Three-layered films enable efficient passive radiation cooling of buildings

2.1 Materials

PDMS, PMMA, curing agent of PDMS, and ethyl acetate were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). Woven cotton fabric (gram weight 112.0 g·m⁻²) was obtained from the local cotton factory. All chemicals were of analytical grade and used as received without further purification.

2.2 Fabrication of three-layered passive radiation cooling materials

The preparation of three-layered passive radiation cooling materials included two steps (Figure 1). First, an ethyl acetate solution with 10% solute mass fraction of PMMA was prepared. A spray gun with 0.2 mm of diameter and 10 cm of height was applied to spray PMMA solution onto a cotton fabric. Second, a solution of PDMS and its curing agent (with mass ratio of 10:1) was scraped on a glass plate and heated at 90°C at a duration of 3 h. Then the formed PDMS film was covered onto the PMMA film to fabricate a three-layered passive radiation cooling composite materials. Given the specific layered structure of multilayered passive radiation cooling fabrics, i.e., PDMS as the top layer, PMMA as the middle layer, and cotton fabric as the bottom layer, the as-prepared multilayered passive radiation fabrics were termed as PPC materials. The thicknesses of PMMA film and PDMS film were adjusted to optimize the passive radiation cooling property of PPC materials.

Figure 1

Structural diagram of three-layered PPC materials.

2.3 Characterization

Scanning electron microscope (SEM; HITACHI, TM1000) was used to characterize the morphology of PPC materials. To enhance its conductivity, gold was sprayed under vacuum for 20 s with a current of 10–15 A. The magnification during testing was regulated from 30 to 5,000 times. The surface hydrophobicity of PPC materials was tested by the contact angle analyzer (DSA30, Kruss, Germany), and the hydrophobicity of PPC material was reflected by the contact angle of surface water. The water drop volume was 5 μL, and the sample was measured at three different positions. The spectral reflectance (ρ(λ)) was recorded by a UV-Vis-NIR spectrophotometer (UV-3600 PLUS, Shimadzu, Japan). Functional groups on PPC surface were analyzed using an ATR-FTIR measurement (Spectrum Two, PerkinElmer, USA).

2.4 Passive radiation cooling performance evaluation

The spectral absorption (α(λ)) and emissivity (ε(λ)) are equal for any object in the thermal equilibrium according to the Kirchhoff’s law. The calculation formula for ε(λ) could be expressed as ε(λ) = 1 – ρ(λ) – τ(λ), where τ(λ) is the transmittance. Fourier infrared spectrometer with an integrating sphere (iS10, Thermo Fisher Scientific, USA) was applied to determine the reflectivity. The emissivity of samples was calculated through the above formula. In the self-cleaning performance experiment, crushed soil was used as the pollution source, which was randomly sprinkled on the surface of PPC materials. The dropper was used to drop water onto the sample to simulate the real situation of rain or accumulated dew, and the removal status of pollutants before and after the application of PPC materials was compared. In the outdoor cooling test experiment, a tin foil covered box was applied to reduce or eliminate the outside impact. A glass plate (5 cm × 5 cm) was applied to hold PPC materials. The temperature of PPC materials was monitored via an attached probe. For the real evaluation of PPC materials, the outdoor test was conducted in Yibin City (Sichuan province of China) at 10:00 on June 12 (36°C of air temperature, 44% of humidity, and 2 m·s⁻¹ of wind). The temperatures of both control and sample groups were recorded every minute.

Moreover, the electrical energy savings were calculated as the following equation, i.e.,

where the irradiated time was 6 h, the roof heat transfer coefficient was 0.64 W·m⁻²·K⁻¹, and the irradiated surface area of house was 98 m². Supposed summer days were 100 days every year, the electricity fee was $0.15 (kW·h)⁻¹, and the coefficient of performance of an air conditioner was 3. CO₂ emission per kW·h was 0.563 kg.

