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PHOTOCATALYST PROCESS FOR MAKING SURACE HYDROPHILLIC
Technical Field
The present invention relates to a method for
hydrophilifying the surface of articles (i.e., rendering the
surface of articles hydrophilic) by the action of a
photocatalyst, and also to a composite with a
hydrophilifiable surface formed of a photocatalyst. It can
be utilized for antifogging, antifouling, and selfcleaning
of articles and other applications.
Background Art
The present inventor has previously proposed a method
for highly hydrophilifying the surface of articles by the
action of a photocatalyst (International Publication No.
W096/29375). According to this method, a coating of a
semiconductor photocatalyst, such as the anatase form of
titania, is provided on the surface of an article.
Photoexcitation of this photocatalytic coating by exposure
to light having a satisfactory intensity for a satisfactory
period of time permits the surface of the photocatalytic
coating to be highly hydrophilified to such an extent as
will make a contact angle with water of about 0°.
As disclosed in W096/29375, the above highly
hydrophilifiable photocatalytic coating is applicable to
various articles for antifoggi.ng, antifouling,
selfcleaning and other various purposes. For example, when
the photocatalytic coating is provided on a transparent
article, such as a windshield for a vehicle, a windowpane
for a building, or an eyeglass lens, or a mirror, the
surface of the coating is highly hydrophilified upon
photoexcitation of the photocatalyst:, preventing the
article from being fogged by moisture oondensate or steam
or from being blurred by water droplets adhering on the
surface thereof. Further, when a building or an article,
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which is disposed outdoors, is provided with the
photocatalytic coating, oil repellent or hydrophobic dust
and contaminants adhering on the hydrophilified surface are
washed away by raindrops every time they are exposed to
rainfall, thus permitting the surface to be selfcleaned.
Various light sources capable of emitting light having
higher energy than the band gap energy of a photocatalyst,
a semiconductor, are utilized for the photoexcitation of
the photocatalyst. In the case of a photocatalyst, of
which the photoexcitation wavelength is in the ultraviolet
region, such as titania, ultraviolet light is necessary for
the photoexcitation of such a photocatalyst. In this case,
when the article is in such a state as will be exposed to
the sunlight, the photocatalyst can be advantageously
photoexcited by ultraviolet light contained in the
sunlight.
So far as the photoexcitation continues, the surface
of the photocatalytic coating permanently retains its
hydrophilic property. Interruption of the photoexcitation
results in gradual decreased hydrophilicity of the surface
of the photocatalytic coating. This is considered
attributable to the fact that the surface of the
photocatalytic coating is gradually contaminated with a
hydrophobic material. As soon as the photocatalyst is
photoexcited again, the hydrophilicity is recovered.
In the case of photoexcitation in a discontinuous
manner like photoexcitation of the photocatalyst by the
sunlight, the hydrophilicity of the surface of the
photocatalytic coating is attenuated upon the interruption
of the photoexcitation, while the resumption of the
photoexcitation results in recovery of the hydrophilicity.
Thus, the attenuation and the recovery of the
hydrophilicity are alternately repeated.
A primary object of the present invention is to
improve the above conventional method, enabling a surface
to be hydrophilified more easily.
Another object of the present invention is to improve
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the above conventional method, enabling a surface to be
highly hydrophilified upon exposure to weaker light, that
is, lower irradiation intensity.
Still another object of the present invention is to
S provide a method which, even when photoexcitation is
interrupted or when an article is placed in the dark,
enables a high level of hydrophilicity imparted to a
surface to be kept for as long a period of time as
possible.
A further object of the present invention is to
provide a method which, when photoexcitation is resumed
after interruption of the photoexcitation, enables the
hydrophilicity of a surface to be recovered upon exposure
to light for a short period of time or upon exposure to
weak light.
A still further object of the present invention is to
provide a composite for use in practice of the above
methods.
Disclosure of Invention
It is known that the surface energy ys in the
interface of a solid and a gas is constituted by three
components, a molecular dispersion force Ysd, a dipole
moment component YSp, and a hydrogen bond component ysh.
The present inventor has found that hydrophilification
of the surface of a photocatalytic coating upon
photoexcitation of a photocatalyst gives rise to a
significant increase in only the hydrogen bond component
ysh among the three components in the surface energy Ys'
The present inventor has further found that the
hydrophilification of the photocatalytic coating is
attributable to the physical adsorption of water on the
surface of the photocatalytic coating by photocatalytic
action.
This invention has been made based on such finding,
and, according to the present invention, a photocataiytic
coating containing a photocatalyst is provided on a
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substrate. Upon photoexcitation of the photocatalyst by
irradiation of the photocatalytic coating with light, the
photocatalytic action brings about an increase in the
hydrogen bond component ysh in the surface energy ys in the
solid/gas interface of the photocatalytic coating,
accelerating the physical adsorption of molecules of water
through a hydrogen bond, which results in the formation of
a high-density physically adsorbed water layer on the
surface of the photocatalytic coating.
Thus, the formation of a physically adsorbed water
layer on the surface of the photocatalytic coating
facilitates a high level of hydrophilification. By virtue
of the presence of the physically adsorbed water layer, the
hydrophilicity of the surface of the photocatalytic coating
is kept for a long period of time even after the
interruption of photoexcitation, minimizing the attenuation
of the hydrophilicity. When the photocatalyst is
photoexcited again, the hydrophilicity of the surface can
be easily recovered upon exposure of the surface to light
for a short period of time or upon exposure of the surface
to weak light.
According to the present inventor's finding, the
hydrophilicity of the surface of the photocatalytic coating
is related to the hydrogen bond component ysh in the
surface energy. Therefore, according to a preferred
embodiment of the present invention, the solid material
that increases a hydrogen bond component ysh in the surface
energy in the solid/gas interface is borne on the
photocatalytic coating.
Since this enhances the hydrogen bond component Y h
S
inherent in the surface energy of the photocatalytic
coating (i.e., during non-excitation of the photocatalyst),
the hydrogen bond component in the surface energy during
photoexcitation of the photocatalyst is further enhanced
accordingly. This in turn permits the formation of the
physically adsorbed water layer by the photocatalytic
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action to be further promoted. In addition, a reduction
in physically adsorbed water layer upon interruption of the
photoexcitation is delayed.
Materials that increase a hydrogen bond component Ysh
5 in the surface energy of the photocatalytic coating include
solid acids which serve as a proton donor (Br~pnsted
acid) or as an electron acceptor (Lewis acid), and solid
bases which serve as an electron donor (Lewis base) or as
a proton acceptor (Br~nsted base). These solid acids or
solid bases per se have a high hydrogen bond component ysh
and, hence, when borne on the surface of the photocatalytic
coating, enhance the hydrogen bond component ysh in the
surface of the photocatalytic coating.
Such solid acids include, for example, metal oxides
bearing sulfuric acid, metal oxides bearing nitric acid,
compound oxides of metals, and A1203~Si02. Compound oxides
of metals include metal oxide superacids,
Ti02/W03,W03/Zr02, and W03/Sn02. The oxide superacid is
defined as a solid oxide having higher acid strength than
100% sulfuric acid and has an acid strength of Ho <_ -11.93
wherein Ho represents the Hammett acidity function.
The present inventor has further found that the
contact angle of the solid surface with an oil in water
increases with increasing the hydrogen bond component Ysh
in the surface energy in the surface of the photocatalytic
coating.
Therefore, according to another aspect of the present
invention, there is provided a method for improving the oil
repellency of the surface of a substrate in water.
According to the present invention, a photocatalytic
coating containing a photocatalyst is provided on a
substrate. Upon photoexcitation of the photocatalyst by
irradiation of the photocatalytic coating with light, the
hydrogen bond component ysh in the surface energy ys in the
solid/gas interface of the photocatalytic coating is
enhanced by photocatalytic action, improving the oil
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repellency of the surface of the substrate in water.
In another aspect, the present invention provides a
method for hydrophilifying the surface the surface of a
substrates comprising the steps of providing a substrate
coated with a solid layer having an interface with air, and
containing a photocatalyst; and photoexciting the
photocatalyst to permit molecules of water to be physically
adsorbed onto the surface of said layer under the
photocatalytic action of said photoctalyst, thereby
hydrophilifying the surface of said substrate.
In another aspect, the present invention provides a
method for enhancing the oil repellency of the surface of a
the substrate in water, comprising the steps of providing a
substrate coated with a solid layer having an interface with
air, and containing a photocatalyst; and photoexciting the
photocatalyst to increase a hydrogen bond component, in the
surface energy in the solid/gas interface of said layer under
the photocatalytic action of said photocatalyst, thereby
enhancing the oil repellency of the surface of the substrate
when placed in water.
In another aspect, the present invention provides a
method for cleaning a substrate comprising the steps of
providing a substrate coated with a solid layer having an
interface with air, and containing a photocatalytic
semiconductor material; photoexciting the photoctalyst to
enhance a hydrogen bond component (75h) in the surface energy
in the solid/gas interface of said layer tinder the
photoctalytic action of said photocatalyst, thereby enhancing
the oil repellency of the surface of the substrate when
placed in water; and immersing the substrate in water or
wetting the substrate with water to release an oil stain
adhering on the surface of the substrate.
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In another aspect, the present invention provides a
composite with a hydrophilifiable surface comprising:
a substrate; a solid layer provided on the surface of the
substrate and having an interface with air, said layer
containing a photocatalyst; and a layer of molecules of water
physically adsorbed onto the surface of said layer containing
a photocatalyst in response to the photoexcitation of the
photocatalyst.
