Building Curtain Wall Calling for Self-cleaning and Automatic Dust Removing Coatings

Long Wenzhi
(Expert Group, Technical Committee for Standardization of Curtain Wall, Door and Window of MOHURD, Beijing 100044)

Abstract: This paper is about the difficulties how to clean the curtain wall by artificial method and about the basic principles and demands of photocatalyst self-cleaning coatings. It points out that building curtain wall and photoelectric curtain wall call for selfcleaning and automatic dust removing coatings.

0 PrefaceThe demand for the Chinese coatings market is growing at about 10% annually, with most of the growth coming from high-tech high-end coatings. There will be 1 billion m2 of building curtain wall from 2010 to 2015; In addition, the “New Energy Industry Revitalization Plan” released recently predicts that China’s photovoltaic power generation capacity will be 20GW in 2020, and the photovoltaic power generation capacity will increase by 143 times compared to 2008 (0.14 GW) Therefore, in the future, the photovoltaic curtain wall and photovoltaic roof glass will also grow significantly. Regardless of whether it is a building curtain wall or a photoelectric curtain wall, the development of the next 15 years will urgently require high-end coatings that are self-cleaning and automatic dust removal.

1 Curtain wall roof surface cleaning has always been a problem

There are many types of panels for curtain wall roofing, including glass, metal plates (aluminum plates, titanium plates, steel plates, copper plates, etc.), natural stone materials (granite, marble, travertine, etc.), and artificial plates (ceramic plates, Trespa, porcelain plates). Etc.), often a project has multiple panels. Moreover, the shape of the curtain wall roof is complex, including rectangular, circular, conical, inverted cone, hyperbolic, and irregular. If the facades of urban buildings are kept tidy, they should be cleaned in accordance with the following requirements: ① glass curtain walls should be cleaned at least once a year; ② metal plate curtain walls should be cleaned at least once every 2 years; ③ wood-based panel curtain walls should be cleaned at least every 2 years 1 time; ④ stone curtain wall, clean at least once every 3 years. However, the cleaning of building curtain wall roofs has always been a difficult problem. Manual cleaning is difficult, and there are large potential safety hazards. It is even more difficult to clean the curtain walls of large high-rise and super high-rise buildings. Taking the cleaning of the glass curtain wall of Burj Khalifa, the tallest building in the world as an example, the difficulty of cleaning the glass curtain wall can be seen. see picture 1.

The tallest building in the world-artificial cleaning of Burj Khalifa

In addition, the manual cleaning of the curtain wall of the Shanghai World Financial Center, currently the tallest building in China (492 meters), the third tallest building in the world, and the world’s tallest flat-roofed building, is also very difficult. See Figure 2.

Manual cleaning of the curtain wall of Shanghai World Financial Center

2 Photocatalyst self-cleaning challenges artificial cleaning

2.1 Types of cleaning coatings

It is difficult to manually clean the surface of the building curtain wall. It is urgent to develop self-cleaning and automatic dust-removing coatings instead of manual cleaning. There are currently three types of cleaning coatings:
(1) General cleaning coatings: Generally used on the surface of small glassware items to play the role of dust and ash removal. Disadvantages: The aging of the coating effect is basically less than 1 a. The building curtain wall is not used, and it is unrealistic to use paint on a large area.(2) Easy-to-clean coatings: coatings whose surfaces can be easily cleaned. The building curtain wall surface is easy to accumulate static electricity. After charging, it is easy to attract nearby dust. Therefore, to prevent the panel from getting dirty from the source, the most important thing is to prevent the panel from being electrostatically charged. Self-cleaning coatings are mainly determined by their anti-static effect. Coatings that have a good anti-static function or can inhibit themselves from becoming dirty are self-cleaning coatings, otherwise they are not.(3) Photocatalyst nano self-cleaning coating: It is an upgraded version of easy-to-clean coating. In addition to being easily cleaned, it has a good antistatic function and has the ability to prevent dirt from becoming dirty.

