Effects of pigment volume concentration on radiative cooling properties of acrylic-based paints with calcium carbonate and hollow silicon dioxide microparticles

Figures & data

Figure 1. SEM images of (a) the solid calcium carbonate and (b) the hollow silica microparticles. The CaCO₃ microparticles are constituted by smaller, box-shaped crystalline components and have an average size of 3.4 ± 0.3 µm. The hollow SiO₂ microparticles are spherical and uniform with an average size of 1.11 ± 0.04 µm. The scales of both images correspond to 10 µm.

Figure 2. Solar-average reflectance, as a function of a total particle volume concentration, of the paint samples with the ratio between CaCO₃ and SiO₂ PVC of 1:0 (black solid squares), 3:1 (blue circles), 1:1 (green triangles), 1:3 (orange stars), and 0:1 (red hollow squares). An increase in the concentration of CaCO₃ generally improves solar reflectance. Solar reflectance also increases as the total PVC increases, up to a critical total PVC. The critical total PVC is dependent on the PVC ratio.

Figure 3. Solar reflectance of the paint samples with a total PVC of 0.5 and CaCO₃:SiO₂ PVC ratios of 3:1 (solid blue line), 1:1 (dotted green line), and 1:3 (dashed orange line). The addition of the CaCO₃ microparticles significantly enhances reflectance in the infrared region. The 1:3 CaCO₃:SiO₂ formulation shows a slight improvement in reflectance at a wavelength of ∼460 nm. Below a wavelength of 450nm, reflectance of the paints sharply decreases due to the intrinsic UV absorption of the acrylic binder.

Figure 4. Thermal emissivity, as a function of total particle volume concentration, of paint samples with ratios between CaCO₃ and SiO₂ PVC of 1:0 (black solid squares), 3:1 (blue circles), 1:1 (green triangles), 1:3 (orange stars), and 0:1 (red hollow squares). The addition of the CaCO₃ microparticles alone reduces the thermal emissivity, while the addition of the hollow SiO₂ improves it and counteracts the negative effect of the CaCO₃ addition. Increasing the SiO₂ concentration above the 1:3 CaCO₃:SiO₂ formulation has a minimal impact on the thermal emissivity.

Figure 5. Outdoor experiment setup. Four samples are placed on four clear polycarbonate towers (insets) and can be simultaneously tested. The cover has four square openings that allow sunlight to reach the samples. Each of the four sides of the test chamber is made of two sets of angled slats that allow air flow while blocking direct sunlight. The sides are coated in white to reduce heat from light absorption. A weather station and a pyranometer are attached to the setup to measure sunlight intensity and ambient temperature.

Figure 6. Measured temperatures of the selected paint samples and the ambient (solid light-blue line) during the daytime. The paint samples are formulated with different total PVCs and CaCO₃:SiO₂ PVC ratios: 0.5, 1:0 (solid black line); 0.5, 3:1 (dotted dark-blue line); 0.45, 1:1 (dashed green line); and 0.5, 1:3 (dotted orange line). The 0.5, 3:1 formulation shows the best performance while having the least total PVC among the four and is the only paint in this study that achieves sub-ambient cooling at 8–10 am.