based on aggregate particle weight.
Description
RELATION TO PREDECESSOR PROVISIONAL PATENT APPLICATIONS
[0001] The present patent application is descended from, and claims
benefit of priority of, U.S. provisional patent applications serial
Nos. 60/216,937 filed on Jul. 10, 2000, for NANOPARTICULATE
TITANIUM DIOXIDE COATINGS AND PROCESS FOR THE PRODUCTION THEREOF
AND USE THEREOF; 60/202,033 filed on May 5, 2000 for ANTIFOULING
PHOTOACTIVE AGGREGATES; 60/188,761 filed on Mar. 13, 2000, for
PHOTOACTIVE ANTIFOULANT AGGREGATES; and 60/170,509 filed on Dec.
13, 1999, for PREPARATION OF COMPOSITE PHOTOCATALYTIC PARTICLES.
All predecessor provisional patent applications are to the selfsame
inventor as the present patent application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] 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.
[0004] 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 (1a) cores nondeleterious to photocatalytic action
coupled with (1b) photocatalytic surfaces; and (2) liquids,
aggregates and solids incorporating such (1) photocatalytic
materials.
[0005] 2. Description of the Prior Art
[0006] 2.1 Photocatalytic Coatings, Especially as May Incorporate
Nanoparticulate Titanium Dioxide
[0007] 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.
[0008] 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.
[0009] 2.1.1 Applications for Titanium Dioxide Nano-coatings
[0010] 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.
[0011] 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.
[0012] A titanium dioxide nano-coating may be applied as a
photocatalytic coating to a waste water treatment apparatus.
[0013] A titanium dioxide nano-coating may be applied to both
powders and continuous surfaces as a coating for absorption of U.V.
radiation,
[0014] A titanium dioxide nano-coating may be applied to a surface
as a flame retardant surface.
[0015] A titanium dioxide nano-coating may be applied to a surface
as a support layer in a dye solar cell.
[0016] 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.
[0017] 2.1.2 Sol/gel Coatings of Nano-particulate TiO.sub.2
[0018] The most important previous methods for the formulation of
nano-particulate TiO.sub.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.sub.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 such
as fusion bonds.
[0019] 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.
[0020] 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.
[0021] 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).sub.4+2H.sub.2O.fwdarw.TiO.sub.2+4 ROH
[0022] 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.
[0023] 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.
[0024] The educts for virtually all sol/gel reactions for the
production of TiO.sub.2 nano-coatings, whether by conventional or
microemulsion methods, are titanium alkoxides (Ti(OR).sub.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.
[0025] 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.
[0026] 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.
[0027] 2.1.3 Chemical Vapor Reaction Processes for the Production
of TiO.sub.2 as May be Used in Nano-Coatings
[0028] 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.
[0029] 2.1.4 Industrial Processes Producing TiO.sub.2 Undesirably
Coarse for Use in Nano-Coatings
[0030] 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.sub.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.
[0031] 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 stoichiometrically
deficient alkaline liquid so that a significant portion,
approximately 50%, of the potential TiO.sub.2 product is left in
solution in the form of titanyl sulfate.
[0032] It is also known from the literature to hydrolyse TiCl.sub.4
under hydrothermal conditions, wherein depending upon the reaction
conditions (concentration, temperature, pH value, mineralisers),
nano-anatases and nano-rutiles are obtained. See H. Cheng et al.,
Chem. Mater. 7 (1995), 663-671. However, due to the complicated
processing requirements, it is doubtful that a commercially viable
product may be obtained using this method.
[0033] 2.1.5 Objects of the Present Invention as Regards the
Production and Use of Coatings, Particularly Nanoparticulate
Titanium Dioxide Coatings
[0034] It is thus a primary object of the invention to produce at
high yield a well-adhering thin, uniform, transparent titanium
dioxide nano-coating--in which nano-coating is present titania
nanoparticles--and to provide a process for the application
thereof. The processes for each of (1) the production and (2) the
application of nano-titanium dioxide coatings should be
economically viable, and would preferably entail only relatively
simple and foolproof conventional processing requirements that,
when conducted at an industrial large scale, will reliably produce
a titanium dioxide nano-coating product fully having the most
favorable thinness, uniformity, and adhesion properties of the best
sol/gel films.
[0035] 2.2 Prior Art Regarding the Application of Photocatalytic
Coatings
[0036] The previous sections 2.1 have discussed prior art, and the
deficiencies of the prior art, in the economical industrial scale
production of photocatalytic coatings particularly including
titanium dioxide nano-coating. As might be expected, the present
invention will teach a solution to this production problem.
[0037] However, the present invention extends further, it having
been recognized that photocatalytic coatings--howsoever
inexpensively obtained--may be beneficially applied in a manner
distinguished over the prior art.
[0038] The prior art for the application of photocatalytic coatings
of any type basically shows a substantially even, uniform and
homogeneous application of these coatings, mostly in the form of
solutions that are applied to surfaces in the manner of paint. The
present invention will soon be seen to teach otherwise, and to
teach that photocatalytic materials are usefully unevenly applied
so as to create "hot spots" of photocatalytic activity, even if and
when the "hot spots" are quite small, having dimensions on the
order of molecules, and occasionally widely dispersed.
[0039] 2.2 Prior Art Regarding the Direct Incorporation of
Photocatalytic Materials In Other Materials for Anti-fouling
Purposes
[0040] Photocatalytic titanium oxides have been the focus of
several efforts to introduce antifouling properties to coatings and
masonry. Examples include Japanese Patent 11 228 204 "Cement
composition containing photocatalyst and construction method using
it"; Japanese Patent 11 061 042 "Highly hydrophilic inorganic
coatings, coated products therefrom and their uses"; and European
Patent EP-A885 857 "Use of a mixture of organic additives for the
preparation of cementitious compositions with constant color, and
dry premixes and cementitious compositions containing the mixture".
Wide-spread commercial use has been limited largely due to the
relatively high cost and poor dispersion characteristics of
commercially available photocatalytic titanium oxide powders. Using
photocatalytic titanium oxide is attractive for an antifouling
product because titanium oxides exhibit robust weatherability and
low toxicity. The anatase crystalline form of titanium dioxide
exhibits high photocatalytic activity and has been the most widely
explored. A problem has been to introduce enough anatase titanium
dioxide into the coating or surface formulation to impart
anti-fouling properties while maintaining an economic advantage
over commercially available leaching-type biocides.
[0041] While prior art techniques attempt to minimize cost
barriers, they are deficient in one or more areas. For example,
extenders have been added to paint formulations to space
photocatalyst particles to preserve photocatalytic efficiency,
however, these extenders are difficult to distribute within the
paint matrix to maximize photocatalytic efficiency. Extenders are
typically larger particles and/or in the form of aggregates and
thus tend to increase the effective photocatalyst volume
concentration and decrease photoactive efficiency as they are added
to replace paint resin content, a phenomena analogous to decreasing
scattering efficiency as described in F. Stieg, "The Effect of
Extenders on the Hiding Power of Titanium Pigments", Official
Digest, 1959, pp. 52-64.
[0042] Titanium oxide particles, especially anatase titanium
dioxide, are difficult to distribute evenly in coating
formulations. Anatase titanium dioxide preferentially agglomerates
due to a relatively large Hamaker constant (6.times.10.sup.-20 J)
that causes individual photocatalyzing particles to clump and
effectively shade each other, reducing photocatalytic efficiency.
It would be desirable for photocatalytic particles to disperse more
easily in slurries and coating formulations.
[0043] A common strategy for improving the dispersion of pigmentary
titanium dioxide is to prepare a composite pigment. U.S. Pat. No.,
5,755,870 to Ravishankar provides a review of such strategies the
teachings of which are incorporated herein by reference. However,
the composite pigments described do not attempt to maximize
photocatalytic activity and indeed often subdue photocatalysis as a
way to. protect paint resin from photodegradation.
[0044] There is a need for a commercially viable photoactive
antifoulant composition that exhibits high photocatalytic activity
and disperses easily in slurries and coating formulations.
SUMMARY OF THE INVENTION
[0045] The present invention contemplates the (i) production and
(ii) application, including at industrial scale, of nanoparticulate
titanium dioxide (TiO.sub.2), and a sol, suitably used as a
coating, made of such nanoparticulate TiO.sub.2.
[0046] The present invention further contemplates composite
photocatalytic materials. The preferred materials consist of (1)
bodies, most preferably in the form of carrier particles, made of
material that is non-photocatalytic and non-interfering with
photocatalytically-induced reactions. These (1) bodies have (2)
surfaces that are photocatalytic, ergo composite photocatalytic
materials.
[0047] The present invention still further contemplates highly
photocatalytic aggregate particles comprised of an extender
particle with discrete photocatalytic titanium oxide particles
exposed on the surface. The aggregates may be used as additives for
making non-toxic, antifouling coatings and building materials. This
invention also includes building materials containing these
aggregates and processes for making the aggregates and slurries of
the aggregates.
[0048] 1. Production and Application of Nanoparticulate Titanium
Dioxide (TiO.sub.2) Coating
[0049] In its aspect concerning the production of nanoparticulate
titanium dioxide (TiO.sub.2), and the use of such TiO.sub.2 in a
sol and as a coating, the preferred particle size distribution of
the nanoparticulate titanium dioxide (TiO.sub.2) is between 1 nm to
100 nm (as determined from scanning electron microscopy) with less
than 0.1 wt. % of carbon in the form of organic compounds or
residues. Prior to application, the nanoparticulate TiO.sub.2
coating has a particle size distribution of between 1 nm to 100 nm
as determined from the absorption onset, a quantum size effect
measurement as described in C. Kormann et al., J. Phys. Chem. 92,
5196 (1988), and a transparency of at least 99% measured in a 5 wt.
% aqueous/hydrochloric acid solution between 400 and 700 nm in
180.degree./d geometry at a layer thickness of 10 .mu.m.
"Monodisperse" means that the collective particles typically have a
range of maximum dimension, or diameter, that varies by less than a
factor of ten (.times.10), and the collective particles will more
typically less than a two times (.times.2) variation in size.
Although not at all necessary for their photocatalytic action, and
not absolutely necessary for the formation of a sol and the use of
same as a coating, it becomes increasingly harder to get uniform
quality results with wide variations in the TiO.sub.2 starting
material, and to that extent some homogeneity is preferred.
[0050] The (nanoparticulate) particles of titanium dioxide (within
the coating according to the invention) may also be themselves
coated with 0.1 to 1000 wt. %, preferably with 5 to 200 wt. %,
relative to the TiO.sub.2, of at least one oxide, hydroxide or
hydrous oxide compound of aluminum, silicon, zirconium, tin,
magnesium, zinc, cerium and phosphorus.
[0051] The present invention also contemplates a transparent
titanium dioxide nanoparticulate liquid coating containing (i) a
sol-forming medium and (ii) a sol-forming amount, not exceeding
about 20 wt. %, of the nanoparticulate titanium dioxide in
accordance with (other aspects of) the invention. The sol-forming
medium preferably comprises (i) water, (ii) an alcohol containing 1
to 10 carbon atoms and at least one hydroxide group per molecule,
or (iii) a mixture thereof.
