A polymer nanocomposite for radiative cooling

Abstract

The present disclosure provides a nanocomposite comprising: a. a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and b. an emissive polymer, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1:1 to 6:1; and the plurality of nanoparticles are dispersed in the emissive polymer. The present disclosure also provides a process of preparing the nanocomposite. The present disclosure further provides a coating composition and an article thereof.

Description

A POLYMER NANOCOMPOSITE FOR RADIATIVE COOLINGFIELD OF INVENTION

[0001] The present disclosure broadly relates to the field of nanocomposites. Particularly, the present disclosure relates to nanocomposites for passive daytime radiative cooling, and a coating composition comprising the nanocomposite.BACKGROUND OF THE INVENTION

[0002] Radiative cooling technologies lower the surface temperature of materials by exceeding the total heat loss with thermal emission over the total heat gain by solar absorption. Therefore, the key requirements of radiative cooling technology are complete reflection of the solar spectrum (0.32-2.5 pm) for minimal sunlight absorption and high thermal emission in the infrared atmospheric window (8-13 pm) for optimum outward thermal radiation.

[0003] Nature has used radiative cooling technologies effectively for millenniums, such as Saharan Silver Ant that survives in hot desert conditions. Persians have also used ice houses utilizing radiative cooling technologies. However, the scientific approach to engineering material properties to achieve radiative cooling has only started over the last three to four decades.

[0004] Photonic multilayer structures were demonstrated to exhibit radiative cooling with a temperature reduction of - 4.9°C below ambient temperature with a cooling power of - 40 Wm'2. Conventional radiative cooling devices consist of complicated multilayer structures backed by a reflective metal which limits their application in everyday household applications. However, single-layer paint polymer nanocomposite coating possessing both high solar reflectivity and high infrared emissivity is the most effective method for building and household cooling applications. The most common way to fabricate polymer nanocomposite is by dispersing solar reflecting metal-oxide nanoparticles (filler) inside a highly emissive polymer matrix (binder). The cooling performances of commercial white paints consisting of TiCh nanoparticles are limited by the solar absorption in ultraviolet regions due to the moderate 3.2 eV band gap of TiCh. To address this problem,wide-bandgap nanoparticles, such as SiCh, BaSCh, and CaCCh are utilized as a filler to minimize solar absorption. However, lower refractive indices of these materials affect the scattering capability. Several attempts have been made to achieve daytime cooling using ultra-white nanocomposites but are not practically applicable either due to the high cost of the raw materials, the frailty of end products or poor radiative cooling performances having lower thermal emissivity.

[0005] Accordingly, there is a dire need to provide a low-cost, scalable ultra- emissive polymer nanocomposite with ultrahigh solar reflectance for practical applications.SUMMARY OF THE INVENTION

[0006] In an aspect of the present disclosure, there is provided a nanocomposite comprising: a) a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and b) an emissive polymer, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0007] In another aspect of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, the process comprising: stirring a predetermined amount of a plurality of nanoparticles in a solution of an emissive polymer, followed by processing to obtain the nanocomposite, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0008] In one another aspect of the present disclosure, there is provided a coating composition comprising the nanocomposite comprising: a) a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and b) an emissive polymer, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0009] The present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended toidentify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following drawings form a part of the present specification and are included to further illustrate aspects of the present disclosure. The disclosure may be better understood by reference to the drawings in combination with the detailed description of the specific embodiments presented herein.

[0011] Figure 1 depicts (a) a schematic representation of daytime radiative cooling; and (b) ideal reflection and emission spectra for radiative cooling.

[0012] Figure 2 depicts a comparison of the refractive index and band gap of different nanoparticles.

[0013] Figure 3 depicts (a) the structure of different polymers (polyvinyl alcohol (PVA), polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF)); (b) FTIR-ATR (Fourier transformed infra-red attenuated total reflectance) absorption spectrum of PVA, PMMA, and PVDF powders; and (c) thermal emission spectrum of free-standing PVA, PMMA, and PVDF films, in accordance with an embodiment of the present disclosure.

[0014] Figure 4 depicts (a) the reflection spectra of MgO (magnesium oxide) and PVDF powders; (b) FTIR-ATR transmission spectrum of MgO nanoparticles; and (c) the thermal emission spectrum of a free-standing PVDF film, in accordance with an embodiment of the present disclosure.

[0015] Figure 5 depicts (a) a schematic representation of the process to prepare the nanocomposite; and (b) an optical image of a MgO-PVDF nanocomposite film on Si wafer (2 cm><2 cm), in accordance with various embodiments of the present disclosure.

[0016] Figure 6 depicts (a) free-standing MgO-PVDF nanocomposite film; (b) an infrared image of the free-standing film; and (c) MgO-PVDF paint coated on a ceramic tile and wood stick, in accordance with an embodiment of the present disclosure.

[0017] Figure 7 depicts (a) X-ray diffraction (XRD) spectrum of the nanocomposite MgO-PVDF nanocomposite; (b) scanning electron microscopy (SEM) images; (c) cross-sectional field emission SEM (FESEM) image; (d) the corresponding elemental mapping of the nanocomposite; (e) reflection spectra of MgO-PVDF coating and a commercial white paint along with AM 1.5 solar spectrum; and (f) thermal emission spectra of MgO-PVDF composite film and commercial paint along with 300K blackbody spectrum and atmospheric transmission profile, in accordance with an embodiment of the present disclosure.

[0018] Figure 8 depicts (a) schematic representation of the device for cooling measurement in the field test of the nanocomposite; (b) optical image of the cooling performance of the nanocomposite in the field test; (c) the average temperature drop obtained by the nanocomposite (MgO-PVDF nanocomposite) compared to the sub- ambient point (substrate) and (d) commercial paint; (e) thermal image of the MgO- PVDF coating and commercial paint; and (f) the calculated cooling power of nanocomposite (MgO-PVDF nanocomposite) for different values of non-radiative heat transfer coefficient, in accordance with an embodiment of the present disclosure.

