HK1199053B - Photocatalytic coating - Google Patents

Description

Photocatalytic coating

The divisional application is based on the divisional application of Chinese patent application with the application number of 200880109024.3, the application date of 2008 about 29 and the name of photocatalytic coating.

Technical Field

The present invention relates to compositions for providing photocatalytic coatings on surfaces. More particularly, the present invention relates to a stain removing, self cleaning paint comprising titanium dioxide particles which does not require pre-activation to achieve high initial photocatalytic activity.

Background

Electrons are transferred from the valence band to the conduction band under the action of Ultraviolet (UV) and near UV radiation, so that the semiconductor material titanium dioxide has photocatalytic properties. The resulting reactive electron-hole pairs migrate to the surface of the titanium dioxide particles where they oxidize the adsorbed water to produce reactive hydroxyl radicalsRadicals and the electrons reduce the adsorbed oxygen to produce superoxide radicals, both of which are capable of degrading NO in air_xAnd Volatile Organic Compounds (VOCs). In view of these properties, photocatalytic titanium dioxide and the like have been used in paints and the like to remove pollutants from the air. Such coatings also have the advantage of being self-cleaning, since dirt (grease, mold, mildew, algae, etc.) is also oxidized on the surface.

Despite the advantages of existing photocatalytic titanium dioxide coatings, there is room for improvement in the art. In particular, it has been observed that the initial activity of conventional photocatalytic titanium dioxide coatings is poor unless the coating has been preactivated, for example by washing with water. While not wishing to be bound by any theory, it is believed that an activation step is required to remove organic components present in the coating composition from the catalyst surface, or may be required to provide a hydrated surface on the titanium dioxide particles, thereby generating reactive free radical species. However, this additional step makes the application of the photocatalytic titanium dioxide coating somewhat inconvenient because it is time consuming and adds additional cost to the application process. It would be desirable to provide photocatalytic titanium dioxide coatings, particularly in the form of paints, that do not require pre-activation (e.g., washing steps or contact with certain elements) to achieve high initial activity levels.

It is also difficult to provide coatings with high photocatalyst content because the catalyst is prone to oxidize and decompose the polymeric binder of the coating. This problem is exacerbated when the coating is exposed to strong UV radiation from direct sunlight, as is the case with exterior wall paints. Such coatings are typically formulated with inorganic binders or with organic polymers that are resistant to photocatalytic oxidation at relatively low catalyst concentrations. However, under low light conditions, the stain removal performance of the coating is less than optimal. It would be desirable to provide coatings for use in low light environments (e.g., indoors) that incorporate high levels of photocatalyst to render soil removal most preferred and which are resistant to degradation, and additionally to provide high catalytic activity under indoor lighting conditions.

It is therefore an object of the present invention to provide coating compositions, especially paint compositions, comprising a titanium dioxide photocatalyst capable of removing contaminants from air, which photocatalyst has a high initial activity without the need for pre-activation. It is a further object of the present invention to provide a durable coating having a high content of photocatalytic titanium dioxide, which coating has a stain removal activity in low light environments, especially in the presence of visible light.

The foregoing discussion is presented merely to provide a better understanding of the nature of the problems faced in the art and should not be construed as an admission that any of the documents cited herein constitute "prior art" to the present application in any way.

Summary of The Invention

In light of the foregoing objects and others, it has surprisingly been found that coatings comprising titanium dioxide having a crystallite size of from about 1nm (nanometers) to about 150nm, more preferably from about 5nm to about 30nm, preferably from about 5 to about 10nm, can achieve high initial levels of photocatalytic activity in the presence of light without the need for pre-activation (e.g., by washing with water). The coatings of the present invention exhibit significant photocatalytic activity in the presence of visible light, making them ideal for use as stain release coatings in low light environments, including indoors.

In one aspect of the invention, the self-cleaning, stain release coating composition is in the form of a water-based paint comprising (i) from about 5% to about 40% by volume of photocatalytic titanium dioxide, preferably in the substantially pure anatase form, characterized by an average crystallite size between about 5nm and about 30nm and having photocatalytic activity in the presence of visible light; (ii) one or more other pigments such that the total Pigment Volume Concentration (PVC) in the paint, including the photocatalytic titanium dioxide, is at least about 65%; and (iii) a styrene acrylic copolymer binder; the paint can reduce NO greatly under the condition of not being pre-activated by water_xA compound is provided.

Another aspect of the present invention provides a substrate having deposited thereon the self-cleaning, stain release coating composition of the present invention, optionally further comprising an overcoat disposed over the paint layer, the overcoat comprising a second photocatalytic titanium dioxide having a grain size in the range of 5nm to 30nm, the overcoat being formed by applying a sol to the paint layer.