[0052] 1.1 Process for the Production of Nanoparticulate Titanium
Dioxide, and a Sol Suitably Used as a Coating
[0053] Therefore, in one of its aspects the present invention is
embodied in a process for the production of the nanoparticulate
titanium dioxide (TiO.sub.2), from which TiO.sub.2 may be produced
a sol suitably used as a coating.
[0054] In the preferred process (i) an alkaline-reacting liquid is
mixed with (ii) an aqueous solution of titanyl sulfate, optionally
containing sulfuric acid, at elevated temperature until the
resultant mixture reacts acidically and is neutralized to a pH of
approximately between 5 and 9, and more preferably approximately
6.5-7.5, forming (or precipitating) flocculates of titanium dioxide
nanoparticles.
[0055] The mixture obtained is cooled. The resulting titanium
dioxide flocculate formed is isolated through separation by
filtration or some other method conventionally recognized in the
art, with the isolated nanoparticulate flocculate washed in water
and then isolated again. This water-washing step is important.
Maximum dispersion into a sol, as will next be discussed, cannot be
obtained but that the titanium dioxide nanoparticulate flocculate
is first washed in water (before being washed in an acid or alkali,
immediately next discussed).
[0056] The isolated and water-washed nanoparticulate flocculate is
then washed in an acid or an alkali, isolating as a product an
acidic or alkaline titania concentrated slurry or cake.
[0057] This isolated titania concentrate is dispersed in a polar
sol-forming medium to make a sol that is suitable as a coating. The
sol is distinguished by, inter alia, being transparent. The sol
also beneficially contains less than 0.1 wt. % of carbon, which is
as good as or better than any titania sol of the prior art.
Finally, this sol will prove to have some very interesting
properties, immediately next discussed, when it is applied to a
surface.
[0058] The transparent titania sol is suitable for application to a
surface, including the surfaces of powders or of granules. After
being coated with the sol, the surface may optionally be prepared
by neutralizing with the required acidic or alkaline reacting
compound, and subsequent washing with water. Notably, and
importantly, neither the titania concentrate nor the TiO.sub.2 of
which it is comprised end up on the surface at anything like
uniformity at the molecular level. Instead, the titania
concentrate, or TiO.sub.2, becomes applied to the surface as
independent nanoparticles or small agglomerations of nanoparticles,
or spots, or islands, that are in size and number dependent upon
(i) the density of the titania concentrate in the sol and (ii) the
area coated. These nanoparticles, or spots, or islands, are
commonly widely separated relative to their own size. Although this
uniformity might initially be perceived to be an undesired
condition, it is in fact beneficial--see the next section 2.
[0059] After being coated with the sol, the surface may further
optionally be coated with 0.1 to 1,000 wt. %, and more preferably
with 5 to 200 wt. %, relative to TiO.sub.2, of at least one oxide,
hydroxide or hydrous oxide compound of aluminum, silicon,
zirconium, tin, magnesium, zinc, cerium and phosphorus. The surface
is still further optionally (i) dried and/or (ii) annealed.
[0060] The polar sol-forming medium preferably comprises water, an
alcohol containing 1 to 10 carbon atoms and at least one hydroxide
group per molecule, or a mixture thereof.
[0061] Perhaps surprisingly, the nanoparticulate TiO.sub.2 coating
according to the invention may be successfully produced within a
large scale industrial process, namely TiO.sub.2 pigment production
using the sulfate process, and is thus very simple and economically
viable.
[0062] The filter residue obtained (after the washings) and the
coating obtained (after application of the sol film) using the
process according to the invention may be inorganically and/or
organically post-treated.
[0063] In principle, any aqueous titanyl sulfate solution is
suitable as the educt. Said solution may optionally contain
sulfuric acid. Contamination by metals which form soluble sulfates
and chlorides, such as for example iron, magnesium, aluminum and
alkali metals do not in principle disrupt the production process,
unless the stated elements have a disadvantageous effect even in
trace quantities in the intended application. It is thus possible
to perform the process according the invention on a large
industrial scale. Black liquor, as is obtained from the sulfate
process by digesting ilmenite and/or titanium slag with sulfuric
acid, dissolving the resultant digestion cake in water and
performing clarification, may for example be used as the educt.
[0064] The production process according to the invention is,
however, not restricted to black liquor as the educt. Examples of
other processes for the production of titanyl sulfate solution
suitable as an educt include:
[0065] 1) dissolution of commercial grade titanyl sulfate in
water;
[0066] 2) dissolution/digestion of titanium dioxide and TiO.sub.2
hydrates, for example orthotitanic acid, metatitanic acid, in
H.sub.2SO.sub.4;
[0067] 3) dissolution/digestion of alkali metal and magnesium
titanates, also in hydrous form, in H.sub.2SO.sub.4;
[0068] 4) reaction of TiCl.sub.4 with H.sub.2SO.sub.4 to form
TiOSO.sub.4 and HCl, as described in DE-A 4 216 122.
[0069] The products, in particular those from 1), 2) and 3), are
preferably used as titanyl sulfate solutions when traces of foreign
metals (for example iron) are not desired in the product according
to the invention.
[0070] In order to achieve economically viable operation, the
titanyl sulfate solutions to be used according to the invention
preferably contain 100 to 300, and more particularly preferably 170
to 230 g of titanium/l, calculated as TiO.sub.2.
[0071] Aqueous solutions of ammonium hydroxide, sodium hydroxide,
or potassium hydroxide are preferably used as the alkaline-reacting
liquid; it is, in principle, also possible to use carbonates of
sodium, potassium and ammonium, but these are less suitable due to
vigorous evolution of CO.sub.2. Ammonium hydroxide solution is
particularly preferred as sodium and potassium ions are not
introduced as a contaminant and is used to illustrate performance
of the process in greater detail.
[0072] The quantity of ammonia should be calculated such that the
reaction medium at the end of step a) has a final pH of
approximately between 5 and 9, and more preferably between 6.5 and
7.5.
[0073] The ammonia is preferably used as an ammonium hydroxide
solution having a concentration of approximately between 1 to 8
molar NH.sub.4OH and more preferably between 1 to 4 molar
NH.sub.4OH.
[0074] The reaction of ammonium hydroxide solution with the titanyl
sulfate solution preferably proceeds in such a manner that the
ammonium hydroxide is added to a solution of titanyl sulfate,
heated to approximately 60 to 1000.degree. C.
[0075] Preferably the reaction of the ammonium hydroxide and
titanyl sulfate solution can also be carried out by adding the two
reactants simultaneously and mixing them with stirring at
temperatures of between 60 and 100.degree. C.
[0076] This reaction of the titanyl sulfate solution should
preferably be performed with vigorous stirring and at temperatures
of 60 to 100.degree. C.
[0077] The addition of the ammonium hydroxide to the titanyl
sulfate solution should preferably take no longer than 30
minutes.
[0078] Once reacted, the resultant mixture should preferably be
quenched to temperatures of below 60.degree. C. and then optionally
stirred for {fraction (1/4)} to 1 hour at this temperature.
[0079] In summary, the production of the sol suitable as a coating,
and the sol so produced, has myriad, and distinguishing,
advantages. The sol is uniquely transparent while achieving the
desirably low carbon of the best prior art titania sols. The yield
in making the sol is unexcelled; virtually 100% of the precipitated
titanium flocculates are taken up into the sol. The process of
making the sol is readily scalable to industrial scale. Finally,
and as a seemingly subtle differentiation in the sol the use and
benefit of which is unanticipated in the prior art, the sol, when
used as a coating, will not deposit its titanium dioxide uniformly
(upon a coated surface, which may be a particle) but will instead
lay down the titanium dioxide in microparticles, or spots, or
islands. The very significant advantage of this is immediately next
discussed in section 2.
[0080] 2. Composite Photocatalytic Materials
[0081] In its aspect concerning the realization of composite
photocatalytic materials, the preferred material of the present
invention includes, as previously stated, (1) bodies that are most
preferably in the form of carrier particles and that are made of
material that do not interfere with photocatalytic activity and do
not adversely interact with other components in an end-use
application. These (1) bodies that are non-deleterious to
photocatalytic reaction have (2) surfaces that are photocatalytic,
forming thus a composite photocatalytic material.
[0082] Moreover, these (2) surfaces are not substantially evenly
possessed of photocatalytic material and photocatalytic action, but
preferably have such photocatalytic material highly specifically
located in "spots", or "islands" that may themselves be either 2 or
3-dimensional.
[0083] To realize these "islands" of photocatalyst, the (2)
surfaces of the (1) bodies, or carrier particles, are not made from
continuous films of photocatalytic material, but are instead made
by attaching discrete nanoparticles of photocatalyst. These
nanoparticles of photocatalyst are preferably smaller--normally
1.times.10-9 to 1.times.10-7 in diameter--than are the carrier
particles themselves, which are commonly about 1.times.10-7 to
1.times.10-2 meters in diameter, depending on application.
[0084] Both the size of the (2) carrier particles, or bodies, and
the density of the spots, or islands, of (1) surface photocatalytic
material are a function of intended application. An exemplary
application of a carrier large particle might be for use in a
gravel-like roof coating where it is substantially desired only
that large, ground-observable, patches of algae should not grow on
the roof. In this application the photocatalytic spots, or islands,
might also be relatively widely separated, the main goal not being
to kill every bacteria or algal cell on the roof, but to prevent
formation of a bio-film. Exemplary applications of small carrier
particles include the lips of a swimming pools, bathroom tiles, and
hospital coatings where it is desired to avoid all bacterial growth
whatsoever. Not only are the carrier particles small, but the
photocatalytic spots, or islands, may be relatively close spaced
(although normally not continuous).
[0085] As an aside, the photocatalyst of the present invention is
generally not intended for use in liquids other than coatings, and
certainly not for antiseptic solutions where photocatalyst
suspensions kill microbes or algae on surfaces. The only time the
inventor has used photocatalyst suspensions was in lab tests
wherein algae was suspended in water and photocatalyst particles
were then introduced into the water to see "for a first glimpse"
whether the photocatalyst killed the algae. However, it is
contemplated that the photocatalyst of the present invention could
be dispersed in water to destroy microbial suspensions. One such
application could be to destroy harmful algae blooms in lakes and
bays. The three main benefits of using photocatalyst of the present
invention in natural waterways would be (i) low toxicity to higher
life forms, (ii) limited persistence in the environment (the
concentrated contaminants of natural water systems tend to foul the
photocatalyst, inactivating it over time), and (iii) excellent
dispersion properties in water (in contrast to poor dispersion for
virgin photocatalyst).