2.2 Photocatalytic nano-self-cleaning coating

The discovery of nanotechnology has brought new changes to the development of special coatings and functional coatings. In particular, the development of nano-photocatalytic technology has greatly promoted the innovation of special functional coatings. Nanophotocatalytic technology, also known as photocatalyst technology, was discovered in the late 1960s. At the time, Akira Fujishima, a graduate student at the University of Tokyo, irradiated the single crystal of titanium oxide with water in an experiment. As a result, it was found that water was decomposed into Oxygen and hydrogen. This effect is known as the “Fondo-Fujishima effect”. Because this is the use of light to promote the oxidative decomposition reaction, the titanium oxide in this phenomenon was later called “photocatalyst”. After 1992, the development of nanotechnology and its recognition of the importance of new materials have led people to re-emphasize this photocatalytic oxidative decomposition effect. Countries around the world have invested a lot of manpower and material resources in the preparation and application of various types of nano-self-cleaning coatings. Due to the excellent self-cleaning effect of nano-TiO2 photocatalyst materials, it will change the difficulty of curtain wall roof cleaning and open up a whole new technical field. The flexible external walls of the Shanghai World Expo China and Japan Pavilions are covered with a layer of titanium dioxide photocatalyst on the surface of the PVC and ETFE membranes. They do not need to be cleaned. As long as it rains, the roof and exterior walls will be exposed to water, and the dust will fall off with clean water. . This is the principle using “photocatalytic” self-cleaning coatings. See Figures 3 and 4.

PVC self-cleaning film for the exterior wall of China Aviation Pavilion in Shanghai World Expo, ETFE self-cleaning film for the exterior wall of Japan Pavilion in Shanghai World Expo

The curtain wall adopts photocatalyst self-cleaning coating, which not only isolates the direct contact between pollutants and the substrate, but does not cause damage to the substrate. In addition to external factors such as man-made destruction, it can always maintain its performance. Self-cleaning products are semi-permanent products. The main component of “photocatalytic” self-cleaning coatings is nano titanium dioxide. Titanium dioxide is a semiconductor that has three crystal structures: anatase, rutile, and brookite. Only anatase has photocatalytic properties. After spraying this liquid on the building curtain wall and roof surface, a layer of quite hard, colorless and transparent coating film will be formed after 5 days, with good scratch resistance and abrasion resistance. It can be permanently attached to the building curtain wall and Roof surface. The photocatalytic self-cleaning curtain wall self-cleaning effect is more than 10 years. The emergence of photocatalytic self-cleaning coating technology has completely changed the problem of manual cleaning. Figures 5 and 6 show the comparison of the effects of photocatalytic self-cleaning coatings.

Comparison of effect before and after applying photocatalyst self-cleaning coating
Whether to use photocatalyst contrast test curve

The nano-photocatalyst directly obtains the self-cleaning effect through the chemical reaction and physical action on the surface. Under the conditions of sunlight and natural rain, it continuously exerts its self-cleaning effect, directly replacing manual operation. Nanophotocatalyst is not a consumable product in itself, itThe reaction plays the role of a catalyst. Its chemical reaction is not just a one-time chemical reaction, but it continuously acts as a catalyst to promote the surface to obtain a hydrophilic effect, and to physically explode the negative oxygen ions and hydroxyl groups to the surface Chemical oxidation-reduction reactionKeep the surface clean at all times. Compared with the effect of manual cleaning, manual cleaning repeats the process from clean to pollution, while the effect of self-cleaning products is to maintain its environmental protection status. Figure 7 shows some photocatalyst effects.

Photocatalyst effect display

3 Self-cleaning principle of “photocatalytic” self-cleaning coatingsThe self-cleaning of the “photocatalytic” self-cleaning coating for curtain wall roofing is due to the combination of two major characteristics of photocatalyst hydrophilic and decomposition of organic pollutants.

3.1 Decomposition principle of organic pollutants for “photocatalytic” self-cleaning coatings (see Figure 8)

Decomposition principle of organic pollutants in "photocatalytic" self-cleaning coatings

The oil pollution decomposition test is shown in Figure 9.