In another aspect of the invention, there is provided the removal of NO from air_xOr other contaminants, comprising applying to a surface such as a wall, floor, ceiling or the like a layer of a decontamination coating of the present invention, with or without pre-activation by washing with an aqueous solvent, preferably without a washing step, said coating being capable of substantially removing contaminants from the air in the presence of UV and/or visible light, preferably in the presence of visible light, and optionally applying over said layer of paint a sol top coating comprising photocatalytic titanium dioxide.

These and other aspects of the invention will be better understood by reference to the following detailed description and drawings.

Brief Description of Drawings

FIG. 1 compares NO for two photocatalytic titanium dioxide coatings that have not been preactivated under various lighting conditions_xWherein "comparative 1" is a coating comprising photocatalytic titanium dioxide powder having an average grain size of about 5 to 10nm, and "comparative 2" is a coating comprising photocatalytic titanium dioxide having an average grain size of about 15 to 25 nm.

FIG. 2 compares NO for various coating systems comprising styrene acrylic photocatalytic paints according to the present invention_xActive, the paints have different photocatalytic titanium dioxide sol surface coatings (B-G) disposed thereon.

Detailed Description

As used herein, unless otherwise indicated, all terms used hereinThere are terms that have their ordinary meaning. All references herein to "wt%" refer to the percentage by mass of the total paint formulation (other than the dried paint) including solvent, unless otherwise specified. Unless otherwise specified, reference to "volume%" or "pigment volume concentration" means the volume% of the dried paint or coating. The term "NO_x"refers to the species NO (nitric oxide) and NO, collectively or individually₂(nitrogen dioxide).

In the broadest sense of the present invention, the self-cleaning, stain release coating composition comprises photocatalytic titanium dioxide particles, an organic binder, and optionally one or more other pigments such as calcium carbonate. The coating may be in the form of a paint (interior or exterior wall), particularly a water-based paint, ideally having a high (e.g. greater than 60%) total Pigment Volume Concentration (PVC).

The coating or paint can greatly reduce NO under the condition of not using water for preactivation_xA compound is provided. It is to be understood that while the coating of the present invention is capable of substantial contaminant reduction without pre-activation with water, it is within the scope of the present invention to activate the coating with water after application to further enhance photocatalytic activity.

When it is stated that a paint has significant "initial" photocatalytic activity without pre-activation with water, it is meant that the paint has significant measurable activity against NOx compounds after the paint coating formed on the substrate has completely dried and/or cured to the extent that such a coating is generally ready for use (e.g., it is tack-free and does not readily transfer upon contact, etc.).

Where reference is made to "removing" contaminants from the air, this is to be understood as including the complete or partial removal of the contaminants from the air. Whether or not "substantial" removal is possible is determined by the methods provided in the examples, where "substantial" removal means that the total concentration of a fixed amount of a given contaminant is reduced by at least about 2.5%, preferably at least about 5%, more preferably at least about 7.5%.

The self-cleaning, stain-removing paint of the present invention comprises photocatalytic titanium dioxide (TiO)₂) Particles capable of generating electron-hole pairs in the presence of electromagnetic radiation, in particular Ultraviolet (UV), near UV and/or visible light. Preferably, the photocatalytic titanium dioxide can be significantly photoactive in the presence of visible light. To this end, it has surprisingly been found that by carefully controlling the crystalline form and particle size of the titanium dioxide, a photocatalyst is provided which is capable of removing contaminants and has significant initial activity in low UV light environments, especially indoor environments, even without washing with a solvent (e.g. water) for activation.

The photocatalytic titanium dioxide particles used in the paint composition are preferably predominantly in the anatase crystalline form because of their higher photoactivity than the rutile form. By "predominantly" is meant that the titanium dioxide particles of the paint have an anatase content of greater than 50 mass%, but preferably have an anatase content of greater than about 80%, more preferably greater than about 90%. In certain embodiments, the photocatalytic titanium dioxide particles of the paint are substantially in the pure anatase form, meaning that the content of rutile crystalline form is less than about 5%, more particularly less than about 2.5%, and still more preferably less than about 1% by mass. In certain embodiments, the photocatalytic titanium dioxide particles will be free of the rutile form, meaning that the rutile crystal form is undetectable using crystallography. In other words, the photocatalytic titanium dioxide particles may comprise 100% of the anatase form. The crystallinity and properties of the crystalline phase were measured using X-ray diffraction.