[0086] Accordingly, by incorporating but minute amounts of
dispersed photocatalytic nanoparticles solely upon the surfaces of
carrier particles--most typically in an amount of less than 20% and
more typically 5% by weight in the composite material--these
dispersed photocatalytic nanoparticles, and diverse surfaces coated
with the composite material, are highly effective in killing
microorganisms, including both algae and bacteria, in the presence
of light in the visible or ultraviolet wavelengths. Indeed, by
attaching microparticles of preferred photocatalytic materials of
titanium dioxide, zinc oxide and tungsten oxide and mixtures
thereof onto the surface of particles of silicate and carbonate
powders and sands, mineral and mineral composites, inorganic
pigments, construction aggregates, polymers and like common
materials in an amount of less than 10% by weight, the composite
particles so formed are at least 50% as effective in killing algae
and bacteria as are the pure photocatalysts themselves.
Accordingly, there is at least a five-to-one (5:1), and more
typically a twenty-to-one (20:1), gain in efficiency in the usage
of the photocatalytic materials--which are greatly more expensive
than are the materials from which the carrier particles are
made.
[0087] The composite photocatalytic materials, preferably
particulate materials, may themselves be combined with any of
dispersants, carriers, binders and the like to make any of aqueous
solutions, coatings, paints and the like as exhibit any of
algicidal, fungicidal, and/or anti-bacterial effects. Liquids,
aggregates and solids incorporating the composite photocatalytic
materials of the present invention may be, for example, coated or
painted onto, by way of example, the interior and exterior surfaces
of buildings and swimming pools.
[0088] Although no theory of the operation of the composite
photocatalytic materials of the present invention is necessary to
make these materials, nor to take advantage of their operational
characteristics, it is possible to speculate on the operation of
the materials of the present invention. It is hypothesized that
only a minute microparticle of pure photocatalytic material such as
titanium dioxide, zinc oxide and tungsten oxide and mixtures
thereof is necessary to adversely affect a much larger bacterium,
or a cell of an algae; that it is not the total amount of
photocatalyst that does the damage to lower life forms, but the
manner in which a photocatalyst is deployed against these life
forms.
[0089] Apparently it is not necessary for control of simple life
forms to expose in the presence of light the entirety of the life
form to a photocatalyst in order to enjoy a prophylactic effect. It
is apparently sufficient for a prophylactic effect to expose only a
minute region of the life form. It may even be the case that a
bacterium or an algae will retreat from an extensive area of
photocatalyst with less damage than it will sustain when exposed,
hypothetically for a longer time, to but a microscopic spot, or
particle, or photocatalyst to which its primitive sensory system is
insufficiently sensitive. The present invention suggests that large
surfaces, such as walls of swimming pools and buildings, should not
have photocatalyst evenly applied so that, at some density of
adjacent bacterial or algal life forms, a bio-film will be formed,
the photocatalyst overwhelmed (including by occlusion of light
energy), and the surface populated. Instead, it may be preferable
that the surface act as a "trojan horse", according areas devoid of
photocatalyst--which areas are sufficient in size to be populated
by one or a few bacteria or algal cells until these bacteria or
algae grow and/or reproduce, forcing members of the incipient
community into damaging contact with minute regions of
photocatalyst. These minute regions, or microdots, or
microparticles, of photocatalyst may, at their high concentrations,
be very effective in promoting electron exchange in the presence of
impinging light. They may become "hot spots" of "stinging" death to
those microorganisms with which they come into contact.
[0090] The mechanism(s) of photocatalytically-induced fungicidal,
bacteriocidal and like effects are poorly understood, but the
present invention suggests that there is more to the conservative
and focused deployment of photocatalysts than simply saving money
by minimizing usage. The present invention suggests that
photocatalyst should be parsimoniously used as a microbial
rapier--the point of which can be deadly to microbial life--instead
of as a bludgeon by which the substantial surface of a microbe is
substantially evenly irritated in a manner that may not prove fatal
to the microbe.
[0091] 2.1 A Composite Photocatalytic Material
[0092] Accordingly, in another of its aspects the present invention
is embodied in a composite body exhibiting a photocatalytic effect.
The body has (i) a core consisting essentially of a material
without deleterious photocatalytic effect on the composite body nor
adverse interaction with other. components in an end-use
application, and (ii) a photocatalytic material upon the surface of
the core. This photocatalytic material is less than 20% by weight
of the combined photocatalytic material and the core.
[0093] The core is a preferably a particle, and more preferably a
particle of less than 1 (one) centimeter in diameter. Meanwhile,
the photocatalytic material is preferably a multiplicity of
particles each of which is preferably of diameter less than one
hundred (100) nanometers. By this construction the composite body
is also a particle.
[0094] The core preferably consists essentially of a material,
nondeleterious to photocatalytic reactions, drawn from the group
consisting of silicates and carbonates, mineral and mineral
composites, metal oxides, inorganic pigments, and construction
aggregates. Alternatively, the core may consist essentially of a
polymer. The polymer core is preferably drawn from the group
consisting essentially of acrylics, acrylonitriles, acrylamides,
butenes, epoxies, fluoropolymers, melamines, methacrylates, nylons,
phenolics, polyamids, polyamines, polyesters, polyethylenes,
polypropylenes, polysulfides, polyurethanes, silicones, styrenes,
terephthalates, vinyls.
[0095] The photocatalytic material is preferably drawn from the
group of metal compound semiconductors consisting essentially of
titanium, zinc, tungsten and iron, and oxides of titanium, zinc,
tungsten and iron, and strontium titanates. This compound
semiconductor photocatalytic material. may be combined with a metal
or metal compound drawn from the group. consisting of nickel,
cobalt, zinc, palladium, platinum, silver, and gold. Most
preferably, the photocatalytic material is drawn from the group of
metal compound semiconductors consisting essentially of anatase
titanium dioxide and zinc oxide.
[0096] The composite photocatalytic material is preferably in the
form of particles having a diameter from 100 nanometers to 1
centimeter, which diameter depends upon the core size selected and
the intended end-use application.
[0097] The weight of the photocatalytic material is preferably less
than 20% of the weight of the core, and more preferably less than
10% of the weight of the core.
[0098] The composite photocatalytic material in accordance with the
present invention is usefully incorporated in other compositions.
When so incorporated, it is preferably so incorporated in amounts
from 0.001% to 85% by volume. The composite photocatalytic material
may be incorporated with, or on, one or more materials from the
group of building materials consisting of concrete, cement, stucco,
masonry, roofing shingles, wall shingles, building siding, flooring
materials and swimming pool surfaces. The composite photocatalytic
material may be incorporated in a composition that is effective as
an anti-fouling coating. For example, it may be incorporated in a
concrete coating effective in killing by contact algae, fungus
and/or bacteria on surfaces.
[0099] Most typically, at a proportion by weight in the composite
particle of less than 10%, the efficacy of the photocatalytic
material within the composite particles to kill by contact both
algae and bacteria upon surfaces is at least one-half (0.5) as good
as is the efficacy of this same photocatalytic material in purest
form to kill. In other words, at least equal killing effect is
realized with at least a five to one (5:1) reduction in the amount
of photocatalytic material used (when this photocatalytic material
is upon the surface of the composite particles).
[0100] 2.2 Methods of Making Composite Photocatalytic Particles
[0101] In yet another of its aspects (concerning the making and use
of photocatalytic materials), the present invention is embodied in
methods of making composite photocatalytic particles.
[0102] In one method an aqueous slurry of first particles--these
particles consisting essentially of a material without deleterious
photocatalytic effect on the composite particle nor adverse
interaction with other components in an end-use application, and
having a size in the range from 100 nanometers to 1 centimeter
diameter--is prepared.
[0103] To this slurry is added a colloidal suspension of 0.1% to
60% by weight second particles, which second particles consist
essentially of photocatalytic material having diameters in the
range from 1 to 100 nanometers. The combined weight of second
particles in the colloidal suspension is less than 20%, and more
preferably less than 10%, of the combined weight of the first
particles that are within the aqueous slurry.
[0104] The aqueous slurry and the colloidal suspension is mixed so
that the photocatalytic material second particles attach through
van der Waals forces or chemical fusion to the nondeleterious
material first particles, forming a slurry of composite particles.
In these composite particles the relatively smaller photocatalytic
material second particles are located upon the surfaces of the
relatively larger, nondeleterious material, first particles.
[0105] The photocatalytic material is in weight preferably less
than 20%, and more preferably less than 10%, of the first
particles. The added colloidal suspension added is preferably from
0.1% to 60% by weight second particles. The colloidal suspension
added is preferably of the highest solids concentration at which
the suspension is stable, normally being in the range from 14% to
50% by weight.
[0106] The pH of the-mixing is often beneficially adjusted so that
both the photocatalytic material second particles and the
nondeleterious material first particles are displaced to the same
direction--whether above or below--from their respective
isoelectric points (those points at which the particles have a
neutral net charge). Furthermore, the nondeleterious material first
particles and the photocatalytic material second particles may also
have opposite charge.
[0107] The adding of the colloidal suspension of second particles,
or the mixing of the aqueous slurry and the colloidal suspension,
or both the adding and the mixing, may optionally transpire in the
presence of at least one dispersant.
[0108] The method may continue with one or more well-known
finishing steps such as filter, wash and/or dry the composite
photocatalytic particles.
[0109] When the aggregation of composite photocatalytic particles
is dried, composite particles with heat resistant cores are then
preferably annealed in a kiln to create stronger fusion bonds
between the photocatalytic material second particles and the
nondeleterious material first particles and/or to improve the
photocatalytic nature of the photocatalyst by changing its
crystalline form. Moreover, the annealed composite photocatalytic
particles are preferably rapidly cooled to ambient room
temperature; this may be simply accomplished by removing the hot
material from the kiln to facilitate heat transfer away from the
material. The time period of this cooling is necessarily dependent,
at least in part, upon the temperature of the annealing and the
amount of the composite photocatalytic particles. However, it is
preferably less than six hours. Since this forced rapid cooling
might normally be considered to induce fracturing in metals, it is
uncommonly applied to the materials (including metal oxides) of the
present invention. However, it has benefit in that it increases
photocatalytic activity.
[0110] 3. Photocatalytic Aggregate Particles
[0111] In still yet another of its aspects, the present invention
contemplates highly photocatalytic aggregate particles comprised of
an extender particle with discrete photocatalytic titanium oxide
particles exposed on the surface. The extender particle reduces the
amount of premium photocatalyst required to achieve desired
photocatalytic activity in a finished product. The discrete nature
of the photocatalytic titanium oxide particles, applied in
sufficient number, increases the photoactivity of the aggregate
particles by increasing their photoactive surface area verses the
surface area provided by a relatively flat continuous coating. The
aggregates of this invention exhibit an inhibitory effect on
surface-borne microorganisms when the mixtures are incorporated
into building materials such as masonry, roofing shingles, siding,
and antifouling coatings. Further, the aggregate particles show
improved handling and dispersion in coating preparations versus
virgin photocatalyst.
[0112] The invention also contemplates processes for making such
aggregates, slurries of the aggregates, coatings, building
materials, and masonry containing the aggregates.