Oil pollution decomposition test

3.2 Hydrophilicity principle of “photocatalytic” self-cleaning coatings (see Figure 10)

Principle of Hydrophilicity of "Photocatalytic" Self-Cleaning Coatings

3.3 Principle of self-cleaningFirst, if a material that is surface-processed with titanium oxide is irradiated with ultraviolet light, an “oxidative decomposition” reaction occurs, and the organic matter attached to the surface is decomposed into water and carbon dioxide. Moreover, when a large amount of water adheres to the surface, a hydrophilic effect occurs: it does not condenseWater droplets, but spread thinly. Through these two phenomena, the dirt on the surface will be cleaned completely. See Figure 11.

Principle of "photocatalytic" self-cleaning coating

The “photocatalytic” self-cleaning coating itself has a very strong decomposition ability. The nano-photocatalyst serves as a catalyst. First, the water molecules in the air are physically exploded into hydroxyl and negative oxygen ions, and these two ions have strong oxidation. The reaction characteristic is that it directly performs redox reaction with organic pollutants, and finally decomposes organic pollutants into water molecules and carbon dioxide. For inorganic pollutants, although its adhesion is very weak, after it is deposited on the surface of a general substrate, the pollution is also very large. After using photocatalyst nanotechnology, the photocatalyst film first isolates the substrate from the pollutants, and it creates a high degree of hydrophilicity through the hydroxyl chain, which makes the contact angle of water droplets and the surface of the photocatalyst very small, so the inorganic pollution on the surface The material and some organic pollutants are easily washed away under the action of natural rain. Using nanotechnology to make titanium dioxide into nano-scale particles, the efficiency of photocatalyst can be improved by greatly increasing the ratio of surface area to volume. After the introduction of photocatalyst nanotechnology, the photocatalyst film not only isolates the direct contact of pollutants with the substrate, prevents it from penetrating the pollutants, and reduces its ability to adhere to the pollutants. In addition, it is obtained through decomposition and self-cleaning A good self-cleaning effect. Using nano-photocatalyst technology has a high one-time investment cost, but it is cost-effective to analyze from cost performance.

3.4 Characteristics of excellent self-cleaning coatings

(1) Use anti-static function to make dirt difficult to access and adhere to the surface. Special nanoparticles contain a large number of free electrons, so the electron resistance value is very low, making the dust, dirt, and dirt in the atmosphere difficult to access or adhere to the treated surface.

(2) Use water and air as catalysts to decompose dirt. Even if there is no light, as long as there is air and water, it can decompose organic matter, and it also has antibacterial, deodorizing, and anti-mold effects, which can maintain a clean and beautiful appearance for a long time.
(3) Using super-hydrophilic nanoparticles and fractal theory to develop super-water and strong adhesion (it will not fall even after being sprayed with water after 10 minutes of construction). And outstanding durability. The adhesion time of similar products in the market generally requires 5 to 24 hours, which causes inconvenience to construction.

(4) Add silver ions to enhance antibacterial function and strongly adhere to the surface. After 10 minutes of construction, it will not fall off even if sprinkled with water, and has excellent durability. 

3.5 Technical characteristics of photocatalytic self-cleaning coatings (see Table 1)

Technical characteristics of photocatalytic self-cleaning coatings

4 Basic requirements for photocatalytic self-cleaning coatings

4.1 With excellent adhesive quality

Nano-titanium dioxide itself does not have any adhesion ability. Without a good adhesive, it will not be able to adhere to the surface of the object. A good photocatalyst adhesive is an adhesive that is modified with inorganic components on the surface of nano-titanium dioxide to make nano-titanium dioxide adhere to the surface of the base material. The appearance of this photocatalyst is usually water, without any stickiness, and without any odor. After spraying the surface of the object, it shows Very strong adhesion. The adhesive is the most critical part of the photocatalyst, and it determines whether the photocatalyst effect is long-term effective. If an organic adhesive is used (such as a low-temperature-curable fluororesin, fine silica gel particles, or a mixture with an organosilicon compound), the nano-titanium dioxide will soon fall off the wall after it decomposes its own adhesive, thereby failing. Photocatalyst products are nano-scale inorganic adhesives, which are attached to the surface of nano-titanium dioxide. When the photocatalyst solution dries, it will adhere firmly to the surface of the object and can be permanently adhered to the surface of the object.

