Every summer, heatwaves keep smashing records. The World Meteorological Organization reported that the past seven years were the hottest ever recorded. As cities keep warming, buildings and people rely more on air conditioning. Yet air conditioners use a lot of energy and produce carbon emissions that fuel climate change even further.
Imagine if staying cool didn’t need any electricity at all. That idea just took a giant leap forward thanks to researchers from Zhengzhou University in China and the University of South Australia (UniSA). They created a biodegradable cooling film that passively keeps surfaces cooler than the surrounding air.
The study, published in Cell Reports Physical Science00263-2?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS2666386425002632%3Fshowall%3Dtrue#fig1), describes this material as a ‘bioplastic metafilm’. It reflects almost 99% of the sun’s rays and can cool surfaces by up to 9.2°C, even under direct sunlight.
“Our metafilm offers an environmentally friendly alternative to air conditioning, which contributes significantly to carbon emissions,” said Yangzhe Hou, a UniSA PhD student involved in the project. Hou, who is also from Zhengzhou University, explained that the material works by reflecting nearly all solar radiation while letting heat from inside buildings escape directly into outer space.
How The Cooling Film Works
Cooling systems usually depend on electricity. Even “passive radiative cooling” technologies, which do not need external power, often use petrochemical-based plastics or ceramics. These materials raise concerns about pollution and long-term sustainability.
This new metafilm is different. It is made from polylactic acid (PLA), a plant-derived bioplastic already used in many packaging products. The film uses a low-temperature separation technique to form a special microstructure that reflects 98.7% of sunlight while allowing heat to radiate away. It requires no electrical input or mechanical systems.
“Unlike conventional cooling technologies, this metafilm requires no electricity or mechanical systems,” said Dr Xianhu Liu from Zhengzhou University. “Most existing passive radiative cooling systems rely on petrochemical-based polymers or ceramics that raise environmental concerns. By using biodegradable PLA, we are presenting a green alternative that offers high solar reflectance, strong thermal emission, sustainability, and durability.”
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The microstructure plays a key role. The film’s pores are just the right size – between 0.5 to 4 micrometers – to scatter and reflect sunlight effectively. This is called Mie scattering resonance, which enhances cooling by bouncing solar radiation back into the atmosphere.
The Science Behind The Film
To create this metafilm, the team developed a low-temperature two-step phase separation method. They dissolved PLA in a solution, cooled it to -20°C for 12 hours, and then let it dry at room temperature. This process caused the material to form a unique bi-continuous pore structure.
The special structure improves both reflectivity and thermal emission. It also increases durability, a common problem with biodegradable materials. Many eco-friendly cooling films degrade too fast in harsh conditions like acid rain or ultraviolet (UV) radiation. However, this film retained its performance even after 120 hours in strong acid and eight months’ worth of UV exposure.
Tests showed it maintained hydrophobic properties, meaning it repelled water even after acid and UV tests. Its cooling effect remained strong, dropping temperatures by up to 6.5°C after exposure to harsh conditions.
Dr Liu explained why this matters: “Our film is scalable, durable and completely degradable.” Co-author Professor Jun Ma from UniSA added, “This research aims to contribute to sustainable development by reducing reliance on fossil fuels and exploring feasible pathways to improve human comfort while minimizing environmental impact.”
Real World Results
Field tests took place in both China and Australia. During the day, the metafilm lowered temperatures by an average of 4.9°C. At night, it maintained an average temperature drop of 5.1°C. These results mean buildings coated with the film could remain cooler than the surrounding air both day and night.
The researchers also used EnergyPlus software to simulate how much energy the metafilm could save in different cities. In Lhasa, China, it cut annual cooling energy use by up to 20.3%. Similar results were predicted for other hot cities worldwide.
The film performed better than many other radiative cooling materials studied so far. Compared to PLA aerogels, cellulose-based papers, and ceramic composites, it showed superior solar reflectivity and lower thermal conductivity. Its ultra-low thermal conductivity of 0.049 W/mK means heat from outside travels into buildings much slower.
Beyond Buildings: Future Applications
The metafilm’s potential goes beyond cooling homes and offices. Because it is biodegradable and can endure harsh conditions, it could be used for cooling farm equipment, cars, and even wound dressings in hospitals. The team is now exploring large-scale manufacturing methods to bring this technology to market.
Yangzhe Hou described it as “a breakthrough in sustainable materials engineering that could help combat rising global temperatures and hotter cities.”
The underlying physics is simple yet powerful. All objects emit heat in the form of infrared radiation. Usually, the heat emitted is balanced by heat absorbed from the sun and the surrounding air. But this metafilm changes the balance by reflecting almost all sunlight while emitting thermal radiation into the cold of outer space. This process lets surfaces stay cooler than the air around them, even under the sun.
The Role of Crystal Structures
Part of the film’s strength comes from its internal crystal structures. PLA can form different types of crystals. In this film, stereo-complex (SC) crystals formed by mixing two kinds of PLA molecules give it extra strength and thermal stability. The SC crystals melt at about 218°C, which is around 50°C higher than normal PLA crystals. This means the film remains strong even in hot weather.
Tests showed that the film had an SC crystallinity of almost 30%, giving it excellent stability. Even after acid immersion and UV exposure, the SC crystallinity increased slightly, making the structure even stronger.
This feature helps overcome a big challenge in the field: combining high cooling performance with long-term environmental durability. Other biodegradable coolers, like those made from gelatin or DNA aerogels, have great cooling abilities but dissolve too quickly in rain or break down under sunlight.
Cooling Power Confirmed
Outdoor tests in Zhengzhou City showed the metafilm could lower temperatures by up to 9.2°C at midday. Its average cooling power was 136 W/m² under peak sunlight of around 945 W/m². At night, its cooling power rose to 149 W/m². This performance beat the two other types of PLA metafilms the team tested.
Tests in Adelaide, Australia, confirmed the film’s durability. After acid and UV aging, the film still reduced temperatures by 5.0°C to 6.5°C during the day. Even under cloudy skies, its temperature stayed close to the air temperature, as clouds block both solar and infrared radiation.
Overall, the film showed impressive and stable cooling performance under a range of real-world conditions. The team believes this innovation could be an eco-friendly solution to help cities fight the effects of rising temperatures.
Toward A Cooler Future
As the climate crisis deepens, solutions that reduce energy use while improving comfort will become essential. This metafilm is a major step toward that goal. By using biodegradable materials, it avoids the pollution problems linked to petroleum-based plastics. By working without electricity, it cuts greenhouse gas emissions linked to power use.
Professor Jun Ma summed it up: “This isn’t just a lab-scale success. Our film is scalable, durable and completely degradable.”
Looking ahead, the team plans to explore commercial production and wider uses. Whether cooling skyscrapers, cars, or electronic devices, the metafilm could become a versatile tool in the fight against climate change.
It’s clear that to build a more sustainable world, innovations like this will be key. Combining simple physics with advanced materials science, researchers are finding ways to keep the planet cool without making it worse.
Note: The article above provided above by The Brighter Side of News.
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