[0113] 3.1 The Preferred Photocatalytic Aggregates
[0114] The preferred aggregate particles of the present
invention--generally comprised of an extender particle with
discrete photocatalytic titanium oxide particles exposed on the
surface, which exhibit antifouling properties and improved
dispersion in slurries and coatings--consist essentially of
photocatalytic titanium oxide, preferably titanium dioxide in the
anatase crystalline form, at less than about 20% by weight,
preferably less than 10% by weight, and more preferably less than
6% by weight, and an extender particle at greater than 20% by
weight. Preferred extender particles include silicate and carbonate
powders, mineral and mineral composites including calcined clay and
wollastonite, metal oxides including zinc oxide, inorganic
pigments, and construction aggregates including roofing
granules.
[0115] In one preferred embodiment, colloidal anatase titanium
dioxide in an amount less than 6 weight % is dispersed on the
surface of crystalline silica powder having an average particle
diameter of 0.7 to 5 microns. In another preferred embodiment,
colloidal anatase titanium dioxide in an amount less than 6 weight
% is dispersed on the surface of zinc oxide powder having an
average particle diameter of 0.7 to 5 microns.
[0116] This invention also includes anti-fouling building products,
including coatings and masonry compositions, comprising aggregate
photocatalytic particles of this invention at a volume
concentration of 0.001% to 85% where the anti-fouling coatings and
masonry resist the growth of microorganisms when U.V. or visible
light energy is present to activate the aggregate photocatalytic
particles. Building products include roofing granules, roofing
shingles, building siding, wall shingles, hard flooring, and
swimming pool surfaces.
[0117] 3.2 Preferred Processes for Producing Photocatalytic
Aggregates
[0118] Several different processes for making the above-described
aggregate photocatalytic materials are preferred. In one
embodiment, an aqueous slurry of extender particles are mixed with
a solution of titanyl sulfate and by the addition of an alkaline
reacting agent, discrete titanium dioxide particles are deposited
onto the extender particles.
[0119] In another embodiment, an alkaline or acidic titania sol is
mixed with extender particles where the particles in the titania
sol have an average diameter size within the range of about 1 to
about 100 nanometers. The solution is maintained such that the
extender particles and the sol particles are both above or below
their respective isoelectric points such that substantially
discrete particles of titanium dioxide are dispersed onto the
surfaces of the extender particles in an amount less than 20 weight
% based on aggregate particle weight.
[0120] These and other aspects and attributes of the present
invention will become increasingly clear upon reference to the
following drawings and accompanying specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0121] FIG. 1, consisting of FIG. 1a through FIG. 1c, are scanning
electron micrographs of silica particles with a coating of
nano-particulate TiO.sub.2 at 4% by wt. silica according to the
invention.
[0122] FIG. 2, consisting of FIGS. 2a through FIG. 2d, are scanning
electron micrographs of silica particles with a coating of
nano-particulate TiO.sub.2 at 0.5% by wt. silica according to the
invention.
[0123] FIG. 3 is a graphical depiction of three example
arrangements of discrete photocatalytic particles, particularly
titanium dioxide particles, on the surface of an extender, or
carrier, or core particle so as to form a photoactive antifouling
aggregate, where FIG. 3a shows discrete particles of titanium oxide
partially covering larger extender particles, FIG. 3b shows
discrete flocculates of titanium oxide particles partially covering
extender particles, and FIG. 3c shows discrete titanium oxide
particles fully covering larger extender particles.
[0124] FIG. 4 is a transmission electron micrograph of a composite
photocatalytic particle having substantially discrete particles of
anatase titanium dioxide dispersed on the surface of a silica
particle created using a compaction milling device.
[0125] FIG. 5 is a bar chart illustrating the algae-inhibiting
effect of photoactive antifouling aggregate comprising 25 weight %
non-colloidal photoactive zinc oxide and 75 weight % colloidal
anatase titanium dioxide.
[0126] FIG. 6 is a bar chart showing the inhibiting effect of an
the aggregate of FIG. 5 on the growth of E. coli bacteria.
[0127] The following examples are intended to illustrate the
invention in greater detail.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0128] The following description is of the best mode presently
contemplated for the carrying out of the invention. This
description is made for the purpose of illustrating the general
principles of the invention, and is not to be taken in a limiting
sense. The scope of the invention is best determined by reference
to the appended claims.
[0129] Although specific embodiments of the invention will now be
described, it should be understood that such embodiments are by way
of example only and are merely illustrative of but a small number
of the many possible specific embodiments to which the principles
of the invention may be applied. Various changes and modifications
obvious to one skilled in the art to which the invention pertains
are deemed to be within the spirit, scope and contemplation of the
invention as further defined in the appended claims.
[0130] 1. Process for the Production of Nanoparticulate Titanium
Dioxide
[0131] It will be recalled that one embodiment of the present
invention is as a process for the production of the nanoparticulate
titanium dioxide coating. The preferred process includes
[0132] a) mixing an alkaline-reacting liquid with an aqueous
solution of titanyl sulfate, optionally containing sulfuric acid,
at elevated temperature until the resultant mixture reacts
acidically and is neutralized to a pH of approximately between 5
and 9, and more preferably approximately 6.5-7.5, forming
flocculates of titanium dioxide nanoparticles;
[0133] b) cooling the mixture obtained in step a);
[0134] c) isolating, through filtration or some other method
conventionally recognized in the art, the resulting titanium
dioxide nanoparticle flocculate formed in step b);
[0135] d) washing said nanoparticle flocculate in water and
isolating again;
[0136] e) washing said nanoparticle flocculate in an acid or alkali
and isolating the product as an acidic or alkaline titania
concentrate;
[0137] f) dispersing said titania concentrate in a polar
sol-forming medium to make a transparent sol;
[0138] g) applying a film of the titania sol to a surface,
including powders or granules;
[0139] h) optionally neutralizing said surface with the required
acidic or alkaline reacting compound and subsequently washing said
surface with water;
[0140] i) optionally coating said titania-coated surface with 0.1
to 1,000 wt. %, preferably with 5 to 200 wt. %, relative to
TiO.sub.2, of at least one oxide, hydroxide or hydrous oxide
compound of aluminum, silicon, zirconium, tin, magnesium, zinc,
cerium and phosphorus;
[0141] j) optionally drying and annealing said surface.
[0142] The sol-forming medium referred to in step f) preferably
comprises water, an alcohol containing 1 to 10 carbon atoms and at
least one hydroxide group-per molecule, or a mixture thereof.
[0143] The nanoparticulate TiO.sub.2 coating according to the
invention may surprisingly also successfully be produced within a
large scale industrial process, namely TiO.sub.2 pigment production
using the sulfate process, and is thus very simple and economically
viable.
[0144] The filter residue obtained (after step d or e) and the
coating obtained (after step g) using the process according to the
invention may be inorganically and/or organically post-treated.
[0145] In principle, any aqueous titanyl sulfate solution is
suitable as the educt. Said solution may optionally contain
sulfuric acid. Contamination by metals which form soluble sulfates
and chlorides, such as for example iron, magnesium, aluminum and
alkali metals do not in principle disrupt the production process,
unless the stated elements have a disadvantageous effect even in
trace quantities in the intended application. It is thus possible
to perform the process according to the invention on a large
industrial scale. Black liquor, as is obtained from the sulfate
process by digesting ilmenite and/or titanium slag with sulfuric
acid, dissolving the resultant digestion cake in water and
performing clarification, may for example be used as the educt.
[0146] The production process according to the invention is,
however, not restricted to black liquor as the educt. Examples of
other processes for the production of titanyl sulfate solution
suitable as an educt are:
[0147] 1) dissolution of commercial grade titanyl sulfate in
water;
[0148] 2) dissolution/digestion of titanium dioxide and TiO.sub.2
hydrates, for example orthotitanic acid, metatitanic acid, in
H.sub.2SO.sub.4;
[0149] 3) dissolution/digestion of alkali metal and magnesium
titanates, also in hydrous form, in H.sub.2SO.sub.4;
[0150] 4) reaction of TiCl.sub.4 with H.sub.2SO.sub.4 to form
TiOSO.sub.4 and HCl, as described in DE-A 4 216 122.
[0151] The products, in particular those from 1), 2) and 3), are
preferably used as titanyl sulfate solutions when traces of foreign
metals (for example iron) are not desired in the product according
to the invention.
[0152] In order to achieve economically viable operation, the
titanyl sulfate solutions to be used according to the invention
preferably contain 100 to 300, particularly preferably 170 to 230 g
of titanium/l, calculated as TiO.sub.2. Aqueous solutions of
ammonium hydroxide, sodium hydroxide, or potassium hydroxide are
preferably used as the alkaline-reacting liquid; it is, in
principle, also possible to use carbonates of sodium, potassium and
ammonium, but these are less suitable due to vigorous evolution of
CO.sub.2. Ammonium hydroxide solution is particularly preferred as
sodium and potassium ions are not introduced as a contaminant and
is used to illustrate performance of the process in greater
detail.
[0153] The quantity of ammonia should be calculated such that the
reaction medium at the end of step a) has a final pH of
approximately between 5 and 9, and more preferably between 6.5 and
7.5.
[0154] The ammonia is preferably used as an ammonium hydroxide
solution having a concentration of approximately between 1 to 8
molar NH.sub.4OH and more preferably between 1 to 4 molar
NH.sub.4OH.
[0155] The reaction of ammonium hydroxide solution with the titanyl
sulfate solution preferably proceeds in such a manner that the
ammonium hydroxide is added to a solution of titanyl sulfate,
heated to approximately 60 to 100.degree. C.
[0156] Preferably the reaction in step a) can also be carried out
by adding the two reactants simultaneously and mixing them with
stirring at temperatures of between 60 and 100.degree. C.
[0157] Step a) should preferably be performed with vigorous
stirring and at temperatures of 60 to 100.degree. C.
[0158] The addition of the ammonium hydroxide in step a) should
preferably take no longer than 30 minutes.
[0159] After step a), the mixture should preferably be quenched to
temperatures of below 60.degree. C. and then optionally stirred for
{fraction (1/4)} to 1 hours at this temperature.
[0160] The resultant mixture is turbid to a greater or lesser
extent and comprised of flocculates of nanoparticlulate
TiO.sub.2.
[0161] After cooling, the flocculate is isolated by filtration or
other conventional separation technique and then washed with water
to remove contaminating sulfur compounds and other water-soluble
contaminants. After isolating the TiO.sub.2 again, the flocculate
is washed with a monobasic acid or alkali to remove further
contaminants and introduce the ions necessary for sol
formation.
[0162] The flocculate is nanoparticulate titanium dioxide having a
particle size of between 1 and 100 nm, containing less than 0.1 wt.
% of carbon and having a transparency of at least 99% (see
above).
[0163] Addition of the ammonium hydroxide in step a) results in an
initial increase in viscosity of the reaction medium as the
resultant bulky flocculates form. Continued stirring distributes
the flocculates more evenly resulting in a decrease in viscosity.
The resulting flocculates may be separated simply by settling, i.e.
standing undisturbed for at least 12 hours and decantation. Due to
their size (preferably greater than 1 micron), the resultant bulky
flocs may readily be centrifuged and filtered.