In another aspect, the present invention provides a
composite with a surface adapted to be rendered oil
repellent, comprising:
a substrate; a solid layer provided on the surface of the
substrate and having an interface with air, said layer
containing a photocatalyst and adapted to increase the
hydrogen bond component (ysh) in the surface energy at said
interface in response to photoexcitation of said
photocatalyst, thereby increasing the oil repellency of the
surface of the composite in water.
In another aspect, the present invention provides a
composite with an easily cleanable surface comprising: a
substrate; and a solid layer provided on the surface of the
substrate and having an interface with air, said layer
containing a photocatalyst, said layer operating to increase
the hydrogen bond component (ysh) in the surface energy at
said interface in response to photoexcitation of the
photocatalyst, thereby enhancing the oil repellency of the
surface of the composite in water, whereby, upon immersion of
the composite in water or wetting of the composite with
water, an oil stain adhering on the surface of said layer is
released therefrom.
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In another aspect, the present invention provides a
method for hydrophilifying the surface of a substrate
comprising the steps of: providing a substrate coated with a
solid layer having an interface with air, and containing a
photocatalyst and a solid acid; and photoexciting the
photocatalyst to permit molecules of water to be physically
adsorbed onto the surface of said layer under the
photocatalytic action of said photoctalyst, thereby
hydrophilifying the surface of said substrate, wherein the
solid acid increases a hydrogen bond component (Ysh) in surface
energy of the interface between said layer and air.
In another aspect, the present invention provides a
method for enhancing the oil repellency of the surface of a
substrate in water, comprising the steps of: providing a
substrate coated with a solid layer having an interface with
air, and containing a photocatalyst and a solid acid; and
photoexciting the photocatalyst to increase a hydrogen bond
component (YS'') , in surface energy in a solid/gas interface of
said layer under the photocatalytic action of said
photocatalyst, thereby enhancing the oil repellency of the
surface of the substrate when placed in. water, wherein the
solid acid increases the hydrogen bond component in the
surface energy.
In another aspect, the present invention provides a
method for cleaning a substrate, comprising the steps of:
providing a substrate coated with a solid layer having an
interface with air, and containing a photocatalytic
semiconductor material and a solid acid; photoexciting the
photocatalyst to enhance a hydrogen bond component (Ygh) in
surface energy in a solid/gas interface of said layer under
the photocatalytic action of said photocatalyst, thereby
enhancing the oil repellency of the surface of the substrate
when placed in water, wherein the solid acid increases a
hydrogen bond component in the surface energy; and immersing
the substrate in water or wetting the substrate with water to
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release an oil stain adhering on the surface of the substrate.
In another aspect, the present invention provides a
composite with a hydroplilifiable surface, comprising: a
substrate; a solid layer provided on the surface of the
substrate and having an interface with air, said layer
containing a photocatalyst and a solid acid; and a layer of
molecules of water physically adsorbed onto the surface of
said layer containing a photocatalyst in response to the
photoexcitation of the photocatalyst, wherein the solid acid
increases a hydrogen bond component (Ysh) in surface energy of
the interface between said layer and air.
This method for improving the oil repellency of a surface
in water can be utilized for cleaning a substrate stained with
an oil. Specifically, when a photocatalytic coating with an
oil adhering thereon is immersed in or wetted with water, the
oil stain is easily released and removed from the surface of
the photocatalytic coating without use of any detergent. The
foregoing and other features and advantages of the present
invention will become more apparent in the
light of the following description of embodiments.
Brief Description of the Drawings
Figs. 1A to 1C are graphs showing a change in contact
angle of the surface of a photocatalytic: coating with water
upon irradiation of the surface of the photocatalytic coating
with ultraviolet rays having different wavelengths as a
function of light irradiation time;
Figs. 2A and 2B, Figs. 3A and 3B, Figs. 4A and 4B, Figs.
5A and 5B, and Figs. 6A and 6B are each an infrared absorption
spectrum of the surface of a photocatalytic coating;
Fig. 7 is a microscopically enlarged, schematic
crosssectional view, of a solid/gas interface of a
photocatalytic coating, illustrating the physical adsorption
of molecules of water on the surface of the photocatalytic
coating by photocatalytic action;
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Fig. 8 is a microscopically enlarged, schematic
crosssectional view of a solid/gas interface of a
photocatalytic coating bearing sulfuric acid;
Fig. 9 is a graph showing a change i.n contact angle of a
photocatalytic coating, bearing sulfuric acid or not bearing
sulfuric acid, with water, when allowed to stand in a dark
place, as a function of standing time in the dark place, in a
certain working example;
Fig. 10 is a graph showing a change in contact angle of a
photocatalytic coating with water upon rephotoexcitation as a
function of light irradiation time;
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Fig. 11 is a diagram, similar to Fig. 8, illustrating
the physical adsorption of molecules of water onto a bridge
OH group present on the surface of titania bearing sulfuric
acid; and
Fig. 12 is a diagram, similar to Fig. 8, illustrating
bonding of molecules of water onto a titanium atom to the
surface of titanic bearing sulfuric acid.
Best Mode for Carrying Out the Invention
The hydrophilic photocatalytic coating may be provided
on various articles according to purposes.
When antifogging is contemplated for
eliminating optical problems caused by adherence of
moisture condensate or water droplets, a hydrophilic
photocatalytic coating may be provided on the following
articles: windowpanes for buildings; windowpanes and
windshields for vehicles and vessels, such as automobiles,
railway vehicles, aircrafts, watercrafts, and submarines;
mirrors, such as rearview mirrors for vehicles, bathroom
or lavatory mirrors, dental mouth mirrors, and reflecting
mirrors such as used in roads; lenses, such as eyeglass
lenses, optical lenses, photographic lenses, endoscopic
lenses, and lighting lenses; shields of goggles or masks
( including diving masks ) for protection and sports; shields
of helmets; and cover glasses for measuring instruments.
Further, provision of a hydrophilic photocatalytic
coating on the surface of a building, a construction,
machinery, or an article, which is disposed outdoors,
permits the surface thereof to be selfcleaned.
Furthermore, provision of a photocatalytic coating on an
article having a fear of coming into contact with dust or
an exhaust gas can prevent the adherence of hydrophobic
dust on the surface of the article.
When utilization of the oil repellency of a
photocatalytic coating in water is contemplated for simply
removing an oil stain, the photocatalytic coating may be
provided on machinery and parts, tableware, kitchen
utensils or other articles which are likely to be stained
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with an oil.
Photocatalyst
Titania (Ti02) is most preferred as t:he photocatalyst for
a photocatalytic coating. Titania is harmless, chemically
stable, and inexpensive. Further, the band gap energy of
titania is so high that titania requires ultraviolet light for
photoexcitation and does not absorb visible light in the
course of photoexcitation, causing no color development
derived from a complementary color component. The
photocatalytic coating using titania as a photocatalyst is
suitable particularly as a coating for transparent members
such as glass, lenses, and mirrors. Although the rutile form
of titania also is usable, the anatase form of titania is
preferred. The anatase form of titania is advantageous in that
a sol containing very fine particles dispersed therein may be
easily commercially available which can easily form a very
thin film. When a highly hydrophilifiable photocatalytic
coating is desired, the use of a nitric <acid peptization type
titania sol is preferred.
Other photocatalysts usable herein include metal oxides
such as ZnO, Sn02, SrTi03, W03, Bi203, and :Ee203. The surface of
these metal oxide photocatalysts is considered to be easily
hydrophilified because, as with titania, each of these metal
oxide photocatalysts has a metal element and oxygen on its
surface.
Formation of Photocatalytic Coating
A photocatalytic coating may be provided on a
substrate by various methods disclosed in W096/29375.
Briefly speaking, one preferred method for the formation of
a photocatalytic coating, possessing excellent abrasion
resistance and highly hydrophilifiable to such an extent as
will make a contact angle with water of 0°, on a
substrate made of a heat-resistant rr~aterial, such as a
metal, a ceramic, or glass, is to fir~~t coat the surface
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of the substrate with amorphous titanic by hydrolysis and
dehydration polycondensation of an organotitanium compound,
for example, tetraethoxytitanium, followed by firing at a
temperature of 400 to 600°C to transform the amorphous
titanic into crystalline titanic (anatase)_
Another preferred method for the formation of a
photocatalytic coating possessing excellent abrasion
resistance and highly hydrophilifiable to such an extent
as will make a contact angle with water of 0° is to
incorporate silica or tin oxide into photocatalytic
titanic.
Still another preferred method for the formation of
a photocatalytic coating superhydrophilifiable to such an
extent as will make a contact angle with water of 0°, on
a substrate made of a non-heat-resistant material, such as
a plastic, or a substrate coated with an organic paint is
to use a composition, for a paint, comprising a coating
forming element of an uncured or partially cured silicone
(organopolysiloxane) or a precursor of a silicone and
photocatalyst particles dispersed in the element.
As disclosed in W096/29375, when the above composition
for a paint is coated on the surface of a substrate to form
a coating which is then subjected to curing of the coating
forming element to form a silicone coating followed by
photoexcitation of the photocatalyst, an organic group
bonded to a silicon atom of the silicone molecule is
substituted by a hydroxyl group through the photocatalytic
action of the photocatalyst, rendering the surface of the
photocatalytic coating superhydrophilic.
