Hierarchical pigment-paint system for rational optical regulation through engineering of nanoparticle additive

Abstract

The present disclosure relates to a method of enhancing optical performance of a radiative cooling paint. The method comprises tuning ultraviolet (UV) reflectivity response of a radiative cooling paint to near-infrared (NIR) reflectivity by rationally selecting one or more components from one or more classes of components within a hierarchical pigment volume concentration (hPVC) system, based on particle size and refractive index of each component, to form a radiative cooling paint with an enhanced optical performance, and introducing sodium polyacrylate into the radiative cooling paint to provide electrostatic repulsion between particles contained in the radiative cooling paint. In various embodiments, the hPVC system comprises an ultraviolet (UV) scattering agent comprising particles having an average particle size of 200 to 400 nm; a visible scattering agent comprising particles having an average particle size of 400 to 600 nm; a near-infrared (NIR) scattering agent comprising particles having an average particle size of 800 nm to 1 µm; and a binder.

Description

HIERARCHICAL PIGMENT-PAINT SYSTEM FOR RATIONAL OPTICALREGULATION THROUGH ENGINEERING OF NANOPARTICLE ADDITIVECROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to the Singapore patent application no. 10202402950S filed on 23 September 2024, the contents of which are hereby incorporated by reference in its entirety for all purposesTECHNICAL FIELD

[0002] This application relates to a method of enhancing optical performance of a radiative cooling paint. In particular, the application relates to a method of enhancing optical performance of a radiative cooling paint by rational selection of one or more components from each class of components within a hierarchical pigment volume concentration (APVC) system, based on particle size and refractive index of each component.BACKGROUND

[0003] With increasing urbanization and climate change, there is a pressing need for sustainable cooling technologies to reduce energy consumption and mitigate rising temperatures. Conventional cooling systems, such as air conditioning, not only consume large amounts of energy but also contribute to greenhouse gas emissions.

[0004] Radiative cooling provides a passive alternative by enabling surfaces to emit long- wavelength infrared (LWIR) radiation through the atmospheric window (8-13 pm) into outer space. To achieve effective sub-ambient cooling under sunlight, coatings must combine high emissivity within this window with high solar reflectivity to minimize heat absorption.

[0005] Radiative cooling paints have emerged as promising solutions due to their simple application on building surfaces and infrastructure. These paints typically incorporateemissive and scattering materials to enhance cooling performance. However, existing approaches such as introducing porosity or dispersing white pigments, often involve tradeoffs, improving optical performance at the expense of mechanical strength and adhesion. In addition, solvent-based formulations release volatile organic compounds (VOCs), raising environmental and health concerns.

[0006] Aqueous radiative cooling paints offer a more environmentally friendly alternative but achieving both high optical and structural performance in such systems remains challenging. Limitations include weak binder efficiency in emulsion-based systems and poor dispersion of nanoparticle additives, which reduce both cooling effectiveness and coating robustness.

[0007] Accordingly, there remains a need for a method and improved radiative cooling paint compositions that seek to address at least some of the problems described hereinabove, or at least to provide an alternative solution.SUMMARY

[0008] According to a first aspect of the present disclosure, a method of enhancing optical performance of a radiative cooling paint is provided. The method comprises tuning ultraviolet (UV) reflectivity response of a radiative cooling paint to near-infrared (NIR) reflectivity by rationally selecting one or more components from each class of components within a hierarchical pigment volume concentration ( / rPVC) system, based on particle size and refractive index of each component, to form a radiative cooling paint with an enhanced optical performance, and introducing sodium polyacrylate into the radiative cooling paint to provide electrostatic repulsion between particles contained in the radiative cooling paint, wherein the / ?PVC system comprises an ultraviolet (UV) scattering agent comprising particles having an average particle size of 200 to 400 nm; a visible scattering agent comprising particles havingan average particle size of 400 to 600 nm; a near-infrared (NIR) scattering agent comprising particles having an average particle size of 800 nm to 1pm; and a binder.

[0009] In some embodiments, the method further comprises adjusting and controlling the particle size and particle size distribution of the UV scattering agent, the visible scattering agent, the NIR scattering agent, or a combination thereof to tune optical properties of the radiative cooling paint across a solar spectrum, including ultraviolet (UV), visible (VIS), and near-infrared (NIR) regions.

[0010] According to a second aspect of the present disclosure, a radiative cooling paint is provided. The radiative cooling paint comprises one or more components rationally selected from each class of components within a hierarchical pigment volume concentration ( / d’VC ) system, wherein the APVC system comprises an ultraviolet (UV) scattering agent comprising particles having an average particle size of 200 to 400 nm; a visible scattering agent comprising particles having an average particle size of 400 to 600 nm, a near-infrared (NIR) scattering agent comprising particles having an average particle size of 800 nm to 1pm; and a binder, and a sodium polyacrylate for providing electrostatic repulsion between particles contained in the radiative cooling paint

