Staying Cool with New Chalk-Coated Fabrics

A few months ago, we learned at ACS Fall 2024 about a new chalk-based fabric coating that could lower the temperature of the air beneath. The researchers have now published their work in ACS Applied Materials & Interfaces, so let's pay it another visit!

Climate change causes long-term shifts in temperature and weather patterns. Although these can be natural—caused by volcanic eruptions or changes in the sun’s activity—the main impact over the past 200 years has been human-led, primarily by the burning of fossil fuels.¹ Steadily climbing temperatures threaten to make some regions uninhabitable. At present, around 30% of people live in regions where they risk being exposed to deadly heatwaves for more than 20 days a year, but with no reduction in our fossil fuel consumption, this is predicted to rise to 74%.²

Since 2014, an additional 15,000 deaths have been reported after heatwaves.³ Such overheating is usually caused by direct exposure to sunlight, or indirect exposure to reflected light from the ground and surrounding environment.^4,5 Many people’s survival in a changing climate could be affected by their ability to maintain personal thermal homeostasis. In principle, our clothes can affect light absorption, transmission, and reflection, and advanced textiles aim to harness this. For cooling, surfaces that are good at selectively reflecting or refracting high-energy sunlight could help maintain cooler body temperatures. But they also need to be able to transmit or emit infrared light from radiant body heat, since most of our bodies' energy release is in the form of heat dissipated through the skin. Previous attempts to create wearable radiative coolers have explored composite fibers, fluorinated polymers, porous polymer membranes, engineered nanomaterials, and noble metal coatings, but these are variously expensive, environmentally harmful, or too complex to be practical for high volume manufacturing and widespread adoption.

This new study instead draws inspiration from limestone plasters that have traditionally been used to cool buildings in hot countries.⁷ With this in mind, the team developed a process that used chemical vapor deposition to apply an ultrathin layer of calcium carbonate and barium sulfate. This technique creates a chalky, matte finish that reflects both ultraviolet and near-infrared light, effectively cooling the fabric. As confirmed by Mie scattering calculations, nanoparticles of these two cheap and biocompatible chemicals served as suitable reflectors for radiative cooling.

Further simulations revealed higher reflectance with surface coatings compared to embedding the same chemicals within the fibers in a polymer matrix. When applied to a polyester poplin fabric, the coating was able to cool by up to 8°C compared to an uncoated sample, achieving a maximum cooling of 6°C below ambient temperature. Wash and durability testing of the lamellar coating revealed no mechanical degradation and no evident attenuation in the material’s performance, suggesting it could deliver long-term effectiveness as a functional textile coating for personal cooling. The group also assessed how well the coated fabrics worked in multiple outdoor environments, including on grass, concrete, and asphalt, and found they could achieve up to 3.4°C of sub-ambient cooling in optically complex built environments. This represents a zero-energy cooling technology.

Calcium carbonate and barium sulfate were chosen because of their low cost, wide availability, biocompatibility, and known near-infrared reflectance. Previous work has explored using ultrawhite paints and films loaded with barium sufate, finding they delivered remarkable daytime subambient radiative cooling.⁸ With appropriate particle size and broad distribution, the resulting nanoparticle film reached an ultrahigh solar reflectance of 97.6%, and the acrylic paint 98.1%. During field tests, the film stayed more than 4.5°C below ambient temperature, with similar cooling performance for the paint.

Polylactic acid is another option. Using this plant-based polymer, other work is exploring electrostatic spinning to generate fabrics with specific porosity. Results so far for personal thermal management suggest a temperature drop of 7.1 °C compared to bare skin under direct sunlight.⁹ Additional groups have looked at radiative cooling meta-fabrics that mimic the natural heat regulation of human skin and are testing possibilities for building in antibacterial and self-cleaning properties.¹⁰

But such passive daytime radiative cooling technology isn’t just for fabric—it could also work when sprayed onto glass or wood, which could be useful for long-term outdoor applications.¹¹ Although these approaches do not solve the climate crisis, they could be a step towards helping us live in our new normal.

References

1. What is Climate Change? United Nations 2024. https://www.un.org/en/.
2. Mora, C. et al. Global risk of deadly heat. Nat. Clim. Chang. 2017, 7, 501–506.
3. Zou, J. et al. Multiscale numerical assessment of urban overheating under climate projections: A review. Urban Clim 2023, 49, 101551.
4. Zhao, H. et al. The effect of solar radiation on pedestrian thermal comfort: A climate chamber experiment. Build. Environ. 2023, 245, 110869.
5. Suhaimi, M.F.B. et al. Impact of solar radiation on human comfort in a vehicle cabin: An analysis of body segment mean radiant temperature. Build. Environ. 2023, 245, 110849.
6. Zhang, X. et al. Scalable Bio-Skin-Inspired Radiative Cooling Metafabric for Breaking Trade-Off between Optical Properties and Application Requirements. ACS Photonics 2023, 10, 5, 1624–1632.
7. Patamia, E.D. et al. Microstructured Reflective Coatings on Commodity Textiles for Passive Personal Cooling. ACS Appl. Mater. Interfaces 2024, 16, 43, 59424–59433.
8. Li, X. et al. Ultrawhite BaSO₄ Paints and Films for Remarkable Daytime Subambient Radiative Cooling. ACS Appl. Mater. Interfaces 2021, 13, 18, 21733–21739.
9. Li, M. et al. Flexible Radiative Cooling Textiles Based on Composite Nanoporous Fibers for Personal Thermal Management. ACS Appl. Mater. Interfaces 2023, 15, 14, 17848–17857.
10. Li, B.-B. et al. Rational Design and Fine Fabrication of Passive Daytime Radiative Cooling Textiles Integrate Antibacterial, UV-Shielding, and Self-Cleaning Characteristics. ACS Appl. Mater. Interfaces 2024, 16, 39, 52633–52644.
11. Wang, H.-D. et al. Superhydrophobic Porous Coating of Polymer Composite for Scalable and Durable Daytime Radiative Cooling. ACS Appl. Mater. Interfaces 2022, 14, 45, 51307–51317.