[0019] Figure 9 depicts (a) the reflection spectra of MgO-PVDF nanocomposite for different weight ratio; (b) the dependence of the thermal emission of the nanocomposite on the weight ratio of components; (c) solar reflectance and thermal emittance values of the nanocomposite for different weight ratio; (d) variation of cooling power with weight ratio of the components in the nanocomposite; (e) comparison of reflection spectra 4:1 MgO-PVDF nanocomposite with 5: 1 MgO- PVDF nanocomposite; and (f) comparison of thermal emission spectra of 4: 1 MgO- PVDF nanocomposite with 5: 1 MgO-PVDF nanocomposite, in accordance with an embodiment of the present disclosure.

[0020] Figure 10 depicts (a) reflection spectra; (b) thermal emission spectra; and (c) rooftop cooling measurements of CBZO-PVA nanocomposite, in accordance with an embodiment of the present disclosure.

[0021] Figure 11 depicts (a) thermal emission spectra; and (b) FESEM image of 0.08: 1 MgO-PVA nanocomposite, in accordance with an embodiment of the present disclosure.

[0022] Figure 12 depicts (a) reflection spectra; (b) thermal emission spectra, and(c) rooftop cooling measurements of MgO-PVA nanocomposite, in accordance with an embodiment of the present disclosure.

[0023] Figure 13 depicts (a) a comparative plot of solar reflectance, thermal emittance, and temperature reduction of nanocomposite (MgO-PVDF nanocomposite) with various polymer nanocomposites; and (b) a comparison of cooling power and radiative cooling figure-of-merit of the nanocomposite (MgO- PVDF nanocomposite), in accordance with an embodiment of the present disclosure.

[0024] Figure 14 depicts (a) nanocomposite MgO-PVDF (MgO-PVDF paint) coated on a ceramic paver and wood stick; (b) a ceramic paver coated with MgO- PVDF paint showing good adhesion and uniformity; (c) photograph of a water droplet on MgO-PVDF coated ceramic paver showing its hydrophobic properties;(d) photograph of a MgO-PVDF nanocomposite coated and an uncoated ceramic paver; thermal images of the MgO-PVDF nanocomposite coated and uncoated pavers at (e) indoor and (f) outdoor, in accordance with an embodiment of the present disclosure.

[0025] Figure 15 depicts theoretically calculated (a) cooling power and (b) attenuation length of MgO-PVDF nanocomposite, in accordance with an embodiment of the present disclosure.

[0026] Figure 16 depicts thermal images of the MgO-PVDF nanocomposite coated and uncoated pavers on (a) day 1 (28 Jan 2023) and (b) after 7 months (24 Aug 2023), in accordance with an embodiment of the present disclosure.DETAILED DESCRIPTION OF THE INVENTION

[0027] Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described. It is to be understood that the present disclosure includes all such variations andmodifications. The disclosure also includes all such steps, features, compositions, and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such steps or features. Definitions

[0028] For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are delineated here. These definitions should be read in the light of the remainder of the disclosure and understood as by a person of skill in the art. The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.

[0029] The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

[0030] The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”.

[0031] Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.

[0032] The term “including” is used to mean “including but not limited to”. “Including” and “including but not limited to” are used interchangeably.

[0033] The term “w / w” means the percentage by weight, relative to the weight of the total composition, unless otherwise specified.

[0034] The term “emissive polymer” refers to a polymer having specific groups providing functional aspects such as higher thermal emittance and solar reflectance. Examples of emissive polymers include, but are not limited to, polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate, or combinations thereof.

[0035] The term “solar reflectance” or “Rsoia ” refers to the property of a substance to reflect and not absorb energy from the sun. The solar reflectance of the nanocomposite in the present disclosure is calculated using the following relation,where, R( ) and 1() are the reflection spectrum of the nanocomposite and AM (air mass coefficient) 1.5 solar spectrum, respectively.

[0036] The term “thermal emissivity” or “sky window emissivity” refers to the effectiveness of the surface in emitting energy as thermal radiation. The sky window emissivity is calculated for the films using the following relation,where, a( ) and B(A) are the emission spectrum of the nanocomposite and the black body spectrum at 300K, respectively. To achieve excellent cooling performance, the thermal emissivity of the nanocomposite in accordance with the present disclosure, should be very high in the atmospheric transmission window (8-13 pm) to maximize the outgoing thermal radiation.

[0037] The term “cooling power” refers to the potential of a substance to dissipate thermal energy through the atmosphere under specific conditions. Various factors such as airflow and humidity greatly affect the cooling capabilities of a substance. To calculate the cooling power, the energy balance of the radiative cooler is analysed as follows. Considering a radiative cooler at temperature Tsand ambient temperature Ta, the net cooling power of the cooler can be written as,where, Prad is the outward thermal radiation from the cooler which depends on the thermal emissivity esas,is the amount of incident atmospheric radiation absorbed by the cooler, B(T, ) is the spectral radiance of a black body at temperature T defined by Planck’s law,where X is the wavelength, fe is the Boltzmann’s constant, c is the speed of light in vacuum. The angle-dependent thermal emissivity of the atmosphere is given bywhere t( ) is the atmospheric transmittance at the zenith angle 0, PSun is the incoming solar power absorbed by the radiative cooler which depends on the solar absorptivity of the cooler asas,where 0Sun is the angle at which the radiative cooler facing the sun and 1( ) is AM1.5 solar spectrum.P non-rad i S the non-radiative heat gain by the cooler from the surrounding environment where h is the non-radiative heat coefficient accounting for both conductive and convective heat transfer. The theoretical cooling power is calculated for different values of non-radiative heat transfer coefficient (h) in the range 0 to 8 WK-’m’2.