[0011] In some embodiments, the radiative cooling paint is a water-based radiative cooling paint.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Various embodiments of the present disclosure are described hereinbelow in the detailed description with reference to the following drawings:FIG. 1 is a schematic illustration of a hierarchical pigment volume concentration (7?PVC)- radiative cooling paint structure obtained by a method in accordance with various embodiments of the present disclosure.FIG. 2 illustrates a plot of particle size related scattering efficiency within the ultraviolet (UV) region of the solar spectrum, that is, between 300 nm and 400 nm. The plot shows that particles having diameters in the range of 200 to 400 nm exhibit high reflectivity in the UV region. FIG. 3A illustrates a plot of full reflective particle size related scattering efficiency within the visible (VIS) region of the solar spectrum, that is, between 400 nm and 800 nm. The plot shows that particles having diameters in the range of 400 to 600 nm exhibit high reflectivity in the VIS region.FIG. 3B illustrates the reflectance spectra of colorful paints comprising iron-based nanoparticle pigments. The spectra show that the colorful paints exhibit high reflectivity in both the VIS and near-infrared (NIR) regions. In the figure, blue corresponds to Prussian blue, red corresponds to FeiOs, and yellow corresponds to FeO(OH).FIG. 4 illustrates a plot of particle size related scattering efficiency within the NIR region of the solar spectrum, that is, between 800 and 2,400 nm. The plot shows that particles having diameters in the range of 800 nm to 1 pm exhibit high reflectivity in the NIR region.FIG. 5 illustrates a broadband solar scattering derived by a / ?PVC-paint system with rational combination of particles with particle size of 250 nm, 500 nm and 1 pm.FIG. 6A is a schematic illustration of a process flow for fabricating a water-based cooling paint in accordance with various embodiments of the present disclosure.FIG. 6B illustrates a simulated scattering efficiency of particles with different sizes.FIG. 6C is a schematic illustration of surface scattering capability provided by different hierarchical pigment volume concentration (APVC) paint systems. In a high pigment volume concentration (PVC) system 610 illustrated in FIG. 6C-(i), a dense particle-occupied surface is formed, which provides sufficient scattering capability. In contrast, a low pigment volume concentration (PVC) system 611 illustrated in FIG. 6C-(ii) results in binder exposure at the surface, leading to insufficient scattering.FIG. 7A illustrates the trend of solar reflectance as a function of pigment volume concentration (PVC). The results demonstrate an increase in reflectance with increasing pigment volume concentration (PVC), which contrasts with previously reported reductions in reflectance over 30% PVC caused by particle aggregation. The thickness of all samples is approximately 200 pm.FIG. 7B shows a scanning electron microscopy (SEM) image of a water-based cooling paint fabricated using an E-assisted dispersion method that provides electrostatic repulsion.FIG. 7C shows an SEM image of a water-based cooling paint fabricated using a water-only dispersion method without electrostatic repulsion.FIG. 7D illustrates a solar reflectance spectrum comparing cooling paints obtained by the E- assisted dispersion method (WCP-10) and water-only dispersion method (water dispersed paint). The water dispersed paint cracked as shown in FIG. 8A. The solar reflectance spectrum was derived from the cracked pieces.FIG. 7E illustrates a solar reflectance spectrum of an optimized water-based cooling paint with a thickness of approximately 200 pm.FIG. 7F is a plot showing the reflectance of a water-based cooling paint (WCP-10) with varying thickness ranging from 100 to 400 pm. Thicknesses below 100pm are not attainable due to viscosity limitations of the high pigment volume concentration (PVC) system.FIG. 7G shows SEM images of WCP-10 samples prepared with different dispersions: a sample without sodium polyacrylate (SPA), a sample containing 0.1g of SPA, and a sample containing 0.2g of SPA. The images indicate that particle dispersion improves progressively with the addition of sodium polyacrylate (SPA), with the 0.2 g sample exhibiting the most homogeneous distribution and the least aggregation.FIG. 7H shows a Fourier-transform infrared (FTIR) spectrum of the WCP-10 sample. The spectrum shows that abundant chemical bond vibrations within the atmospheric window range provides sufficient thermal emission capability.FIG. 71 shows an infrared (IR) emittance spectrum of the optimized WCP-10. The inset illustrates an infrared image of an aluminum plate coated with WCP-10 in an “NTU” pattern, the plate being heated on a hot plate at 40 °C.FIG. 8A is a photograph showing a comparison of paint films formed at approximately 70% pigment volume concentration (PVC) using the E-assisted dispersion method 801 and the water-only dispersion method 802.FIG. 8B shows cross-sectional SEM images of dried paints prepared using the E-assisted dispersion method and the water-only dispersion method. The SEM image of the water-only dispersion sample, taken at a cracking edge, reveals structural non-uniformity that results in severe cracking during drying, in contrast to the more uniform structure of the E-assisted dispersion sample.FIG. 8C shows SEM images of a silicon dioxide (SiCh) paint and a zirconium dioxide (ZrCb) paint prepared using different dispersion methods. The formulation of the SiCb paint comprises 0.3 g of S i O2 particles having an average particle size of 20 nm, 2 mL of water, 2 mL of silicone acrylic emulsion (SAE), 0 g / 0.1 g of sodium polyacrylate (SPA). The formulation of the ZrCb paint comprises 4 g of ZrCb particles having an average particle size of 300 nm, 2 mL of water, 2 mL of silicone acrylic emulsion (SAE) and 0 g / 0.1 g of sodium polyacrylate (SPA)FIG. 8D is a photograph showing the adhesion of WCP-10 on various construction materials, including concrete 808, glass 809, and metal 810, as well as on a commercial primer (sealant) 807, demonstrating its wide application as a coating layer.FIG. 8E is a photograph showing a water-based cooling paint applied on an aluminum plate, subjected to a bending test at approximately 45°. The absence of cracking under the bending condition demonstrates the strong adhesion and flexibility of the coating.FIG. 8F is a photograph showing a water permeability test conducted for dust pick-up evaluation. The evaluation is based on the change in sample weight, which reflects the extentof water penetration into the coating structure and simulates dust accumulation through rainwater diffusion.FIG. 8G is a graph showing the optical performance comparison before and after a one-month outdoor exposure test. The negligible change in the spectrum under alternating rainy and sunny conditions demonstrates the excellent anti-dirt pick-up performance of the coating.FIG. 8H is a graph showing the optical performance comparison of the coating upon sequential chemical treatments, each treatment lasting for one hour. The optical performance is evaluated before and after the sequenti l chemical treatments.FIG. 9 is a graph showing a thermogravimetric analysis (TGA) of WCP-10 for thermal stability evaluation.FIG. 10 is a graph showing an abrasion test comparison between WCP-10 and a commercial radiative cooling paint (Nippon Cool -Tech®). The initial weights of the paints are 5.91 g for the commercial paint and 5.46 g for WCP-10. The test was conducted on an aluminum substrate.FIG. 11A shows an experimental setup for testing radiative cooling paint, conducted at the applicant’s premises under controlled outdoor conditions.FIG. 11B is a graph showing the cooling performance of WCP-10 on a typical sunny day in Singapore (September 8, 2023), the testing and measurements being carried out at the applicant’s premises under controlled outdoor conditions.FIG. 11C is a graph showing a day-long cooling performance comparison between WCP-10 and a commercial radiative cooling (commercial -RC) paint, tested at the applicant’s premises in Singapore (April 6-7, 2023) under controlled outdoor conditions. The thicknesses of both paints are approximately 200pm.FIG. 11D is a graph showing the solar reflectance spectrum of the commercial-RC paint, measured in a comparative field test conducted at the applicant’s premises under controlled outdoor conditionsFIG. HE is a graph showing the long-wave infrared (LWIR) emittance spectrum of the commercial -RC paint, measured in a comparative field test conducted at the applicant’s premises under controlled outdoor conditions.FIG. HF is a graph showing the radiative cooling performance test of the WCP-10 samples with different thicknesses (200 pm, 300 pm and 400 pm).FIG. 11G is a graph showing the results of an experimental cooling power test of WCP-10 conducted on a typical clear night in Singapore (September 12, 2023), the test being carried out at the applicant’s premises under controlled outdoor conditions.FIG. 11H is a graph showing the theoretical cooling power of WCP-10 under various sunlight conditions for evaluating its cooling potential. The ambient daytime and nighttime temperatures are 34 °C and 29 °C, respectively.FIG. HI is a graph showing the outdoor cooling performance test of WCP-10 conducted in Illinois USA on May 25, 2023, under controlled outdoor conditions.DETAILED DESCRIPTION

[0013] The following detailed description is made with reference to the accompanying drawings, showing details and embodiments of the present disclosure for the purposes of illustration. Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments, even if not explicitly described in these other embodiments. Additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0014] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0015] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance as generally understood in the relevant technical field, e.g., within 10% of the specified value.

[0016] As used herein, the term “and / or” includes any, and all combinations of one or more of the associated listed items.

[0017] As used herein, “comprising” means including, but not limited to, whatever follows the word “comprising” Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present

[0018] As used herein, “consisting of’ means including, and limited to, whatever follows the phrase “consisting of’. Thus, use of the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0019] A detailed description of various embodiments will be described below with reference to the drawings.

[0020] The present disclosure relates to a method of enhancing optical performance of a radiative cooling paint. It relates to engineering of inorganic nanoparticles or particles to modulate their optical properties to deliver excellent solar reflectivity and thermal emissivity while maintaining adhesion, low-VOC properties and desired applicability of the radiative cooling paint. By rational selection of specific nanoparticle or particle materials and adjusting their dimensions, the hiding power and scattering efficiency of the radiative cooling paint can be tailored within the solar spectrum, including ultraviolet (UV), visible (VIS), and nearinfrared (NIR) regions.

[0021] As used herein, the term “solar spectrum” refers to electromagnetic radiation emitted by the sun and incident on the Earth’s surface. Unless otherwise specified, the solar spectrum encompasses three principal wavelength regions, namely, the ultraviolet (UV) region: about 280 to 400 nm; the visible (VIS) region: about 400 to 780 nm; and the nearinfrared (NIR) region: about 780 to 2500 nm.

[0022] In a first aspect, various embodiments of a method of enhancing optical performance of a radiative cooling paint are provided. The method comprises tuning ultraviolet (UV) reflectivity response of a radiative cooling paint to near-infrared (NIR) reflectivity by rationally selecting one or more components from each class of components within a hierarchical pigment volume concentration (7?PVC) system, based on particle size and refractive index of each component, to form a radiative cooling paint with an enhanced optical performance, wherein the APVC system comprises: an ultraviolet (UV) scattering agent comprising particles having an average particle size of 200 to 400 nm; a visible scattering agent comprising particles having an average particle size of 400 to 600 nm; a nearinfrared (NIR) scattering agent comprising particles having an average particle size of 800 nm to 1 pm; and a binder.

[0023] As used herein, the term “component” refers to any material, particle, pigment, fdler, binder, additive, or agent that is included in or incorporated into a paint composition. The term “component” encompasses both particulate and non-particulate materials and is not limited by chemical composition, physical form, or function, unless otherwise specified.