Bearing of Surface Energy Enhancer
A solid acid or a solid base may be borne on the
photocatalytic coating to enhance the hydrogen bond
component in the surface energy of the photocatalytic
coating, thereby accelerating the physical adsorption of
water.
For example, sulfuric acid or nitric acid may be borne
as the solid acid by previously providing a photocatalytic
CA 02241059 1998-06-19
titania coating on a substrate, coating sulfuric acid or
nitric acid on the photocatalytic titania coating, and
conducting heat treatment at a temperature of about 400 to
600 ° C. This causes a sulfonic or nitric group to be bonded
S to a titanium atom present on the surface of titanic,
enhancing the hydrogen bond component in the surface
energy. Sulfonic acid or picric acid may be used instead
of sulfuric acid or nitric acid.
Alternatively, a compound oxide of metals or
10 A1203~Si02 may be borne as the solid acid. In this case,
Ti02/WO~, when fired at a temperature of 600 to 800°C,
exhibits the highest acidity and, at that time, has
a Hammett acidity function Ho of -13 to -14; W03/SnO~, when
fired at a temperature of 900 to 1100°C, exhibits the
highest acidity and, at that time, has a Hammett acidity
function Ho - -13 to -14; W03/Zr02, when fired at a
temperature of 700 to 900°C, exhibits the highest acidity
and, at that time, has a Hammett acidity function Ho of -13
to -15; W03/Fe~O~, when fired at a temperature of 600 to
800°C, exhibits the highest acidity and, at that time, has
a Hammett acidity function of Ho of -12 to -13; and
A1203~SiO~, when fired at a temperature of 400 to 600'C,
exhibits the highest acidity and, at that time, has
a Hammett acidity function Ho of -12 to -13.
Therefore, TiO~/W03, W03/ZrO~, and W03/SnO~ are
preferred from the viewpoint of the magnitude of the
acidity.
On the other hand, when the formation of a
photocatalytic coating on a glass substrate is
contemplated, preferred is A1~03~SiO~ which, when fired at
a temperature of 400 to 600°C which does not significantly
soften the glass substrate, exhibits the highest acidity.
When a compound oxide of metals is borne as the solid
acid, it is possible to use oxide particles or a metallic
acid containing at least part of metallic elements
constituting the compound oxide of metals. In this case,
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a photocatalytic titanic coating is previously provided on
a substrate, the oxide particles or metallic acid is coated
on the photocatalytic coating, and firing is performed at
such a temperature that the compound oxide of metals
S exhibits high acidity.
Light Source for Excitation of Photocatalyst
Photoexcitation of a photocatalyst in the
photocatalytic coating requires irradiation of the
photocatalyst with light at a wavelength having higher
energy than the band gap energy of the photocatalyst which
is a semiconductor. Ultraviolet light is necessary for the
excitation of some photocatalysts. For example,
photoexcitation is possible with ultraviolet light at a
wavelength of not more than 387 nm for the anatase form of
titanic, at a wavelength of not more than 413 nm for the
rutile form of titanic, at a wavelength of not more than
344 nm for tin oxide, and at a wavelength of not more than
387 nm for zinc oxide.
In the case of a photocatalyst, of which the
excitation wavelength is in the ultraviolet region, such
as titanic, sources usable for ultraviolet light include
an ultraviolet lamp, a mercury lamp, and a metal halide
lamp. Further, weak ultraviolet light contained in light
emitted from room lamps, such as fluorescent lamps and
incandescent lamps, also can excite the photocatalyst.
In the case of articles in such a state as will be
exposed to the sunlight, such as windowpanes for buildings,
rearview mirrors, and articles which are disposed outdoors,
the photocatalyst can be advantageously photoexcited by
taking advantage of ultraviolet light contained in
sunlight.
Hydrophilification of Surface
Photoexcitation of a photocatalyst by irradiation of
the photocatalytic coating with light permits the surface
3S of the photocatalytic coating to be highly hydrophilified.
Interruption of the photoexcitation results in gradual
attenuation of the hydrophilicity of the photocatalytic
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coating, and the hydrophilicity is recovered upon re-
photoexcitation. For example, in the case of excitation
of the photocatalyst in the photocatalytic coating provided
on the surface of an article by the sunlight, the surface
of the photocatalytic coating is highly hydrophilized
during exposure to the sunlight in the daytime, the
hydrophilicity lowers but is retained on a certain
satisfactory level in the nighttime, and when the sun is
up again, the hydrophilicity is recovered. Thus, the
surface of the article retains a high level of
hydrophilicity.
Therefore, when the photocatalytic coating is provided
on articles, such as windowpanes for buildings, windowpanes
and windshields for vehicles and vessels, mirrors, lenses,
shields of goggles and helmets, or cover glasses for
measuring instruments, condensation of moisture or steam
in air does not result in the formation of light scattering
fog on the surface of these articles, because the condensed
water spreads into an even film without forming discrete
water droplets.
Likewise, exposure of windowpanes,
rearview mirrors for vehicles, windshields for vehicles,
eyeglass lenses, and shields of helmets to rainfall or a
spray of water does not result in the formation of discrete
water droplets which obstruct the view, because water
droplets adhered on the surface of these articles rapidly
spread into an even water film.
This permits a high level of view and visibility to
be ensured, which in turn ensures traffic safety for
vehicles and improves the efficiency of various works and
activities_
When the photocatalytic coating is provided on
machinery and articles which are disposed outdoors, the
photocatalyst is photoexcited during exposure to the
sunlight in the daytime, rendering the surface of the
photocatalytic coating hydrophilic. These machinery and
articles are sometimes exposed to rainfall. In this case,
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since the hydrophilified surface gets intimate with water
rather than hydrophobic dust and contaminants, the
hydrophobic dust and contaminants deposited on the surface
of the machinery and articles are separated from this
surface upon contact of the surface with water. Therefore,
every time when the hydrophilified surface is exposed to
rainfall, the dust and contaminants deposited on the
surface are washed away by raindrops, permitting the
surface to be selfcleaned. Further, lipophilic dust is
less likely to adhere onto the surface of the hydrophilic
photocatalytic coating.
Provision of a photocatalytic coating on machinery and
parts, tableware, kitchen utensils or other articles which
are likely to be stained with an oil, results in improved
oil repellency of the surface of these articles in water
by virtue of photocatalytic action. Therefore, when these
articles with the photocatalytic coating, stained with an
oil or fat are immersed in or wetted or rinsed with water,
the surface of the photocatalytic coating repels the oil
and fat, permitting the oil and fat to be released and
easily removed from the surface of the articles. Thus,
articles stained with an oil or fat can be cleaned without
use of any detergent.
RXAMPT,F~
The present invention will be described from various
viewpoints with reference to the following examples_
Example 1
Hydrophilification by Photocatalytic Action
Tetraethoxysilane Si(OC~HS)4 (manufactured by Wako
Pure Chemical Industries, Ltd., Osaka) (6 parts by weight),
6 parts by weight of pure water, and 2 parts by weight of
36% hydrochloric acid as a tetraethoxysilane hydrolysis
rate modifier were added to and mixed with 86 parts by
weight of ethanol as a solvent to prepare a silica coating
solution. Since the mixing was exothermic, the mixed
solution was allowed to stand for about one hr, thereby
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cooling the solution. The solution was flow-coated on the
surface of a 10-cm square soda-lime glass plate, and the
coating was dried at a temperature of 80°C. The drying
first caused hydrolysis of tetraethoxysilane to give
silanol Si(OH)4 which then underwent dehydration
polycondensation to form a thin film of amorphous silica
on the surface of the glass plate.
Then, 0.1 part by weight of 36o hydrochloric acid as
a hydrolysis rate modifier was added to a mixture of 1 part
by weight of tetraethoxytitanium Ti(OC~H~)4 (manufactured
by Merck) with 9 parts by weight of ethanol to prepare a
titanic coating solution which was then flow-coated on the
above glass plate in dry air. The coverage of the solution
was 45 ug/cm~ in terms of titanic. Due to very high
hydrolysis rate of tetraethoxysilane, part of
tetraethoxytitanium was hydrolyzed in the stage of coating,
initiating the formation of titanium hydroxide Ti(OH)4.
The glass plate was held for 1 to 10 min at about
150°C to complete the hydrolysis of tetraethoxytitanium
and, at the same time, to conduct dehydration
polycondensation of the resultant titanium hydroxide,
thereby giving amorphous titanic. Thus, a glass plate
bearing a base coat, of amorphous silica, having thereon
a top coat of amorphous titanic was prepared as a sample.
The sample was fired at 500°C to transform the
amorphous titanic into the anatase form of titanic to
prepare sample n1.
The sample r1 was allowed to stand for several days
in a dark place, and the surface of the sample n1 was then
irradiated with ultraviolet light using a 20-W black light
blue (BLB) fluorescent lamp (FL20BLB, manufactured by
Sankyo Denki ) at an irradiation intensity of 0 . 5 mW/cm~ ( in
terms of the irradiation intensity of ultraviolet light
having higher energy than band gap energy of the anatase
form of titanic, i.e., ultraviolet light at wavelengths
shorter than 387 nm) for about one hr, thereby preparing
CA 02241059 1998-06-19
a sample #2.
For comparison, a glass plate not coated with silica
and titanic was allowed to stand in a dark place for
several days and then used as a sample #3.