3.1 Morphology and microstructure of PPC materials

White is the most popular color for today’s industrial products. Nature offers a variety of white surfaces that are built without pigment particles. Such natural white colors are caused by scattering layers which consist of highly disordered nanostructures. The emissivity of materials is related to the functional groups of their surfaces, while the reflectivity of materials is closely dependent on the microstructure of materials. In this work, the specific microstructure (Figure 2a–d) enabled the white color of sprayed PMMA film (Figure 2f). After the PMMA solution was spun with a pen, the ethyl acetate solvent which had certain viscidity and water absorption characteristic would evaporate relatively slowly, allowing the formation of disordered filaments. These filaments displayed an unsmooth surface after being enlarged. Compared with smooth filaments, irregular filaments were more available for the refection of sunlight because of their larger specific surface areas. The physical drawings of PPC materials at different stages are also presented in Figure 2e–h. First, a clean cotton fabric was placed, and then PMMA solution was sprayed on the cotton fabric to form PMMA–cotton composite. The prepared pure PDMS film was covered on PMMA–cotton composite to form the target radiation PPC material. In the practical application process, it is necessary to fix around the material to ensure a long-term usage.

Figure 2

SEM images of PMMA film with four magnifications including ×50 (a), ×100 (b), ×200 (c), and ×1,000 (d). Photographs of samples at different preparing stages including cotton (e), PMMA–cotton (f), PDMS (g), and PCC (h).

3.2 Structural optimization of PCC materials as a function of film thickness

Since the thickness of radiation cooling materials presented a great influence on the radiation cooling performance, the emissivity of PDMS film as a function of thickness is shown in Figure 3a and b. The influence of PDMS thickness on emissivity was slight in the thickness range of 0.5–2 mm. As shown in Figure 3a, the emissivity values (8–14 µm of the atmospheric window) of PMMA film of 1.5 and 2.0 mm presented higher values at 363–556 and 1,209–1,295 nm, while the emissivity values (8–14 µm of the atmospheric window) of PMMA film of 0.5 and 1.0 mm presented higher values at 320–363, 560–763, and 1,312–1,428 nm. The average emissivity (8–14 µm of the atmospheric window) of PMMA films showed variable trends for different wavelengths with the thickness of films. As shown in Figure 3c, PDMS (1.5 mm) + PMMA (1.5 mm) presented a high average emissivity value of 93.4% (8–14 μm of atmospheric window). The high emissivity of PDMS was mainly attributed to the telescopic vibration of numerous Si−O−Si bonds in PDMS (34). Since the volume of liquid storage tank of the pen was fixed, PMMA films with three thicknesses were prepared with the addition of 1, 2, and 3 cans of PMMA solution, respectively. In the process of jet spinning, PMMA filaments float too lightly and were greatly influenced by the airflow, which led to the disproportionate thickness of final film formed.

Figure 3

Emissivity of PPC components as a function of film thickness: (a) PDMS films, (b) PMMA films, and (c) PDMS–PMMA composite films. (d) Reflectivity curves of PMMA films as a function of film thickness.

PDMS films with different thicknesses were compounded with PMMA film (1.5 mm). The emissivity curves of these composite films are shown in Figure 3c. The average emissivity values of these composite materials (8–14 μm of atmospheric window) were higher than that of PMMA film on the PDMS film. Among these, the average emissivity of PDMS film (1.5 mm) combined with PMMA film was the highest. Therefore, PDMS film of 1.5 mm was appropriate as the top layer of PPC materials. After determining the top layer of three-layered composite materials, it is necessary to optimize the second layer to improve the overall reflectivity. As displayed in Figure 3d, the reflectivity of PMMA film increased with the rising thickness of PMMA. The reflectivity could reach >94% from 500 to 2,000 nm for 3.5 mm of PMMA film. As the thickness of PMMA film increased to 6.5 mm further, its reflectivity value improved with a percentage of 5%. Considering the cost and performance of passive radiation cooling materials comprehensively, 3.5 mm of thickness for PMMA film was appropriate as the optimized thickness in the following experiments.