[0024] FIG. 1 is a schematic illustration of a AP VC-radiative cooling paint structure 100 obtained in accordance with various embodiments of the method of the present disclosure FIG. 1 shows a / zPVC-radiative cooling paint 101 coated onto a construction surface 102 The Z / PVC-radiative cooling paint 101 comprises particles of a UV scattering agent 103, particles of a visible scattering agent 104, particles of an NIR scattering agent 105 and a binder 106.

[0025] All three scattering agents have different particle sizes and distinct refractive indices, such that each scattering agent predominates in influencing the optical properties within a particular wavelength range of the radiative cooling paint.

[0026] As used herein, the term “optical properties” refers to characteristics of a material, formulation or composition in relation to interaction with solar radiation. Unless otherwise specified, the optical properties include one or more of scattering, reflectance, and emissivityacross the solar spectrum, including ultraviolet (UV), visible (VIS), and near-infrared (NIR) regions.

[0027] The small particles of the UV scattering agent, having a wide bandgap greater than 4. MeV, exhibit high VIS transparency and strong UV scattering capability. As a result, they enhance UV resistance, contributing to about 6% of the total solar energy rejection and improve mechanical strength without adversely affecting the color appearance of the radiative cooling paint. In various embodiments, the UV scattering agent comprises particles selected from, but not limited to, barium sulfate, zirconium oxide, calcium carbonate, yttrium oxide, aluminium oxide and zinc oxide. FIG. 2 is a plot showing particle size related scattering efficiency within the UV region of the solar spectrum. The plot shows that particles having diameters in the range of 200 to 400 nm is crucial for achieving high reflectivity in the UV region for the listed UV scattering agents.

[0028] The visible scattering agent, comprising particles having an average particle size of 400 to 600 nm and a wide bandgap of greater than 4. MeV, avoids UV absorbance while exhibiting strong Mie scattering in the VIS range of the solar spectrum, which accounts for about 50% of the total solar energy. In addition, the visible scattering agent has a high refractive index, of at least about 1.6, to provide strong hiding power. For aesthetic purposes, the visible scattering agent may be classified into two types of pigments, namely, a white pigment and a colored pigment.

[0029] White pigment with broadband high reflectivity typically comprises color-neutral inorganic particles. FIG. 3A shows that particles having diameters in the range of 400 to 600 nm provide sufficient scattering in the VIS range for heat rejection. In various embodiments, the visible scattering agent comprises particles selected from, but not limited to, barium sulfate, titanium oxide, titanium dioxide, zirconium oxide, yttrium oxide and aluminium oxide.

[0030] Colored pigment is essential for colorful paint with desired appearance. With a selected particle size in the range of 400 to 600 nm and a narrow size distribution, the colored pigment provides a distinct absorption peak while maintaining high reflectivity to other solar wavelengths. For example, FIG. 3B shows colorful radiative cooling paints possessing high VIS and NIR reflectivity using iron-based particle pigments. In various embodiments, the iron-based particle pigments are selected from, but are not limited to, iron(III) oxide (FezCh), iron(II, III) oxide (TesCh), iron(III) oxyhydroxide (FeO(OH)) and iron(III) ferrocyanide, also known as Prussian Blue

[0031] The NIR scattering agent, comprising particles having an average particle size of 800 nm to 1 pm, provides strong scattering of NIR energy from sunlight, as illustrated in FIG.4 which shows particle-size-dependent scattering efficiency within the NIR region of the solar spectrum. The relatively large particles interact extensively with NIR light, which accounts for about 44% of the total solar energy. The NIR scattering agent has a high refractive index of at least about 1.6 and high whiteness, enabling NIR energy rejection while minimizing effects on the optical properties in the VIS and UV ranges. In various embodiments, the NIR scattering agent comprises particles selected from, but not limited to, aluminum oxide, zirconium oxide, calcium carbonate, and barium sulfate

[0032] By adjusting and controlling the particle size and particle size distribution of the UV scattering agent, the visible scattering agent, the NIR scattering agent, or a combination thereof to tune the optical properties of the radiative cooling paint across the solar spectrum, an optimal design of solar reflectivity can be obtained without compromising thermal emission, thereby maximizing the cooling performance of the radiative cooling paint.

[0033] In some embodiments, the method of the present disclosure may be applied for controlling the reflectivity of a white radiative cooling paint. In other embodiments, the method may be applied for controlling the reflectivity of a colored radiative cooling paint, which is needed for niche applications, such as those need for matching specific color schemeor military color code. The method may further comprise introducing a color pigment into the / ?PVC system to obtain the colored radiative cooling paint.

[0034] In some embodiments, iron-based particles such as iron oxide may be employed as a color pigment. The particle size and the particle size distribution of the iron oxide particles may be adjusted accordingly to provide specific colorful appearance and high NIR reflection. The rational selection and combination of non-ab sorptive particles with visibly absorptive pigments in paint formulations enable desired color, high UV and high NIR reflectivity, as well as sufficient VIS reflection to be achieved, thereby maximizing the solar energy rejection without adversely affecting the appearance of the colored radiative cooling paint.

[0035] Apart from particle size and particle size distribution, refractive index is also a factor to consider as it is directly related to the hiding power. Currently, titanium oxide is widely used as a white pigment due to its high refractive index of about 2.7. While it exhibits strong hiding power in the VIS range, its insufficient bandgap (between 3.0 to 3.2 eV) results in strong UV absorbance. This leads to a solar heating effect that is undesirable for cooling paint applications. In addition, UV-triggered catalytic activity of titanium oxide may accelerate degradation of polymeric binder, which in turn causes the degradation of the polymeric cooling paint.

[0036] Thus, inorganic particles with a high refractive index (of at least about 1 .6) and an adequate bandgap (greater than 4.14eV) serve as ideal candidates for solar reflectance. For example, zirconium oxide, with a refractive index (n) of about 2.2, and barium sulfate, with a refractive index (n) of about 1.68, provide strong NIR scattering and negligible UV absorption, which means they can provide sufficient reflectivity in colored radiative cooling paints through high reflectivity in the invisible range. Iron oxide particle pigments, with a refractive index (n) of about 2.6, exhibit tunable VIS absorbance with distinct absorption peak. It is also capable of providing NIR reflectivity while minimizing undesired absorption in theVIS and UV ranges.

[0037] By optimizing pigment design, improved cooling performance can be imparted to the colored radiative cooling paint without adversely affecting its other properties.

[0038] In various embodiments, the method of the present disclosure provides a radiative cooling paint with high reflectivity across the full solar spectrum. In some embodiments, this is achieved by rationally selecting the one or more components from each class of components within the bPVC system to obtain a radiative cooling paint comprising different components with different particles sizes In other words, the UV scattering agent, the visible scattering agent and the NIR scattering agent are provided by a different component, such that the UV scattering agent, the visible scattering agent and the NIR scattering agent are distinct.

[0039] In other embodiments, the UV scattering agent, the visible scattering agent, and the NIR scattering agent are provided by the same component. In such cases, the component is present in different particle size ranges, with each particle size range corresponding to a respective class of components within the APVC system.

[0040] In certain embodiments, the UV scattering agent, the visible scattering agent and the NIR scattering agent are provided by the same component comprising barium sulfate, the barium sulfate being present in different particle size ranges, each particle size range corresponding to a respective class of components within the APVC system In an exemplary embodiment, the barium sulfate may comprise particles with particle sizes of 250 nm, 500 nm and 1 pm. A radiative cooling paint comprising particles with different particle sizes provides broadband solar scattering that is significantly higher than that achieved using particles of a single size, as illustrated in FIG. 5. It is further noted that certain components, for example, barium sulfate and calcium carbonate, exhibit strong bonding vibration within the atmospheric window range, thereby contributing to thermal emission into the surroundings.

[0041] Despite the contribution from the scattering agents, polymer binders are intrinsically highly emissive within the atmospheric window range as well, which further ensures effective thermal emission. In various embodiments, the polymer binder is selectedfrom, but not limited to, an acrylic solution, an acrylic emulsion, a polyurethane emulsion, and a polyvinylidene fluoride (PVDF) resin.