5 The contact angle of the sample #2 and the sample #3
with water was measured with a contact angle
goniometer (Model CA-X150, manufactured by Kyowa Interface
Science Co., Ltd., Asaka-shi, Saitama-ken). The detection
limit on the low-angle side of the contact angle goniometer
10 was 1°. The contact angle was measured 30 sec after
dropping a water droplet through a microsyringe on the
surface of the sample. This measuring method was used also
in the following examples. The goniometer reading of the
contact angle of the surface of the sample #2 with water
15 was 0°, indicating that the surface of this sample was
superhydrophilic. By contrast, the contact angle of the
sample #3 with water was 30 to 40°.
This suggests that the surface of the titanic coating
has been highly hydrophilified by the photocatalytic action
of titanic.
Likewise, the surface of a soda-lime glass plate was
coated with a thin film of amorphous titanic, and the
coated glass plate was fired at 500°C to transform the
amorphous titanic into the anatase form of titanic, thereby
preparing a sample #4. The sample #4 was placed in a
desiccator (temperature 24°C, humidity 45-500) and
irradiated with ultraviolet light at an irradiation
intensity of 0.5 mW/cm~ until the contact angle of this
sample with water became 3°.
The sample #4 was then allowed to stand in a dark
place and taken out of the dark place at different time
intervals. In this case, each time when the sample was
taken out of the dark place, the contact angle of the
sample with water was measured. A change in contact angle
with the elapse of time is tabulated in the follocaing Table
1.
CA 02241059 1998-06-19
16
Table 1
Sample Contact angle with water (' )
Sample #4
after (immediately 3.0
irradiation)
Sample #4 (after 3 hr) 5.0
Sample #4 (after 6 hr) 7.7
Sample #4 (after 8 hr) 8.2
Sample #4 (after 24 hr) 17.8
Sample #4 (after 48 hr) 21.0
Sample #4 (after 72 hr) 27.9
As shown in Table l, interruption of the
photoexcitation of the photocatalyst causes the
hydrophilicity of the sample to attenuate with the elapse
of time. This is probably because the surface of the
photocatalytic coating is contaminated with a hydrophobic
material.
Example 2
Influence of Excitation Wavelength
A titania (anatase form) sol (STS-11, manufactured by
Ishihara Sangyo Kaisha Ltd. , Osaka ) was spray-coated on the
surface of a 15-cm square glazed tile (AB02E01,
manufactured by TOTO, LTD.), and the coating was fired for
10 min at 800°C to prepare a sample #1. This sample and
a comparative glazed title not coated with the titania were
allowed to stand in a dark place for 10 days, and the
sample and the comparative sample were irradiated with
monochromatic ultraviolet light using an Hg-Xe lamp under
conditions specified in the following Table 2, and a change
in contact angle with water as a function of the
irradiation time was determined.
CA 02241059 1998-06-19
17
Table 2
Irradiation
Wavelength of intensity of Density of photon
UV light (nm) UV 1i ht (mW/cm2) ( hoton/sec/cm2)
313 10.6 1.66 x 1016
365 18 3.31 x 1016
405 6 1.22 x 1016
The results are shown as graphs in Figs. 1A to 1C.
In these graphs, values plotted by open circles represent
the contact angle of the sample #1 with water, and values
plotted by closed circles represent the contact angle of
the glazed tile not coated with titanic.
As can be seen from the graph shown in Fig. 1C, the
irradiation of the sample with ultraviolet light having
lower energy than that at a wavelength of 387 nm
corresponding to the band gap energy of the anatase form
of titanic (i.e., ultraviolet light having a longer
wavelength than 387 nm) results in no hydrophilification.
On the other hand, as shown in graphs of Figs. 1A and
1B, in the case of irradiation of the sample with
ultraviolet light having higher energy than the band gap
energy of the anatase form of titanic, the surface of the
sample is hydrophilified in response to the ultraviolet
light irradiation.
From the above results, it has been confirmed that the
hydrophilification of the surface does not occur without
the photoexcitation of the semiconductor photocatalyst and
is attributable to the photocatalytic action.
The reason why the contact angle of the sample, in
this example, with water did not reach 0° is considered to
reside in that, unlike the sample of Example 1, the sample
of this example has no silica layer interposed between the
glass substrate and the titanic layer, causing an alkaline
network-modifier ion, such as sodium ion, to be diffused
CA 02241059 1998-06-19
18
from the glaze into the titanic coating during firing at
800°C, which inhibits the photocatalytic activity of the
anatase.
Example 3
Physical Adsorption of Water by Photocatalytic Action
A titanic (anatase form) powder (P-25, manufactured
by Nippon Aerosil Co., Ltd.) was pressed to prepare three
disk samples. These samples were subjected to the
following tests 1 to 3, and the surface of each of the
samples was analyzed by Fourier transform infrared
spectroscopy (FT-IR) using a Fourier transform infrared
spectrometer (FTS-40A). In each test, an ultraviolet lamp
(UVL-21 ) at a wavelength of 366 nm was used for ultraviolet
irradiation.
In the analysis of the infrared absorption spectrum,
absorption bands provide the following information.
Sharp absorption band at wavenumber 3690 cm l:
stretching of OH bond in chemically adsorbed water
Hroad absorption band at wavenumber 3300 cm 1:
stretching of OH bond in physically adsorbed water
Sharp absorption band at wavenumber 1640 cm-l: bending
of HOH bond in physically adsorbed water
Absorption bands at wavenumbers 1700 cm-1, 1547 cm 1,
1475 cm-l, 1440 cm-1, and 1365 cm-l: a carboxylate complex
produced by the adsorption of a contaminant onto the
surface of the sample
Test 1
At the outset, the titanic disk immediately after
pressing was analyzed by infrared spectroscopy. An
absorption spectrum for the disk immediately after pressing
is shown as a curve n1 in Figs. 2A and 2B.
The titanic disk was stored for 17 hr in a dry box
containing silica gel as a desiccant, stored for 17 hr, and
analyzed by infrared spectroscopy to provide an infrared
absorption spectrum. The absorption spectrum thus obtained
is shown as a curve 2 in Figs. 2A and 2B. Comparison of
CA 02241059 1998-06-19
19
the spectrum fil with the spectrum n2 shows that, for the
spectrum #2, a dramatic reduction in absorption is observed
at wavenumber 3690 cm-1, indicating reduced chemically
adsorbed water. Further, for the spectrum ~2, a dramatic
reduction in absorption is observed also at wavenumbers
3300 cm-1 and 1640 cm-1, indicating that the physically
adsorbed water as well has been reduced. Thus, it is
apparent that storage in dry air for 17 hr resulted in a
reduction in both chemically adsorbed water and physically
adsorbed water. When the above procedure is repeated
except for the use of the anatase form of titania as a thin
film instead of the pressed disk, an increase in contact
angle of the sample with water is observed.
On the other hand, an increase in absorption at
wavenumbers 1300 to 1700 cm-1 attributable to a carboxyl ate
complex is observed, suggesting that the above substance
was adsorbed on and contaminated the surface of the sample
during storage of the sample.
Then, the titania disk was placed in the dry box and
irradiated with ultraviolet light at an irradiation
intensity of about 0.5 mW/cm~ for about one hr, followed by
infrared spectroscopic analysis to provide an infrared
absorption spectrum. The absorption spectrum thus obtained
is shown as a curve #3 in Figs. 2A and 2B.
As can be seen from the spectrum 3, the absorption
at wavenumber 3690 cm-1 returned to substantially the same
level of absorption as observed in the initial state.
Further, the absorption at wavenumbers 3300 cm-1 and 1640
cm-1 also returned to the same level of absorption as
observed in the initial state. These results show that
ultraviolet irradiation brings both the amount of the
chemically adsorbed water and the amount of the physically
adsorbed water to those observed in the original state.
As can be expected from the results of Example 1, it
is considered that, when the above procedure is repeated
except for the use of a thin film instead of the pressed
CA 02241059 1998-06-19
disk, the surface of the thin film is hydrophilified to
decrease the contact angle of the thin film with water.
Thereafter, the sample was stored for 24 hr in a dark
room communicating with the air and then analyzed by
5 infrared spectroscopy to provide an infrared absorption
spectrum. In order to avoid excessive complication of the
diagram, the absorption spectrum thus obtained is shown as
a curve n4 in Figs. 3A and 3B, separately from Figs_ 2A and
2B. For comparison convenience, the spectrum #2 is
10 reproduced in the graph of Figs. 3A and 3B. As can be seen
from the spectrum ~4, only a slight reduction in absorption
is observed at wavenumbers 3690 cm-1 and 1640 cm-1. This
demonstrates that storage of the sample, after ultraviolet
irradiation, in a dark room in the presence of moisture in
15 the air results in slight reduction in chemically adsorbed
water and physically adsorbed water. However, an increase
in absorption is observed at wavenumbers 1300 cm-1 and 1700
cm-1, indicating further adherence of the carboxylate
complex on the disk. When the above procedure is repeated
20 except for use of a thin film instead of the pressed disk,
an increase in contact angle of the thin film with water
is observed.
Finally, the titania disk was again irradiated with
ultraviolet light in the dark room communicating with the
air at an irradiation intensity of 0.5 mW/cm~ for about one
hr and then analyzed by infrared spectroscopy. The
absorption spectrum thus obtained is shown as a curve n5
in Figs . 3A and 3B . As can be seen from the graph, no
change is observed in absorption at wavenumber 3690 cm l,
whereas the absorption at wavenumber 3300 cm-1 was markedly
increased with the absorption at wavenumber 1640 cm 1 being
increased. These results show that re-irradiation of the
sample with ultraviolet light resulted in an increase in
the amount of the physically adsorbed water with the amount
of the chemically adsorbed water remaining unchanged. The
amount of the carboxylate complex (contaminant) remained
CA 02241059 1998-06-19
21
unchanged, indicating that the carboxylate complex was not
removed by ultraviolet irradiation. When the above
procedure is repeated except for use of a thin film instead
of the pressed disk, a decrease in contact angle of the
thin film with water is observed.