The photocatalytic titanium dioxide particles used in the paint composition generally have an average particle size such that the particles absorb light predominantly, rather than scattering it. As the particle size becomes very small, the band gap between the valence band and the conduction band becomes small. Thus, when the particle size is sufficiently small, it has been observed that the titanium dioxide particles are capable of absorbing light in the visible spectrum. The carbon dioxide particles included in the paint of the present invention generally have a particle size of between about 1nm and about 150 nm. More typically, the particle size is between about 5nm and about 20nm, 25nm, or about 30 nm. In a preferred embodiment, the particle size of the titanium dioxide in the paint is between about 5nm and about 15nm, more particularly between about 5 and about 10 nm. It is to be understood that reference herein to the size of the carbon dioxide particles (or grains) is to the average particle size of the titanium dioxide particles. When the term "about" is used to modify particle size, it should be understood to include particle sizes that are slightly larger or smaller than the indicated values, as there is inherent experimental error in the measurement and variability between different methods of measuring particle size, as will be apparent to those skilled in the art. The diameter can be measured by, for example, Transmission Electron Microscopy (TEM) and XRD.

Alternatively, the particles are characterized by surface area. Generally, the surface area of the powdered titanium dioxide photocatalyst will be greater than about 70m, as measured by any suitable method, including the 5-point BET method²A/g, more typically greater than about 100m²Per g, and preferably greater than about 150m²(ii) in terms of/g. In certain embodiments, the surface area of the titanium dioxide photocatalyst will be greater than about 200m²A/g of greater than about 250m²A/g, even greater than about 300m²/g。

It has been found that photocatalytic titanium dioxide available from the company inorganic chemicals of the american federate under the names PCS300 and PC500 is particularly suitable for inclusion in the paint according to the invention. PCS300 is a 100% anatase titania powder having an average grain size between about 5nm and about 10 nm. PC500 is also 100% anatase titanium dioxide powder, TiO₂Between about 82 wt.% and about 86 wt.%, and having a surface area of from about 250 to about 300m2/g as measured by the same 5-point BET method, corresponding to an average particle size of from about 5nm to about 10 nm. A product also from inorganic chemical company of america under the name PC105 may also be used in certain embodiments of the present invention. This photocatalytic powder comprises more than 95% by weight of titanium dioxide, 100% anatase and having an average crystallite size of from about 15nm to about 25nm, a surface area of about 80 and about 100m²Between/g.

The photocatalytic titanium dioxide typically comprises from about 2 to about 40 volume percent of the paint formulation. More typically, the photocatalytic titanium dioxide comprises from about 5% to about 20%, preferably from about 7.5% to about 15% by volume of the paint. In a representative embodiment, the photocatalytic titanium dioxide comprises about 10% by volume of the paint formulation. The foregoing amounts represent the volume of photocatalyst in the final paint formulation (e.g., including solvent) rather than the volume percent in the dried paint coating. Typically, the weight percent of titanium dioxide in the paint formulation is between about 1wt% and about 20wt%, more typically between about 5wt% and about 10wt%, preferably about 7.5 wt%.

It is within the scope of the present invention to provide a paint having two or more different titanium dioxide photocatalysts wherein at least one, and preferably each, titanium dioxide photocatalyst material meets the above specifications. Thus, for example, the present invention includes the use of bimodal photocatalytic titanium dioxide materials formed by combining two different titanium dioxide powders or sols together, wherein the particle size and/or surface area of at least one, and preferably both, is as defined above. In other embodiments, the photocatalyst will "consist essentially of" the particular titanium dioxide material described herein, which means that any other photocatalyst having a significantly different activity is excluded, or the amount of other photocatalyst that can significantly affect paint durability, stain removal performance, or self-cleaning performance is excluded.

The paint of the present invention comprises an organic binder. In the broadest aspect of the present invention, it is contemplated that any polymeric binder may be used. In one embodiment, the polymer binder is a water-dispersible polymer including, but not limited to, latex binders such as natural latex, neoprene latex, nitrile latex, acrylic latex, vinyl acrylic latex, styrene-butadiene latex, and the like. Exemplary polymers for use in these compositions include, but are not limited to, methyl methacrylate, styrene, 2-hydroxyethyl methacrylate polymers (CAS # 70677-00-8); acrylic acid, methyl methacrylate, styrene, hydroxyethyl acrylate, butyl acrylate polymer (CAS # 7732-38-6); butyl acrylate, methyl methacrylate, hydroxyethyl acrylate polymer (CAS # 25951-38-6); butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, acrylic acid polymer (CAS # 42398-14-1); styrene, butyl acrylate polymer (CAS # 25767-47-9); butyl acrylate, 2-ethylhexyl acrylate, methacrylic acid polymer C (CAS # 31071-53-1); an acrylic polymer; and carboxylated styrene butadiene polymers, to name a few. It is further contemplated that combinations of more than one organic binder may also be used in the practice of the present invention.