[0164] The precipitate is then washed with water, preferably by
dispersing the precipitate in 3 to 10 times its weight in water,
and then isolating the precipitate through filtration or other
conventional separation method.
[0165] The said precipitate is then washed in a monobasic acid or
alkali solution by preferably dispersing the precipitate in 1 to 6
times its weight in acid or alkali and then isolating the
precipitate through filtration or other conventional separation
method as is know in the art. The preferred washing agent is
hydrochloric acid, which is used to illustrate the further
processing in greater detail. The same procedure should be used
with other acids and alkali.
[0166] The HCl concentration in the hydrochloric acid should
preferably be no less than 3 molar, preferably 3 to 6 molar, and
particularly preferably 4 to 6 molar.
[0167] Depending upon the filter unit and starting material, the
acid or alkali-washed titania concentrates typically contain 4 to
40 wt. % of TiO.sub.2, the remainder being wash acid or wash
alkali, moisture and possibly small quantities of contaminants. The
nanoparticles may be stored as acidic or alkaline concentrates in
air-tight containers at room temperature without change for some
weeks, and as necessary, suspended in a sol-forming medium for
producing sol coatings.
[0168] Once redispersed in water, the titania concentrates yield
"solutions" (sol coatings) which, apart from slight opalescence
(Tyndall effect), are clear, transparent and colorless or nearly
colorless. The TiO.sub.2 is present in these sol coatings
exclusively as nano-particles having a diameter of between 1 and
100 nm.
[0169] It is possible in this manner to produce strongly acidic or
strongly alkali, virtually completely transparent (water-clear) sol
coatings containing up to approximately 20 wt. % of TiO.sub.2. At a
concentration of 5 wt. % of TiO.sub.2, the transparency of the sol
coatings is at least 99% over the visible range of the spectrum
from 400 nm to 700 nm wavelengths (measured in 180.degree./d
geometry at a layer thickness of 10 .mu.m).
[0170] Generally, a sol coating may be created my combining 2 to 3
parts by weight water with one-part by weight acidic or alkaline
concentrate. Such sol coatings are also generally stable for some
weeks. As much as 10 to 20 parts additional water may be added to
further dilute the sol coating.
[0171] Similar sol coatings my also be produced in polar organic
solvents, primarily in mono- and polyhydric short-chain alcohols,
such as for example ethanol and 1,4-butanediol. The alcohols
preferably contain 1 to 10 carbon atoms per molecule.
[0172] An alternative method of carrying out the invention is
forming an aqueous colloidal coating by combining water with the
acidic or alkali titania concentrate of this invention and adding
at least one dispersant. The dispersant may also be added
simultaneously with the water. The dispersant can be selected from
those described in U.S. Pat. No. 5,393,510, the teachings of which
are incorporated herein by reference. Examples of dispersants
include alcohol amines such as 2-amino-2-methyl-1-propanol,
2,2',2"-nitrilotrisethanol, 2,2'-iminobisethanol, 2-aminoethanol
and the like, and 1-amino-2-propanol, polyacrylates, citric acid
and tetrapotassium pyrophosphate (TKPP) and the like. Typically a
combination of the above dispersants is preferred in an amount of
about 0.05 to about 5% based on TiO.sub.2 weight, or based on total
solids weight when the coating is mixed with powders or
granules.
[0173] Spread thinly onto a surface, the nano-particulates of the
sol coatings will be attracted to the surface by van der Waals'
forces and may be further anchored to the surface material by
stronger chemical bonds such as fusion bonds. Coatings may be
applied to continuous solid surfaces by dip-coating, rolling,
brushing, or other such application procedure. Coatings may be
applied to particles, such as powders and granules, by direct
mixing, fluid bed application, or other suitable application
procedure. It has been found that uniform surface coatings of
nano-particulate TiO.sub.2 on powders and granules is best achieved
by maintaining the to-be-coated particles and the colloidal
particles at both above or below their respective isoelectric
points such that substantially discrete particles of titania are
evenly dispersed onto the surfaces of the target particles. In one
preferred embodiment of this invention, titania suspended in a sol
medium containing HCl is added to particulates pre-wetted with a
solution of HCl resulting in evenly dispersed nanoparticles of
TiO.sub.2 on the particulates.
[0174] Where acidic or alkali residue may impact the performance of
the nano-coating, the coated surface may be further washed with a
neutralizing agent (such as a dilute ammonium hydroxide solution
when the residue is acidic or a dilute solution of HCl when the
residue is alkali) and then the resulting surface washed with water
to remove any remaining contaminants.
[0175] In the event that a reduction in photoactivity is desired,
the nanoparticles may be inorganically coated (post-treated),
wherein, as with pigment TiO.sub.2, coating is performed-with
oxides, hydroxides or hydrous oxides of one or more of the
following elements: Al, Si, Sn, Mg, Zn, Ce, P. The quantities to be
used amount to 0.1 to 1000, preferably to 5 to 200 wt. %, relative
to TiO.sub.2.
[0176] Inorganic post-treatment is not necessary, and generally
undesirable, if the product is used as a catalyst for the
photochemical degradation of organic compounds (polymers,
pollutants) or as a support for dye solar cells. However,
surprisingly it has been found that a coating of silicate
precipitated onto the nano-coating from a solution of sodium
silicate has a limited impact on photocatalytic activity when the
amount of silicate precipitated is approximately less than 5 times
the amount of TiO.sub.2 in the nano-coating. The silicate is
preferably precipitated from a solution of sodium silicate
containing 0.05% to 2% silica by wt. Precipitation is accomplished
by titrating the sodium silicate solution with an acid, such as
HCl, to a neutral pH of about 7. The surface is then preferably
washed to remove contaminants. Such silicate coatings may be
desired to further enhance the adhesion of the nano-coating to a
surface.
[0177] As a final step in the process for making the nano-coating,
the coated surface may be dried and annealed to drive off moisture,
crystallize the TiO.sub.2 and better fuse the nanoparticulate
TiO.sub.2 to the surface. The photocatalytic activity of the
coating may be optimized by annealing the coating at a temperature
of approximately between 400.degree. C. and 650.degree. C. for 30
minutes to 5 hours. Photocatalytic activity may be reduced by
annealing at a temperature above 700.degree. C. which temperature
induces a crystalline phase change in the TiO.sub.2 from the
anatase form to the less photocatalytic rutile form. Annealing and
its effect on photocatalytic activity is discussed in further
detail in L. Gomathi Devi's "Photocatalytic degradation of
p-amino-azb-benzene and p-hydroxy-azo-benzene using various heat
treated TiO.sub.2 as the photocatalyst", J. of Photochem. and
Photobio. A: Chem. 121 (1999), 141-145.
[0178] In applications in which acid excesses have a disruptive
effect, the sol coatings according to the invention may
subsequently be stabilized in the neutral pH range in a manner
known in principle, for example with acetylacetone (WO 93/05875) or
with hydroxycarboxylic acids (EP-A 518 175).
[0179] The coating of nanoparticulate titanium dioxide is used as a
photocatalyst to prevent fouling from microorganisms on surfaces,
as a U.V. screening agent, and as a flame retardant.
[0180] 1.1 Example of the Process for the Production of
Nanoparticulate Titanium Dioxide Coatings
[0181] An example of the process of the invention for the
production of nanoparticulate titanium dioxide coatings is as
follows:
[0182] Recommended Laboratory supplies and equipment for laboratory
preparation of nanoparticulate titanium dioxide coatings include
(i) a fume hood, (ii) 2 heated stir plates, (iii) a glass stir rod,
(iv) 100, 250 and 1000 ml liter beakers, (v) a 1000 ml filtration
flask, (vi) 10 ml and 100 ml graduated cylinders, (vii) cellulose
nitrate filtration paper, 90 mm circles, 0.45 micron, (viii) teflon
coated magnetic stir bars, (ix) an aspirator or other vacuum source
for filtration (x) lab balances (+-1 mg and +-0.1 mg), (xi) a
container for the ice bath, (xii) a 1 liter filtration flask
(Erlenmeyer with a sidearm), (xiii) a Coors-type ceramic Buchner
funnel with fixed plate for 90 mm filtration paper, (xiv) a rubber
gasket for the filtration flask, (xv) a mortar and pestle (100 ml
minimum size for combining sol with silica), (xvi) a drying oven
(to 130.degree. C.), (xvii) a ceramic or pyrex vessel for
annealing, (xviii) an annealing oven (to 650.degree. C.), (xix) 10
ml pipettes, (xx) a pH meter or pH paper (pH 7), (xxi) a
thermometer (to 100.degree. C.), (xii) a squirt bottle for water,
and (xiii) a non-metallic spatula for removing filter cake from the
filter. A 1 liter vessel with temperature control and stir
capability is optional.
[0183] Required chemicals include (i) deionized water, (ii)
ammonium hydroxide, aq (29.6%), (iii) hydrochloric acid, aq (37%),
(iv) TiOSO4 (Noah Technologies), and (v) water ice.
[0184] 210 ml water is mixed with 100 g TiOSO.sub.4 (Noah
Technologies, comprising 80.3% TiOSO.sub.4.cndot.2H.sub.2O, 8.3%
free acid sulfuric, 11.4% moisture) and heated to 85.degree. C.
while stirring in a jacketed glass vessel using a mechanical
stirrer. 270 ml NH.sub.4OH 1.91 M is slowly added over 10 minutes
with continued stirring causing titania to precipitate from the
solution. The stirring continues until the viscosity of the
solution thins and stabilizes. The solution is then neutralized to
about pH 7 with the addition of 14 ml NH.sub.4OH 3.81 M and stirred
for an additional 15 minutes at 85.degree. C. The suspension is
then quenched to 28.degree. C. over 20 minutes and the precipitate
filtered using a 0.45 micron nitrocellulose filter. The white
precipitate is then re-suspended in 1 liter water to rinse the
flocculates and then filtered again. The resulting filter cake is
re-suspended in 250 ml HCl 6 M and filtered again. The resulting
acidic titania cake is comprised of nanoparticulate titania. The
cake may be used immediately for making a colloidal titania coating
or stored in an air-tight container for later use. To make a
transparent colloidal coating, a quantity of the acidic titania
cake (about 9% by wt. TiO.sub.2) is dispersed in three times its
weight in water. The stable pH range for titania sol (for sol
containing 4.6% TiO2 by wt.; in the method described in this
example, the sol contains 2.3% TiO2 by wt.) is 1.1 (+-0.2) -(+-0.2)
pH. The titania completely precipitates from the sol at 5.2 (+-0.2)
pH.
[0185] FIG. 1a through FIG. 1c are scanning electron micrographs
showing silica particles with a coating of nano-particulate
TiO.sub.2 at 4% by wt. silica according to the above process. FIGS.
2a through FIG. 2d, are similar scanning electron micrographs of
silica particles with a coating of nano-particulate TiO.sub.2 at
0.5% by wt. silica according to the above process.
