JP3348613B2 - Photocatalytic hydrophilic coating composition - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrophilic coating composition for forming a coating film capable of making and maintaining the surface of a member highly hydrophilic. More specifically, the present invention provides a highly hydrophilic surface of a mirror, glass, lens, prism or other transparent member to form a coating film capable of preventing clouding and water droplet formation of the member. The present invention relates to an antifogging coating composition. The present invention also prevents the surface from being stained by highly hydrophilizing the surface of a building, a window glass, a mechanical device or an article,
Alternatively, the present invention relates to an easy-cleaning coating composition for forming a coating film whose surface can be cleaned using water.

[0002]

BACKGROUND OF THE INVENTION It is often experienced that in cold weather, windshields and glazings of automobiles and other vehicles, glazings of buildings, lenses of eyeglasses, and cover glass of various instrument panels are fogged by condensed moisture. . In addition, mirrors and eyeglass lenses in bathrooms and washrooms are often fogged by steam. The fogging on the surface of the article is that if the surface is placed at a temperature below the dew point of the atmosphere, the moisture in the atmosphere will condense and condense on the surface,
This is because they grow like water drops. If the condensed water droplets are sufficiently fine and their diameter is about half the wavelength of visible light, the water droplets will scatter light and the glass or mirror will be apparently opaque, again losing visibility. If the condensation of moisture progresses further and the fine condensed water droplets fuse together and grow into larger discrete water droplets, their surfaces will be refracted by the refraction of light at the water droplet-surface interface and the water droplet-air interface. Dark, blurred, mottled or cloudy. as a result,
In a transparent article such as glass, the perspective image is distorted and the transparency is reduced, and the reflected image is disturbed by a mirror. Furthermore, windshields and window glasses of vehicles, window glasses of buildings, back mirrors of vehicles,
If eyeglass lenses, masks, or helmet shields are subjected to rainfall or splashes, and a large number of discrete water droplets adhere to the surface, those surfaces may become dark, blurred, mottled, or cloudy, and still lose visibility. Will be As used herein, the term "anti-fog" broadly refers to a technique for preventing optical hindrance due to such fogging, growth of condensed water droplets, and adhesion of water droplets. Needless to say, the above "clouding" has a profound effect on safety and efficiency of various operations. For example, if the windshield or window glass of the vehicle or the back mirror of the vehicle is shaded or clouded in cold weather or rainy weather, it is difficult to secure a view, and traffic safety is impaired. If the focusing lens of the endoscope lens, dental mirror, or dental laser treatment device becomes cloudy, accurate diagnosis,
It will hinder surgery and treatment. If the cover glass of the instrument panel becomes fogged, it becomes difficult to read the data.

On the other hand, in the field of construction and paints, stains on building exterior materials, outdoor buildings and their coatings have become a problem with environmental pollution. Dust and particles suspended in the air accumulate on the roof and outer walls of buildings in fine weather. Sediment is washed away by rainwater as it rains and flows down the building's outer walls. Furthermore, in the rain, the floating dust is carried by the rain and flows down on the outer wall of the building or the surface of the outdoor building. As a result, pollutants adhere to the surface along the path of rainwater. When the surface dries, striped stains appear on the surface. Soil on building exterior materials and coatings consists of combustion products such as carbon black and inorganic contaminants such as urban dust and clay particles. It is considered that the diversity of such contaminants complicates antifouling measures (Yoshinori Tachibana, "Method of Accelerated Testing of Contamination of Exterior Wall Finishing Materials", Journal of Architectural Institute of Japan, Structural Transactions, No. 404, October 1989, pp. 15-2
4). According to conventional wisdom, polytetrafluoroethylene (PTFE) is used to prevent the above-mentioned dirt on the building exterior.
Although water-repellent paint such as was considered preferable,
Recently, for urban dust containing many hydrophobic components,
It is considered that the surface of the coating film should be made as hydrophilic as possible (Polymer, vol. 44, May 1995, p. 3).
07). Therefore, it has been proposed to paint a building with a hydrophilic graft polymer (newspaper "Chemical Industry Daily",
Jan. 30, 1995). According to reports, this coating exhibits a hydrophilicity of 30 to 40 ° in contact angle with water.
However, the contact angle of inorganic dust represented by clay minerals with water is 20 ° to 50 °, and the contact angle with water is 3 °.
Having an affinity for a graft polymer of 0 to 40 °;
It is considered that the coating of the graft polymer cannot prevent contamination by inorganic dust because it easily adheres to the surface.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hydrophilic coating having excellent storage stability for forming a coating film capable of maintaining and maintaining the surface of a member to a high degree of hydrophilicity.
The object of the present invention is to provide a sting composition. Another object of the present invention is to form a coating film capable of preventing clouding and water droplet formation of the member by making the surface of a mirror, glass, lens, prism or other transparent member highly hydrophilic. ,
An object of the present invention is to provide an antifogging coating composition having excellent storage stability. Another object of the present invention is to make the surfaces of buildings, windowpanes, machinery and articles highly hydrophilic,
It is an object of the present invention to provide an easy-to-clean coating composition excellent in storage stability for preventing a surface from being soiled or forming a coating film whose surface can be cleaned with water. .