[0042] The / / PVC-radiative cooling paint of the present disclosure effectively regulates the optical performance across the broadband solar spectrum without a significant increase in the pigment volume concentration (PVC). As the proposed components are readily available in the industry, the method of the present disclosure serves as an effective and generic strategy for enhancing radiative cooling paint design and performance Moreover, such a hierarchical particle distribution not only helps to achieve a dense paint structure (as smaller particles occupy the voids among larger particles), it is also compatible with industrial paint production processes.

[0043] As used herein, the term “pigment volume concentration” refers to the ratio of the volume of solid pigment particles to the total volume of non-volatile components in a paint or coating formulation, expressed as a percentage. Unless otherwise specified, the abbreviation “PVC” shall be used throughout the description, claims and drawings to denote pigment volume concentration.

[0044] In some embodiments, the method may further include introducing the one or more components rationally selected from the 71PVC system into a paint matrix to tune the optical properties of the radiative cooling paint across the solar spectrum. Tn certain embodiments, the one or more components rationally selected from the / ?PVC system are introduced into an existing paint formulation to tune the optical properties of the existing paint, thereby enhancing the optical performance of the existing paint.

[0045] In various embodiments, the method further comprises introducing sodium polyacrylate into the radiative cooling paint to provide electrostatic repulsion between the particles contained in the radiative cooling paint. This step, when incorporated into the method of the present disclosure, is referred to as the electrostatic-assisted (E-assisted) dispersion method. The E-assisted method relies on the intrinsic electrostatic repulsion between thesodium polyacrylate particles and the water molecules present in the aqueous solution to achieve better dispersion of the particles in the aqueous solution without affecting the optical performance of the radiative cooling paint. The E-assisted method can be applied to systems of various particle types and concentrations. In some embodiments, the sodium polyacrylate is present in an amount sufficient to provide the electrostatic repulsion between the particles contained in the radiative cooling paint. In certain embodiments, the sodium polyacrylate is present in an amount ranging from 0.5 to 1.5 wt%, based on the total weight of the radiative cooling paint.

[0046] In the embodiments where the U V scattering agent, the visible scattering agent and the NIR scattering agent are provided by barium sulfate, the binder may comprise silicone acrylic emulsion.

[0047] In some embodiments, the barium sulfate is present in an amount ranging from 40 to 70 wt%, based on the total weight of the radiative cooling paint.

[0048] In various embodiment, the radiative cooling paint is a water-based radiative cooling paint.

[0049] As used herein, the term “water-based radiative cooling paint” will be referred to as “water-based cooling paint” or “WCP” throughout the description, claims and drawings, unless otherwise specified. This terminologies are adopted for convenience and consistency, and no limitation on the scope of the disclosure is intended by such usage.

[0050] In various embodiments, the method of the present disclosure may be employed for preparing a radiative cooling paint with enhanced performance useful for application on a variety of surfaces to provide a cooling roof, colorful high reflective cooling fagade, colorful cooling textile, colorful cooling infrastructures such as chemical storage tank, food distribution facilities, signal base stations and data centres, and for use as a colorful cooling car paint.

[0051] In a second aspect of the present disclosure, a radiative cooling paint is provided.In various embodiments, the radiative cooling paint comprises one or more components rationally selected from each class of components from a hierarchical pigment volume concentration (hPVC) system, wherein the / ?PVC system comprises an ultraviolet (UV) scattering agent comprising particles having an average particle size of 200 to 400 nm; a visible scattering agent comprising particles having an average particle size of 400 to 600 nm; a near-infrared (NIR) scattering agent comprising particles having an average particle size of 800 nm to 1 pm; and a binder.

[0052] In various embodiments, the radiative cooling paint further comprises sodium polyacrylate for providing electrostatic repulsion between the particles contained in the radiative cooling paint.

[0053] In various embodiments, the UV scattering agent comprises particles selected from, but not limited to, barium sulfate, zirconium oxide, calcium carbonate, yttrium oxide, aluminium oxide and zinc oxide.

[0054] In various embodiments, the visible scattering agent comprises particles selected from, but not limited to, barium sulfate, titanium oxide, titanium dioxide, zirconium oxide, yttrium oxide and aluminum oxide.

[0055] In various embodiments, the NIR scattering agent comprises particles selected from, but not limited to, aluminum oxide, zirconium oxide, calcium carbonate and barium sulfate.

[0056] In various embodiments, the binder is selected from the group consisting of an acrylic solution, an acrylic emulsion, polyurethane emulsion and polyvinylidene fluoride resin.

[0057] In various embodiments, the UV scattering agent, the visible scattering agent and the NIR scattering agent are provided by a different component, such that the UV scattering agent, the visible scattering agent and the NIR scattering agent are distinct. In someembodiments, the UV scattering agent, the visible scattering agent, and the NIR scattering agent are provided by the same component. In such cases, the component is present in different particle size ranges, with each particle size range corresponding to a respective class of components within the APVC system.

[0058] In certain embodiments, the UV scattering agent, the visible scattering agent and the NIR scattering agent are provided by the same component comprising barium sulfate. In these embodiments, the binder may comprise silicone acrylic emulsion.

[0059] In some embodiments, the amount of barium sulfate present in the radiative cooling paint ranges from 40 to 70 wt%, based on the total weight of the radiative cooling paint.

[0060] In some embodiments, the sodium polyacrylate is present in an amount sufficient to provide electrostatic repulsion between the particles contained in the radiative cooling paint. In certain embodiments, the sodium poly aery late is present in an amount ranging from 0.5 to 1.5 wt%, based on the total weight of the radiative cooling paint.

[0061] In some embodiments, the radiative cooling paint is a colored radiative cooling paint comprising an iron-based particle pigment. The iron-based particle pigment is selected from, but not limited to, iron(III) oxide (Fe2O3), iron(II, III) oxide (Fe^Cri). iron(III) oxyhydroxide (FeO(OH)) and iron(III) ferrocyanide, also known as Prussian Blue (Fe4[Fe(CN)6]3).

[0062] In some embodiments, the radiative cooling paint is a water-based radiative cooling paint (hereinafter referred to as “water-based cooling paint” (WCP)).

[0063] The water-based cooling paint is robust, contains a low content of volatile organic compounds (VOCs), and adhere to a variety of surfaces and commercial facade materials. In some embodiments, the water-based cooling paint has a pigment volume concentration (PVC) of about 70%. In various embodiments, the water-based cooling paint exhibits enhanced optical performance, including a solar reflectance of about 97% and a long-wave infrared (LWIR) emissivity of about 95%.

[0064] In some embodiments, the water-based cooling paint comprises barium sulfate, silicone acrylic emulsion (SAE) and sodium polyacrylate (SPA).

[0065] Barium sulfate is adopted as the scattering agent due to its negligible absorbance in the UV range, attributed to its large bandgap of about 7.2 eV, as well as its cost-effectiveness.

[0066] The water-based silicone acrylic emulsion (SAE) is adopted to provide binding and to increase surface adhesion through its abundant Si-0 bonds. It has low-VOC of less than 50 g / L, and the low-VOC content renders the silicone acrylic emulsion more environmentally friendly in the paint market.

[0067] Further, sodium polyacrylate (SPA), a cost-effective softener in water treatment, is adopted to provide electrostatic repulsion between the particles contained in the radiative cooling paint or the water-based cooling paint for better dispersion without affecting the optical performance (i.e. remaining non-absorptive in the solar spectrum), and for enhancing the connectivity of the polymeric chains contained in the radiative cooling paint or the waterbased cooling paint. The issue of structure robustness resulting from high pigment volume concentration (PVC) levels is alleviated by the low molecular weight of sodium polyacrylate.

[0068] Compared to a cooling paint obtained using the water-only dispersion system, the water-based cooling paint of the present disclosure obtained using the E-assisted dispersion method exhibits not only enhanced optical properties, but also great robustness, strong interfacial adhesion over a wide range of surfaces, and good water resistance. The water-based cooling paint is also relatively more cost-effective, with an estimated unit expense substantially lower than that of commercially available cooling paints.