In particular, the organic binder may be chosen from copolymers of styrene/butadiene and polymers and copolymers of acrylic esters, in particular copolymers of polyvinyl acrylate and styrene/acrylic esters. In the present invention, the styrene acrylic copolymer includes a styrene/acrylate copolymer thereof. Has been found to trade name ACRONAL^TMThe styrene acrylic emulsion sold at 290d (basf) is particularly suitable for use as the organic binder in the paints of the invention.

In certain embodiments, the organic binder in the paints of the present invention "consists essentially of a" preferred styrene acrylic binder, "which means excluding other organic binders that are present in an amount that significantly reduces the durability of the paint coating on the substrate, as compared to an otherwise identical paint coating that contains only a styrene acrylic binder as the organic binder.

In certain embodiments, the paints of the present invention are substantially free of inorganic binders, meaning that the inorganic binder is present in an amount insufficient to form a coherent adherent film on a substrate in the absence of an organic binder. In representative embodiments, the paint comprises less than 0.5wt%, preferably less than about 0.2wt%, and more preferably less than about 0.1wt% of the inorganic binder. In certain embodiments, the paints of the present invention are free of inorganic binders. Inorganic binders include, but are not limited to, alkali metal silicates such as potassium silicate, sodium silicate, and/or lithium silicate.

The paints of the invention also comprise one or more pigments. The term "pigment" is intended to include, but is not limited to, pigments used as colorantsCompounds, including white pigments; and ingredients commonly known in the art as "opacifying agents" and "fillers". Included are any particulate organic or inorganic compounds capable of providing hiding power to the coating, particularly at least one inorganic compound such as non-photocatalytic titanium dioxide. Such titanium dioxide pigments without photoactivity are disclosed in us patent 6342099 (riti inorganic chemical company), the disclosure of which is incorporated herein by reference. In particular, the titanium dioxide pigment may be Tiona sold by inorganic chemical company of American Union^TM595 by granulation. Pigments also include calcium carbonate, which is typically added to paints as a filler. One suitable calcium carbonate material is known under the trade name Setacarb^TMMaterials sold under 850oc (omya).

The Pigment Volume Concentration (PVC) in the paint of the present invention is typically, but not necessarily, between about 60% and about 90%, more typically between about 65% and about 80%, and preferably between about 70% and about 75%. The term "pigment volume concentration" refers to the total volume percentage of all pigments in the composition, wherein the term "pigment" includes all forms of titanium dioxide, whether photocatalytic (e.g., PC500) or non-photocatalytic (e.g., Tiona)^TM595) And any other component commonly recognized in the art as a pigment including, but not limited to, calcium carbonate and other particulate fillers.

Various other compounds may be added to the compositions of the present invention if desired, but preferably such addition does not impair the shelf life, photoactivity, durability or non-staining properties of the resulting coating. Examples of such other compounds include: fillers such as quartz, calcite, clay, talc, barites and/or Na-Al-silicates, etc.; pigments such as TiO₂Lithopone and other inorganic pigments; dispersants such as polyphosphates, polyacrylates, polyphosphonates, cycloalkane sulfonates, and lignosulfonates, to name a few; wetting agents including anionic, cationic, amphoteric and/or nonionic surfactants; defoamers such as silicone emulsions, hydrocarbons, and long chain alcohols; stabilizers, including, for example, macrogolA partially cationic compound; coalescing agents including, but not limited to, alkali-stable esters, glycols, and hydrocarbons; rheological additives such as cellulose derivatives (e.g. carboxymethyl cellulose and/or hydroxyethyl cellulose), xanthan gum, polyurethanes, polyacrylates, modified starches, organic bentonites and other layered silicates; water-proofing agents such as alkyl silicates, silicones, wax emulsions, fatty acid Li salts; and conventional fungicides or bactericides.

Example 1

The ability of the coatings of the present invention to remove NOx contaminants, their self-cleaning properties and their durability were studied by preparing three water-based styrene acrylic paints. The comparative samples "control 1" and "control 2" each contained 10 vol% photocatalytic titanium dioxide, while no photocatalyst was present in the control sample. The photocatalytic titanium dioxide used in comparative example 1 was PCS300 from inorganic chemicals america. PCS300 is a photocatalytic titanium dioxide powder having an average grain size of about 5 to about 10nm (nanometers). The photocatalytic titanium dioxide used in comparative example 2 was PC105, also available from inorganic chemical company of America, and had an average grain size of about 15 to 25 nm. Both PCS300 and PC105 had an anatase content of about 100%. The complete paint formulation is provided in table 1.

