Cambridge researchers made cellulose acetate films that absorbed less than 5% of sunlight; coated surfaces stayed about 5°C below the surrounding air without power

Summer afternoons can turn roofs, cars and other outdoor surfaces into heat traps that absorb and retain heat well after sunset. Even air conditioning, although effective, comes with added costs and extra strain on the electricity network. Researchers from the University of Cambridge and their collaborators set out to address that gap in a joint project. The solution, reported in the journal Advanced Science, came from a familiar material: cellulose acetate. This thin, porous film is made by chemically modifying wood pulp and is already used in products such as eyeglass frames and cigarette filters; it absorbed less than 5% of incoming sunlight.

That figure is important because limiting solar absorption helps reduce the amount of heat the material gains from sunlight. The researchers also tested the films outdoors, where surfaces coated with the material reached temperatures about 5°C below the surrounding air under suitable conditions.

How does the film keep its cool without using electricity?


The film cools itself through its structure rather than through a special coating. The cellulose acetate is processed into a porous film with many tiny air spaces. Light entering the material is scattered by these pores rather than simply being absorbed and turned into heat. In addition, the material can emit infrared heat to the atmosphere through a process known as radiative cooling.

The research team designed two films: one about 30 microns thick and another about 300 microns thick, roughly the thickness of a few sheets of paper stacked together. The thinner film offered greater light transmission and could therefore be considered for applications where some light needs to pass through, while the thicker film was more suitable as a standalone layer over a surface. The paper found that surfaces coated with the film stayed about 5°C cooler than the surrounding air without fans, compressors, or electricity. The authors said earlier cellulose-based attempts were difficult to make because of complex preparation methods or unavailable materials.

Why such a cooling technology can be useful in daily life

The researchers' results point to possible uses on surfaces that receive substantial sunlight. Roofs, vehicles, storage facilities, greenhouses, and packaging for perishable cargo are among the kinds of applications where passive cooling could potentially be useful. The film's ability to limit solar heat gain while releasing heat through radiative cooling could make it relevant to situations where reducing surface temperatures without additional electricity is desirable.

That possibility is particularly relevant as demand for cooling continues to grow in warmer conditions. A material that can lower surface temperatures without consuming electricity could potentially complement conventional cooling systems and reduce some of the heat absorbed by exposed surfaces. Cellulose acetate is also a familiar material that can be produced using established industrial processes, which could be useful if the technology eventually moves toward larger-scale applications.

The results should nevertheless be viewed as an early-stage demonstration rather than a finished cooling product. The material would need further testing for long-term durability, larger-scale production, and practical methods of applying it to different surfaces before its broader usefulness could be established. The study's outdoor results show that porous cellulose acetate can combine low solar absorption with radiative cooling under tested conditions, but translating that performance into everyday products would require additional development.

Anyone who has touched a car hood in July knows how useful a cooler surface could be. The Cambridge study reports that its porous cellulose acetate film can keep coated surfaces several degrees cooler than the surrounding air without electricity under outdoor test conditions. With rising temperatures increasing the need for cooling, materials that can manage solar heat without adding to power demand are an area researchers are continuing to explore.