[0186] A perhaps more understandable view of an entire surface
coating of nano-particulate TiO.sub.2 in accordance with the above
process of the present invention is within the graphical depiction
of FIG. 3. FIG. 3 diagrammatically shows three example arrangements
of discrete photocatalytic particles, particularly titanium dioxide
particles, on the surface of an extender, or carrier, or core
particle so as to form a photoactive antifouling aggregate. FIG. 3a
shows in the direction of the arrow the accumulation of discrete
particles 11 of titanium oxide--by action of a sol coating--so as
to partially cover larger extender particles 21. FIG. 3b shows in
the direction of the arrow the accumulation of irregularly-shaped
discrete flocculates 12 of titanium dioxide particles--again by
action of a sol coating--so as to partially cover extender
particles 21. Finally, FIG. 3c shows agglomerations 13 of discrete
titanium dioxide particles 11 to fully cover the larger extender
particles 21. When it is remembered that even the smallest titanium
dioxide particles--the discrete particles 11 of FIG. 3a--contain
many molecules of TiO.sub.2, normally more than one hundred, it is
clear that the titanium dioxide is agglomerated as nanoparticles,
or spots, or islands. Particularly obvious in FIGS. 3a and 3c--but,
technically, also in FIG. 3c--the coating is not even, and is not
uniform.
[0187] 1.2 Example of the Application of a Nanoparticulate Titanium
Dioxide Coating, Particularly to Silicon Powder
[0188] An example of the process of the invention for the
application of a nanoparticulate titanium dioxide coating is as
follows. The example is for the application of nanoparticulate TiO2
coating to silica powder.
[0189] Additional required chemicals include (vi) Min-U-Sil 5
Silica, U.S. Silica.
[0190] 2.5 ml of HCl 0.15 M is mixed with 5 g silica powder
(Minucel 5 from U.S. Silica, avg. particle size 1.4 microns) to
create a slurry. 2.22 g titania sol from Example 1 is then added to
the slurry. 10 ml NH.sub.4OH 0.1 M is then stirred into the
titania-coated silica slurry to neutralize it to pH 7. The
resulting slurry is then filtered, re-suspended in 25 ml water to
rinse, and then filtered again. The resulting cake is then dried at
130.degree. C. for 30 minutes and then annealed at 650.degree. C.
for 4.5 hours. The resulting powder is silica coated with
approximately 1% by weight nanoparticulate TiO.sub.2. The powder is
photocatalytic which may be measured by the decolorization of the
textile dye Reactive Black 5 as described in I. Arslanin's
"Degradation of commercial reactive dyestuffs by heterogenous and
homogenous advanced oxidation processes: a comparative study" Dyes
and Pigments 43 (1999) 95-108. Examination of the powder using
scanning electron microscopy demonstrates a well-dispersed coating
of nanoparticulate TiO.sub.2 having particle sizes of about 1 nm to
100 nm adhering to the silica particles. For example, see FIG. 4
which is a transmission electron micrograph of a composite
photocatalytic particle having substantially discrete particles of
anatase titanium dioxide dispersed on the surface of a silica
particle created using a compaction milling device.
[0191] 1.3 Example of the Process of Scaling-Up for the Production
of Composite Photocatalytic Particles Containing Nanoparticulate
Titanium Dioxide Upon Their Surface
[0192] An example of the process of the invention for scaling-up
the production of composite photocatalytic particles containing
nanoparticulate titanium dioxide upon their surface is as
follows:
[0193] Scaling up this process for making composite photocatalytic
particles containing nanoparticulate titanium dioxide upon their
surface (hereinafter called Catalytic Power) requires that the
process be made volume efficient, and thus cost efficient. To do
so, washing steps can be modified from a single step into several
steps of smaller charges with intermediate filtering. The main
point is to wash the slurry to remove salts and other contaminants.
This can be broken into smaller washings as necessary.
[0194] Filtering the material from the 6 M HCl creates 2 potential
problems: The first is to find large-scale corrosion resistant
filtering equipment with the necessary personal safety
considerations. The second is how to handle the waste stream.
Typically, in industrial processes, waste streams are neutralized
before going down the sewer so when it hits the waste treatment
plant, they have only small pH adjustments to make and it has
minimal impact on the "bugs".
[0195] To address this problem, an alternative to filtering is to
use a settling tank wherein settled material is drawn from the
bottom of the tank. The time for settling is variously between 12
hours and 36 hours, and most often overnight. It is also possible
to reuse a portion of the HCl (perhaps 50-90% of it) to reduce the
waste stream.
[0196] Additionally, in order to minimize the time on the HCl
filtration step (where the small particle size leads to long
filtration times), one could use an idea analogous to affinity
chromatography. One fills a column with glass beads and pours the
acidic suspension of titania down through it. For small enough
beads and a long enough column, the titania would filter out and
stick to the beads. A pressure gradient through the column would
assist the separation. Once the liquid has passed through, the
beads would then be emptied into a container and tumbled with water
to create the desired sol. The beads would then be removed through
a coarse filter, left to dry, and then reused for the next
separation. The column itself could be coated with teflon to
minimize sticking of titania.
[0197] It has been found that dilute sols (around 1% TiO.sub.2)
lead to greater photocatalytic activity on the coated silica than
more concentrated sols (around 2.3% TiO.sub.2). The trade off is in
manufacturing cost (the amount of waste water generated). A variant
of this method adds a dispersant to the acidic titania sol in order
to improve the distribution of the nanoparticulates on the core
particles. Indeed, the reason the more dilute sols seem to increase
photocatalytic activity (see the next section 2.) may be due to
better distribution of the nanoparticulates on the core
particles.
[0198] The desired % of water in the final filter cake (5%
TiO.sub.2 on Silica) prior to drying is typically 30%+-7%. The
variance is caused by variability in filtration times and pressure
gradient across the filter media: more filtration time or greater
gradient makes the cake drier, less filtration time or less
gradient, wetter. Less moisture is desirable to minimize energy
costs from drying.
[0199] The annealing phase of the process may also be optimized for
economic benefit. Annealing time need be no longer, and temperature
no higher, than required to achieve satisfactory photocatalytic
activity in the finished Catalytic Powders.
[0200] 2. Composite Photocatalytic Particles
[0201] It will be recalled that the present invention has separate,
and severable, aspects relating to composite photocatalytic
particles comprised of a particle core with substantially discrete
photocatalytic particles dispersed onto the surface of the particle
core. Suitable core particles include silicate and carbonate sands
and powders, inorganic pigments, mineral and mineral composites,
construction aggregates including roofing granules, polymeric
granules and mixtures thereof. The photocatalytic particles have an
average diameter size within the range of about 1 nm to 100 nm and
are dispersed on the surfaces of the core particles in an amount of
less than 20 wt. % based on total particle weight. The scope of the
present invention also includes building materials containing these
composite photocatalytic particles and processes for making these
composite particles.
[0202] 2.1 Preparation of Composite Photocatalytic Particles
[0203] The core particles used to make the composite photocatalytic
particles of the present invention can be varied. They may be
rounded, polyhedral, or irregular shaped and produced through
mining, crushing of aggregates, or a manufacturing process for
making polymeric granules or composite polymeric and mineral-based
granules, such as roofing granules. Preferably, the core particles
do not interfere with the photocatalytic action of the composite
particle and do not adversely interact with other components in an
end-use application. One important aspect is the size of the core
particle. It is desirable that the core particle be larger than the
photocatalyst particles. Typically, the average size of the core
particle is within the range of 100 nanometers to 1 centimeter in
diameter, the size being determined by the end-use of the composite
photocatalytic particle.
[0204] Examples of core particles include, but are not limited to
polymer granules and powders such as: acrylics, acrylonitriles,
acrylamides, butenes, epoxies, fluoropolymers, melamines,
methacrylates, nylons, phenolics, polyamids, polyamines,
polyesters, polyethylenes, polypropylenes, polysulfides,
polyurethanes, silicones, styrenes, terephthalates, vinyls; and
inorganic particles of the following, including those in hydrated
form: oxides of silicon, titanium, zirconium, zinc, magnesium,
tungsten, iron, aluminum, yttrium, antimony, cerium, and tin;
sulfates of barium and calcium; sulfides of zinc; carbonates of
zinc, calcium, magnesium, lead and mixed metals, such as naturally
occurring dolomite which is a carbonate of calcium and magnesium,
CaMg(CO.sub.3).sub.2; nitrides of aluminum; phosphates of aluminum,
calcium, magnesium, zinc, and cerium; titanates of magnesium,
calcium, strontium, and aluminum; fluorides of magnesium and
calcium; silicates of zinc, zirconium, calcium, barium, magnesium,
mixed alkaline earths and naturally occurring silicate minerals and
the like; aluminosilicates of alkali and alkaline earths, and
naturally occurring aluminosilicates and the like; aluminates of
zinc, calcium, magnesium, and mixed alkaline earths; hydroxides of
aluminum, diamond; feldspars; or the like and above mixtures or
composites thereof. As used herein, mixtures refer to a physical
mixture of core particles containing more than one type of
particulate form. As used herein, composites refer to intimate
combinations of two or more core materials in a single particle,
such as an alloy, or any other combination wherein at least two
distinct materials are present in an aggregate particle.
[0205] The photocatalyst particles used to make the composite
particles of this invention can be varied. Typically, the average
size of the photocatalyst particle is within the range of 1
nanometer to 100 nanometers, preferably about 1 nanometer to 50
nanometers, and more preferably about 1 nanometers to 10
nanometers. In accordance with the present invention, the
photocatalyst particles form a noncontinuous coating of a discrete
particulate form and can be observed and measured by electron
microscopy such as transmission electron microscopy.
[0206] The photocatalytic particles used to coat the surfaces of
the core particles include one or a combination of two or more of
known metal compound semiconductors such as titanium oxides, zinc
oxides, tungsten oxides, iron oxides, strontium titanates, and the
like. Particularly titanium oxides which have a high photocatalytic
function, a high chemical stability and no toxicity is preferred.
In addition, it is preferred to include inside said photocatalyst
particles and/or on the surfaces thereof at least one metal and/or
a compound thereof selected from the group consisting of V, Fe, Co,
Ni, Cu, Zn, Ru, Rh, Si, Sn, Pd, Ag, Pt and Au as a second component
because of the higher photocatalytic function of the resulting
photocatalyst particles. The aforementioned metal compounds
include, for example, metal oxides, hydroxides, oxyhydroxides,
sulfates, halides, nitrates, and even metal ions. The content of
the second component may vary depending upon the kind thereof.
Preferred photocatalyst particles which may contain the
aforementioned metals and/or metal compounds are of titanium
oxide.
[0207] Preferred photocatalyst particles are anatase titanium
dioxide, zinc oxide, tungsten trioxide, and the above mixtures or
composites thereof. More preferred photocatalyst particles are
mixtures, composites, or alloys of the above oxides with silica
dioxides and tin oxides.
[0208] The amount and size of photocatalyst particles will
influence the surface area and thus impact the oil absorption of
the final composite particle, as described hereinbelow. For
example, larger size photocatalyst particles within the above
prescribed ranges and/or fewer photocatalyst particles can be used
to minimize oil absorption. Typically, the amount of photocatalyst
particles is less than about 20 weight %, based on the total weight
of the composite particle, preferably less than about 10 weight %,
and more preferably less than about 6 weight %. The shape of the
photocatalyst particles can be spherical, equiaxial, rod-like or
platelet. Preferably, the photocatalytic particle is equiaxial or
spherical to minimize oil absorption.