[0038] The term “radiative cooling figure-of-merit” in the context of the present disclosure is defined as,where ssky is the sky window emissivity, Rsoiar is the solar reflectance and r is the ratio of the incoming solar power over blackbody emission through atmospherictransmission window and is used to compare different cooling paints irrespective of weather conditions.

[0039] The term “drop casting” refers to a method of forming or casting a thin solid film of a material on a substrate such as silica wafer, cement, ceramic wood, and the like by dropping a mixture of material contained in a suitable solvent on to the substrate followed by subsequent evaporation of the solvent.

[0040] Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, weight ratio in the range of 1 : 1 to 6: 1 should be interpreted to include not only the explicitly recited limits of 1:1 and 6:1 but also to include sub-ranges, such as 2:1 to 6:1, 1:1 to 6:1, and so forth, as well as individual amounts, including fractional values, within the specified ranges, such as 1.5:1 to 6:1, 1:1 to 4.5:1, and 2:1 to 5.5:1.

[0041] The present disclosure is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally equivalent products, compositions, formulations, and methods are clearly within the scope of the disclosure, as described herein.

[0042] As discussed in the background, the radiative cooling performances of currently available commercial paints are limited by solar absorption in ultra-violet regions due to moderate band gaps of the fillers. Wide-band gap nanoparticles such as SiCh, BaSC , and CaCCh are utilized as fillers to minimize solar absorption, but lower refractive indices of these materials affect their scattering capability. Accordingly, the present disclosure provides a low-cost scalable ultra-emissive nanocomposite comprising a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and an emissive polymer, with ultrahigh solar reflectance. The nanocomposite of the present disclosure demonstrated an extremely high radiative cooling performance and is suitable for practical applications.

[0043] It is to be noted that to keep a surface cool under hot sunlight by means of radiative cooling, a nanocomposite material should reflect all the incoming irradiation from the sun and emit infrared radiation inside the atmospheric transmission window (8-13 pm). A schematic of the daytime radiative cooling is shown in Figure 1(a). Figure 1(b) shows the ideal conditions to achieve cooling below ambient i.e. solar reflectance and thermal emittance should be 1 in the solar region (320-2500 nm) and transmission window, respectively.

[0044] Also, in order to reflect the whole solar spectrum, the nanocomposite should possess a very high electron band gap to reduce ultraviolet absorption and a large refractive index to enhance sunlight scattering. A comparative plot of the refractive index and band gap of MgO with different nanoparticles (TiCh, ZnO, CaCCh, BaSC , AI2O3, SiCh) is presented in Figure 2. It is to be noted that a higher band gap value is needed for reducing solar absorption while a higher refractive index is necessary to enhance sunlight reflection. As can be seen, SiCh having a higher band gap exhibits a smaller refractive index value while TiCF possesses a higher refractive index with a lower band gap. MgO with its large band gap value of 7.8 eV, higher than most of the earlier reported cooling materials (BaSO4, ZrO2, ZnO, TiO2, CaCOs), is noted to absorb UV light below 200 nm. Additionally, MgO exhibits a higher refractive index and phonon resonance at 11.7 pm due to Mg-O- Mg bonding vibration that leads to strong absorption in the mid-infrared region. Therefore, MgO nanoparticles can act as fillers for the nanocomposite to reflect the full solar spectrum, in accordance with various embodiments of the present disclosure.

[0045] The choice of the emissive polymer for the nanocomposite in accordance with the present disclosure for daytime radiative cooling is based on its lower solar absorbance and higher infrared emittance. As is known, for infrared absorption and emission, various functional groups such as O-H, C=C, C=O, C-Cl are responsible because these groups have different vibrational modes, such as stretching and bending vibrations, with different vibrational frequencies. In the fingerprint region (6.7 - 16.7 pm), various C-X groups, where X represents halogen elements such as F, Cl, and Br, have strong absorption along with C-H, C-O, and C-N vibrations. Asthe atmospheric transmission window (8-13 pm) overlaps with the fingerprint region of the polymers, daytime radiative cooling requires a polymer with above mentioned vibrational groups. Among various polymers, polyvinyl alcohol (PVA), poly methyl methacrylate (PMMA), and polyvinylidene fluoride (PVDF) having the structures depicted in Figure 3(a), are most preferred due to several advantages. PVA and PMMA are visibly transparent while PVDF is semi-transparent. The solar absorptivity of PVA and PMMA is 5%, whereas the same for PVDF is 3%. Attenuated total reflectance measurements indicate that PVA has C-H, C-0 groups, PMMA has C-H, C=O, C-0 groups, and PVDF has C-H, C-F groups inside the fingerprint region (Figure 3(b)). In the atmospheric window, PVDF has an average emissivity of 97%, and that for PVA and PMMA are 52% and 81% respectively (Figure 3(c)). Owing to lower solar absorptivity or high solar reflectivity, and high thermal emissivity in the atmospheric window (8-13 pm), an emissive polymer is chosen, to enhance the infrared emission properties of the nanocomposite in accordance with various embodiments of the present disclosure.

[0046] MgO and PVDF powders independently exhibited high solar reflectance values of 99.7% and 97.9%, respectively as can be seen from Figure 4(a), which lead to strong solar reflection of the MgO-PVDF nanocomposites in accordance with various embodiments of the present disclosure. FTIR-ATR transmission spectrum of MgO powders presented in Figure 4(b) additionally showed that a dip occurs at 11.7 pm originating from Mg-O-Mg stretching vibration. Also, as observed from the thermal emission spectrum of a free-standing PVDF film (Figure 4(c)), PVDF was found to be suitable for exhibiting very high thermal emission due to various bonding and stretching vibrations (such as C-H, C-F, C-C bond). It was also observed that owing to the high thermal emission of PVDF and phonon resonance of MgO an ultrahigh emissive MgO-PVDF nanocomposite for various applications was developed for passive day time radiating cooling. Accordingly, the present disclosure provides a nanocomposite for passive daytime radiative cooling comprising a plurality of nanoparticles with an emissive polymer.