3.3 Optical properties of PPC materials

Since pure PMMA film is easy to be broken and deformed, it is urgently necessary to provide a supporter for the integrity of PPC materials. Herein, the designed three-layered structure of PPC materials was proposed as displayed in Figure 1. The emissivity property of PPC materials should be attributed to PDMS film component. The reflectivity property of PPC materials should be attributed to PMMA film component. The supporter of PDMS and PMMA should be ascribed to cotton fabric. From the ATR-FTIR spectra illustrated in Figure 4, the stretching vibration absorption band of C–H bond (alkyl group) and Si−O−Si were observed at 2,963 and 1,010 cm⁻¹, respectively. The bending vibration absorption band of Si–CH₃ and out of the plane of C–H were displayed at 1,257 and 786 cm⁻¹ (35). Taken together, the preparation of PPC materials conformed to the ideal concept of heat emitter in the selection of chemical bonds.

Figure 4

Infrared absorption spectrum curve of PPC material.

For materials used in the building cooling, the emissivity and the reflectivity are the two most critical cooling properties to be evaluated (Figure 5a). As shown in Figure 5b and c, PPC material presented a high average emissivity values of 93.4% (8–14 μm of atmospheric window) and high reflectivity values over 94% in the sunlight, indicating that PPC was an ideal material for the passive radiation cooling application.

Figure 5

Schematic diagram of building cooling (a). Emissivity curve (b) and reflectance curve (c) of PPC material.

3.4 Flexibility and hydrophobic self-cleaning performance of PPC materials

Since polymers containing one or more of these bonds have a certain radiation effect in the atmospheric window, most radiation cooling materials have two major problems in the practical application with the deepening of the research. First, small water droplets are easily formed on the surface of radiation cooling materials, which seriously affects the cooling capacity in hot and humid circumstance. Second, the surface of radiation cooling materials is easily polluted by dust and other pollution sources. A relatively simple and effective solution to solve these two problems is to endow the radiation cooling material with a hydrophobic surface. The hydrophobic surface can not only make small droplets roll down automatically, but also take away some surface dirt when the droplets roll down, enabling the self-cleaning characteristic of radiation cooling materials. The flexibility of PPC is shown in Figure 6a–d. After being bent for many times, the surface of PPC would not crack, and could still be quickly returned to pristine matrix without external applied force. As shown in the photographs of Figure 6e–h, a droplet of water flowed smoothly and automatically through the surface of PPC material as the included angle between the inclined plane and the horizontal plane was set to 45°. The contact angle of PPC material was ∼117°, which was larger than 90°, indicating that the surface of PCC material was hydrophobic (36). PDMS is sticky and easily absorbs dust. However, comparing with the coverage of dust on the surface of PPC materials, the force induced by dropping or flowing water would be enough to remove most of the dust as shown in Figure 6. The dirt or soil located on the surface of PPC could be wiped off easily with the flow of water droplets as illustrated in Figure 6i–l, presenting a self-cleaning performance. The surface hydrophobicity of PPC material could prevent dew or rain from damaging PMMA film, as well as prevent the dust effectively. The dust fell onto PPC would be wiped off through the naturally falling raindrops. The hydrophobic characteristic of PPC was expected to solve the problem of poor cooling ability in high temperature and high humidity environment, and prolong the service life of radiation cooling materials.

Figure 6

Pictures of PPC flexibility including before bending (a), bending (b) and (c), and after bending (d). Pictures of falling liquid droplet (e)–(h). Pictures of self-cleaning phenomenon for dust washed away by running water (i–l).