[0209] It is desirable to have a substantially uniform distribution
of the photocatalyst particles on the surfaces of the core
particles. The photocatalyst particles will be attracted to the
core particle surfaces by van der Waals' forces and may be further
anchored to the core particle surfaces by chemical bonding and/or
by hydrous oxide bridges, if hydrous oxides are present on the core
particles as a topcoat.
[0210] Aggregates or agglomerates of photocatalyst particles are
preferably broken down to primary particles to maximize surface
area of the photocatalyst and minimize the amount of photocatalyst
used. Aggregates are distinguished from agglomerates in that
aggregates are held together by strong bonds such as fusion bonds
and cannot be fragmented easily, while agglomerates are weakly
bonded and can be broken up by high energy agitation.
[0211] The composite photocatalyst particles of this invention can
be prepared by a variety of processes. In one process, an aqueous
slurry of core particles is prepared. A colloidal suspension of
photocatalyst particles, i.e., a sol is added to the aqueous core
particle slurry with sufficient mixing. Mixing can be carried out
by any suitable means at a ratio of core particles to
photocatalytic particles which achieves the desired weight % of
discrete particles in the final composite particle product. "Sol"
is defined herein as a stable dispersion of colloidal particles in
a liquid containing about 0.1 to 60% by weight photocatalyst
particles as a dispersion in a liquid typically water. "Colloidal"
is used herein to refer to a suspension of small particles which
are substantially individual or monomeric particles and small
enough that they do not settle. For purposes of this invention, it
is important that the average size of the photocatalytic particles
in the colloidal suspension (i.e., sol) be within the range of
about 1 to about 100 nm (0.001-0.1 microns) in diameter, preferably
about 1 to about 50 nm (0.001-0.05 microns), and more preferably
about 1 to about 50 nm (0.001-0.01 microns). These photocatalytic
particles sizes are generally the same sizes in the final composite
particle product. It is preferred that the colloidal suspension be
at the highest solids concentration at which the suspension is
stable, typically about 14 to 50 wt. % solids. These colloidal
suspensions (sols) can be prepared as known in the art, such as
described in Yasuyuki Hamasaki's "Photoelectrochemical Properties
of Anatase and Rutile Films Prepared by the Sol-Gel Method," 1994,
J. Electrochem. Soc. Vol. 141, No. 3 pp 660-663 and Byung-Kwan
Kim's "Preparation of TiO2-SiO2 powder by modified sol-gel method
and their photocatalytic activities," 1996, Kongop Hwahak, 7(6), pp
1034-1042.
[0212] It has been found that both the particles in the core
particle slurry and the photocatalyst particles in the colloidal
suspension should be preferably both above or both below their
respective isoelectric points to achieve a substantially uniform
surface coating. The "isoelectric point" is used herein to refer to
the pH at which particles have a neutral net charge. The core
particles in the slurry and the photocatalyst particles in the
colloidal suspension may also have opposite charges. Additionally,
if the mixture of core particle slurry and colloidal photocatalyst
particles have low ionic strength and the pH is such that both the
core particles and the photocatalyst particles are both above or
below their isoelectric points, then it is useful to adjust the pH
of the mixture so that either the core particles or the
photocatalyst particles approach their respective isoelectric
points. This additional pH adjustment will generally be necessary
whenever the ionic strength of the mixture is low.
[0213] Alternatively, core particles may be combined with a
reaction mixture which is a precursor for forming a colloidal
suspension of photocatalyst particles. The nano-size photocatalyst
particles are then formed in the presence of the core particles and
deposit onto the core particles. For example, reference U.S. Pat.
No. 5,840,111 wherein a precursor solution comprising sulfuric acid
and titanyl sulfate is combined at elevated temperature to an
alkaline-reacting liquid until the resultant mixture reacts
acidically and forms titanium dioxide nanoparticles.
[0214] Optionally, photocatalyst particles may be adhered to the
core particle by a hydrous oxide bridge. Such hydrous oxides are
silica, alumina, zirconia, and the like. In this process, a dry mix
of core particles containing one or more soluble forms of silica,
alumina, zirconia, and the like, such as sodium silicate, potassium
silicate and sodium aluminate, are combined with an acidic
colloidal suspension of photocatalyst. Suitable acids include HCl,
H.sub.2SO.sub.4, HNO.sub.3, H.sub.3PO.sub.4 or the like.
Alternatively, an alkali colloidal suspension of photocatalyst may
be used in which case the core particles contain aluminum sulfate,
aluminum chloride or other alkali-neutralized soluble forms of
silica, alumina, zirconia, and the like. Suitable bases include
NaOH and KOH. Core particles are added to the colloidal suspension
with high shear mixing. In carrying out the mixing, a high shear
mixer such as a Waring blender, homogenizer, serrated disc type
agitator or the like can be used. Specific speed characteristics
depend on equipment, blade configuration, size, etc., but can be
determined readily by one skilled in the art. The total solids
content (i.e., core and photocatalyst particles) of the resulting
slurry is above about 25% by weight, and above 50% by weight is
preferred. The resulting slurry is then dried.
[0215] Optionally, photocatalyst particles may be adhered to the
core particle by a calcium oxide bridge. In this process, a dry mix
of core particles containing Portland cement, or other similar
cement, in the particle is combined with an acidic colloidal
suspension of photocatalyst. Mixing may be accomplished with a
rotary cement mixer as used by building contractors in the field.
The total solids content (i.e., core and photocatalyst particles)
of the resulting slurry is above about 25% by weight, and above 50%
by weight is preferred. The resulting slurry may then be dried or
used directly as the wet aggregate component for addition to cement
or concrete mixes as known in the art.
[0216] An alternative method of carrying out the invention is
forming an aqueous mixture by combining water with the colloidal
suspension of photocatalyst particles as described above in the
presence of at least one dispersant. The dispersant can be either
added simultaneously with the water or subsequently to the addition
of photocatalyst particles. The dispersant can be selected from
those described in U.S. Pat. No. 5,393,510, the teachings of which
are incorporated herein by reference. Examples of dispersants
include alcohol amines such as 2-amino-2-methyl-1-propanol,
2,2',2"-nitrilotrisethanol, 2,2'-iminobisethanol, 2-aminoethanol
and the like, and 1-amino-2-propanol, polyacrylates, citric acid
and tetrapotassium pyrophosphate (TKPP) and the like. Typically a
combination of the above dispersants is preferred in an amount of
about 0.05 to about 5% based on the core particle weight. The
concentration of photocatalyst particles in the colloidal
suspension is from about 0.1 to 60 weight % preferably about 14 to
50 wt %. It is preferable that the photocatalyst colloidal
particles be well dispersed and not in an aggregate or flocculated
form. As described above, both positive or both negative charges of
the photocatalyst particles in the colloidal suspension and the
core particles are preferred to achieve a substantially uniform
surface coating. Core particles are added to this aqueous mixture
with high shear mixing as described above. The total solids content
(i.e., core and photocatalyst particles) of the resulting slurry is
above about 25% by weight, and above 500 by weight is
preferred.
[0217] The conventional finishing steps such as filtering, washing,
and drying the composite photocatalyst particles are known and are
subsequently carried out. The resulting product is a dry, finished
composite photocatalyst particle which is useful for end-use
applications and/or can be used to prepare a slurry useful for
end-use applications. For example, slurries of silica or carbonate
sands coated with photocatalyst particles can be combined with
Portland cement, or other similar cement, for preparing stucco as
known in the art.
[0218] The resulting composite photocatalyst particles of this
invention are suitable for use as aggregates and fillers for
creating microbe-resistant building products. For example, building
products that may use composite particles of this invention include
stucco, precast concrete, structural cement, swimming pool cement,
cementatious coatings, grout, roofing shingles, textured and
abrasion resistant coatings, and other building products. The
enhanced microbe resistance is demonstrated under conditions where
light is present.
[0219] To give a clearer understanding of the invention, the
following Examples are construed as illustrative and not limitative
of the underlying principles of the invention in any way
whatsoever.
[0220] 2.2 First Example of A Composite Photocatalytic Particle
[0221] A pure strain of green algae was inoculated into liquid
growth media with 5% by weight 1.4 micron average diameter silica
powder (the control) and also into identical media mixed with 5% by
weight silica powder coated with 5% by weight nanoparticulate
anatase titanium dioxide. The composite photocatalytic particle was
prepared using the method detailed in Comparative Example 1.2. The
mixtures were placed in two stirred flasks and exposed for three
days under cool white fluorescent light at 450 foot-candles. The
amount of algae growth in each flask was then measured using
absorbance normalized at 480 nm. Normalized on a 0 to 1 scale,
absorbance at 480 nm averaged 0.08 for the media containing
photocatalytic powder verses 1 for the media containing regular
powder.
[0222] A bar chart illustrating the algae-inhibiting effect of
photoactive antifouling aggregate comprising 25 weight %
non-colloidal photoactive zinc oxide and 75 weight % colloidal
anatase titanium dioxide is shown in FIG. 5.
[0223] A bar chart showing the inhibiting effect of an the
aggregate of FIG. 5 on the growth of E. coli bacteria is shown in
FIG. 6.
[0224] 2.3 Second Example of A Composite Photocatalytic
Particle
[0225] E. coli was inoculated onto a polyester resin coating mixed
with 20% by weight solids 1.4 micron average diameter silica powder
(the control) and also onto an identical coating mixed with 20% by
weight solids silica powder prepared as in Comparative Example 2.2.
After twenty-four hours of exposure under cool white fluorescent
light at 450 foot-candles, the polyester films were imprinted onto
agar plates and the agar left to colonize over 12 hours. The number
of colonies that grew on the agar plates were then counted.
Normalized on a 0 to 1 scale, the number of E. coli colonies
observed averaged 0.03 for the treated polyester resin versus 1 for
the untreated resin.
[0226] 3.0 Photocatalytic Aggregates
[0227] The extender particles used to make the composite aggregate
particles of this invention can be varied. They may be rounded,
polyhedral, or irregular shaped and produced through mining,
grinding of minerals, or synthetic methods. Preferably, the
extender particles do not interfere with the photocatalytic action
of the composite aggregate and do not adversely interact with other
components in an end-use application. One important aspect is the
size of the extender particle. It is desirable that the extender
particle have an average size of 100 nanometers to 1 centimeter and
that the extender particle be larger than the photocatalyst
particles.