Test 2
At the outset, the titanic disk immediately after
pressing was analyzed by infrared spectroscopy to provide
an infrared absorption spectrum (a spectrum ~1 in a graph
shown in Figs. 4A and 4B). Thereafter, the disk was
irradiated with ultraviolet light for one hr at an
irradiation intensity of about 0.5 mW/cm~ and then analyzed
by infrared spectroscopy to provide an infrared absorption
spectrum (a spectrum ~2 in Figs. 4A and 4B). The disk was
further irradiated with ultraviolet light at the same
irradiation intensity for additional one hr (2 hr in
total), further additional one hr (3 hr in total), and
further additional two hr ( 5 hr in total ) and then analyzed
by infrared spectroscopy to provide infrared absorption
spectra (spectra T3, #4, and ~5 in Figs. 5A and 5B).
Comparison of the spectrum #1 with the spectrum n2
shows that the first ultraviolet irradiation caused an
increase in both the amount of chemically adsorbed water
and the amount of physically adsorbed water. During the
ultraviolet irradiation, the amount of a carboxylate
complex adhered onto the sample was slightly increased.
When the above procedure is repeated except for use of a
thin film instead of the pressed disk, the contact angle
of the thin film with water is decreased upon ultraviolet
irradiation.
Ultraviolet irradiation for additional one hr {2 hr
in total) resulted in a slight decrease in amount of the
chemically adsorbed water with the amount of the physically
adsorbed water remaining unchanged (compare the spectrum
tt2 with the spectrum n3). The amount of the carboxylate
complex was slightly increased. No change in the amount
CA 02241059 1998-06-19
22
of the physically adsorbed water is considered to be
attributable to the saturation of the amount of the
physically adsorbed water. It is considered that when the
above procedure is repeated except for use of a thin film
instead of the pressed disk, the contact angle of the thin
film with water remains unchanged.
Ultraviolet irradiation for further additional one hr
(3 hr in total) and for further additional 2 hr (5 hr in
total) resulted in a further decrease in amount of the
chemically adsorbed water with the amount of the physically
adsorbed water remaining unchanged ( see spectra #4 and n5 ) .
The amount of the carboxylate adhered onto the sample was
increased. It is considered that when the above procedure
is repeated except for use of thin film instead of the
pressed disk, the contact angle of the thin film with water
remains unchanged.
Test 3
This test is similar to the test 1. A major
difference between the test 1 and the test 3 was to
decrease the irradiation intensity of ultraviolet light.
At the outset, the titanic disk immediately after
pressing was analyzed by infrared spectroscopy to provide
an infrared absorption spectrum (a spectrum #1 in Figs. 6A
and 6B). Thereafter, it was stored for 34 hr in a dark
room communicating with the air and then analyzed by
infrared spectroscopy to provide an infrared absorption
spectrum (a spectrum n2 in Figs. 6A and 6B). The titanic
disk was then irradiated, in the same dark room, with
ultraviolet light at an irradiation intensity of 0.024
mW/cm~ for about 2 hr, followed by infrared spectroscopic
analysi s to provide an infrared absorption spectrum (a
spectrum n3 in Figs. 6A and 6H).
As can be seen from the graph, standing of the disk
in a dark room in the presence of moisture in the air
results in a decrease in both the amount of the chemically
adsorbed water and the amount of the physically adsorbed
CA 02241059 1998-06-19
23
water. An increase in amount of the carboxylate complex
adhered onto the disk was observed. It is considered that,
when the above procedure is repeated except for use of a
thin film instead of the pressed disk, the contact angle
of the thin film with water is increased.
The amount of the chemically adsorbed water slightly
increased in response to ultraviolet irradiation, and the
ultraviolet irradiation brought the amount of the
physically adsorbed water to the same level of absorption
as observed in the initial state. During the ultraviolet
irradiation, the amount of the carboxylate complex adhered
onto the disk slightly increased. It is considered that,
when the above procedure is repeated except for use of a
thin film instead of the pressed disk, the contact angle
of the thin film with water is increased.
Evaluation
The test results are summarized in the following Table
3.
CA 02241059 1998-06-19
24
b ~ b
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a~ a~ u
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x ~n cn ro ro
~ x ro ro ro C ~ +~ ro ro a~ a~
a~ ro ro
N o a~ a~ a~ ro .C .~ a~ a~ N N
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tn 1-~ U U U U
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+~ U +~ ~ .4~ +~ ~.-~.~ N
U ro U ro ro ro ro ro N U ro
ro -..Iro -r.l -r.l-~ -'.I-~ = ro -r.l
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rl 'O N 'O ' 'L3 'L7 U r1 b
E a, ro n, ro E ro ro ro ro ~ a.
la U N fa to N a
~ ~
~ \ ' to x N w N to N Sa E .. N
~ 'U ~ ~
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t
(fI 3 . .-1 to -.~3 -ri-.-I-.-I-~I t-I -.-
. t-r ~ o
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~ E ca ro O E c ro
' ~ 1~ +~ ;.~ N 'b ~ ~
-C
E-~ 'b +~ O ~ v N N N O 41N
~ -C C1
tn N ~ U X17 ~ i C t-i~
fn
C ~-I C .i ~ .-1 O . O -ftf O
f-I u1 O
O -,i O -~ O O O O o E
~ ~ v O v
to -~I ~ -ri-~ -r-I--1 -~i --1
f-1 = > > ~ a >
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.~ > o > > > ro ro c~ a~ ~nro
-~I -a a ; ro
.-Ia~ ro a~ ro N ro ro N S O tr
ro ro m ro ~
to t-i to to N S-~ S-a ~ ~ - = :-~
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O +> O ~ 1~ ~ ; + N ~ :~.-I
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In :~ -I 1~ H (n r--IH r-1 .-I e~ cn ro
S-I t-I ;-~ =
a~ ~n ~ cn ~ ~ ~ ~ ~ ~ _ I I
b b ro :- ~ I 1 I 1
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r I I I I
CA 02241059 1998-06-19
As can be fully understood from Table 3, the amount
of the physically adsorbed water increases in good response
to the ultraviolet irradiation.
In this connection, it is considered that, as shown
5 in the upper part of Fig. 7, in the crystal face of a
titanic crystal in a titanic coating 10, a terminal OH
group 12 is bonded to one titanium atom, a bridge OH group
14 is bonded to adjacent two titanium atoms, and these OH
groups constitute a layer of chemically adsorbed water.
10 As shown in the lower part of Fig. 7, ultraviolet
irradiation in the presence of moisture in the air results
in the physical adsorption of molecules of water onto the
hydrogen atom in the terminal OH group and the bridge OH
group through a hydrogen bond to form a layer 18 of
15 physically adsorbed water.
As described above, the amount of the physically
adsorbed water increases in good response to the
ultraviolet irradiation, and, hence, this example
demonstrates that the formation of the physically adsorbed
20 water layer 16 is induced by the photocatalytic action of
titanic. It is understood that the presence of the
physically adsorbed water layer 16 results in improved
hydrophilicity of the surface of titanic.
On the other hand, the amount of the carboxylate
25 complex adhered onto the surface of the sample appears to
increase with increasing the time of contact with the air.
It is considered that photoexcitation of the photocatalyst
would result in improved hydrophilicity of the surface of
the sample despite the increased amount of the carboxylate
complex adhered onto the sample.
EXAMPLE 4
Surface Energy and Hydrophilicity
Tetraethoxysilane (manufactured by Wako Pure Chemical
Industries, Ltd.) (6 parts by weight), 6 parts by weight
of pure water, and 2 parts by weight of 36% hydrochloric
acid as a tetraethoxysilane hydrolysis rate modifier were
added to and mixed with 86 parts by weight of ethanol as
CA 02241059 1998-06-19
26
a solvent to prepare a silica coating solution. The mixed
solution was allowed to stand for about one hr and then
flow-coated on the surface of a soda-lime glass to prepare
two glass plates coated with an amorphous silica base coat .
Then, 0.1 part by weight of 36% hydrochloric acid as
a hydrolysis rate modifier was added to a mixture of 1 part
by weight of tetraethoxytitanium (manufactured by Merck)
with 9 parts by weight of ethanol to prepare a titanic
coating solution which was then flow-coated on the surface
of the above glass plates in dry air. The coverage of the
solution was 45 ug/cm~ in terms of titanic.
Thereafter, the glass plates were held for 1 to 10 min
at about 150°C in dry air to prepare two glass plates
coated with a top coat of amorphous titanic.
Further, these glass plates were fired respectively
at 440 ° C and 550 ° C to transform the amorphous titanic into
the anatase form of titanic, thereby preparing a sample ~1
and a sample n2.
The contact angle of the surface of these samples with
water was measured. Further, formamide, ~i-thiodiglycol,
ethylene glycol, a-bromonaphthalene, hexachlorobutadiene,
and methylene iodide were selected as liquids with known
components of the surface energy, and the contact angle of
the surface of the samples with these liquids was measured.