[0047] In an embodiment of the present disclosure, there is provided a) a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and b) an emissivepolymer, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer. In another embodiment of the present disclosure, the plurality of nanoparticles have a band gap in a range of 6 to 8 eV. In one another embodiment of the present disclosure, the plurality of nanoparticles have band gap in a range of 7 to 8 eV. In one another embodiment of the present disclosure, the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1. In yet another embodiment of the present disclosure, the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 5: 1. In one another embodiment of the present disclosure, the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 4: 1.

[0048] In an embodiment of the present disclosure, there is provided a nanocomposite as disclosed herein, wherein the nanoparticle has a refractive index in a range of 1.5 to 2. In another embodiment of the present disclosure, the nanoparticle has a refractive index in a range of 1.6 to 1.8.

[0049] In an embodiment of the present disclosure, there is provided a nanocomposite as disclosed herein, wherein the plurality of nanoparticles is selected from magnesium oxide (MgO), calcium-barium-zinc-aluminium oxide (CBZO), or combinations thereof. In another embodiment of the present disclosure, the plurality of nanoparticles is magnesium oxide (MgO). In yet another embodiment of the present disclosure, the plurality of nanoparticles is calcium- barium-zinc-aluminium oxide (CBZO).

[0050] In an embodiment of the present disclosure, there is provided a nanocomposite as disclosed herein, wherein the emissive polymer is selected from polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate, or combinations thereof. In another embodiment of the present disclosure, the emissive polymer is polyvinylidene fluoride. In one another embodiment of the present disclosure, the emissive polymer is polyvinyl alcohol. In yet another embodiment of the present disclosure, the emissive polymer is polymethyl methacrylate.

[0051] In an embodiment of the present disclosure, there is provided a nanocomposite as disclosed herein, wherein the emissive polymer is selected from polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate, or combinations thereof; and the plurality of nanoparticles is selected from magnesium oxide (MgO), calcium-barium-zinc oxide (CBZO), titanium dioxide (TiCh), alumina (AI2O3), or combinations thereof.

[0052] In an embodiment of the present disclosure, there is provided a nanocomposite as disclosed herein, wherein the nanocomposite has a particle size in a range of 30 to 80 nm. In another embodiment of the present disclosure, the nanocomposite has a particle size in a range of 40 to 60 nm.

[0053] In an embodiment of the present disclosure, there is provided a nanocomposite as disclosed herein, wherein the nanocomposite exhibits a solar reflectance in a range of 82 to 96.3% and an atmospheric window thermal emissivity in a range of 0.88 to 0.99. In another embodiment of the present disclosure, the nanocomposite exhibits a solar reflectance in a range of 82 to 96.3% and an atmospheric window thermal emissivity in a range of 0.89 to 0.99. In one another embodiment of the present disclosure, the nanocomposite exhibits a solar reflectance of 96.3% and an atmospheric window thermal emissivity of 0.985.

[0054] In an embodiment of the present disclosure, there is provided a nanocomposite as disclosed herein, wherein the nanocomposite has a cooling power in the range of 90 to 150 Wm'2. In another embodiment of the present disclosure, the nanocomposite has a cooling power in the range of 95 to 125 Wm'2. In one another embodiment of the present disclosure, the nanocomposite has a cooling power of 100 Wm'2.

[0055] In an embodiment of the present disclosure, there is provided a nanocomposite comprising: a) a plurality of nanoparticles selected from magnesium oxide (MgO), calcium-barium-zinc oxide (CBZO), or combinations thereof having band gap in a range of 5 to 8 eV; and b) an emissive polymer selected from polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate or combinations thereof, wherein the nanoparticle and the polymer are in a weightratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0056] In an embodiment of the present disclosure, there is provided a nanocomposite comprising: a) a plurality of nanoparticles selected from magnesium oxide (MgO), calcium-barium-zinc oxide (CBZO), or combinations thereof having band gap in a range of 5 to 8 eV; and b) an emissive polymer selected from polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate or combinations thereof, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; the plurality of nanoparticles are dispersed in the emissive polymer; the nanocomposite has a particle size in a range of 30 to 80 nm; and the nanocomposite exhibits a solar reflectance in a range of 82 to 96.3 % and an atmospheric window thermal emissivity in a range of 0.88 to 0.99.

[0057] In an embodiment of the present disclosure, there is provided a nanocomposite comprising: a) a plurality of nanoparticles selected from magnesium oxide (MgO), calcium-barium-zinc oxide (CBZO), or combinations thereof having band gap in a range of 5 to 8 eV; and b) an emissive polymer selected from polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate or combinations thereof, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; the plurality of nanoparticles are dispersed in the emissive polymer; the nanocomposite has a particle size in a range of 30 to 80 nm; and the nanocomposite exhibits a solar reflectance in a range of 82 to 96.3 %; an atmospheric window thermal emissivity in a range of 0.88 to 0.99; and a cooling power in the range of 90 to 150 Wm'2.

[0058] In an embodiment of the present disclosure, there is provided a nanocomposite comprising: a) magnesium oxide (MgO) nanoparticles having band gap in a range of 7.7 eV; and b) polyvinylidene fluoride as an emissive polymer, wherein the nanoparticles and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and MgO nanoparticles are dispersed in polyvinylidene fluoride. In another embodiment of the present disclosure, the nanoparticles and the polymer are in a weight ratio in a range of 1 : 1 to 5: 1. In one another embodiment of the presentdisclosure, the nanoparticles and the polymer are in a weight ratio in a range of 1 : 1 to 4: 1.