TABLE 1

The remaining components of table 1 are as follows: the thickener is known under the name Natrosol^TMA 3% solution of hydroxyethylcellulose sold by 250MR (Hercules). Antifoaming agent Foammaster^TMNXA is a proprietary product sold by Henkel Corp. Setacarb^TM850OG is a calcium carbonate filler from Omya. Antiprex^TMA is a water-soluble polymeric dispersant from Ciba specialty Chemicals. Tiona^TMT595 is pigmentary titanium dioxide from the company America inorganic Chemicals. Acronal^TM290D is a styrene acrylic copolymer latex from BASF used as an organic binder. Acronal^TM290D contain 50wt% solids in water. Texanol^TMAn ester alcohol coalescing solvent sold by Eastman Kodak. Acticide SPX is a germicide available from Actichem Specialties Inc.

The ingredients of part a and part B were mixed separately under high shear mixing. Then, part a was added to part B under high shear mixing to form the final paint. On the substrate at 770g/m²Coverage (based on the dried weight of the coating) various paint samples were applied and the substrates were subjected to the following tests.

I-determination of NOx removed by coating

A complete method for determining NOx removal is described in U.S. patent publication 2007/0167551, the contents of which are incorporated herein by reference. Briefly, the sample is placed in an airtight sample chamber and sealed. The sample chamber communicates with a three-channel gas mixer (Brooks Instruments, Holland) through which NO (nitric oxide), NO are mixed₂(nitrogen dioxide) and compressed air containing water vapor are introduced into the chamber at a predetermined level. Using a source with model number VL-6LM365&An ultraviolet lamp with a wavelength of 312 nanometers (BDH) and a wavelength of 8W/m within the range of 300-400 nm²The initial and final values of NOx (after five minutes of irradiation) were measured by a nitrogen oxide analyzer (Monitor Europe) model ML9841B connected to the sample cell the percentage reduction of NOx was measured as (△ NOx/initial NOx) × 100 the samples were studied without pre-activation and pre-activation (after washing with water) the results are summarized in table 2.

TABLE 2

The results show that the paint containing photocatalytic titanium dioxide powder having an average crystallite size of from about 5 to about 10nm (comparative 1) exhibits surprisingly high NOx activity even without pre-activation of the photocatalyst by a conventional washing step. In contrast, comparative 2, which contained titanium dioxide powder having an average grain size of from about 15nm to about 25nm, exhibited a much smaller degree of NOx reduction in the absence of the pre-activation step. Both comparative 1 and comparative 2 exhibit excellent NOx removal performance after washing to pre-activate the catalyst. However, unexpectedly, even in the case of pre-activation of the control 2 sample, control 1, which was not pre-activated, was superior to control 2.

II-determination of the photoactivity of the coating on methylene blue

Photoactivity of methylene blue was determined using a method similar to that described in U.S. patent publication 2007/0167551, the disclosure of which is incorporated herein by reference and modified as described herein. The self-cleaning properties of various paint samples were studied based on their ability to degrade the organic dye methylene blue. Loss of color is observed due to degradation of the pigment to water, carbon dioxide and nitrogen-containing species. The optical activity was monitored by measuring L (brightness). The procedure was as follows:

films of the paint are prepared on suitable substrates such as polyester films, aluminium panels or glass plates. The thickness of the film when dried should be similar to the thickness used in the end application and is typically no less than 25 microns thick. The paint film was allowed to dry at least overnight.

0.3739g of methylene blue was dissolved in 1 liter of water so that the concentration was 1mmol/L, thereby preparing a solution of methylene blue in water. The methylene blue solution was poured into a suitable tray and the paint film was dipped therein. Soaking the paint film in the methylene blue solution for 30-60 minutes to ensure that methylene blue is chemically absorbed into TiO₂On the surface of (a).

The paint film was removed from the solution and the excess solution was removed using an absorbent tissue. The paint film was thoroughly dried and then measured for brightness (L) value using a colorimeter or spectrophotometer.

For example, the paint film is exposed to UV light in an Atlas Suntest cabinet for a period of 18 to 48 hours at an intensity of 30 to 60W/m²(wavelength of 300 to 400 nm).

The L values are re-measured. The difference between the initial and final L-measurements is a measure of the self-cleaning ability of the coating. The larger the difference between the values of L, the greater the self-cleaning effect. The results for each paint after 18 and 36 hours of irradiation are shown in table 3 below.

TABLE 3

The results show that the paint containing photocatalytic titanium dioxide powder having an average grain size of about 5 to about 10nm (comparative 1) exhibits significantly greater self-cleaning activity after 18 hours and 36 hours of irradiation than the comparative 2 sample.

III-determination of the durability of the coating

A complete method for determining the durability of a paint is described in U.S. patent publication 2007/0167551, the disclosure of which is incorporated herein by reference. The method involves emitting 550W/m at 340nm²A paint film of 20-50 microns thickness on a stainless steel substrate was weathered in a Ci65A weatherometer (Atlas electric devices, Chicago) with a UV xenon source of 6.5kW at an accelerated rate. The samples were heated to about 63 ℃ and sprayed with water every 120 minutes for 18 minutes without a dark cycle. Durability was measured as a function of weight loss after sample exposure.