[0069] FIG. 6A illustrates an exemplary facile all-in-one fabrication flow 600 that is compatible with industrial paint processing. In this process, sodium polyacrylate 601 is first introduced into water 602 and ionizes under stirring at room temperature to form polymeric chains bearing negative charges, as shown in FIG. 6A-(i). Upon subsequent addition of barium sulfate particles 603 (under stirring at room temperature) to mixture 604 (containing waterand sodium polyacrylate), the sodium polyacrylate 601 begins to bond to particle surfaces of the barium sulfate, thereby imparting electrostatic repulsion 605 (FIG. 6A-(ii)) to suppress particle clustering. Thereafter, silicone acrylic emulsion 606 is introduced into mixture 607 (which contains water, sodium polyacrylate and barium sulfate particles) under stirring at room temperature. The silicone acrylic emulsion 606 provides steric hindrance through the polymeric chains, as illustrated in FIG. 6A-(iii), thereby further suppressing particle aggregation arising from Brownian motion. Stirring continues under room temperature until a water-based cooling paint 608 is formed. Compared with a water-only dispersion system, sodium polyacrylate successfully releases water molecules otherwise trapped on the particle surfaces. This contributes to the free water for particle dispersion. The electrostatic repulsion provided by the sodium polyacrylate at the particle surface of barium sulfate enables homogeneous dispersion with lower water content. This further increases crystallization of the silicone acrylic emulsion and results in a higher particle density after drying.

[0070] It will be appreciated that the sequence of mixing the components may be varied according to practical considerations such as to accommodate mixing efficiency or large-scale processes, and such variations would be understood by a person skilled in the art to fall within the scope of the present disclosure For example, in some embodiments, sodium polyacrylate, silicone acrylic emulsion and water may be mixed prior to the addition of barium sulfate to the resulting mixture.

[0071] For solar reflection, the particles are adopted as the solar scattering additives, and their dimensions determine scattering efficiency. Strong Mie scattering occurs on particle surfaces when the particle size is comparable to the solar wavelength. Simulated scattering efficiency, as shown in FIG. 6B, directly demonstrates that particles with particle size ranging from 300 to 400 nm perform the best, as most of the energy lies in the short wavelength range (0.3 to 2pm).

[0072] The scattering effect of particles of different sizes within the solar spectrum was simulated using the Finite-Difference Time-Domain (FDTD) method. Barium sulfate, with a refractive index (n) of about 1.68 was used in the simulation. The scattering cross section of a single nanoparticle surrounded by air was calculated under total-field scattered-field (TFSF) and perfect matching layer (PML) boundary conditions, based on the ratio of the cross- sectional length to the incident field. To obtain a scattering map correlating particle size with solar wavelength, a sweeping analysis was conducted using a two-dimension matrix of particle diameters (D = 0.1 to 1 pm) and wavelengths (W = 0.3 to 2.5pm). The results show that particles with sizes in the range of 300 to 400 nm exhibit the broadest Mie scattering effectiveness within the solar spectrum.

[0073] As a result of the simulation, barium sulfate particles with a dimension of 300 to 400 nm was selected. It was selected also for its bandgap suitability (negligible absorbance of UV that accounts for about 6.2% of solar energy in the tropical region) and broad scattering wavelength range. Although barium sulfate exhibits lower hiding power due to lower refractive index than titanium dioxide and zirconium dioxide, it can be up to 4 to 20 times more cost-effective upon mass production. As the refractive index difference between the binder .4) and the barium sulfate particles (n^~1.7) is smaller, this leads to weaker interfacial scattering. To maximize solar scattering under such conditions, densely packed particles on the paint surface are required, which in turn necessitates a higher pigment volume concentration (PVC) system 610 (FIG. 6C-(i)). On the other hand, in a low pigment volume concentration (PVC) system 611, insufficient particle occupation leads to binder exposure (FIG. 6C-(ii)). In such cases, solar reflection will be compromised, as the crystalized binder network is partially absorptive in the NIR region and transmissive in the VIS region.

[0074] To assess compositional effect, optical performances in high PVC range of the water-based cooling paints prepared by the procedure described in Example 1 were evaluated. The results show that when pigment volume concentration (PVC) increases from 50% to 70%(i.e., WCP-4 to WCP-10) using the E-assisted dispersion method, the solar reflectance of the water-based cooling paint (WCP) increases up to about 97%. This breaks the commonly accepted critical pigment volume concentration (PVC) (about 30%) limitation on optical performances with particle clustering.

[0075] With a pigment volume concentration (PVC) of about 70%, the water-based cooling paint already exhibits optical performance comparable to that of intrinsic commercial barium sulfate particles (about 98%). Further increases in the pigment volume concentration (PVC) may, however, result in structural defects after drying, such as chalking and cracking.

[0076] In fact, sodium polyacrylate (SPA), which provides electrostatic repulsion between particles contained in the water-based radiative cooling paint, enables homogeneous dispersion and strong particle compaction with low water content. This leads to a densely packed surface free of particle aggregation issues.

[0077] FIG. 7B and FIG. 7C are scanning electron microscopy (SEM) images of the waterbased cooling paint fabricated with electrostatic repulsion (FIG. 7B) and without (FIG. 7C) electrostatic repulsion. The images show obvious structural differences. FIG. 7B shows that the water-based cooling paint fabricated using the E-assisted dispersion method with sodium polyacrylate achieves homogeneous dispersion and strong particle compaction at low water content, without any particle aggregation issues In contrast, FIG. 7C shows obvious particle aggregation 701 in the water-only dispersion system, even when an excess amount of water was used. It is worth noting that the water-only dispersion paint cracked after drying, which was attributed to poor binder crystallization resulting from the excess water used. Although a high PVC confers good reflectance in the VIS range (attributed to the strongest scattering efficiency shown in FIG. 6B), insufficient light scattering in the UV and NIR ranges is still observed due to particle aggregation, highlighting the importance of homogenous particle distribution in high PVC system (FIG. 7D). Furthermore, a homogenous high PVC system obtained through E-assisted dispersion enables sufficient solar scattering at small coatingthickness (about 200 pm, FIG. 7E and FIG. 7F), which is comparable to the application standards of commercial exterior paints. A comparison among samples with different amounts of sodium polyacrylate added (FIG. 7G) demonstrates that the E-assisted method achieves homogenous dispersion with a small amount of water and without compromising the film quality. In contrast, an excessive amount of sodium polyacrylate (the 0.2 g SPA sample) does not lead to improve dispersion but instead introduces potential structural weaknesses and poor water resistance.

[0078] In addition to solar reflection, thermal emission capability plays a significant role in passive heat dissipation. As emissivity in the LWIR range is associated with molecular vibrations, appropriate chemical bonds provide sufficient infrared emissivity within the atmospheric window. Through the FITR transmittance spectrum (see FIG. 7H), it can be observed that the sample, WCP-10 exhibits abundant chemical bonds, including C-O, C=O, - SO4, Si-O-Si and Si-C E which strongly vibrate within this range. This vibrational behavior is attributed to the properties of acrylic, silicone and barium sulfate particles. Consistently, WCP-10 was tested to exhibit an emittance of about 95% within the atmospheric window, comparable to that of state-of-the-art radiative coolers (FIG. 71) To further demonstrate its high emissivity rather than transmissivity, infrared image (inset of FIG. 71) was obtained to evaluate the surface temperature of a water-based cooling paint (“NTU” pattern) coated on an aluminum plate placed over a heater. The contrasted temperature profile between the waterbased cooling paint and the aluminum substrate, which has very low emissivity, suggests that the water-based cooling paint is highly thermal emissive. Further tests were carried out, the details of which are described in the Examples section below .

[0079] The technology of the present disclosure provides several technical benefits over conventional radiative cooling paint formulations. Through rational selection and design of the particles, the optical properties of the radiative cooling paint may be tuned to achieve customized cooling performance and desired appearance under specific environmentalconditions and architectural requirements. The optimization of solar reflectivity further enhances the cooling efficiency of the radiative cooling paint, thereby improving energy savings and providing better thermal comfort in buildings.