[0059] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, the process comprising: stirring a predetermined amount of a plurality of nanoparticles in a solution of an emissive polymer, followed by processing to obtain the nanocomposite, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0060] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein the solution of an emissive polymer is obtained by adding the emissive polymer in a solvent selected from A-methyl-2-pyrrolidone, dimethyl formamide, dimethyl sulfoxide, dimethyl acetamide or combinations thereof.

[0061] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein the solution of an emissive polymer is obtained by adding the emissive polymer in a solvent selected from A-methyl-2-pyrrolidone, dimethyl formamide, dimethyl sulfoxide, dimethyl acetamide or combinations thereof, wherein the emissive polymer is selected from polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate, or combinations thereof; and the plurality of nanoparticles is selected from magnesium oxide (MgO), calcium-barium-zinc oxide (CBZO), titanium dioxide (TiCh), alumina (AI2O3), or combinations thereof.

[0062] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein stirring is carried out in a magnetic stirrer for a time period in a range of 30 to 90 minutes. In another embodiment of the present disclosure, stirring is carried out in a magnetic stirrer for a time period in a range of 40 to 70 minutes. In another embodiment of the present disclosure, stirring is carried out in a magnetic stirrer for a time period of 60 minutes.

[0063] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein stirring is carried out ata temperature in the range of 30 to 50°C. In another embodiment of the present disclosure, stirring is carried out at a temperature in the range of 35 to 45°C. In one another embodiment of the present disclosure, stirring is carried out at a temperature of 40°C.

[0064] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein the processing comprises homogenizing by sonication, stirring or combinations thereof. In another embodiment of the present disclosure, the processing is carried out for a time period in a range of 30 to 90 minutes.

[0065] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein the nanocomposite is further drop-casted on a substrate followed by heating, to obtain a thin film of nanocomposite. In another embodiment of the present disclosure, wherein heating is carried out at a temperature in a range of 30 to 50°C for a time period in a range of 60 to 180 minutes. In one another embodiment of the present disclosure, wherein heating is carried out at a temperature in a range of 35 to 45°C for a time period in a range of 100 to 150 minutes. In one another embodiment of the present disclosure, heating is carried out at a temperature of 40°C for a time period of 120 minutes.

[0066] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein the nanocomposite is further drop-casted on a substrate followed by heating at a temperature in a range of 30 to 50°C for a time period in a range of 60 to 180 minutes, to obtain a film of nanocomposite. In another embodiment of the present disclosure, the nanocomposite is further drop-casted on a substrate followed by heating at a temperature of 40°C for a time of 120 minutes, to obtain a film of nanocomposite.

[0067] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein the film of nanocomposite has a thickness in a range of 50 to 300 pm. In another embodiment of the present disclosure, the film of nanocomposite has a thickness in a range of 100 to 250 pm. In one another embodiment of the present disclosure, the film of nanocomposite has a thickness in a range of 200 to 250 pm. In more embodimentof the present disclosure, In another embodiment of the present disclosure, the film of nanocomposite has a thickness of 214 pm.

[0068] In an embodiment of the present disclosure, there is provided a process of preparing the nanocomposite as disclosed herein, wherein the substrate is selected from silica wafer, cement, ceramic, magnesium oxide, glass, wood, or combinations thereof. In another embodiment of the present disclosure, wherein the substrate is selected from silica wafer, cement, ceramic, wood or magnesium oxide. In one another embodiment of the present disclosure, the substrate is silica wafer. In another embodiment of the present disclosure, the substrate is magnesium oxide which is a single crystalline thin solid magnesium oxide.

[0069] In an embodiment of the present disclosure, there is provided a process to prepare the nanocomposite as disclosed herein, wherein the film of nanocomposite provides a cooling effect to the substrate by reducing temperature in a range of 7 to 14°C with respect to ambient temperature.

[0070] In an embodiment of the present disclosure, there is provided a film of the nanocomposite comprising: a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and an emissive polymer, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6:1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0071] In an embodiment of the present disclosure, there is provided a process to prepare a film of the nanocomposite as disclosed herein, the process comprising: stirring a predetermined amount of a plurality of nanoparticles in a solution of an emissive polymer, followed by processing to obtain the nanocomposite; and dropcasting the nanocomposite on a substrate followed by heating, to obtain a film of nanocomposite, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 :1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0072] In an embodiment of the present disclosure, there is provided a coating composition comprising the nanocomposite comprising a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and an emissive polymer, wherein thenanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0073] In an embodiment of the present disclosure, there is provided a coating composition as disclosed herein, wherein the coating composition further comprises additives, fillers, adhesives, binders, colorants, or combinations thereof.

[0074] In an embodiment of the present disclosure, there is provided a coating composition comprising a) the nanocomposite comprising a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and an emissive polymer; b) additives; c) fillers; d) adhesives; e) binder; and f) colorants, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

[0075] In an embodiment of the present disclosure, there is provided an article comprising the coating composition comprising: a) a nanocomposite comprising a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and b) an emissive polymer. In another embodiment of the present disclosure, the article is a substrate coated with the coating composition as disclosed herein. In one another embodiment of the present disclosure, the article comprises the coating composition and the substrate selected from silica wafer, cement, ceramic, magnesium oxide, glass, wood, or combinations thereof.

[0076] Although the subject matter has been described in considerable detail with reference to certain examples and implementations thereof, other implementations are possible.EXAMPLES

[0077] The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, theexemplary methods, devices, and materials are described herein. It is to be understood that this disclosure is not limited to the particular methods, and experimental conditions described, as such methods and conditions may apply.Materials and methods

[0078] For the purpose of the present disclosure, the following raw materials were used.

[0079] Magnesium oxide (MgO) powder (Sigma Aldrich, 99.99%), polyvinylidene fluoride (PVDF) (Sigma Aldrich), A-methyl-2-pyrrolidone (NMP) (Sigma Aldrich, 99.5%), Silicon (Si) wafer (2 cm><2 cm) (Prolyx Microelectronics Pvt. Ltd.), CaBaZmALO? powder (CBZO, as-prepared), polyvinyl alcohol (PVA, Sigma Aldrich).EXAMPLE 1Preparation of MgO-PVDF nanocomposite

[0080] The nanocomposites in accordance with the present disclosure were prepared by the process as illustrated in Figure 5(a).