[0005]

SUMMARY OF THE INVENTION The present invention is based on the discovery that, in a member having a surface layer containing a photocatalyst formed thereon, when the photocatalyst is photoexcited, the surface of the member is highly hydrophilized. This phenomenon is considered to proceed by the following mechanism. That is, when light having energy equal to or more than the energy gap between the upper end of the valence band and the lower end of the conduction band of the photocatalyst is irradiated on the photocatalyst, the electrons in the valence band of the photocatalyst are excited, and the conduction electrons and holes are generated. Produced and either or both actions impart polarity to the surface. As a result, water is chemically adsorbed on the surface, and a physically adsorbed water layer is formed thereon to increase. As a result, the surface exhibits a high degree of hydrophilicity with a water wetting angle of 10 ° or less.

In the present invention, the average composition formula R _p SiX _q
O _{(4-pq) / 2} (wherein, R is a functional group composed of one or more monovalent organic groups, or two or more selected from monovalent organic groups and hydrogen groups) X is an alkoxy group or a halogen atom, and p and q are 0 <
a film-forming element composed of a siloxane represented by the following formula: p <2, 0 <q <4), which forms a silicone resin film when cured; pH
A titanium oxide particle coated on the surface with inorganic oxide particles of 5 or less, wherein the titanium oxide particles are dispersed in the coating film forming element and are exposed to silicon molecules on the surface of the coating film by photoexcitation. Photocatalytic titanium oxide particles for at least partially replacing an organic group bonded to a silicon atom with a hydroxyl group by photocatalysis and further hydrophilizing the surface of the coating in response to photoexcitation; or an isoelectric point of 9 or more at pH a inorganic oxide particles by coating the surface with a photocatalytic oxidation of titanium particles, the coating film is dispersed forming element in silico of the surface of the coating by light excitation - organic bonded to the silicon atom in emissions molecule The group is at least partially replaced by a hydroxyl group by photocatalysis,
Further, there is provided a photocatalytic hydrophilic coating composition comprising: photocatalytic titanium oxide particles for hydrophilizing the surface of a film in response to photoexcitation. Such a core
When a coating film is formed on the surface of the substrate with the composition, the curing reaction of the silicone forms a surface layer containing the photocatalytic oxide particles and the silicon on the surface of the substrate. In this case, the organic group bonded to the silicon atom in the silicon molecule is at least partially replaced by a hydroxyl group by photocatalysis by the photoexcitation of the photocatalyst, and a physically adsorbed water layer is formed thereon in response to the photoexcitation of the photocatalyst. By doing so, the surface exhibits hydrophilicity close to the water wetting angle of 0 °, and the hydrophilicity retention when held in a dark place is improved. In the present invention, the photocatalytic titanium oxide particles are further coated with an inorganic oxide particle having an isoelectric point of 5 or less or an inorganic oxide particle having an isoelectric point of 9 or more. The titanium oxide particles can exhibit excellent dispersion stability even in a neutral liquid. By the way, the siloxane polymer component represented by (a) of claims 1 and 2 has the best dispersion stability in a neutral liquid, so that the photocatalytic hydrophilic coating composition is coated by the coating. Under the neutral condition, good storage stability is exhibited. The reason is considered as follows.
That is, in the case of only the photocatalytic titanium oxide particles, the isoelectric point is around 7, and under neutral conditions, the isoelectric point on the particle surface becomes almost zero and the particles are easily aggregated. It is considered that by coating with the above-mentioned substance, the isoelectric point of the particle surface shifts to the acidic side or the basic side so that excellent dispersion stability can be exhibited.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, photocatalytic titanium oxide refers to light (excitation light) having an energy (ie, a short wavelength) larger than the energy gap between the conduction band and the valence band of the crystal. Irradiates a titanium oxide capable of generating conduction electrons and holes by excitation of electrons in the valence band (photoexcitation) upon irradiation with, for example, an anatase type titanium oxide and a rutile type titanium oxide. Crystalline titanium oxide can be suitably used. The high hydrophilicity in the present invention means a state exhibiting water wettability of 10 ° or less, preferably 5 ° or less in terms of a contact angle with water.