[0080] In addition, the disclosed radiative cooling paint employs inorganic particles that are environmentally friendly and abundantly available, contributing to the sustainability and cost-effectiveness of the technology for large-scale implementation. The approach is also versatile, being applicable to paints for a wide range of surfaces and substrates, including rooftops, faqades, pavements, and vehicles, thereby expanding the potential for deployment in both urban and rural environments. Furthermore, by regulating the optical and thermal performances through rational selection and design of the particle properties, the cooling performance of the radiative cooling paint can be improved without necessarily increasing the particle loading, thus maintaining both cost efficiency and the robustness of the radiative cooling paint.

[0081] Paints play an important role in building industries. Most reported radiative cooling paints using organic solvents have high volatile organic compounds (VOC) emission, resulting in poor industry acceptance and environmental concerns. Although high PVC aqueous paint addresses the volatile organic compounds (VOC) issue, its mechanical and optical properties are inferior due to challenging dispersion of nanoparticle additives. The proposed solution effectively addresses this issue, not only enhances the applicability of radiative cooling paint, but also provides insights on design of water-based paints for other purposes, offering solutions for mitigating environmental impacts and achieving low carbon and sustainable built environments.

[0082] To facilitate a better understanding of the present disclosure, the following examples of specific embodiments are given. In no way should the following examples be read to limit or define the entire scope of the invention. One skilled in the art will recognizethat the examples set out hereinbelow are not an exhaustive list of the embodiments of the present disclosure.EXAMPLESExample 1

[0083] Fabrication of Radiative Cooling Paint

[0084] This example describes a method for fabricating a water-based radiative cooling paint (WCP) in accordance with some embodiments of the present disclosure

[0085] Prior to the fabrication of the water-based radiative cooling paint (WCP) and water- dispersed paint (without E-assisted method) slurries, barium sulfate particles were washed with ethanol to remove surface contaminants introduced during manufacturing.

[0086] For the WCP slurry, 0.1 g of sodium polyacrylate, 2 mL of silicone acrylic emulsion and 2 mL of water were mixed and stirred for 1 h to obtain a homogenous solution. Barium sulfate particles were then slowly added into the solution to form the WCP slurry. With a fixed solution volume, different amounts of barium sulfate were added to prepare various pigment volume concentration (PVC) WCP. A sample with Xg particle addition is referred to as WCP-X, where X = 10, 8, 6 or 4. The PVC values of WCP-10, WCP-8, WCP-6 and WCP- 4 were calculated from the dried paints and found to be about 70%, about 65%, about 58%, and about 50%, respectively.

[0087] For the pure water-dispersed paint slurry, sodium poly acrylate was replaced with an additional 2 mL of water (total of 4 mL) to ensure proper workability, followed by 1 h of stirring to maximize uniformity.

[0088] Once both slurries were prepared, the slurries were coated onto aluminum plates and dried under identical ambient conditions (approximately 27 °C, Relative Humidity (RH) of about 60%) for 2 days.Example 2

[0089] Material Characterizations

[0090] Structural images were taken by Scanning Electron Microscopy (JEOL 7600). Reflectance spectra were tested by an UV-vis-NIR spectrometer (Lambda 950, Perkin Elmer), equipped with Spectrolon® coated integrating sphere (150 mm InGaAs). Emittance spectra were measured using a Fourier-transform infrared (FTIR) spectrometer (Frontier, Perkin Elmer) equipped with a gold-coated integrating sphere (Broker). Infrared (IR) images were captured using an infrared camera (FLIR, E60), following a black surface calibration. Th erm ogravi metric curves were obtained using a th erm ogravi metric analyzer (TGA, Q500). Abrasion testing was conducted using an abrasion tester (Taber), wherein an H-10 abrading wheel was employed as the abrader.Example 3

[0091] Adhesion and Robustness Tests

[0092] In addition to optical performance, adhesion and robustness tests were conducted to further evaluate the contribution of the E-assisted dispersion method in a high-PVC system.

[0093] In the water-based high-PVC system, water serves as the dispersion medium to promote unifonnity and workability. However, the incorporation of high particle loading requires an increased amount of water to maintain homogeneity. Structural formation during the crosslinking stage is directly influenced by the water and binder ratio, as both the binder concentration and water evaporation play critical roles in the crosslinking process.

[0094] The adhesion test was carried out by preparing radiative cooling paints comprising barium sulfate with a pigment volume concentration (PVC) of about 70% using the E-assisted dispersion method and the water-only dispersion method. In the cooling paint prepared by the water-only dispersion method, 4 mL of water was required to disperse 10 g of barium sulfate particles while maintaining desired workability. As shown in FIG. SA, a distinct contrast is observed between radiative cooling paints prepared with and without the E-assisted dispersion method. The cooling paint prepared by the E-assisted dispersion method shows a smoothsurface, while the radiative cooling paint prepared by the water-only dispersion method shows visible cracking 803 and poor adhesion 804, which indicates inadequate binder crystallization during curing. This is attributed not only to weak polymeric network resulting from insufficient binder concentration, but also to non-uniform binder penetration caused by particle aggregation. These findings further demonstrate the importance of employing an appropriate dispersion method for water-based paints with increasing pigment volume concentration (PVC). Verified by the SEM images as shown in FIG. 8B, particle aggregation 805 and structural non-uniformity hinder the binder crystallization, leading to severe cracking. In contrast, the increased binder concentration and the homogeneous distribution of binder among the particles in the radiative cooling paint prepared by the E-assisted dispersion method ensure adequate interfacial bonding. Notably, the dispersion related structural enhancement imparted by sodium polyacrylate is applicable to paints comprising various inorganic particles (see FIG. 8C), as carboxylate groups have been demonstrated to be attachable to a wide range of particle surfaces. FIG. 8C shows SEM images of silicon dioxide (SiCh) paint (FIG. 8C-(i), (ii)), and zirconium dioxide (ZrCh) paint (FIG. 8C-(iii), (iv)) obtained using the two different dispersion methods. Both silicon dioxide ('SiCh) and zirconium dioxide (ZrO?) paints without sodium polyacrylate (SPA) show cracking (see 806) besides large clusters of particles.

[0095] FIG. 8D shows the adhesion of the WCP-10 sample on various construction materials. The abundant Si-0 and hydrogen bonds provided by silicone acrylic emulsion impart strong adhesion to a wide range of building facade materials, including a sealing layer (primer) 807, concrete 808, glass 809, and metal such as aluminum 810. Robust adhesion is maintained even when WCP-10 is subjected to a 45° bending test (see FIG. 8E). Notably, the enhanced interfacial bonding achieved through silicone acrylic emulsion as a binder (primer) further broadens the range of application scenarios.

[0096] Next, the robustness of WCP-10 was investigated in various aspects. A 2-hour water permeability test (see inset of FIG. 8F) was carried out to evaluate the dirt pick-upproperty, as dust typically diffuses into surface-level porous defects along with rainwater and becomes trapped therein upon drying. Negligible weight change, as shown in FIG. 8F indicates excellent resistance to dust diffusion, which is attributed to the dense structure. Furthermore, a one-month outdoor exposure test (from July 24 to August 24 in Singapore, with the sample positioned at a 45° angle facing the sky) was conducted under a tropical climate and controlled outdoor conditions to investigate potential contaminant-induced optical degradation The reflectance spectra derived from the test, as shown in FIG. 8G, reveal that the WCP-10 surface remains optically stable and dust-free, which is attributed to its homogeneous and dense surface with good anti-diffusion properties. Notably, strong UV reflectance (greater than 90%) is advantageous not only for rejecting solar heating, but also for minimizing UV-induced binder degradation. In the water-based cooling paint of the present disclosure, the combination of strong Mie scattering and the large bandgap of barium sulfate acts synergistically to suppress photoexcitation, thus leading to improved UV resistance.