Table 4 summarizes the results of the durability tests of comparative 1 and comparative 2 at various time intervals of up to 1551 hours.

TABLE 4

As shown in table 4, the durability of the comparative 2 paint was substantially equivalent to the durability of the less photoactive comparative 1 paint after about 1000 hours of exposure. This result is unexpected because it would have been expected that the more photoactive paint of comparative 2 would deteriorate significantly faster than comparative 1, which is less active, under these conditions. Note that the weight loss percentage of the more active comparative 1 paint was more or less greater during 765 hours of exposure, with the largest difference observed after about 451 hours. This may be due to the fact that: in comparison to comparative 2, comparative 1 has a much higher initial activity without pre-activation (see table 2). However, during weathering, both paints were fully activated due to the presence of water, and at longer intervals, it was seen that the weight loss percentages tended to converge. Throughout the accelerated weathering, comparative 1 demonstrates superior durability comparable to comparative 2.

III-determination of NOx removal under different light sources

The respective abilities of paint samples, comparative 1 and comparative 2, to remove NOx under different light sources were determined using the procedure described above for determining NOx removal in section I of the example. In addition to UV, low intensity fluorescent strip lighting, daylight (filtered through glass) and Osram incandescent light sources were used. In each case, the paints were tested without pre-activation. The results are prepared in the following table (table 5) and shown in fig. 1.

TABLE 5

UV light source is VL-6LM365&A 312 nanometer wavelength (BDH) UV lamp, as used in section I of this example. Fluorescence is the light produced by illumination of a conventional indoor fluorescent strip. Sunlight is filtered through glass toProviding 2.4. mu.W/cm²The strength of (2). Incandescent light is provided by Osram incandescent lamps.

The results shown in Table 5 show that the comparative 1 paint, which was not preactivated, exhibited significant NOx removal activity under various lighting conditions, whereas the comparative 2 paint, which was not preactivated, exhibited no activity under fluorescent or incandescent light irradiation and was in sunlight (2.4. mu.W/cm)²) Has no substantial activity. The superior performance of the comparative 1 paint under these ultra-low UV irradiation conditions is believed to be due to the ability of the PCS300 photocatalyst to absorb the visible spectrum. Without wishing to be bound by any particular theory, it is believed that the very small grain size (e.g., about 5-10 nm) results in a reduction in the band gap between the valence and conduction bands, thereby allowing the particles to generate electron-hole pairs in the presence of visible light.

Example 2

Although paints with photocatalysts having a crystallite size between about 5 and about 15nm represent a preferred embodiment of the present invention, including photocatalytic TiO having a particle size of about 5 to 10nm, designated as comparative 1 in example 1, for example₂The benefits of high PVC (pigment volume concentration) that can be achieved by using a styrene acrylic binder can also be seen, although milder with the less preferred titanium dioxide grain size (i.e., about 15 to about 50 nm). Paints, for example, using high levels of PC105 photocatalyst (grain size of about 15nm to about 25 nm) have also been found to be useful in NOx removal coatings.

This example shows the efficacy of the paint designated as comparative 2 in example 1 in removing contaminants under "real world" conditions. Sealing the corners of the parking lot by constructing two walls to provide 917m³And a ceiling height of 2.85 m. 322m was coated with comparative 2 paint from example 1²While covering the walls with nylon (both existing and artificial). The photocatalytic paint was not pre-activated by washing with water. During the NOx removal test, the symmetry was maintained by 20cm from the ceilingTwenty fixed UV lamps irradiate the enclosed space to provide 1W/m²Total UV irradiance of.

Exhaust gas from a vehicle placed outside the enclosed space is connected to the enclosed area by a duct so that the exhaust gas is released to 4.74m inside the enclosed area. Ventilation (inlet and outlet) is provided in the room through artificial walls to increase the concentration of contaminants near the ceiling and to provide airflow rates and velocities of 566m each³H and 14.3 m/h. The flow rate and the flow velocity of the exhaust gas of the automobile were estimated to be 50.6m, respectively³H and 2m/s, and is therefore maintained at a positive pressure in the enclosed space to avoid air flowing into the enclosed space from the outside.

A portable gas analyzer was used to continuously measure the NOx exhaust of the automobile. In addition, NOx measurements were continuously taken at the inlet and outlet of the ventilator and at a third sampling point about 15m from the ventilator outlet and near the ceiling.