The silicone described in the first and second aspects (a) has an average compositional formula of R _p SiO _{(4-p) / 2} (where R is one or two monovalent organic groups) X is an alkoxy group or a halogen atom, and p is 0 <p <2. Which satisfies the following formula). This resin can be obtained by partially hydrolyzing and dehydrating polycondensation of a silicone precursor described later.

Here, the precursor of the silicone includes an average compositional formula RpSiXqO (4-pq) / 2 (where R is a functional group comprising one or more monovalent organic groups, or X is an alkoxy group or a halogen atom, and p and q are 0 <p <2, 0 <q <4. A film-forming element comprising a siloxane represented by the following formula: or a general formula RpSiX4-p (wherein R is a functional group comprising one or more monovalent organic groups, or Functional group consisting of two or more selected from a valence organic group and a hydrogen group,
X is an alkoxy group or a halogen atom, and p is 1
Or 2), which is preferably composed of a hydrolyzable silane derivative represented by the formula (1).

Here, as the coating film forming element comprising the above hydrolyzable silane derivative, methyltrimethoxysilane,
Methyltriethoxysilane, methyltripropoxysilane, methyltributoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltripropoxysilane, ethyltributoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenyltripropoxysilane, Phenyltributoxysilane,
Dimethyldimethoxysilane, dimethyldiethoxysilane, dimethyldipropoxysilane, dimethyldibutoxysilane, diethyldimethoxysilane, diethyldiethoxysilane, diethyldipropoxysilane, diethyldibutoxysilane, phenylmethyldimethoxysilane, phenylmethyldiethoxysilane, phenyl Methyldipropoxysilane, phenylmethyldibutoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, n-propyltripropoxysilane, n-propyltributoxysilane, γ-glycoxydoxypropyltrimethoxysilane, γ -Acryloxypropyltrimethoxysilane and the like can be suitably used.

The film-forming element composed of the above-mentioned siloxane includes partial hydrolysis and dehydration-condensation polymerization of the above-mentioned hydrolyzable silane derivative, or partially hydrolyzate of the above-mentioned hydrolyzable silane derivative, tetramethoxysilane and tetramethoxysilane. It can be produced by dehydration polycondensation with a partial hydrolyzate such as ethoxysilane, tetrapropoxysilane, tetrabutoxysilane, diethoxydimethoxysilane and the like.

The isoelectric point described in claim 1 (b) is p
As the inorganic oxide particles having an H of 5 or less, silica, aluminosilicate and the like can be suitably used. Claim 2 (b)
As the inorganic oxide particles having an isoelectric point of 9 or more at pH described in (1), zirconia and the like can be suitably used.

[0013] the method of fixing the coating to the photocatalytic oxidation of titanium particles, for example, spray - co - plating method, a dip - coating method, flow - co - plating method, a spin -
After coating by a coating method, a roll coating method, brush coating, sponge coating or the like, it is fixed by a method such as heat treatment.

The coating composition includes Ag, Cu, Z
A metal such as n can be added. When the metal is added, the surface layer formed by the coating film can kill bacteria and fungi attached to the surface even in a dark place.

[0015] The coating composition includes Pt, Pd, R
A platinum group metal such as u, Rh, Ir, Os can be added. When the metal is added, the surface layer formed by the coating film can enhance the oxidation-reduction activity of the photocatalyst, and can improve the decomposability of organic contaminants and harmful gases and odors.

