Cooling a building without using electricity may seem far‑fetched, but over several years, a research team at the Chinese University of Hong Kong (CUHK) has shown that a coat of paint can provide a surprising amount of cooling. Radiative cooling has been used as an energy-free process that reflects sunlight off surfaces while allowing for releasing their thermal energy into space. However, the main difficulty of implementing such a paint was its affordability.In 2023, the same CUHK research team that had previously worked on radiative‑cooling paints published ‘Cheaper Radiative Cooling Paint by Milling Larger Glass Bubbles’ in the journal Energy and Buildings, showing that just five minutes of ball milling could raise the solar reflectance of a glass‑bubble based paint from 75.8% to 95.8%. As a result of outdoor tests carried out at noon, the unmilled coating ran about 9.4°C hotter than the surrounding air, while the milled coating's surface temperature stayed comparable to ambient air, effectively eliminating the heat buildup seen in the unmilled version.What glass bubbles are doing inside the paint Radiative cooling paints generally function on the principle of loading a polymeric base with particles that diffuse any incoming light before it is absorbed and converted into heat energy. One material that shows promise in this respect is glass bubbles, microscopic hollow glass spheres, owing to the fact that their size can be controlled during processing and that they scatter light very effectively. This is subject to the problem that large and less expensive glass bubbles do not scatter light as well as small ones.The study that made large, cheap bubbles work However, the CUHK team opted for an alternative method. Rather than starting with small glass bubbles, the researchers began with larger, hollow glass spheres (median size 47.9 μm), significantly cheaper than the small ones typically used and subjected them to five minutes of ball milling. This mechanical grinding fragmented the bubbles, reducing the median particle size to 9.7 μm, close to that of the high‑reflectance filler used in their previous radiative‑cooling paint, according to the above-mentioned 2023 study in Energy and Buildings.The results achieved were quite impressive. The paint produced with 47.9-μm bubbles was able to reflect only 75.8% of sunlight. After being subjected to five-minute ball milling, the reflectivity rate of the paint rose to 95.8%. According to the field tests that accompanied the research, unground bubbles showed a temperature that was higher by 9.4°C compared to the ambient air at noon, whereas milled bubbles maintained a surface temperature comparable to that of the air and practically nullified the heat absorption by the paint. In addition to the optical gains, the bigger bubbles used in this study cost substantially less, reported as roughly 40% of the price of the smaller, conventionally sized bubbles, as per the previously mentioned 2023 study in Energy and Buildings.Why this builds on earlier work from the same lab This was not the first time the CUHK team tried improving the performance of glass-bubble paints using ball-milling. Their previous research, 'Enhanced Radiative Cooling Paint with Broken Glass Bubbles,' published in the journal Renewable Energy, found that ball-milling small glass bubbles could increase reflectivity from 93.3% to 97.3%, with field tests on CUHK rooftops showing the midday cooling gap between the paint surface and ambient air increase from 1.8 to 3.5°C.133673968However, what the earlier paper could not solve was the high price point: small glass bubbles are costly to produce, so although ball-milling improved the paint's performance, cost remained a fundamental problem. This is where the 2023 paper helps address the problem, showing that the same ball-milling process can also be used with large glass bubbles with equal or better effect.How this compares with ordinary cool roof coatings These figures should be compared with those of current products on the market. According to the Strategic Environmental Research and Development Program and Environmental Security Technology Certification Program (SERDP/ESTCP), U.S. Department of Defense programs that support environmental and energy resilience research, a standard cool roof coating or white paint typically has a solar reflectance of around 75%-85% when freshly applied, with many conventional coatings dropping to roughly 60%-80% after months of exposure to sun, rain and dirt. A figure of 95.8% reflectivity of the milled glass-bubble paint is even higher than the average reflectivity when first applied, which explains why the coating could maintain surface temperatures close to the air.What still needs to be worked out This does not imply that such paints can immediately replace all other white reflective roof coatings. The experiments conducted by researchers at the Chinese University of Hong Kong (CUHK) were performed as field tests on rooftops and, while they demonstrate strong cooling at noon, the team itself notes that more work is needed to test the paint in real buildings over longer periods, leaving open questions about how well the milled glass bubbles will retain their reflectiveness after years of exposure to UV rays, rainfall, and dust. Scaling production of the paints and ensuring that the milling process itself does not create additional energy or equipment costs that can nullify the cost-effectiveness of the new materials become the key practical problems.Even so, the finding could be valuable for the field, which often assumes that better performance means higher prices. Just five minutes in a ball mill brought the paint's reflectance close to that of higher-cost radiative cooling coatings that use expensive particles.