[0081] In an example, 1.028 g of poly vinylidene fluoride (PVDF) was dissolved in 20 mL A-methyl-2-pyrrolidone (NMP), stirred for 1 hour at 40°C and a homogeneous solution of the polymer (solution of the emissive polymer, PVDF solution) was obtained. MgO nanoparticles were then added to PVDF solution with different weight ratios (MgO : PVDF, 1 : 1, 2: 1, 3: 1, 4: 1, 5: 1) under stirring for 1 hour. To disperse the MgO nanoparticles into the PVDF polymer matrix homogeneously, the solution was sonicated for another 1 hour and a nanocomposite solution was obtained. The resulting nanocomposite solution was then drop casted on a cleaned silicon (Si) wafer (2 cm><2 cm) and was baked at 40 °C for 2 hours to obtain a uniform thin film of MgO-PVDF nanocomposite. Figure 5(b) shows the optical image of the MgO-PVDF nanocomposite film on a 2 cm><2 cm silicon wafer. The optical image and infrared image of the as-prepared free standing thin film of the MgO-PVDF nanocomposites are depicted in Figures 6(a) and 6(b), respectively. Also the optical image of the MgO-PVDF nanocomposite coated on a ceramic tile and wood stick is shown in Figure 6(c).

[0082] Similarly, CBZO-PVA nanocomposites and thin film of CBZO-PVA nanocomposites in varying weight ratios of CBZO and PVA (5: 1 and 6: 1), were prepared by the process as explained above. Further nanocomposites of varying weight ratios of MgO and PVA were prepared by the same process as described herein.EXAMPLE 2Characterization and testing of nanocomposites

[0083] Structural characterizations of the nanocomposites as prepared in Example 1 were performed using high-resolution X-ray diffraction and field-emission scanning electron microscopy. Reflection measurements were carried out with Cary 5000 Agilent UV-Vis-NIR spectrophotometer from 200 to 2500 nm spectral range, whereas a Bruker Fourier transformed infrared (FTIR) spectrometer was used to characterize the infrared absorptivity of the nanocomposites. An experimental field test was also performed on the flat roof of a three-storey building. a. X-ray diffraction spectroscopy

[0084] X-ray diffraction (XRD) analysis of the MgO-PVDF nanocomposite as prepared in Example 1 was carried out and the resulting spectrum for the MgO- PVDF nanocomposite in 4: 1 weight ratio is shown in Figure 7(a). The peaks at 20 equal to 18.5° and 38.1° correspond to the formation of a (020) and y (211) phases of the PVDF polymer matrix, respectively. The peaks at 36.9°, 42.9°, 62.3°, 74.6°, and 78.6° confirmed the presence of MgO with orientations (111), (200), (220), (311), and (222), respectively. Since the nanocomposite was deposited on a silicon wafer as the substrate, a prominent diffraction peak at 69.2° corresponding to Si (100) was also observed. b. Scanning electron microscopy (SEM) analysis

[0085] Scanning electron microscopy image of the MgO-PVDF nanocomposite as prepared in Example 1 above is represented in Figure 7(b). From Figure 7(b) it could be noted that MgO nanoparticles are were found to be as randomly distributed aggregated particles of different sizes. However, elemental mapping (Figure 7(c)) confirmed that MgO nanoparticles were uniformly distributed inside PVDFemissive polymer matrix. The broader size distribution and intrinsic band gap enabled MgO nanoparticles to effectively scatter all the incoming solar radiation. Additionally, the thickness of the optimized MgO-PVDF nanocomposites with a dielectric particle size of ~ 50 nm, was measured with field emission scanning electron microscopy (FESEM) and is illustrated in Figure 7(d). The thickness of the film was found to be ~ 214 pm. However, the thickness of the coating composition was optimized by using suitable additives along with nanocomposite. c. Solar reflectance of the nanocomposite

[0086] Solar reflectance of the MgO-PVDF nanocomposite as prepared in Example 1 was measured. As can be seen from Figure 7(e), the solar reflectance of the MgO- PVDF nanocomposite was found to be ~ 96.3% which was significantly higher compared to a commercial TiO2 based white paint having solar reflectance of ~ 77%. This was because MgO nanoparticles acted as an excellent scatterers of the sunlight which gave rise to very high solar reflectance value (t) the nanocomposite. Such a high value of solar reflectance also led to ultra-white appearance of the film of the nanocomposite. d. Thermal emission of the MgO-PVDF nanocomposite

[0087] Figure 7(f) shows the thermal emission spectrum of the MgO-PVDF nanocomposite as prepared in Example 1. As evident from the Figure 7(f), the thermal emission of the MgO-PVDF nanocomposite in accordance with the present disclosure was near unity value (1) in the atmospheric window. This high emission of this nanocomposite was attributed to the phonon resonance of MgO nanoparticles and the bonding and stretching vibration of the PVDF polymer matrix. e. Field test for cooling performance of the nanocomposites

[0088] Experimental field test for the cooling performance of the MgO-PVDF (in weight ratio of 4: 1) nanocomposite as prepared in Example 1, was studied on a flat roof using a device shown in Figure 8(a). An optical image of the field set up is illustrated in Figure 8(b). The device was covered with an aluminum foil to reduce the solar absorption. Two thermocouples were attached to measure the temperatures of the cooling material and a sub-ambient point respectively. Practically, convective heat transfer occurs due to airflow. Therefore, no windshield was used to cover thesample for the practical realization of its cooling capability. As can be seen from Figure 8(c), the average temperature drop obtained by the MgO-PVDF nanocomposite (sample as shown in Figure 8(c)) as cooling material was about ~ 7°C compared to that of the sub-ambient point i.e., the substrate(sub-ambient as shown in Figure 8(c)). Further, the maximum temperature drop of about 13°C was obtained. Also, as can be seen from Figure 8(d), the temperature of the MgO-PVDF nanocomposite stayed ~ 3 °C below on average in comparison to a commercial paint and further a maximum temperature drop of ~ 4.5°C was obtained by the MgO- PVDF nanocomposite.