[0097] Apart from dirt pick-up and optical stability, chemical stability was also evaluated to assess the capability of WCP-10 under extreme weather conditions during long-term usage. The evaluation was performed by sequentially treating WCP-10 with common organic, acidic, and alkaline solutions on the same panel surface (as in the permeability test shown in FIG 8F), and comparing the optical performance before and after the treatment. Because surface structural changes generally result from particle movement within the paint, the unchanged optical spectrum (see FIG. 8H) indicates a stable particle arrangement maintained by the polymeric matrix. This stability is attributed to the inertia nature of both the barium sulfate particles and the crosslinked silicone acrylate.Example 4

[0098] Thermo gravimetric Treatment

[0099] Given that thermal degradation presents a major challenge for pigment volume concentration (PVC) coatings, thermogravimetric analysis under air flow (FIG. 9) was conducted to evaluate the heat resistance of the water-based cooling paint. Negligible water loss 901 confirms the presence of a dense structure, while the gradual degradation of the acrylic component 902 and the degradation of silicone component 903 indicate good thermal stability under normal operating temperatures. Notably, at 800 °C, approximately 90% of the residues correspond to particles as barium sulfate particles remain stable at temperatures exceeding 1,000 °C.Example 5

[0100] Mechanical Stability and Adhesion Strength

[0101] Lastly, mechanical stability and adhesion strength were examined by severe abrasion test (FIG. 10), where a 1.25 kg load and rough wheel (Taber H-10 Abrader) were adopted for 1,000 cycles (at 60 cycle / min) on both the water-based cooling paint and the commercial radiative cooling paint (Nippon Cool-Tech®, Water-based Cooling Paint). This test condition resembles cases where severe frictions are experienced. The lower weight loss of the water-based cooling paint compared to the commercial paint in the abrasion test indicates an improvement of about 15% in anti-friction capability, which reflects stronger adhesion between the water-based cooling paint and the metal substrate.Example 6

[0102] Outdoor Cooling Performance Test

[0103] The outdoor cooling performance test was conducted in Singapore (a typical tropical region with high relative humidity (RH) and strong sunlight), under controlled outdoor conditions.

[0104] FIG. 11A shows an experimental setup for carrying out the test. In particular, aWCP-10 sample 112 was coated onto a 10 cm x 10 cm aluminum plate 114 with a thicknessof 5 mm. The aluminum plate was placed above a K-type self-adhesive thermocouple (SA3-K-120, Omega) 116. A high density expanded polystyrene foam (EPS) 118 (white color and thermal insulative) was cut to the same size with a thickness of approximately 7 cm for thermal insulation. Between the aluminum and the EPS, a double-sided tape (not shown) was used to seal the air gap and to ensure stable adhesion. Before testing, all the thermocouples were calibrated with K-type standard error. A datalogger (NI DAQ module) was connected to the thermocouple 116, a relative humidity (RH) sensor 120, a pyranometer 122 (LP PYRA 10, Delta Ohm) and a pyrgeometer 124 to extract data, such as temperatures, RH and sunlight intensity. The ambient temperature sensor 126 and the RH sensor 120 were both covered with a professional solar shield to avoid atmospheric effects. The cooling power test was conducted by attaching a self-adhesive electrothermal heater (not shown) under the aluminum plate. Starting from the static state, the heater was powered to keep the sample at ambient temperature, while the power consumed was extracted to be the effective cooling power.

[0105] As shown in FIG. 11B, WCP-10 achieved up to 1.5 °C sub-ambient cooling performance under intensive sunlight (about 800 to 1,000 W / m2) at noon on a sunny day, while the cooling performance was further enhanced to about 3 °C at sunset (200 to 400 W / m2). The hindered noon time cooling performance comes from the narrowed atmospheric windows by high humidity as well as intensive sunlight in tropical regions. The improved cooling performance observed at sunset is primarily attributed to the decrease in solar irradiance and the corresponding reduction in incident thermal energy. Day-long effective cooling test was conducted to evaluate the long-term cooling performance of WCP-10 (FIG. 11C). WCP-10 achieved all-day sub-ambient cooling in cloudy weather, which is attributed to the high reflectance and thermal emittance in the atmospheric window. Compared to waterbased commercial -RC (Nippon Cool-Tech®, FIG. HD and FIG. HE), WCP-10 exhibits enhanced cooling performance, particularly under direct sunlight, demonstrating the effectiveness of the optical design resulting from the excellent dispersion and packing ofbarium sulfate particles in WCP-10. The similar nighttime temperatures arise from the comparable average thermal emissivity (about 95%) of the two samples. It is further noted that negligible differences in cooling performance were observed within the rational paint thickness range, which may be attributed to strong convection effects and the negligible heat capacity exhibited by the thin coating layers (FIG. 11F). As indicated by the nighttime temperature recording as shown in FIG. 11C, 3.0 °C to 3.5 °C sub-ambient cooling reveals the best cooling performance of the radiative cooling paint under tropical climate. The nighttime cooling power was experimentally investigated by compensating temperature gap with an electrothermal heater. As shown in FIG. 11G, WCP-10 was heated (from 3 °C below ambient temperature) and maintained at ambient temperature for 25 min under 57 W / m2power input. This indicates the cooling power of 57 W / m2, approaching the theoretical cooling power limit of 70 W / m2in the tropical regions. The lower value compared to reported radiative cooler at other regions is due to the high relative humidity and stronger downwelling radiation. Consistent with theoretical analysis (see Example 9) based on heat balance (FIG. 11H), obvious sub-ambient cooling performance could be achieved both at night and on cloudy days. Under typical sunny daytime conditions with intensive sunlight, near ambient cooling can be achieved, which corresponds closely to the experimental results shown in FIG. 11B. In contrast to the ideal thermal exchange assumed in energy-balance-based theoretical analysis, inevitable contact resistance and the effects of finite heat capacity influence the experimental results, leading to a possible overestimation of the nighttime cooling power (about 13 W / m2) (see Example 10). In apparent contrast to tropical regions, WCP-10 works quite well in Illinois, United States of America with up to 3 °C sub-ambient cooling under high solar intensity (900 W / m2) (FIG. 111).Example 7

[0106] Theoretical heat transfer model and cooling power simulation

[0107] The theoretical simulation of the radiative cooler is evaluated using an energybalance model, in which the energy difference between the input and output powers is considered as the radiative cooling performance. The energy balance for a radiative cooler at temperature Tsunder direct sunlight includes contributions from solar heat gain, atmospheric radiation, and thermal convection / conduction, and can be expressed as:PcoolVs) ~ Prad (Ts) Patm(Tamb) Psolar Pconv+cond(Ts> Tc!iam) (1 1)

[0108] where Tsis the radiative cooler surface temperature, Pcooiis the net cooling power, Pradis the power emitted out by the coating, Patmis the absorbed power from downwelling atmospheric radiation, Psoiaris the absorbed power from solar irradiance, and PCOnv+cond is considered as parasitic heat exchange with surroundings.

[0109] The details of each item are as follows:

[0110] Here, IBis the intensity of black-body radiation at the surface temperature byPlanck’s law, which can be expressed aswhere h, c, A, kBare Planck’s constant, velocity of light, wavelength and Boltzmann constant, respectively. f(A, 0) is the measured spectral emissivity of radiative cooler. It is worth noting that the downwelling atmospheric radiation in tropical regions is 40% higher than that in temperate regions, leading to lower cooling power.

[0111] where Tambrepresents the ambient air temperature. IB(Tamb,X) = 2hc21'sAe intensity of black-body radiation at ambient temperature TambbyPlanck’s law. Ea£m(A, 0) represents the atmospheric emissivity, which can be calculated using atmospheric transmittance t (2) as:And t (A) can be modeled from software MODTRAN.(1.4)

[0112] where Isoiar is local solar illumination.

[0113] Furthermore, the parasitic heat transfer with ambient air is non-negligible. The corrected convection component can be expressed as:(1 -5)

[0114] where hcis the parasitic heat transfer coefficient due to heat convection and conduction.