After allowing the exhaust gas to reach a steady state (about 3 hours) in the enclosed space, the UV lamp was turned on for four or five hours. Mixing NO with NO₂The decrease in (c) is measured as the difference between the steady state concentration and the final concentration after irradiation. Reduction of NO concentration in exhaust gas of automobile and NO₂The value of the concentration rise as a function of the test time was corrected to isolate the contribution of the photocatalytic paint to the total reduction of these contaminants. The experiment was repeated over three consecutive days. On the fourth day, control measurements were made in the absence of UV irradiation. The results are shown in Table 6 (percent photocatalytic degradation of NO) and Table 7 (NO)₂Percent photocatalytic degradation).

TABLE 6

TABLE 7

It is apparent from the data in tables 6 and 7 that styrene acrylic paints containing 10 volume percent (about 8 wt%) photocatalytic titanium dioxide crystallites having an average particle size of about 15 to 25nm are effective in reducing NOx pollutants from air even without pre-activation. In addition, this example highlights the usefulness of the paint coating of the present invention when applied to such parking lot interiors where it is desirable to remove heavy contaminants from the air.

Example 3

A styrene acrylic paint was prepared essentially as described in example 1, except that PCs300 was replaced with a similar 100% anatase photocatalytic titanium dioxide available from american union inorganic chemicals under the trade name PC 500. The surface area of PC500 was about 300m²(ii)/g, corresponding to an average grain size of about 5 to about 10 nm. PC500 was included in the paint at a level of 8% by volume, with the styrene acrylic paint base constituting about 50% by volume. The ability of the paint to remove NOx without pre-activation was investigated according to the procedure described in example 1 above, as being at 0.5W/m²～8W/m²UV intensity in the intensity range. The results are given in Table 8

TABLE 8

UV intensity (W/m)2)	Percentage of NOx reduction
		0.5	31.3
1	37.1
		2	40.6
3	44.2
		4	45.5
5	46.4
		6	46.9
7	46.9
		8	47.3

These results show that the paint of the invention provides high contaminant removal even at very low UV intensities, even without pre-activation. In fact, the difference in NOx reduction is only 16% (from 47.3% to 31.3%) when the UV intensity increases by more than an order of magnitude.

Utilizing various photocatalytic TiO's as set forth in Table 9₂The sol was coated on top of PC500 paint to investigate if further improvement in NOx removal performance could be obtained.

TABLE 9

Sample (I)	Sol surface coating
		A	Is free of
B	S5300A
		C	SP300N
D	S5300B (23.6% w/w TiO)2)
		E	S5300B (10.0% w/w TiO)2)
F	S5300B (5.0% w/w TiO)2)
		G	AW1610(0.24% w/w TiO)2)

Sample a represents a styrene acrylic paint containing a PC500 photocatalyst and without any sol surface coating. Samples B-G are the paints of sample A on which the sol surface coatings shown were applied. S5300A is a photocatalytic titanium dioxide sol obtained from inorganic chemicals of ritie union. It is a superfine TiO colloid gelled with an acid at a pH of about 1.1 (+ -0.4)₂(anatase) aqueous colloidal dispersion having a titanium dioxide content of about 20 (+ -2) wt%, a density of about 1.2g/ml and a surface area of more than 250m by 5-point BET method (measured on dry product)²(ii) in terms of/g. S5300B is also available from America inorganic chemistry, Inc. as alkali at about 1Ultra-fine TiO gellable at pH of 1.4 (+ -1)₂(anatase) aqueous colloidal dispersion having a titanium dioxide content of about 17.5 (+ -2.5) wt%, a density of about 1.1g/ml and a surface area of more than 250m by 5-point BET method (measured on dry product)²(ii) in terms of/g. Various S5300B sols shown in table 9 were modified to have the indicated titania contents by weight. AW1610 is a photocatalytic TiO containing an average grain size of about 3.6nm₂At a pH of 9.2 and a density of about 1.00g/ml, TiO₂The content is about 0.25%. SP300N is a photocatalytic TiO with an average grain size of about 5-10 nm₂(about 17wt%) of a slurry, pH 7.0, density about 1.15 g/ml.

The ability of various coating systems (paint + sol) to remove NOx was investigated as ranging from 0.5W/m²To 8W/m²As a function of UV light intensity. The results are shown in fig. 2. It can be seen that coating system D, which comprises a PC500 paint and has an S5300B overcoat (23.6% w/w TiO) exhibits unexpectedly superior NOx removal capability over the entire UV intensity range with minimal variation in the percent NOx reduction within that range₂)。

All documents, including patent applications and publications cited herein, are hereby incorporated by reference in their entirety and for all purposes to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference in its entirety. It will be apparent to those skilled in the art that many modifications and variations of the present invention can be made without departing from the spirit and scope thereof. The specific embodiments described herein are offered by way of example only, and the invention is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims (22)

1. A self-cleaning, stain-removing paint comprising:

(i) photocatalytic titanium dioxide in the anatase form in a proportion of between 5% and 40% by volume, wherein the content of the rutile crystalline form is less than 5% by mass, said photocatalytic titanium dioxide being characterized by an average crystallite size between 5nm and 30nm and by photocatalytic activity in the presence of visible light;

(ii) one or more other pigments such that the total pigment volume concentration in the paint, including the photocatalytic titanium dioxide, is at least 65%; and

(iii) a styrene acrylic copolymer binder;

the paint is capable of substantial NO reduction immediately after the paint forms a dry coating on a substrate without pre-activation with water_xA compound is provided.