[0089] Thermal image shown in Figure 8(e) additionally indicated the lower temperature of the nanocomposite compared to the commercial paint and the hot roof under direct sunlight. The clouds appeared to be the most challenging atmospheric factor for getting higher cooling as they block the transmission window, however, the MgO-PVDF nanocomposite film provided superior cooling performance and confirmed its great potentiality for cooling performance in practical applications.

[0090] The cooling power of the MgO-PVDF (4: 1) nanocomposite as prepared in Example 1 as a function of temperature difference for daytime conditions with ambient temperature fixed to be at 300 K and incident solar irradiation of 850 Win2were also calculated and are represented in Figure 8(f). It could be observed that under extreme sunlight, MgO-PVDF nanocomposite had the capability to achieve maximum cooling power of - 100 Wm'2(when the surface temperature of the cooler became same as ambient temperature i.e., Ta- Ts=0 as shown in Figure 8f). When the cooling power (h) was zero, the temperature value corresponded to maximum temperature reduction under different non-radiative heat transfer coefficient. The maximum temperature reduction values were found to be 16, 12, and 10°C for h = 4, 6, 8 WK4m'2, respectively. Therefore, it was observed that the cooling power was significantly affected by the non-radiative heat transfer coefficient.EXAMPLE 3Varying weight ratio of the nanoparticles and the polymer in the nanocomposite

[0091] Experiments to study the impact on the cooling performance of the nanocomposites having different weight ratios of the nanoparticles to that of the emissive polymer were also performed. Nanocomposites comprising varied weight ratios of MgO as nanoparticles and PVDF as emissive polymer such as 1 : 1, 2:1, 3: 1, 4: 1, and 5: 1 prepared as explained in Example 1, were tested for their cooling performances.

[0092] It could be observed that the volume fraction of MgO had high impact on the solar reflectance value. As shown in Figure 9(a), when the weight ratio of MgO nanoparticles to PVDF emissive polymer increased from 1 : 1 to 4:1, the solar reflectance value of the MgO-PVDF nanocomposite increased from 82 % to 96.3 % (Figure 9(b)). Additionally, it could be determined, that a nanocomposite based film having a higher weight ratio of MgO-PVDF as 5 : 1 , resulted in solar reflectance (86.75%) (Figure 9(c)) and thermal emittance (0.96), (Figure 9(d)) when compared to the MgO : PVDF nanocomposite in 4:1 weight ratio which exhibited a solar reflectance 96.3 % and thermal emittance 0.985. Figure 9(e) depicts the solar reflectance and thermal emittance value of the different weight ratios of the nanocomposites.

[0093] Further, the theoretical cooling power of the different nanocomposites having different weight ratios of the nanoparticles and emissive polymer were calculated for a constant value of h = 6.9 WK_1m'2with ambient temperature and solar intensity fixed at 300K and 850 Wm'2, respectively. The results as illustrated by Figure 9(f), suggested that as the weight-ratio of the nanoparticles to the emissive polymer in nanocomposite increased, the cooling power increased and at 4: 1 weight-ratio high solar reflectance and thermal emittance values were observed.

[0094] On the other hand, nanocomposites comprising polyvinyl alcohol (PVA) as a polymer matrix were prepared, owing to its (PVA) high transparency (95%) in the visible region and high emissivity in atmospheric transparent window. As a dielectric nanoparticle, calcium-barium-zinc-aluminium oxide (CBZO) was usedbecause of its high solar reflection (94 %). Two CBZO-PVA nanocomposites having weight ratios of 5: 1 and 6: 1 in a manner as explained in Example 1, were prepared and their cooling performances were determined. The CBZO-PVA nanocomposites showed solar reflectance and thermal emittance of 86 % (87 %) and 0.895 (0.9), respectively for weight ratios of 5:1 (6:1). The results achieved are indicated in Figures 10(a) and (b), respectively. Further, cooling measurement of the 6: 1 CBZO- PVA nanocomposite provided 5°C temperature reduction with respect to a subambient temperature (Figure 10(c)).

[0095] Nanocomposites comprising varied weight ratios of MgO as nanoparticles and PVAas emissive binder as 0.08: 1, 1 : 1, 2: 1, 3:1, 4:1, and 5: 1 in a manner similar as explained in Example 1 above were prepared and tested for their cooling performance, results of which are illustrated by Figures 11 and 12.

[0096] As observed from Figure 11(a), MgO-PVA nanocomposite with weightratio of 0.08: 1 exhibited thermal emittance of 0.84 only.

[0097] Figures 12(a) and (b) depict that for a nanocomposite having MgO and PVA in a weight ratio of 5: 1, the solar reflectance and thermal emittance values were found to be 92 % and 0.98, respectively. Further, as can be observed from cooling measurements depicted in Figure 12(c), the surface coated with MgO : PVA (5: 1) nanocomposite (sample) remained average 6°C below (cooler) compared to a subambient condition.EXAMPLE 4Comparative cooling performance of the nanocomposite and polymer composites

[0098] The comparative plot of solar reflectance, thermal emittance, and temperature reduction of various polymer composites and the MgO-PVDF nanocomposite of the present disclosure is depicted in Figure 13(a). As can be seen from Figure 13(a), MgO-PVDF nanocomposite exhibited the highest thermal emittance along with high solar reflectance which led to better temperature reduction compared to other previously reported radiative cooling nanocomposites.