The surface of the samples was then irradiated with
ultraviolet light using a black light blue fluorescent lamp
(FL20HLB) at an irradiation intensity of 0.5 mW/cm~ for
about one hr. Thereafter, the contact angle of the surface
of the samples with water, formamide, (3-thiodiglycol,
ethylene glycol, a-bromonaphthalene, hexachlorobutadiene,
and methylene iodide was measured again.
The results of the contact angle measurement before
and after the ultraviolet irradiation are tabulated in the
following Table 4. It is particularly noteworthy that the
sample n1 was highly hydrophilified to give a contact angle
thereof with water of 0°.
CA 02241059 1998-06-19
27
a~
C
N
-i o~ ~ m c~
N
?c N C'~ f'~ C'7
't7
.C
-rI
O
O
~
.i
O
C
I
<v
O
.-I
to N O ~ O
b
O
c0
.-i
~
S
C
U
.L7
O
C
N
c0
I CO O~ !t7 tD
~
N N .-1
E
-C.
O
N
W
C
C
al
.1
r1 o~ CT7 O t!~
O
?, W n
U
N W
.fl
E-~O
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O N u7
27 ~D .-~ ~ .-I
O
L
E
N
'>7
-rl tf7 In
E
rt7 ~ C.~ CO t'7
E c~ e~ ..-i
N
O
w
N
O N O f'~ ('~
u7 vD
b
3
C C C C
N O N O
O O
La N tr to
-rl -.-I-.~
O ~ O N
;~ ;~ .~ +~
w ~ W ;~
~0 <J 0 c0
C~J ~ w O w
-rl -.~ -.I -r1
.-I .L1 0 17 ~0
'W ~ ~p T1
.r ~. ~. .J
c0 ~ ~ Ip
to t, to to
cD .-i .i N N
4r ~r fa Sa
U7 ~ ~ ~ ~
-.i -ri .i
CA 02241059 1998-06-19
28
Based on the contact angles thus measured, the surface
free energy of the titanic coating was determined by the
following method.
It is known that the following Young's equation (1)
is established for the contact angle, 8, that, when a
liquid droplet is put on the surface of a solid, the
droplet makes with the solid surface:
YL-cose = YS - YSL ( 1 )
wherein YS represents the Gibbs free energy in the
interface of the solid and the gas, YL represents the Gibbs
free energy in the interface of the liquid and the gas, and
YSL represents the Gibbs free energy in the interface of
the solid and the liquid.
~o
The Gibbs free energy released upon contact between
the liquid and the solid is called the "work of adhesion"
(WSL) and given by the following Dupre's equation (2):
WSL =_ YS + YL _ YSL (2)
Further, according to the extended Fowkes' equation
established by Hata and Kitazaki (Journal of The Adhesion
Society of Japan 8, 131, 1972), YSL is given by the
following equation:
YSL - YS ~ YL 2 Y YS YL 2 Y YS YL 2 JYS YL ~3)
wherein superscript d represents the molecular dispersion
force of the surface energy, superscript p represents the
dipole moment component of the surface energy and
superscript h represents the hydrogen bond component of the
surface energy.
From the equations (1) and (2),
WSL = (1 + cosh)-YL (4)
From the equations (3) and (4),
- y+ 2se~y~
In a liquid L, when the molecular dispersion force
yLd, the dipole moment component yLP, and the hydrogen bond
CA 02241059 1998-06-19
29
component yLh in the surface energy are known and when the
contact angle 8 is known, three parameters, ysd, YSp, and
ysh, for a certain solid S can be determined from the
equation (5) by the method of least squares.
For the surface energy of water, formamide, R
thiodiglycol, ethylene glycol, a-bromonaphthalene,
hexachlorobutadiene, and methylene iodide, three
components, yLd, YLP, and yLh, are known to be as given in
the following Table 5 (Yuji Harada, "WAKARIYASUI COATING
GIJUTSU", RIKO SHUPPAN, p.93)
Table 5
Liquid YL YLd YLP YLh
Water 72.8 29.1 1.3 42.4
Formamide 58.2 35.1 1.6 21.5
a-Thioglycol 54.0 39.2 1.4 13.4
Ethylene glycol 47.7 30.1 0 17.6
a-Bromonaphthalene 44.6 44.4 0.2 0
Hexachlorobutadiene 36.0 35.8 0.2 0
Methylene iodide 50.8 46.8 4.0 0
(erg/cm')
The measured values of the contact angle 8 given in
Table 4 and the known three components, yLd, yLP, and yLh,
of the surface energy of various liquids given in Table 5
were inserted into the equation (5), and each of the
components, Ysd, ysP, and Ysh, of the surface energy in the
titania coating was calculated by the method of least
squares. The results are summarized in Table 6.
CA 02241059 1998-06-19
Table 6
Surface
free
energy
(erg/cm2)
Sample YSd YSP YSh YS
~1 (before
5 UV irradiation) 29.14 25.21 8.01 62.36
n1 ( after
W irradiation) 31.81 16.46 23.26 71.53
n2 (before
W irradiation) 32.60 12.46 5.51 50.56
10 n2 (after
UV irradiation) 31.61 18.51 22.11 72.23
As can be seen from Table 6, it has been found that
for both the sample #1 and the sample #2, the ultraviolet
15 irradiation results in markedly increased hydrogen bond
component YSh in the surface energy. On the other hand, no
clear change is observed in the molecular dispersion force
component YSd and the dipole moment component YSP in the
surface energy.
20 From the above results, it is considered that the
photocatalytic action caused by the photoexcitation of the
photocatalyst enhances the hydrogen bond component YSh in
the surface energy of the photocatalytic coating,
accelerating the physical adsorption of molecules of water,
25 which results in increased amount of the physically
adsorbed water to highly hydrophilify the surface of the
coating.
Example 5
Oil Repellency in Water
30 This example demonstrates that the oil repellency of
the surface of a photocatalytic coating in water improves
with increasing the hydrogen bond component YSh in the
surface energy.
The oil repellency in water of the surface of the
sample l and the sample 2 prepared in Example 4 was
CA 02241059 1998-06-19
31
observed before and after the ultraviolet irradiation. For
this purpose, according to a conventional method, methylene
iodide was selected as a liquid representative of an oil.
Before and after the ultraviolet irradiation, methylene
iodide was dropped on the surface of the sample #1 and the
sample #2, and the samples were then immersed, while
keeping the surface horizontal, into water contained in a
water tank.
As a result, in the sample #1 and the sample n2 before
the ultraviolet irradiation, methylene iodide remained
adhered in a lens form in the interface of the
photocatalytic coating and water. This state was
photographed from the side, and the contact angle of the
surface of the photocatalytic coating with methylene iodide
in water was measured based on the photograph and found to
be 80° for the sample #1 and 70° for the sample n2.
When the sample #1 and the sample #2 after the
ultraviolet irradiation were immersed in water, the surface
of the photocatalytic coating well repelled methylene
iodide which then became spherical, soon separated from the
surface of the sample and floated on the surface of water,
making it impossible to measure the contact angle.
Therefore, the contact angle of the sample with
methylene iodide in water was determined by the following
calculation.
In general, for the contact angle, e, that, when an
oil droplet is put on the surface of a solid followed by
immersion of the solid in water, the oil droplet makes with
the solid surface (that is, the contact angle of the solid
surface with the oil in water), the following equation
based on the Young's equation is applicable:
YLW - cose = YSW - YSL ( 6 )
wherein YSw represents the Gibbs free energy in the
interface of the solid and water, yL~, represents the Gibbs
free energy in the interface of the oil and water, and YSL
represents the Gibbs free energy in the interface of the
CA 02241059 1998-06-19
32
solid and the oil.
Accordingly,
cosh = ( YSW - YSL ) ~YLW
Further, according to the extended Fowkes equation
established by Mr. Hata and Mr. Kitazaki, YSW~ YSL~ and
YLW are given respectively by the following equations:
Ys,~y = YS ~ Ys~ ' 2 ~YS 'Y~ ' 2 JYS Y ~ ' 2 ~ys yF~ (8)
ySI, = yS ~ yL - 2 ~~ yL - 2 YS yL - 2 ~'YS yL (9)
YLW = YL ~ Y,N - 2 V YL Y~ - 2 Y YL Y'~~V - 2 JYL Y'~~V (10)
wherein YW represents the Gibbs free energy in the
interface of water and the gas.
Therefore, if the kind of the oil is determined, YSW,
YSL, and YLW could be determined from the equations (8),
(9), and (10). These values may be substituted for YSW
YSL, and YLW in the equation (7) to calculate the contact
angle 8 of the solid surface with the oil in water.
For the sample #1 and the sample n2 prepared in
Example 4, data before and after the ultraviolet
irradiation described in the above Table 5 and the known
values of the components in the surface energy of water and
methylene iodide given in Table 4 were inserted into the
equations (8) to (10) to determine the free energies YSW
YLW, and YSL~ and the contact angle 8 of the surface of
the photocatalytic coating with methylene iodide in water
is calculated by the equation (7). The results are
summarized in the following Table 7.
CA 02241059 1998-06-19
33
Table 7
Sample YSW YSL YLW COS6 a
#1 (before
UV irradiation) 28.8 19.4 45.2 0.21 78
#1 ( after
UV irradiation) 11.5 28.9 45.2 -0.39 113
#2 .( before
UV irradiation) 23.2 9.0 45.2 0.31 72
n2 ( after
UV irradiation) 13.4 28.8 45.2 -0.34 110
As is apparent from Table 7, both the sample n1 and
the sample n2 after the ultraviolet irradiation had a
markedly increased contact angle 8 with methylene iodide.
