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   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Colorful films could help buildings, cars keep their cool    
      
     Date:   
         March 27, 2023   
     Source:   
         American Chemical Society   
     Summary:   
         The cold blast of an air conditioner can be a relief as   
         temperatures soar, but 'A/C' units require large amounts of   
         energy and can leak greenhouse gases. Today, scientists report   
         an eco-friendly alternative - - a plant-based film that cools   
         when exposed to sunlight and comes in many textures and bright,   
         iridescent colors. The material could someday keep buildings,   
         cars and other structures cool without requiring power.   
      
      
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   FULL STORY   
   ==========================================================================   
   The cold blast of an air conditioner can be a welcome relief as   
   temperatures soar, but "A/C" units require large amounts of energy   
   and can leak potent greenhouse gases. Today, scientists report an   
   eco-friendly alternative -- a plant-based film that gets cooler when   
   exposed to sunlight and comes in a variety of textures and bright,   
   iridescent colors. The material could someday keep buildings, cars and   
   other structures cool without requiring external power.   
      
      
   ==========================================================================   
   The researchers will present their results at the spring meeting of the   
   American Chemical Society (ACS).   
      
   "To make materials that remain cooler than the air around them during the   
   day, you need something that reflects a lot of solar light and doesn't   
   absorb it, which would transform energy from the light into heat," says   
   Silvia Vignolini, Ph.D., the project's principal investigator. "There are   
   only a few materials that have this property, and adding color pigments   
   would typically undo their cooling effects," Vignolini adds.   
      
   Passive daytime radiative cooling (PDRC) is the ability of a surface   
   to emit its own heat into space without it being absorbed by the air or   
   atmosphere. The result is a surface that, without using any electrical   
   power, can become several degrees colder than the air around it. When   
   used on buildings or other structures, materials that promote this effect   
   can help limit the use of air conditioning and other power-intensive   
   cooling methods.   
      
   Some paints and films currently in development can achieve PDRC, but   
   most of them are white or have a mirrored finish, says Qingchen Shen,   
   Ph.D., who is presenting the work at the meeting. Both Vignolini and Shen   
   are at Cambridge University (U.K.). But a building owner who wanted to   
   use a blue-colored PDRC paint would be out of luck -- colored pigments,   
   by definition, absorb specific wavelengths of sunlight and only reflect   
   the colors we see, causing undesirable warming effects in the process.   
      
   But there's a way to achieve color without the use of pigments. Soap   
   bubbles, for example, show a prism of different colors on their   
   surfaces. These colors result from the way light interacts with differing   
   thicknesses of the bubble's film, a phenomenon called structural   
   color. Part of Vignolini's research focuses on identifying the causes   
   behind different types of structural colors in nature. In one case,   
   her group found that cellulose nanocrystals (CNCs), which are derived   
   from the cellulose found in plants, could be made into iridescent,   
   colorful films without any added pigment.   
      
   As it turns out, cellulose is also one of the few naturally occurring   
   materials that can promote PDRC. Vignolini learned this after hearing   
   a talk from the first researchers to have created a cooling film   
   material. "I thought wow, this is really amazing, and I never really   
   thought cellulose could do this."  In recent work, Shen and Vignolini   
   layered colorful CNC materials with a white- colored material made from   
   ethyl cellulose, producing a colorful bi-layered PDRC film. They made   
   films with vibrant blue, green and red colors that, when placed under   
   sunlight, were an average of nearly 40 F cooler than the surrounding   
   air. A square meter of the film generated over 120 Watts of cooling   
   power, rivaling many types of residential air conditioners. The most   
   challenging aspect of this research, Shen says, was finding a way to   
   make the two layers stick together -- on their own, the CNC films were   
   brittle, and the ethyl cellulose layer had to be plasma-treated to get   
   good adhesion. The result, however, was films that were robust and could   
   be prepared several meters at a time in a standard manufacturing line.   
      
   Since creating these first films, the researchers have been improving   
   their aesthetic appearance. Using a method modified from approaches   
   previously explored by the group, they're making cellulose-based cooling   
   films that are glittery and colorful. They've also adjusted the ethyl   
   cellulose film to have different textures, like the differences between   
   types of wood finishes used in architecture and interior design, says   
   Shen. These changes would give people more options when incorporating   
   PDRC effects in their homes, businesses, cars and other structures.   
      
   The researchers now plan to find ways they can make their films even   
   more functional. According to Shen, CNC materials can be used as sensors   
   to detect environmental pollutants or weather changes, which could be   
   useful if combined with the cooling power of their CNC-ethyl cellulose   
   films. For example, a cobalt-colored PDRC on a building fac,ade in a   
   car-dense, urban area could someday keep the building cool and incorporate   
   detectors that would alert officials to higher levels of smog-causing   
   molecules in the air.   
      
   The researchers acknowledge support and funding from Purdue University,   
   the American Society of Mechanical Engineers, the European Research   
   Council, the Engineering and Physical Sciences Research Council, the   
   Biotechnology and Biological Sciences Research Council, the European   
   Union and Shanghai Jiao Tong University.   
      
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   ==========================================================================   
   Story Source: Materials provided by American_Chemical_Society. Note:   
   Content may be edited for style and length.   
      
      
   ==========================================================================   
      
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/03/230327114920.htm   
      
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