By Kikuko Tagawa
The word “photocatalyst” may be unfamiliar to you, but it is mentioned frequently in Japan’s business media; it is now well known by most business people in Japan.
Photocatalyst treatments are probably in your environment already. Do the mirrors on your car doors ever fog up? If not, the mirrors could be coated with a photocatalyst. Most 2004 model Japanese cars have photocatalyst-coated door mirrors as standard equipment, as do an increased number of 2005 European models. And a mirror manufacturer with a large share of the United States market has just started producing photocatalyst-coated mirrors.
A few weeks ago, a paint salesperson was urging me to paint my house exterior with his photocatalytic paint. He drove me around to show me houses painted with photocatalytic paint several years ago. He said, “Seeing is believing. You see no rain prints, no dirt, and yet it does not harm humans and plants. No smell even when you paint. With light and rain, the dirt on the wall just disappears, and it is perfectly environmentally friendly.”
What is photocatalyst?
In 1967, at the laboratory of Prof. Kenichi Honda, University of Tokyo, and Prof. Akira Fujishima, University of Tokyo, (who was then a postgraduate student), discovered that the electric poles of titanium dioxide (TiO2) and platinum decompose water into oxygen and hydrogen when exposed to UV irradiation (the Honda-Fujishima effect). It is a chemical reaction similar to photosynthesis. In this reaction, TiO2 plays a catalytic role, so it was dubbed photocatalyst. It came into the limelight as a possible solution to energy issues—it was thought it could be used to produce the clean energy of hydrogen with light energy. However, it was found that the reaction was too moderate to assist in using generated hydrogen as a fuel.
In 1990, Dr. Kazuhito Hashimoto, chief of the Research Centre for Advanced Science and Technology at the University of Tokyo (and an assistant to Dr. Fujishima), discovered that the photocatalyst has strong oxidative decomposition power, so that a TiO2 photocatalyst surface has antifouling, antimicrobial and deodorizing properties.
In 1995, Fujishima, Hashimoto and Dr. Toshiya Watanabe, professor at the Research Centre for Advanced Science and Technology, discovered the super-hydrophilic properties of the TiO2 photocatalyst. When the photocatalytic surface is irradiated by light, it gets super-hydrophilic. The dirt on the surface rises as water comes between it and the surface, and the dirt is easily removed. Also, this super-hydrophilicity means that water droplets cannot rest on the coating surface, a property that prevents fogging.
Based on the research and development, in 1994 TOTO Ltd., Fukuoka, Japan, released antifouling, antimicrobial and deodorizing tiles with photocatalyst technology. TOTO and the University of Tokyo also filed a joint application with the Patent Office for photocatalyst technology, and the patent was later granted.
Self-cleaning fabric structures
“It was 1995 when I first came across photocatalyst,” says Takashi Nohmura, chief research and development officer for Taiyo Kogyo Corp., Tokyo, and chairman of the Japanese Association of Photocatalytic Products. “We asked Nippon Soda Co. Ltd. to paint photocatalyst manually onto a PVC fabric sample. We put it in an outdoor exposure test for several months and found that it showed an overwhelming antifouling property which I had never seen before. The surface kept changing every day, getting a little dirtier (although it is much cleaner than the other materials) then cleaner, back and forth, depending on the amount of sun and rainfall. It seemed as if someone secretly had come and wiped off the dirt.
“Now it is well known that it has this so called self-cleaning effect,” says Nohmura.However, at the time it was too amazing and unbelievable to us all. We even thought we might have been cheated somehow.”
With acrylic or fluoride polymer coatings, PVDF and PVF film laminate on PVC, manufacturers have made extensive efforts to achieve soil resistance in architectural fabrics. But none of them was sufficient, and clients kept complaining.
“Fabric structures get dirty easily and look terrible. Finally we found the solution to the problem, I thought,” says Nohmura. Successfully getting the photocatalyst onto the fabrics required a process of trial and error.
After some research and development collaboration, in 1998 Taiyo Kogyo and Nippon Soda released a photocatalyst-treated PVC fabric for architectural applications that was based on the TOTO patent. As of December 31, 2003, Taiyo Kogyo itself had consumed 9,041,760 sq. yds of photocatalytic PVC fabric in installed projects in China, Taiwan and Thailand. Taiyo Kogyo plans to market the product to other countries.
The company offers a sub-license on photocatalytic PVC fabric to some fabric manufacturers for awnings. Kuraray Co. Ltd., Osaka, Japan, is one of them. Atsumi Adachi, a manager of Kuraray’s Fibers and Industrial Materials Co. says, “We developed our photocatalytic awning fabric Excellence®, based on the Taiyo and Nippon Soda photocatalytic license, and released it to the Japanese market in January 2004. It has excellent resistance to dirt, and our clients are really happy about it. It offers the aesthetic advantage to awnings and cuts the cost of cleaning and maintenance significantly, too. Because the TiO2 works as a catalyst, it stays chemically intact, so the antifouling properties last as long as the TiO2 on the fabric.”
Yamaguchi Sangyo Co. Ltd., Saga, Japan, tried another way—painting photocatalyst-coating onto PVC fabric structures after fabrication but before installation. Atsuki Yamaguchi, president of Yamaguchi Sangyo, says, “We coated the photocatalytic material onto the fabric in our plant using special equipment. The coating material was developed by a company in Saga based on the patent owned by Saga Prefecture. We installed the first project at the Atagohama Nursery School in April 2004 and another project at Kitakyushu Museum in September. We are excited about this new technology.”