[0115] In principle, the net cooling power obtained from Eq. (1.1) when Ts— Tambcorresponds to the effective cooling power. Also, the steady-state temperature is the solution to Eq. (1.1) when Pcooi— 0. All equations were analysed and solved using MATLAB to obtain the cooling power and steady-state temperature.Example 8

[0116] Overestimation of cooling power in experimental test[001 17] Cooling capability of a radiative cooler is usually evaluated by its cooling power under specific climatic conditions. In tropical regions, the narrower atmospheric transparency window and stronger downwelling radiation cause a greater discrepancy between the theoretical and experimental cooling power. During the cooling power test, an electrothermal plate was attached beneath the aluminum plate, with a thermocouple positioned in between. The thermocouple wire inevitably created a partial gap at the interface. In addition, intrinsic thermal contact resistance was present, leading to heater power loss. Furthermore, the thermalinsulator (EPS) employed in the experiment possesses a thermal conductivity of approximately 0.03 W / m-K, which caused direct power loss at the bottom interface. These experimental factors collectively contributed to an overestimation of the measured cooling power.

[0118] The method of the present disclosure enhances the optical performance of the radiative cooling paint by tuning its UV reflectivity response to NIR reflectivity through particle engineering. By rationally selecting and controlling the combination of the particle dimensions and type, the solar reflection of the radiative cooling paint can be maximized while maintaining desired paint features such as color and robustness, and paint properties such as scattering, reflectivity and emissivity, leading to improved cooling capabilities and widened applicability.

[0119] Electrostatic repulsion has been proven to be effective on particle dispersion in high PVC water-based paint system. It not only contributes to homogenous particle distribution, but also to significantly reduce the water / binder ratio to keep smooth and uniform binder crystallization.

[0120] With an ultrahigh pigment volume concentration (PVC) of about 70%, the derived water-based cooling paint exhibits a dense and uniform surface while maintaining robust mechanical, chemical, and thermal stabilities, as well as universal adhesion. In contrast to the cracking and poor adhesion observed in conventional water-dispersed paints, the effective E- assisted dispersion method employed herein minimizes particle aggregation and allows thorough penetration of binder among dispersed particles, thereby forming a homogeneous network across the surface, inner structure, and bottom interface. As a result, essential paint features such as a uniform, non-cracking structure, strong adhesion across diverse substrates, and a robust surface are achieved in a green aqueous system. The optimized high-PVC waterbased cooling paint not only demonstrates excellent resistance to weathering and thennal aging but also exhibits superior thennal stability and outstanding optical perfonnance,including a solar reflectance (Rsoiar) of about 97% and a long-wave infrared emissivity (ELWIR) of greater than 95%. These properties are comparable to those of state-of-the-art radiative coolers, ensuring effectiveness under various climates and promising long-term outdoor cooling applications. Furthermore, from a practical standpoint, strong interfacial adhesion to a wide variety of building facade materials and primer layers ensures broad applicability for both fresh applications and repainting works.

[0121] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding, and are not intended to be limiting or exhaustive. Modifications may be made by one of ordinary skill in the art without departing from the scope of the claimed invention.

Claims

Claims:

1. A method of enhancing optical performance of a radiative cooling paint, the method comprising: tuning ultraviolet (UV) reflectivity response of a radiative cooling paint to nearinfrared (NIR) reflectivity by rationally selecting one or more components from each class of components within a hierarchical pigment volume concentration (APVC) system, based on particle size and refractive index of each component, to form a radiative cooling paint with an enhanced optical performance; and introducing sodium polyacrylate into the radiative cooling paint to provide electrostatic repulsion between particles contained in the radiative cooling paint, wherein the / ?PVC system comprises: an ultraviolet (UV) scattering agent comprising particles having an average particle size of 200 to 400 nm, a visible scattering agent comprising particles having an average particle size of 400 to 600 nm; a near-infrared (NIR) scattering agent comprising particles having an average particle size of 800 nm to 1pm; and a binder.

2. The method of claim 1, further comprising: adjusting and controlling the particle size and particle size distribution of the UV scattering agent, the visible scattering agent, the NIR scattering agent, or a combination thereof to tune optical properties of the radiative cooling paint across the solar spectrum, including ultraviolet (UV), visible (VIS), and near-infrared (NIR) regions.

3. The method of claim 2, wherein the optical properties of the radiative cooling paint comprise one or more of scattering, reflectance, and emissivity.

4. The method of claim 1, wherein the UV scattering agent comprises particles selected from the group consisting of barium sulfate, zirconium oxide, calcium carbonate, yttrium oxide, aluminium oxide and zinc oxide.

5. The method of claim 1 , wherein the visible scattering agent comprises particles selected from the group consisting of barium sulfate, titanium oxide, titanium dioxide, zirconium oxide, yttrium oxide and aluminum oxide.

6. The method of claim 1, wherein the NIR scattering agent comprises particles selected from the group consisting of aluminum oxide, zirconium oxide, calcium carbonate and barium sulfate.

7. The method of claim 1, wherein the binder is selected from the group consisting of an acrylic solution, an acrylic emulsion, polyurethane emulsion and polyvinylidene fluoride resin.

8. The method of claim 1, wherein the UV scattering agent, the visible scattering agent and the NIR scattering agent are provided by the same component, the same component being present in different particle size ranges, each particle size range corresponding to a respective class of components within the APVC system.

9. The method of claim 8, wherein the said same component comprises barium sulfate.

10. The method of claim 9, wherein the binder comprises silicone acrylic emulsion.

11. The method of claim 9 or 10, wherein the radiative cooling paint is a water-based radiative cooling paint.

12. The method of claim 1, further comprising: introducing a color pigment comprising an iron-based particle pigment into the APVC system to obtain a colored radiative cooling paint.

13. The method of claim 1, further comprising: introducing the one or more components selected from the APVC system into a paint matrix, the components being configured to tune optical properties of the radiative cooling paint across the solar spectrum.

14. A radiative cooling paint comprising: one or more components rationally selected from each class of components within a hierarchical pigment volume concentration (TzPVC) system to form a radiative cooling paint, wherein the APVC system comprises: an ultraviolet (UV) scattering agent comprising particles having an average particle size of 200 to 400 nm; a visible scattering agent comprising particles having an average particle size of 400 to 600 nm; a near-infrared (NIR) scattering agent comprising particles having an average particle size of 800 nm to 1 pm; and a binder, anda sodium polyacrylate for providing electrostatic repulsion between particles contained in the radiative cooling paint.

15. The radiative cooling paint of claim 14, wherein the UV scattering agent comprises particles selected from the group consisting of barium sulfate, zirconium oxide, calcium carbonate, yttrium oxide, aluminium oxide and zinc oxide.

16. The radiative cooling paint of claim 14, wherein the visible scattering agent comprises particles selected from the group consisting of barium sulfate, titanium oxide, titanium dioxide, zirconium oxide, yttrium oxide and aluminum oxide.

17. The radiative cooling paint of claim 14, wherein the NIR scattering agent comprises particles selected from the group consisting of aluminum oxide, zirconium oxide, calcium carbonate and barium sulfate.

18. The radiative cooling paint of claim 14, wherein the binder is selected from the group consisting of an acrylic solution, an acrylic emulsion, polyurethane emulsion and polyvinylidene fluoride resin.

19. The radiative cooling paint of claim 14, wherein the UV scattering agent, the visible scattering agent and the NIR scattering agent are provided by the same component, the same component being present in different particle size ranges, each particle size range corresponding to a respective class of components within the / ?PVC system.

20. The radiative cooling paint of claim 19, wherein the said same component comprises barium sulfate.

21. The radiative cooling paint of claim 20, wherein the binder comprises silicone acrylic emulsion.

22. The radiative cooling paint of claim 21, wherein the radiative cooling paint is a waterbased radiative cooling paint.

23. The radiative cooling paint of claim 22, wherein the water-based radiative cooling paint having a pigment volume concentration of 70%.

24. The radiative cooling paint of claim 22, wherein the water-based radiative cooling paint exhibits enhanced optical performance including a reflectance of 97% and a long-wave infrared emissivity of 95%.

25. The radiative cooling paint of claim 14 or 21, wherein the sodium polyacrylate is present in an amount ranging from 0.5 to 1.5 wt%.

26. The radiative cooling paint of claim 14, wherein the radiative cooling paint is a colored radiative cooling paint comprising an iron-based particle pigment.

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