This fact is in well agreement with the fact that, in the
above experiment, the surface of the photocatalytic coating
well repelled methylene iodide in water. Further, this
means that immersion of a photocatalytic coating having an
oil stain in water or wetting of such a photocatalytic
coating with water permits the oil stain to be simply
removed from the coating.
This phenomenon will be discussed. As is apparent
from the above Table 5, the hydrogen bond component among
the components of the surface energy constitutes the
largest difference between water and the oil.
Specifically, in the oil typified by methylene iodide, the
hydrogen bond component YLh in the surface energy is
generally small and close to zero, while in water, the
hydrogen bond component yWh in the surface energy is as
large as 42_4. Therefore, in the equation (8), increasing
h h
ySh results in a marked increase in ~7Sy~ and a decrease in
YSW.
On the other hand, in the equation (9), increasing ySh
results in an increase in YS by a magnitude corresponding
to the increase in ySh. Since, however, yLh is close to
CA 02241059 1998-06-19
34
Yh Y6
zero, 5 L does not change, resulting in increased YSL'
Therefore, it is considered that, as is apparent from
the equation (7), the contact angle A of the solid surface
with the oil in water increases with increasing the
hydrogen bond component Ysh in the surface energy of the
photocatalytic coating, rendering the surface of the
photocatalytic coating oil-repellent in water.
Example 6
Oil repellency in Water - Oleic Acid
A glass plate coated with a base coat of amorphous
silica and a top coat of amorphous titanic was prepared in
the same manner as in Example 4. The glass plate was fired
at 475°C to transform the amorphous titanic into the
anatase form of titanic.
Subsequently, the surface of the sample was irradiated
with ultraviolet light using a black light blue fluorescent
lamp (FL20BLB) at an irradiation intensity of 0.5 mW/cm~
for about one hr.
Oleic acid was dropped on this sample and a soda-lime
glass plate with no photocatalytic coating in air, and the
contact angle in air of the surface of the samples with
oleic acid was measured. As a result, for both samples,
the contact angle in air with oleic acid was 35V .
Thereafter, each sample was immersed in water, and the
contact angle in water of the samples with oleic acid was
then measured. As a result, the contact angle was 85° for
the glass plate with a photocatalytic coating and 38 _ 5 ° for
the glass plate with no photocatalytic coating. Thus, it
was confirmed that the photoexcited photocatalytic coating
exhibits oil repellency in water_
Example 7
Oil Repellency of Photocatalyst-Containing Silicone Coating
" wato,-
A 10-cm square aluminum plate was provided as a
substrate. The substrate was previously coated with a
CA 02241059 2005-04-12
silicone resin coating to smoothen the surface thereof.
Subsequently, trimeth.oxymethylsilane (liquid B of
"Glasca*" a paint composition manufactured by Japan'
Synthetic Rubber Co., Ltd.), a precursor of a silicone, was
5 added to a nitric acid peptization-type titanic (anatase
form) sol (available form Nissan Chemical Industries Ltd.
under the designation TA-15, average particle diameter 0.01
~.m). In this case, the titanic sol was added in such an
amount that the proportion of titanic to the total weight
10 on a solid basis of titanic and silicone was 50% by weight.
The mixture was diluted with propanol, and a curing agent
was added to prepare a titanic-containing silicone paint.
The titanic-containing silicone paint was coated on
an aluminum plate, and the coating was cured at 150°C to
15 form a top coat with particles of ti.tania in an anatase
form dispersed in a silicone coating.
This sample was irradiated with ultraviolet light
using a black light blue fluorescent lamp (FL20BLB) at an
irradiation intensity of 0 _ 5 mW/cm~ for about one day. The
20 water absorption of the sample was less than lo.
A salad oil was dropped on this sample and an aluminum
plate with no titanic-containing silicone coating in air,
and the contact angle of the surface of these samples with
the salad oil was measured. The contact angle of the
25 samples with the salad oil in air was 22° for the sample
having a titanic-containing silicone coating and 39° for
the sample not having any titanic-containing silicone
coating.
Thereafter, each sample was immer~>ed in water, and the
30 contact angle of each sample with the salad oil in water
was measured. The contact angle of the samples with the
salad oil in water was 105° for the glass plate with a
titanic-containing silicone coating and 35° for the glass
plate with no titanic-containing silicone coating.
35 When the sample coated with a titanic-containing
silicone was vibrated by flicking the sample with a finger,
* Trade-mark
CA 02241059 1998-06-19
36
the salad oil adhered onto the surface of the sample
separated from the surface of the sample and floated on the
water. On the other hand, for the sample with no titania-
containing silicone coating, the salad oil remained in an
adhered state and spread over the surface of the sample.
From the above results, it is considered that
photoexcitation of a photocatalyst by irradiation of a
silicone coating containing a photocatalyst causes the
surface of the coating to be hydrophilified by the
photocatalytic action to increase the hydrogen bond
component in the surface energy, increasing the oil
repellency of the coating in water.
Example 8
Photocatalytic Coating Bearing Sulfuric Acid
Two soda-lime glass plates coated with a base coat of
amorphous silica and a top coat of amorphous titania were
prepared in the same manner as in Example 1.
The surface of one of the glass plates was coated with
about 0.8 ml of a 5 wt% aqueous sulfuric acid solution to
form a coating which was then fired at about 525°C to
prepare a sample n1. The firing transforms the amorphous
titania into the anatase form of titania and, at the same
time, as shown in Fig. 8, causes a sulfonic group to be
bonded to the titanium atom on the surface of titania to
form a titania bearing sulfuric acid.
For comparison, the other glass plate was fired,
without coating of sulfuric acid, at about 525°C, the same
temperature as used in the case of the above substrate, to
transform the amorphous titania into the anatase form of
titania, thereby preparing a sample #2_
Each sample was allowed to stand in a dark place for
two days, during which time the contact angle of the
surface of the samples with water was measured. A change
in contact angle with the elapse of time is shown in Fig.
9. From the graph in Fig. 9, it is noteworthy that as
compared with the sample n2 not bearing sulfuric acid, the
sample ?1 having a surface bearing sulfuric acid, when
CA 02241059 1998-06-19
37
allowed to stand in a dark place, exhibits a lower degree
of increase in contact angle, that is, better retention of
hydrophilicity observed immediately after the firing,
meaning that the time taken for the surface of the sample
to be rendered hydrophobic is increased.
After standing in the dark room for two days, the
surface of the sample #1 and the sample #2 was irradiated
with ultraviolet light using a black light blue fluorescent
lamp (FL20BLB) at an irradiation intensity of 0.5 mW/cm~,
and a change in contact angle of the surface of the samples
with water as a function of the irradiation time was
determined. The results are shown in Fig. 10.
As is apparent the graph in Fig. 10, the sample r2
not bearing sulfuric acid requires 2 hr for the sample to
be hydrophilified to a contact angle thereof with water of
less than 3°, whereas for the sample #1 having a surface
bearing sulfuric acid, only one hr is required for the
hydrophilification to the same level of contact angle. The
above results show that bearing sulfuric acid on the
surface of the coating accelerates the hydrophilification
of the photocatalytic coating upon photoexcitation of the
photocatalyst after standing in a dark place.
It is considered that, as shown in Fig. 11, the
presence of a sulfonic group permits the hydrogen atom in
a bridge OH group on the surface of titania functions as
a Br~nsted acid site ( a proton donating site ) to accelerate
the physical adsorption of molecules of water, or
otherwise, as shown in Fig. 12, the titanium atom on the
surface of the titania functions as a Lewis acid site (an
electron accepting site) to accelerate the physical
adsorption of molecules of water, increasing the amount of
the physically adsorbed water on the surface of the
coating.
Example 9
Photocatalytic Coating Bearing TiO~/WO
Ammonia peptization-type anatase sol (STS-11,
CA 02241059 1998-06-19
38
manufactured by Ishihara Sangyo Kaisha Ltd.) (1 g) was
mixed with 2 g of 25% aqueous ammonia containing tungstic
acid dissolved therein, and 2 g of distilled water was
added to the mixture to prepare a coating liquid. The
molar ratio of the titanic particles to the tungstic acid
in the coating liquid was 10 . 1.
The coating liquid was coated on a glazed title
(AB02E11, manufactured by TOTO, LTD.) having a size of 5
x 10 cm, and the coating was fired for 30 min at 700°C to
prepare a sample #1 with a coating, of the anatase form of
titanic, bearing TiO~/W03. No color development derived
from the surface coating was observed.
For comparison, the titanic sol (STS-11) was coated
on the same type of glazed title (AB02E11) as used above,
and the coating was fired for 30 min at 700°C to prepare
a sample #2 with a coating of titanic alone.
Immediately after the firing, the contact angle of
each sample with water was measured. As a result, the
contact angle was 9° for the sample #2, while the sample
#1 was as low as 1 ° . This indicates that immediately after
the firing, a titanic coating bearing TiO~/WO~ can exhibit
high hydrophilicity.
Each sample was then allowed to stand for one day in
a dark place to determine a change in contact angle of the
surface of the samples with water. As a result, the
contact angle of the sample #2 with water was increased to
40°, whereas the sample #1 retained the low contact angle,
i.e., had a contact angle of less than 5°. After standing
of the sample #1 in a dark place for additional 4 days, the
sample #1 retained the contact angle on a low level of
about 5°.