The coating composition may further contain a solvent such as water, ethanol, propanol, hydrochloric acid, nitric acid, sulfuric acid,
Catalysts that promote hydrolysis of silicone precursors such as acetic acid and maleic acid, basic compounds such as tributylamine and hexylamine, and acidic compounds such as aluminum triisopropoxide and tetraisopropyl titanate For example, a catalyst for curing the precursor of the silicone such as silane coupling agent or a surfactant for improving the dispersibility of the coating liquid such as a silane coupling agent may be added.

The method of using the coating composition of the present invention is basically based on applying the coating composition to the surface of a substrate and curing the coating composition to form a coating film.

The coating method of the above coating composition includes spray coating, dip coating, flow coating, spin coating, roll coating, brush coating, and sponge. A method such as coating can be suitably used. As a curing method, it can be carried out by polymerizing by heat treatment, standing at room temperature, ultraviolet irradiation, or the like.

When a coating film is formed on the member surface by the above method,
The surface of the member becomes hydrophilic in response to the photoexcitation of the photocatalyst. Here, in order for the substrate surface to be highly hydrophilized by photoexcitation of the photocatalyst, the illuminance of the excitation light must be 0.001.
mW / cm2 or more may be sufficient, but it is preferably 0.01 mW / cm2 or more, and more preferably 0.1 mW / cm2 or more. Light sources used for photoexcitation of the photocatalyst include sunlight,
Interior lighting, fluorescent lamp, mercury lamp, incandescent lamp, xenon lamp, high-pressure sodium lamp, metal halide lamp, B
An LB lamp or the like can be suitably used.

The thickness of the surface layer formed by the coating film on the member surface is preferably 0.4 μm or less. Then, cloudiness due to irregular reflection of light can be prevented, and the surface layer becomes substantially transparent. Furthermore, the thickness of the surface layer is
It is more preferable that the thickness be 0.2 μm or less. Then, it is possible to prevent the surface layer from being colored by light interference. Also, the thinner the surface layer, the better its transparency.
Further, when the film thickness is reduced, the wear resistance of the surface layer is improved.

As shown in PCT / JP96 / 00733, if the surface of the member is not more than 10 ° in terms of the contact angle with water, even if moisture or steam in the air is condensed, condensation occurs. Water tends to form a uniform water film without forming individual water droplets. Therefore, the tendency that light scattering fogging does not occur on the surface becomes remarkable. Similarly, when a window glass, a vehicle back mirror, a vehicle windshield, an eyeglass lens, or a shield of a helmet is subjected to rainfall or splashing, discrete unsightly water droplets are not formed, and high visibility and visibility are achieved. As a result, the effects of ensuring safety, ensuring the safety of vehicles and traffic, and improving the efficiency of various tasks and activities are greatly improved. Similarly, as shown in PCT / JP96 / 00733, if the surface of the member is less than or equal to 10 °, preferably less than or equal to 5 ° in terms of the contact angle with water, exhaust from city dust, automobiles, etc. Combustion products such as carbon black contained in gas, hydrophobic contaminants such as oils and fats, sealant eluting components, etc., and inorganic clay contaminants are unlikely to adhere to each other. It is in a state where it can be dropped.

If the surface of the member exhibits the above-mentioned high hydrophilicity and maintains the state, the anti-fogging effect, the surface cleaning effect, the antistatic effect (dust adhesion preventing effect), the heat insulating effect, The effect of preventing bubbles from adhering in water, the effect of improving efficiency in a heat exchanger, the effect of improving biocompatibility, and the like are exhibited.