3.5 Cooling performance of PPC materials

Infrared radiography was applied to monitor the heat-up situation of wooden roof covered with or without PPC materials in the simulated extra ventricular test (Figure 7). With the increase of irradiation time from 0 to 120 min, PPC covered wooden roof presented a slight temperature enhancement at a low level. It is worth noting that the bare wooden roof presented a significant heat-up tendency from the initial 9.7°C to the final 43.6°C. During the irradiation period, the temperature of some bare wooden roofs reached above 50°C at 30 and 60 min, and then the temperature dropped. This is because the light intensity of the sun changed obviously during the outdoor experiment. The constantly shifting sunlight directly affected the angle of sunlight irradiating cooling materials, thus affecting the temperature change of the roof surface.

Figure 7

Images of PPC materials taken with an infrared camera in the outdoor simulation application.

The physical diagram of devices used in the radiation cooling exponent is provided in Figure 8a. To eliminate the influence of circumstance efficiently during the whole experimental process, a tin foil was fully wrapped around the box, and a plastic coverage was wrapped on the top surface of the box, which allowed the full transmission of sunlight as well as the negative influence of environment. The passive radiation cooling effect of PPC material was monitored for the variation of solar intensity from 10:00 to 12:00 in Yibin City (Sichuan province of China) on June 12, 2024. The sunshine intensity rose continuously from 433 to 505 W·m⁻² at the investigated time period. As shown in Figure 8d, the temperature of atmosphere-1 sample (i.e., glass plate without the coverage of PPC) increased to 55°C rapidly in the first 18 min, and then rose to 62.3°C slowly until 60 min, achieving the equilibrium of thermal energy. For other references, a glass was placed onto the polyethylene film directly as the atmosphere-2 sample and the Al foil was placed onto the polyethylene film directly as the atmosphere-3 sample. The temperatures of these two atmosphere samples increased to 64.2°C and 69.6°C, respectively, arriving at the thermal equilibrium of solar energy. For the glass plate covered with PPC, the temperature increased obviously in the first 25 min, and then rose to 54.6°C with the equilibrium of thermal energy. On the one hand, the final temperature of glass plate with PPC coverage was distinctly lower than that of glass without PPC coverage. On the other hand, the heating rate of glass plate with PPC coverage was also significantly higher than that of glass without PPC coverage. The above results demonstrated that PPC material had an excellent cooling ability, and the cooling effect was about 7.7–15.0°C. This cooling effect was in the middle-upper position among the numerous reported radiation cooling materials. Given the efficient heat absorption and conduct of PPC-covered glass plate under the sunlight irradiation, it was urgently necessary to apply the cooling materials for glass buildings in summer.

Figure 8

Picture and illustration of radiation cooling devices (a) and (b). Sunlight intensity (c). Temperature curves of cooling measurement (d).

According to the estimation of power saving and corresponding carbon dioxide emission reduction of medium-sized houses, the wooden roof temperature under the sunlight could reach about 65°C, and that of PPC-covered roof could stay stable at about 55°C. The circumstance of Yibin City (Sichuan province China) was 34°C. To maintain the indoor temperature at 20°C, the electricity consumption for a medium-sized house was about 574 kW·h/year, while that for PC-covered medium-sized house was 474 kW·h/year. Given that, PPC-covered wooden roof for medium-sized houses could reduce the electricity consumption efficiently. According to the equation listed in Table 1, the calculated reduced ratio of CO₂ emission was 17%/year.

The three-layered films had a good durability. The accelerated weathering test of the three-layered films was carried out for 20 days (4 h of UV irradiation with intensity of 0.71 W·m⁻² at 310 nm and 4 h of water spray at 60°C on a continuous cycle). The solar reflectance spectrum showed that the reflectance of the film was slightly reduced after a long period of accelerated aging, but still over 90% of incident sunlight is reflected after 20 days’ accelerated weathering treatment. The long-wave infrared emissivity was almost unaffected (Figure 9).

Figure 9

Reflectance and emissivity spectra of PPC materials with accelerated weathering time. (a) Reflectance spectra and (b) emissivity spectra.