4.2 Nanometer titanium dioxide particles are uniform

Titanium dioxide in the photocatalyst solution requires uniform particles. Industrial titanium dioxide powder is very cheap, but the particle diameter is not uniform, which is not suitable for making photocatalyst liquid.

4.3 Dispersion technology is better

It is difficult to evenly disperse titanium dioxide in water and keep it for a long time without precipitation. Because titanium dioxide is a solid with a larger specific gravity than water, it will precipitate and agglomerate when the amount is large. If the photocatalyst liquid has delamination, it means that the dispersion technology is not good. From a micro perspective, the titanium dioxide agglomerates together, and the effect will be greatly reduced.

4.4 Modification of titanium dioxide

To improve the photocatalytic reaction of the photocatalyst liquid to visible light, it is not possible to use titanium dioxide alone, but to introduce foreign substances into the photocatalyst liquid for modification (such as metal ion doping, semiconductor coupling method, etc.) to increase the sensitivity to visible light.

5 Photovoltaic curtain wall and photovoltaic roof glass need self-cleaning and automatic dust removal coating

5.1 Photoelectric glass urgently needs self-cleaning coating technology

The solar panel lighting glass of the photovoltaic screen roof is exposed to sunlight, rain and other environmental pollution for a long time outdoors. Like the glass of a building, a layer of dirt will form on the glass surface after a period of time. Dirt grows thicker and thicker at any time (see Figure 12), which in turn causes the dirt layer to affect the transmittance of light. Solar packaging is commonly used with 3.2 mm ultra-white rolled glass with a transmission ratio of about 92%. After testing, the dirt accumulated in the plain area of ​​China in 3 months can block the sunlight from 3% to 5%. The longer the time, the thicker the dirt. Even the glass with anti-reflection treatment and good optical properties may have better initial performance. The surface of the glass can be difficult to keep clean under long-term outdoor use. A small amount (0.005 kg / m2) of dust can reduce the energy conversion rate of solar panels by 40%, and the amount of power generated due to glass pollution is considerable every year.

Contrast before and after photoelectric glass pollution

In order to improve the conversion efficiency of solar cells, a large amount of manpower must be used to clean the surface of the solar panel glass (see Figure 13). Photoelectric glass also urgently needs self-cleaning coating technology.

Manual cleaning of photoelectric glass

5.2 The United States Develops Automatic Dust Removal Coatings for Solar PanelsIn February 2004, when Opportunity just started its Mars exploration mission, its 1.3-meter-long solar panel could provide 900 W · h of electricity per day, but less than half a year, with the solar energy of Opportunity The Martian dust slowly gets on the panel, and the function of the solar panel is greatly reduced. The “culprit” of all this is that the dust on Mars covers the solar panel, which greatly reduces the function of the solar panel. U.S. scientists have used the new automatic dust removal technology developed for Mars exploration to propose a solution-self-cleaning solar panels.This automatic dust removal technology mainly relies on a layer of transparent, electrically sensitive coating on the glass on the surface of the solar panel. The sensor monitors the thickness of the dust on the surface of the solar panel at any time. Once the deposited dust reaches a critical level, the sensor will inject electrical energy into the coating. The input power causes the surface material to send dust-repellent microwaves, which will roll up the surface dust to the edge of the solar panel. This technology can remove 90% of the dust deposited on the solar panel in 2 minutes, and the entire cleaning process requires only a small amount of electricity.

6 Conclusion There are
too many low-end products and too few high-end products in the domestic architectural coatings market. Chinese architectural curtain walls, photovoltaic curtain walls, and photovoltaic glass call for domestic high-quality self-cleaning high-end coatings and automatic dust-removing high-end coatings.

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