Thereafter, the surface of the sample #1 was
irradiated with ultraviolet light using a black light blue
fluorescent lamp (FL20BLB) at an irradiation intensity of
0.5 mW/cm~ for about 2 hr. As a result, the surface of
the sample #1 was superhydrophilified to a contact angle
CA 02241059 1998-06-19
39
thereof with water of 0°.
Oleic acid was coated on the surface of the sample n1
and the sample #2, and these samples were then rubbed with
a neutral detergent, rinsed with tap water and distilled
water, and dried in a drier for 30 min at 50°C to
intentionally contaminate the surface of the samples. As
a result, the contact angle of the samples with water was
increased to 30 to 40°.
The surface of the sample #1 was then irradiated with
ultraviolet light using a black light blue fluorescent lamp
at an irradiation intensity of 0.3 mW/cm2 for about 2 hr.
This resulted in superhydrophilification of the sample to
a contact angle of the sample with water of 0 ° . On the
other hand, the surface of the sample ~2 was irradiated
with ultraviolet light at an irradiation intensity of 0.3
mW/cm~ for about one day. After the ultraviolet
irradiation, the contact angle of the sample #2 with water
was 9°.
Example 10
Firing Temperature of Coating Bearing TiO~/WO.~
Two tiles with a coating, of the anatase form of
titania, bearing TiO~/W03 were prepared in the same manner
as in Example 9, except that the ffiring temperature was
varied. The firing temperature of the sample #1 was 600°C,
and the firing temperature of the sample #2 was 750°C.
Immediately after the firing, the contact angle of
both the samples with water was as low as 1°. These
samples were then allowed to stand in a dark place for one
day, and the contact angle of the surface of each sample
with water was measured again. As a result, the samples
retained the contact angle on a low level of less than 5°.
Thereafter, these samples were intentionally
contaminated with oleic acid and a neutral detergent in the
same manner as in Example 9. As a result, the contact
angle with water increased to 50° for the sample ~1 and to
60° for the sample tt2.
CA 02241059 1998-06-19
The samples were then irradiated with ultraviolet
light at an irradiation intensity of 0.3 mW/cm~ for about
2 hr. This resulted in superhydrophilification of both the
samples to a contact angle with water of 0°.
5 Example 11
Ratio of TiO~ to W0~ in Coating Bearing TiO~/WO~
In the same manner as in Example 9, four coatings with
the tungstic acid being varied were prepared and then
coated on tiles, followed by firing to prepare four tiles
10 #1 to #4 with a coating of the anatase form of titania
bearing TiO~/W03_ The molar ratio of the titanic particles
to the tungstic acid in the coating liquid used was 20 .
1 for the sample #1, 100 . 1 for the sample #2, 200 : 1 for
the sample #3, and 1000 . 1 for the sample #4. For all the
15 samples, the firing was performed at 700°C.
Immediately after the preparation, the samples were
intentionally contaminated with oleic acid and a neutral
detergent in the same manner as in Example 9. They were
then irradiated with ultraviolet light at an irradiation
20 intensity of 0.3 mW/cm2 for about one day. As a result,
after the ultraviolet irradiation, all the samples had a
low contact angle with water of 1°.
Thereafter, the samples were allowed to stand in a
dark place for one day, and the contact angle of the
25 surface of the samples with water was measured. As a
result, the samples retained the low contact angle with
water, that is, had a contact angle with water of less than
IO° for the samples #1 and #2, 8° for the sample #3, and
9° for the sample #4.
30 This example demonstrates that, when TiO~/WO~, a
composite of metal oxides, is borne on a photocatalytic
coating, the hydrophilicity is retained after the
interruption of the photoexcitation of the photocatalyst.
Example 12
35 Formation of Photocatalytic Coating by Sputtering
An amorphous titanic film was formed on the surface
CA 02241059 1998-06-19
41
of a 10-cm square soda-lime glass plate by electron beam
deposition, and the titanic film was fired at 500°C to
crystallize the amorphous form of titanic, thereby
producing the anatase form of titanic. The thickness of
the titanic (anatase form) film was 100 nm.
Tungstic acid dissolved in 25% aqueous ammonia was
coated on the titanic (anatase form) film at a coverage of
0.6 ug/cm2 in terms of the weight of tungstic acid, and the
coating was fired at 500°C to prepare a sample.
Immediately after the firing, the contact angle of the
surface of the sample with water was as low as 2°.
The sample was then allowed to stand in a dark place
for one day, followed by measurement of the contact angle
of the surface of the sample with water to determine a
change in contact angle of the sample with the elapse of
the time. As a result, the sample retained the contact
angle on a low level, that is, had a contact angle with
water of 9°.
Thereafter, the surface of the sample was irradiated
with ultraviolet light at an irradiation intensity of 0.3
mW/cm~ for one day. After the ultraviolet irradiation, the
contact angle of the sample with water was measured and
found to be 0°, indicating that the surface of the sample
was highly hydrophilified.
. Example 13
Formation of Photocatalytic Coating Using Alkoxide
Tetraethoxysilane as a precursor of silica and
ethanolamine as a hydrolysis inhibitor were added to
ethanol to prepare a silica coating solution having a
tetraethoxysilane concentration of 3_5% by weight. A 10-cm
square soda-lime plate was immersed in this solution and
then pulled up at a rate of 24 cm per min to dip-coat the
surface of the glass plate with the above solution,
followed by drying. This first caused tetraethoxysilane
to be hydrolyzed to give silanol which subsequently
underwent dehydration polycondensation to form a thin film
CA 02241059 1998-06-19
42
of amorphous silica on the surface of the glass plate.
Separately, tetraethoxytitanium as a precursor of
titania and ethanolamine as a hydrolysis inhibitor were
added to ethanol to prepare a titania coating solution
having a tetraethoxytitanium concentration of 3.5% by
weight. The glass plate with the film of amorphous silica
previously coated thereon was immersed in the titanic
coating solution and then pulled at a rate of 24 cm per min
to dip-coat the surface of the coated glass plate with the
titanic coating solution, followed by drying. This first'
caused tetraethoxytitanium to be hydrolyzed to give
titanium hydroxide which then underwent dehydration
polycondensation to form an amorphous titanic film
( thickness : about 50 nm ) on the surface of the coated glass
plate.
The glass plate was then immersed in an aqueous
solution of tungstic acid dissolved in a concentration of
0.25% by weight in a 25o aqueous ammonia solution and
pulled up at a rate of 24 cm per min to dip-coat the
surface of the glass plate with the solution, followed by
firing at 500°C to prepare a sample n1. The firing
resulted in the crystallization of the amorphous titanic
to produce the anatase form of titanic. It is considered
that a double oxide of Ti02/W0~ also is produced.at least
in the interface of the titanic film and the tungsten film.
For comparison, a glass plate not coated with the
aqueous tungstic acid solution was fired at 500°C to
prepare a sample #2. The firing resulted in the
crystallization of the amorphous titanic to produce the
anatase form of titanic.
Immediately after the firing, the surface of the
sample n1 and the sample ~2 was coated with oleic acid,
rubbed with a neutral detergent, rinsed with tap water and
distilled water, and dried in a drier for 30 min at 50°C
to intentionally contaminate the surface of the samples.
Thereafter, the surface of the samples was irradiated with
CA 02241059 1998-06-19
43
ultraviolet light at an irradiation intensity of 0.3 mW/cm~
for one day, and the contact angles of the surface of the
samples with water was measured. As a result, for both the
samples, the contact angle of the surface with water was
as low as 1°.
Subsequently, the samples were allowed to stand in a
dark place for 6 hr, and the contact angle of the surface
of the samples with water was measured again to determine
a change in contact angle of the samples with water. As
a result, the contact angle of the sample #2 with water was
increased to 22°, whereas the sample #1 retained the low
contact angle, i.e., had a contact angle of 7°.
Example 14
Photocatalytic Coating Bearing Sulfuric Acid and TiO~/WO.~
In the same manner as in Example 13, a soda-lime glass
plate was coated first with a thin film of amorphous silica
and then with a thin film of amorphous titania.
The coated glass plate was then immersed in a to
aqueous ammonia solution containing 0.25% by weight of
tungstic acid and 0.33% by weight of ammonium sulfate
dissolved therein and pulled up at a rate of 24 cm per min
to dip-coat the surface of the glass plate with the
solution, followed by firing at 500°C to prepare a sample.
The firing resulted in the crystallization of amorphous
titanic to form a photocatalytic coating of anatase on the
surface of the plate. It is considered that, in addition
to the formation of the photocatalytic coating of anatase,
a double oxide of TiO~/WO~ also is produced on the surface
of the photocatalytic coating and, further, the sulfonic
group is bonded to the titanium atom on the surface of the
titanic.
Immediately after the firing, the surface of the
sample was intentionally contaminated with oleic acid and
a neutral detergent in the same manner as in Example 13.
This caused the contact angle of the sample with ;.cater to
be increased 35'.
CA 02241059 1998-06-19
44
Thereafter, the surface of the sample was irradiated
with ultraviolet light at an irradiation intensity of 0.3
mW/cm2 for one day, and the contact angle of the sample
with water was measured again and found to be as low as 0 ° .
The sample was then allowed to stand in a dark place
for one day, and the contact angle of the surface of the
sample with water was measured again to determine a change
in contact angle. As a result, the surface of the sample
retained the contact angle on a low level, i.e., had a
contact angle with water of 9°.