Photocatalysts and PTFE
PTFE-coated fiberglass fabric for large-scale sport or commercial facilities has a high soil resistance due to its water repellency. It also has good weatherability and fire retardancy that lasts long term, but it cannot cope with soot and smoke in urban areas. As a result, its soil resistance is inferior to photocatalyst-treated PVC, which is less expensive. Taiyo Kogyo developed photocatalyst-treated PTFE fiberglass fabric and introduced it to the market in August 2003.
“It is a breakthrough product that joins photocatalysis to the PTFE fabric by mixing the photocatalyst material with resin in the top surface layer of the PTFE,” says Nohmura. “Direct coating of the photocatalyst to PTFE is difficult.”
The new material was shown to have strong oxidative decomposition power towards air pollutants in the Japanese Industrial Standards (JIS) R1701-1 experiment conducted in 2004. The experiment showed that 9,794,.33 sq.m of photocatalyst-coated fabric decomposes the major air pollutant nitrogen dioxide (NO2) expelled by 1.7 four-ton trucks. The presence of photocatalytic membrane structures reduces NO2 and sulfur dioxide in the surrounding environment.
“Membrane structures can not only offer flexible and unique designs and large bright inner spaces but also contribute to the improvement of the environment,” says Nohmura. “And with its high fire retardancy, chemical stability, tensile strength, and durability, photocatalytic PTFE fiberglass fabric is expected to be the leading player in future large-scale membrane structure projects.”
It’s natural that photocatalyst exterior building materials have come first to the market; they can get sufficient sunlight and rain to activate the photocatalyst’s properties. Photocatalyst coating is available on other exterior materials, such as tiles, glass, curtain-wall and exterior panels, which are referred to as self-cleaning materials. According to Nohmura, Taiyo Kogyo will have a photocatalyst demonstration/experiment pavilion at World Expo 2005 in Aichi, Japan.
Photocatalysts, cooling and air quality
Recently, photocatalysts were attracting attention again as a solution to energy issues. The enormous increase in power consumption in the summer has become a problem in cities, where traditional building cooling systems consume large amounts of electricity. You may recall the summer 2003 blackout in the New York City region caused by great demands on the power supply. Cities become uncomfortably hot in the summer as air conditioning systems give off heat outside buildings—the so-called heat-island phenomenon.
Due to photocatalyst super-hydro-philicity, a building exterior treated with photocatalyst will be covered with a thin water film, if water exists at the surface. Water just spreads over the surface of the photocatalyst and never forms water droplets. The heat evaporation from this process can lower the air-conditioning load of the building. A national project by the New Energy and Industrial Technology Development Organization (NEDO, www.nedo.go.jp), Kanagawa, Japan, aims at not only saving energy in each building but also reducing the heat-island phenomena in cities, using photocatalyst technology.
Several companies are participating, including Taiyo Kogyo and Izumi-Cosmo Co. Ltd., Tokyo. Izumi-Cosmo developed the building cooling system utilizing photocatalytics and conducted an experiment at Nissan Shatai Co. Ltd. in 2003 and 2004.
Polyester mesh fabric coated with TiO2 photocatalyst on top of fluorocarbon polymer was used to “clothe” a three-story rectangular building of ferro-concrete construction. Thirty-six panels 1.49-by-7.28m were installed on the long, exterior east and west walls. A continuous spray of water—220 cc per minute per mesh panel—was supplied by hoses at the top of the system-conveying pump.
The surface temperature of the photocatalyst mesh on days when water was sprayed 45.34C was 7.60 degrees lower than on days with no spray 52.94C and was 2.94 degrees lower than the water temperature 48.28 degrees C. Pictures taken with an infrared camera show the effect of the spray with color differences. Room temperatures were lowered by .99–1.99 degrees.
“The Aqua Wall® system showed a good building cooling effect, and the workers all commented that they felt the difference inside when the Aqua Wall was operating,” says Haruhisa Hirata, chief of the Industrial Materials Department of Izumi-Cosmo Co. Ltd.
In another fabric-product breakthrough, photocatalystic filters are used in air cleaners. Takashi Yoshimura of N.I. Teijin Shoji Co. Ltd., Osaka, Japan, says, “We are now developing an air cleaner using photocatalytic filters. It has antibacterial properties and removes almost all mold spores and bacteria in the air as well as odors. In the air cleaner, special inverter lamps radiate nonhazardous UV to activate the photocatalyst.”
The technology can also be used in water purification. It decomposes staphylococcus aureus, so it is widely used on tiles and other surfaces in hospitals. Other applications include transparent sound-proof walls, lighting (fluorescent lamps), curtains, blinds, apparel, artificial flowers, small ornaments, and more.
Nohmura says, “Photocatalyst technology is continuously developing. The photocatalyst itself has been enjoying accelerated technological progress in recent days and is expected to develop further. It is now a pressing need to have universal quality standards and testing methods for photocatalytic products, for the wider applications and global development.”
As we can see, photocatalytics have great possibilities in various fields, including many segments of the specialty fabrics industry. Various conferences have been held and many in the industry and in academia are conducting research on the technology and its applications now. One such event is the Global Market for Photocatalysis, Japan 2005 (www.ipie-expo.com/en/index.html). To promote our industry and enlarge our market, and to work on new technologies like this and others will be essential.