The substrate to which the present invention can be applied is a transparent member when an anti-fogging effect is expected, and glass, plastic, or the like can be suitably used as the material. Speaking of applicable base materials, mirrors such as rearview mirrors for vehicles, mirrors for bathrooms, mirrors for toilets, dental mirrors, road mirrors; spectacle lenses, optical lenses, illumination lenses, semiconductor lenses Lenses such as copier lenses, rear view camera lenses for vehicles; prisms; windows of buildings and towers; automobiles, railway vehicles, aircraft, ships, submersibles, snowmobiles, lowway gondola, amusement parks Gondolas, vehicle windows such as spaceships; cars, motorbikes, railway vehicles, aircraft, ships, submarines, snowmobiles, snowmobiles, lowway gondola, amusement park gondola, space Windshield for vehicles such as ships; protective goggles, sports goggles, protective mask shield, sports mask shield, helmet shield, frozen food display Case glass, Chinese bun, etc. Glass for display case of warm food; Cover for measuring equipment, Cover for rear view camera lens for vehicle, Focusing lens for laser dental treatment device, etc., Laser light detection for inter-vehicle distance sensor, etc. Cover for an infrared sensor, a cover for an infrared sensor, a filter for a camera, and a film, a sheet, a seal, and the like to be attached to the surface of the article.

When a surface cleaning effect is expected as a substrate to which the present invention can be applied, the material may be, for example, metal, ceramic, glass, plastic, wood, stone, cement, concrete, fiber, Fabrics, combinations thereof, and laminates thereof can be suitably used. Speaking of applicable base materials, building materials, building exteriors, building interiors, window frames, windowpanes, structural members, vehicle exteriors and coatings, machinery and articles exteriors, dustproof covers and coatings, traffic signs, Various display devices,
Advertising towers, noise barriers for roads, noise barriers for railways, bridges, exterior and coating of garages, tunnel interiors and coatings, insulators, solar battery covers, solar water heater collector covers, vinyl houses, vehicle lighting Light covers, housing equipment, toilets, bathtubs, washbasins, lighting fixtures, lighting covers, kitchen utensils, dishes, dishwashers, dish dryers, sinks, cooking ranges, kitchen hoods
And a film, a sheet, a seal, and the like for attaching to the surface of the article.

When the antistatic effect is expected as a substrate to which the present invention can be applied, the material is, for example, metal,
Ceramic, glass, plastic, wood, stone, cement, concrete, fiber, fabric, combinations thereof, and laminates thereof can be suitably used. Speaking of applicable substrates, cathode ray tubes, magnetic recording media, optical recording media, magneto-optical recording media, audio tapes, video tapes, analog records, housings and parts for household electrical appliances, Exterior and painting, OA equipment product housing and parts and exterior and painting, building materials, building exterior, building interior, window frames, window glass, structural members, vehicle exterior and painting, mechanical equipment and articles exterior, dustproof cover and Includes films, sheets, seals, etc. for painting and sticking to the surface of the article.

[0026]

EXAMPLES The photocatalytic oxidation of titanium particles produced by a chlorine method is surface-treated with γ- glycoxide trimethoxysilane, silica was prepared by heating at 500 ° C. under Kutsugaehikari catalyst
Sex acid titanium particles, dispersed in water of pH 7. After mixing the dispersion and the siloxane resin coating, the mixture was diluted with ethanol and stirred for 3 hours to obtain a coating liquid. This coating liquid was left at 17 ° C. for one month, but the dispersibility was good. When the coating liquid was applied to a quartz glass plate and cured at 150 ° C., the sample was irradiated with a BLB lamp having an ultraviolet illuminance of 0.5 mW / cm ² for 3 days.
The contact angle with water became 0 °.

[0027]

The photocatalytic hydrophilic coating composition of the present invention has excellent long-term storage stability and can maintain and maintain the surface of the member to a high degree of hydrophilicity.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C09D 183/04 C09C 3/12 C09D 5/00 C09D 7/12 C09K 3/18 104

Claims (6)