[0228] Examples of extender particles include, but are not limited
to inorganic particles of the following, including those in
hydrated form: oxides of silicon, titanium, zirconium, zinc,
magnesium, tungsten, iron, aluminum, yttrium, antimony, cerium, and
tin; sulfates of barium and calcium; sulfides of zinc; carbonates
of zinc, calcium, magnesium, lead and mixed metals, such as
naturally occurring dolomite which is a carbonate of calcium and
magnesium, CaMg (CO.sub.3).sub.2; nitrides of aluminum; phosphates
of aluminum, calcium, magnesium, zinc, and cerium; titanates of
magnesium, strontium, calcium, and aluminum; fluorides of magnesium
and calcium; silicates of zinc, zirconium, calcium, barium,
magnesium, mixed alkaline earths and naturally occurring silicate
minerals and the like; aluminosilicates of alkali and alkaline
earths, and naturally occurring aluminosilicates and the like;
aluminates of zinc, calcium, magnesium, and mixed alkaline earths;
hydroxides of aluminum, diamond; feldspars; natural and synthetic
clays; wollastonite; or the like and above mixtures or composites
thereof. As used herein, mixtures refer to a physical mixture of
extender particles containing more than one type of extender
material form. As used herein, composites refer to intimate
combinations of two or more extender materials in a single extender
particle, such as an alloy, or any other combination wherein at
least two distinct materials are present in an aggregate extender
particle.
[0229] The photocatalytic titanium oxide is exposed on the surface
of the extender particle in the form of discrete particles. The
discrete particles may form small agglomerates, such as flocculated
particles, on the surface of the aggregate particle, but this is
less desirable because some discrete particles will then be shaded.
The discrete particles typically have an average size within the
range of 1 nanometer to 100 nanometers, preferably about 1
nanometers to 50 nanometers, and more preferably about 1 nanometers
to 10 nanometers. The discrete particles can be observed and
measured by electron microscopy such as scanning electron
microscopy.
[0230] The photocatalyst used to make the composite aggregate
particles of this invention are titanium oxides which have a high
photocatalytic function, a high chemical stability and no toxicity.
More particularly preferred is the anatase crystalline form of
titanium dioxide.
[0231] It is desirable to have a substantially uniform, although
not necessarily continuous, distribution of discrete photocatalyst
particles on the surfaces of the aggregate particles. Typically,
the amount of photocatalyst is less than 20 weight % based on the
total weight of the aggregate material, preferably less than 10
weight %, and more preferably less than 6 weight %.
[0232] The photocatalyst material will be attracted to the extender
particle surfaces by van der Waals' forces and may be further
anchored to the extender material surfaces by stronger chemical
bonds such as fusion bonds. It has been found that flocculation of
photocatalyst particles reduces photocatalytic efficiency, likely
due to optical crowding effects, and is generally undesirable.
[0233] The aggregates of this invention generally disperse easily
in aqueous and solvent-based slurries, coatings, and solutions.
Unlike virgin photocatalyst, dispersion does not generally require
the use of chemical dispersing aides or aggressive agitation or
milling.
[0234] 3.1 Preparation of Photoactive Antifoulant Aggregates
[0235] The photoactive antifoulant aggregates of this invention can
be prepared by a variety of processes. In one process, an aqueous
slurry of extender particles is prepared. To this slurry is added,
with sufficient mixing, a colloidal suspension, i.e. a sol, of
titanium oxide particles. Mixing can be carried out by any suitable
means at a ratio of extender particles to photocatalytic particles
which achieves the desired weight % of premium photocatalyst in the
final aggregate. "Sol" is defined herein as a stable dispersion of
colloidal particles in a liquid containing about 0.1 to 60% by
weight particles as a dispersion in a liquid typically water.
"Colloidal" is used herein to refer to a suspension of small
particles which are substantially individual or monomeric particles
and small enough that they do not settle. The photocatalyst
particle sizes are generally the same sizes at the start of the
process as in the final aggregate particle product. It is preferred
that the colloidal suspensions of photocatalyst be at the highest
solids concentration at which the suspension is stable, typically
about 14 to 50 weight % solids. These colloidal suspensions (sols)
can be prepared as known-in the art, such as described in U.S. Pat.
No. 5,840,111; Yasuyuki Hamasaki's "Photoelectrochemical properties
of anatase and rutile films prepared by the sol-gel method," 1994,
J. Electrochem. Soc. Vol. 141, No. 3 pp 660-663; and/or Byung-Kwan
Kim's "Preparation of TiO2-SiO2 powder by modified sol-gel method
and their photocatalytic activities," 1996, Kongop Hwahak, 7(6), pp
1034-1042.
[0236] It has been found that the particles in the extender
particle slurry and the photocatalyst particles in the colloidal
suspension should both be preferably above or below their
respective isoelectric points to achieve a substantially uniform
surface coating of the smaller colloidal particles on the larger
slurry particles. The "isoelectric point" is used herein to refer
to the pH at which particles have a neutral net charge. The
particles in slurry form and the particles in colloidal suspension
may also have opposite charges. Additionally, if the mixture of
slurry and colloidal particles have low ionic strength and the pH
is such that the extender particles and photocatalyst particles are
both above or below their isoelectric points, then it is useful to
adjust the pH of the mixture so that one of the particles
approaches its isoelectric point. This additional pH adjustment
will generally be necessary whenever the ionic strength of the
mixture is low.
[0237] In applications in which acid excesses have a disruptive
effect, the colloidal suspensions according to the invention may
subsequently be stabilized in the neutral pH range in a manner
known in principle, for example with acetylacetone (see, e.g.,
WO-93/05875) or with hydroxycarboxylic acids (see, e.g., EP-A518
175).
[0238] In an alternative preparation process, extender particles
may be added to a solution containing a soluble form of a titanium
oxide precursor and then an acid or base added to reactively coat
the extender particles in situ with discrete photocatalyst
particles to make the aggregate particles of this invention. For
example, in U.S. Pat. No. 5,840,111 Wiederhoft describes a
precursor solution comprising sulfuric acid and titanyl sulfate.
Extender particles may be added to this precursor solution and then
an alkaline-reacting liquid added, with sufficient mixing, until
the resultant mixture reacts acidically and forms a coating of
discrete titanium dioxide particles on the extender particles.
[0239] The conventional finishing steps such as filtering, washing,
drying and grinding the aggregate antifouling product are known and
are subsequently carried out. The resulting product is a dry,
finished aggregate photocatalyst particle which is useful for
end-use applications and/or can be used to prepare a slurry useful
for end-use applications. Methods of preparing particulate slurries
are known in the art, for example, as described in Canadian Patent
935,255.
[0240] Alternatively, titanium oxide particles may be adhered to
the extender particle by stronger chemical bonds such as fusion
bonds. In one embodiment of this process, a dry mix of extender
particles containing one or more soluble forms of silica, alumina,
zirconia, and the like, such as sodium silicate, potassium silicate
and sodium aluminate, are combined with an acidic colloidal
suspension of photocatalyst, such as the titania sol described
earlier. Suitable acids include HCl, H.sub.2SO.sub.4, HNO.sub.3,
H.sub.3PO.sub.4 or the like. Alternatively, a basic colloidal
suspension of photocatalyst may be used in which case the extender
particles contain aluminum sulfate, aluminum chloride or other base
neutralized soluble forms of silica, alumina, zirconia, and the
like. Suitable bases include NaOH and KOH. Extender particles are
added to the colloidal suspension with sufficient mixing. The total
solids content (i.e., extender and titanium oxide particles) of the
resulting slurry is above about 25% by weight, and above 50% by
weight is preferred.
[0241] An alternative method of carrying out the invention is
forming an aqueous mixture by combining water with the colloidal
suspension of titanium oxide in the presence of at least one
dispersant. The dispersant can be either added simultaneously with
the water or subsequently to the addition of titanium oxide
particles. The dispersant can be selected from those described in
U.S. Pat. No. 5,393,510, the teachings of which are incorporated
herein by reference. Examples of dispersants include alcohol amines
such as 2-amino-2-methyl-1-propanol, 2,2',2"-nitrilotrisethanol,
2,2'-iminobisethanol, 2-aminoethanol and the like, and
1-amino-2-propanol, polyacrylates, citric acid and tetrapotassium
pyrophosphate (TKPP) and the like. Typically a combination of the
above dispersants is preferred in an amount of about 0.05 to about
5% based on the aggregate particle weight. The concentration of
particles in colloidal suspension is from about 0.1 to 60 weight %,
preferably about 14 to 50 weight %, and in slurry form above 25
weight %, and above 50 weight % preferred. It is preferable that
the particles be well dispersed and not in an aggregate or
flocculated form. As described above, all positive or all negative
charges of the titanium oxide particles and the extender particles
are preferred to achieve a substantially uniform surface coating.
Extender particles are added to this aqueous mixture with high
shear mixing or milling as described in greater detail in Canadian
Patent 935,255, U.S. Pat. Nos. 3,702,773 and 4,177,081, the
teachings of which U.S. patents are incorporated herein by
reference. In carrying out the mixing, a high shear mixer or mill
such as a Waring.TM. blender, homogenizer, serrated disc type
agitator, ball mill, sand mill, disc mill, pearl mill, high speed
impeller mill or the like can be used. (Waring.TM. is a registered
trademark of the Waring Corporation.) Specific speed
characteristics depend on equipment, blade configuration, size,
etc., but can be determined readily by one skilled in the art. The
total solids content (i.e., extender and photocatalyst particles)
of the resulting slurry is above about 25% by weight, and above 50%
by weight is preferred.
[0242] 3.2 Action of the Antifouling Aggregates So Produced
[0243] The resulting improved photoactive antifoulant aggregate
products of this invention are suitable for use in coatings and
building products, for example, in antifoulant coatings, stucco,
swimming pool cement, grout, concrete, wall shingles, hard
flooring, and roofing granules. The antifouling activity is best
demonstrated in products where the surface concentration of exposed
photoactive aggregate is greater than 1%, preferably greater than
5%, and more preferably greater than 10%. Surface concentration is
expressed as a percentage and represents the volume of the
photoactive aggregate at the active surface divided by the sum of
the volumes of the photoactive aggregate at the active surface and
the carrier at the active surface. Antifouling activity is observed
only when U.V. or visible light is present to expose the
photoactive aggregate. Photoactive aggregate present in the body of
the coating or building product but not exposed at the surface does
not contribute to antifouling activity. Polymeric binders subject
to photocatalytic attack, such as acrylic and polyester resin,
chalk over time from contact with the photoactive aggregates of
this invention in the presence of U.V. or visible light.
Photocatalytic chalking from photoactive pigments is well known in
the painting industry, and such chalking exposes pigment particles
in the paint. In the present invention, chalking exposes more
antifouling aggregate and thus improves the antifouling activity of
the coating. Where chalking is undesirable in the coating,
alternative resins may be employed such as silicones and
fluoropolymers as described in further detail in U.S. Pat. Nos.
5,547,823 and 5,616,532, the teachings of which are incorporated
herein by reference.
[0244] In accordance with the preceding explanation, variations and
adaptations of the method of producing and of using a
nanoparticulate titanium dioxide coating in accordance with the
present invention will suggest themselves to a practitioner of the
chemical arts.
[0245] In accordance with these and other possible variations and
adaptations of the present invention, the scope of the invention
should be determined in accordance with the following claims, only,
and not solely in accordance with that embodiment within which the
invention has been taught.
* * * * *