2. The paint according to claim 1, wherein the photocatalytic titanium dioxide has an average grain size between 5nm and 10 nm.

3. The paint according to claim 1, wherein said photocatalytic titanium dioxide comprises between 7% and 15% by volume of said paint.

4. The paint according to claim 1, wherein said photocatalytic titanium dioxide comprises 10% by volume of said paint.

5. The paint of claim 1, wherein the one or more other pigments comprise non-photocatalytic titanium dioxide.

6. The paint of claim 1, wherein the one or more other pigments comprise calcium carbonate.

7. The paint of claim 1, wherein the one or more other pigments comprise non-photocatalytic titanium dioxide and calcium carbonate, and wherein the total pigment volume concentration is between 70% and 75%.

8. The paint of claim 1, wherein the paint is free of inorganic binders.

9. The paint of claim 1, further comprising one or more ingredients selected from the group consisting of solvents, thickeners, dispersants, coalescents, antifoaming agents, biocides, and combinations thereof.

10. A method of forming a self-cleaning, stain-release coating on a substrate, the method comprising:

(a) applying to the substrate a paint composition comprising:

(i) 5% to 40% by volume of a photocatalytic titanium dioxide characterized by an average grain size between 5nm and 30nm and having photocatalytic activity in the presence of visible light;

(ii) one or more other pigments such that the total pigment volume concentration in the paint, including the photocatalytic titanium dioxide, is at least 65%; and

(iii) a styrene acrylic copolymer binder; and

(b) optionally, applying a surface coating comprising a titanium dioxide sol on the paint,

the titanium dioxide sol comprises photocatalytic titanium dioxide particles;

wherein the coating is capable of substantially reducing NO without pre-activation with water_xA compound is provided.

11. The method according to claim 10, wherein the paint composition comprises photocatalytic titanium dioxide having an average crystallite size between 5nm and 10 nm.

12. The method according to claim 10, wherein said photocatalytic titanium dioxide comprises from 7% to 15% by volume of said paint composition.

13. The method according to claim 10, wherein said photocatalytic titanium dioxide comprises 10% by volume of said paint composition.

14. The method according to claim 10, wherein the one or more other pigments comprise non-photocatalytic titanium dioxide.

15. The method according to claim 10, wherein the one or more other pigments comprise calcium carbonate.

16. The method according to claim 10, wherein the one or more other pigments comprise non-photocatalytic titanium dioxide and calcium carbonate, and wherein the total pigment volume concentration in the paint composition is between 70% and 75%.

17. The method according to claim 10, wherein the paint composition is free of inorganic binder.

18. The method according to claim 10, wherein the paint composition further comprises one or more ingredients selected from the group consisting of solvents, thickeners, dispersants, coalescents, antifoaming agents, biocides, and combinations thereof.

19. A substrate having a coating system applied thereto, comprising:

(a) a stain release paint layer formed by applying to the substrate a paint composition comprising:

(i) photocatalytic titanium dioxide in the anatase form in a proportion of between 5% and 40% by volume, wherein the content of the rutile crystalline form is less than 5% by mass, said photocatalytic titanium dioxide being characterized by an average crystallite size between 5nm and 30nm and by photocatalytic activity in the presence of visible light;

(ii) one or more other pigments such that the total pigment volume concentration in the paint, including the photocatalytic titanium dioxide, is at least 65%; and

(iii) a styrene acrylic copolymer binder; and

(b) optionally, a topcoat disposed on the layer of stain removing paint, the topcoat being formed by applying to the paint layer a sol comprising an aqueous colloidal dispersion of photocatalytic ultrafine titanium dioxide in the anatase crystalline form and having a surface area greater than 250m²In terms of the specific surface area, measured by the 5-point BET method,

wherein the coating is capable of significantly reducing NO without pre-activation with water_xA compound is provided.

20. The substrate according to claim 19, wherein said paint composition comprises photocatalytic titanium dioxide having an average crystallite size between 5nm and 10nm, and wherein said photocatalytic titanium dioxide comprises from 7% to 15% by volume of said paint composition.

21. The paint according to claim 1, wherein the paint comprises from 0wt% to less than 0.2wt% of the inorganic binder.

22. The method according to claim 10, wherein the paint comprises from 0wt% to less than 0.2wt% of an inorganic binder.