(57) [Claims] 1. A film forming element comprising a siloxane which forms a silicone resin film when cured, and a photocatalytic substance dispersed in the film forming element and hydrophilizing the surface of the film in response to photoexcitation. A photocatalytic hydrophilic coating composition containing titanium oxide, wherein the film-forming element comprising the siloxane comprises a hydrolyzable silicone.
Partial hydrolysis and dehydration polycondensation of orchid derivative or hydrolysis
Partial hydrolyzate of a decomposable silane derivative and tetramethoxy
Silane, tetraethoxysilane, tetrapropoxysila
, Tetrabutoxysilane, diethoxydimethoxysila
Produced by dehydration-condensation polymerization with a partial hydrolyzate of
Xysilane, methyltriethoxysilane, methyltrip
Lopoxysilane, methyltributoxysilane, ethylto
Limethoxysilane, ethyltriethoxysilane, ethyl
Tripropoxysilane, ethyltributoxysilane,
Phenyltrimethoxysilane, phenyltriethoxysila
, Phenyltripropoxysilane, phenyltribute
Xysilane, dimethyldimethoxysilane, dimethyldie
Toxysilane, dimethyldipropoxysilane, dimethyl
Dibutoxy silane, diethyl dimethoxy silane, diethyl
Ludiethoxysilane, diethyldipropoxysilane, di
Ethyl dibutoxy silane, phenyl methyl dimethoxy
Orchid, phenylmethyldiethoxysilane, phenylmethyl
Ludipropoxysilane, phenylmethyldibutoxysila
N-propyltrimethoxysilane, n-propyl
Liethoxysilane, n-propyltripropoxysila
, N-propyltributoxysilane, γ-glycoxy
Doxypropyltrimethoxysilane, γ-acryloxy
Any one of propyltrimethoxysilane, and the photocatalytic titanium oxide has a surface coated with inorganic oxide particles having an isoelectric point of 5 or less at pH. 2. A film-forming element comprising a siloxane which forms a silicone resin film when cured, and a photocatalytic substance dispersed in the film-forming element for hydrophilizing the surface of the film in response to photoexcitation. A photocatalytic hydrophilic coating composition containing titanium oxide, wherein the film-forming element comprising the siloxane comprises a hydrolyzable silicone.
Partial hydrolysis and dehydration polycondensation of orchid derivative or hydrolysis
Partial hydrolyzate of a decomposable silane derivative and tetramethoxy
Silane, tetraethoxysilane, tetrapropoxysila
, Tetrabutoxysilane, diethoxydimethoxysila
Produced by dehydration-condensation polymerization with a partial hydrolyzate of
Xysilane, methyltriethoxysilane, methyltrip
Lopoxysilane, methyltributoxysilane, ethylto
Limethoxysilane, ethyltriethoxysilane, ethyl
Tripropoxysilane, ethyltributoxysilane,
Phenyltrimethoxysilane, phenyltriethoxysila
, Phenyltripropoxysilane, phenyltribute
Xysilane, dimethyldimethoxysilane, dimethyldie
Toxysilane, dimethyldipropoxysilane, dimethyl
Dibutoxy silane, diethyl dimethoxy silane, diethyl
Ludiethoxysilane, diethyldipropoxysilane, di
Ethyl dibutoxy silane, phenyl methyl dimethoxy
Orchid, phenylmethyldiethoxysilane, phenylmethyl
Ludipropoxysilane, phenylmethyldibutoxysila
N-propyltrimethoxysilane, n-propyl
Liethoxysilane, n-propyltripropoxysila
, N-propyltributoxysilane, γ-glycoxy
Doxypropyltrimethoxysilane, γ-acryloxy
Any one of propyltrimethoxysilane, and the photocatalytic titanium oxide has a surface coated with inorganic oxide particles having an isoelectric point of 9 or more at pH. 3. The coating, the surface of which has been hydrophilized in response to photoexcitation, allows the condensation of moisture and / or water droplets adhering to the surface to spread on the surface of the coating, whereby the coating The photocatalytic hydrophilic coating composition according to claim 1 or 2, wherein the substrate coated with the composition is prevented from being clouded or overcast by moisture condensed water and / or water droplets. 4. The coating, the surface of which has been hydrophilized in response to photoexcitation, enables attached sediments and / or contaminants to be washed away by rainwater when exposed to rainfall, whereby the surface self The photocatalytic hydrophilic coating composition according to claim 1 or 2, wherein purification is possible. 5. The coating whose surface has been hydrophilized in response to photoexcitation prevents the contaminant from adhering to the surface when rainwater containing the contaminant comes into contact. Or the photocatalytic hydrophilic coating composition according to item 2. 6. The coating, the surface of which has been hydrophilized in response to photoexcitation, releases adhering deposits and / or contaminants when immersed in or wetted with water, so that the surface is exposed to water. The photocatalytic hydrophilic coating composition according to claim 1 or 2, wherein the composition is easily washed with water.