[0099] The comparative plot of cooling power and radiative cooling figure of merit, of various polymer composites and the MgO-PVDF nanocomposite is presented in Figure 13(b). From Figure 13(b), higher value of cooling power of MgO-PVDF nanocomposites indicated its supremacy over other composites. Also, considering a standard surface temperature of 300K and r as 10 (the ratio of the incoming solar power over blackbody emission through the atmospheric transmission window), the standard figure-of-merit of the MgO-PVDF nanocomposite reached 0.62 which was higher than other reported cooling paints. Thus, with higher cooling power and higher figure-of-merit, the MgO-PVDF nanocomposite proved to be a great potential for use as cooling paint for buildings.EXAMPLE 5Compatibility and hydrophobicity hydrophobicity of MgO-PVDF nanocomposite

[0100] To check the compatibility of the MgO-PVDF (4: 1 weight ratio) nanocomposite coating on different substrates, the nanocomposite coating, also referred to as paint, was coated on a ceramic paver and wood stick. Figure 14a shows that the paint was easily coatable on the pavers and wood stick with high adherence. A fully coated ceramic paver as shown in Figure 14b indicated its highly bright appearance and uniformity. The contact angle of water in air on the coating surface was around -110° which suggested that the nanocomposite is hydrophobic in nature and possessed water-resistant properties for building (Figure 14c) applications. To show the cooling capability of the MgO-PVDF coating on ceramic paver, an infrared camera was used to capture the thermal images of a nanocomposite coated and uncoated paver kept on a table (Figure 14d). At indoor environment, the temperature of the nanocomposite coated surface became ~ 2°C lesser compared to the uncoated surface (Figure 14e). However, the temperature reduction reached ~ 10°C when the two pavers were placed at outdoor environments under the bright sunshine (see Figure 14f), which clearly showed the remarkable cooling capability of the MgO-PVDF nanocomposite. With mechanical stability,water-resistance property, scalability, and lower cost, MgO-PVDF paint was perfectly suitable for building, paves, tiles, and other household applications.Thickness-dependent Cooling Power

[0101] The thickness of the nanocomposite coating film played a crucial role in radiative cooling as the solar reflectivity and infrared emissivity strongly depend on the material quantity. With increasing thickness, both reflectivity and emissivity increased and reached a maximum. However, a higher thickness of the film would introduce additional costs for practical applications. Hence, an optimized thickness was preferable for radiative cooling. For efficient solar reflection, the material should have a thickness greater than the attenuation length of light which is defined as the length at which the light intensity decreases to 1 / e of its value at the surface. To calculate the attenuation length and the thickness-dependent cooling power of the MgO-PVDF nanocomposite, an optical theory based on thickness-dependent Fresnel equation was adopted.

[0102] First, the effective permittivity of the MgO-PVDF nanocomposite film using Maxwell-Garnett effective medium approximation was extracted,where, EMg0and EPVDFare the dielectric permittivity of the MgO nanoparticles and PVDF polymer, respectively. The complex refractive index n = n + ik = ^Eeff ■> where n and k are the real part of refractive index and extinction coefficient, respectively. The emissivity of the nanocomposite was calculated using the following equation,

[0103] The theoretical cooling power was then calculated using the radiative heat transfer model utilizing the thickness-dependent emissivity of the MgO-PVDFnanocomposite and shown in Figure 15a. Results showed that MgO-PVDF nanocomposite with a thickness greater than 50 pm provides high cooling power.

[0104] The attenuation length was calculated as Lattenuation= and plotted inFigure 15b. The result suggested that to effectively scatter all the incident sunlight, the composite thickness should have a thickness greater than 150 pm. Therefore, the thickness of 200 pm for MgO-PVDF nanocomposite satisfied both requirements.Durability and Stability of the MgO-PVDF Nanocomposite

[0105] To determine the durability and stability of the MgO-PVDF nanocomposite, and the impact of ageing on the cooling performance, the amount of temperature reduction under direct sunlight was measured on a nanocomposite coated surface at nearly eight-month intervals. The coated surface was kept inside a laboratory during these eight months. Though exposure to the dust was minimal, the surface was aircleaned before the measurement after eight months. Figure 16 (a and b) depicts the thermal images of the nanocomposite coated and uncoated pavers captured on 28 January 2023 and 24 August 2023, respectively. On both dates, the coated paver exhibited ~ 10°C lowered temperatures than the uncoated paver (or “sub-ambient temperature”). Therefore, these results proved that the cooling performance of the MgO-PVDF nanocomposite did not degrade, and could be used for a long time.Adherence Strength and Smoothness of the MgO-PVDF nanocomposite

[0106] MgO-PVDF nanocomposite was coated on various substrates, e.g., Si, ceramic pavers, wood sticks, etc. The adherence of the coating on these substrates is an important parameter. The adherence strength was determined in two ways. Firstly, the coated surfaces were rubbed with bare hands with reasonable force to see if the coating stuck to the surface or came off. On this front, no delamination or scratch was observed on the surfaces. Secondly, the water resistance of the coating was checked by pouring water into the coated surfaces. Here also, the coating stuck to the surface and didn’t become watery which indicated excellent adherence strength.EXAMPLE 6A coating composition

[0107] A coating composition comprising the nanocomposite of the present disclosure along with suitable additives was prepared.

[0108] Further to superior performance, the nanocomposite of the present disclosure provides an economically viable paint or coating composition. From the commercial aspect, the coating composition of the present disclosure incurred much less cost compared to the commercially existing cooling paints. For an average coating thickness of 50 pm, the price to coat 1 square-feet area with commercial paint was double the price of MgO-PVDF paint. Therefore, the low-cost nanocomposite with remarkable radiative cooling performances was found to be a perfect choice for cooling paints for building.ADVANTAGES OF THE PRESENT INVENTION

[0109] The present disclosure provides a nanocomposite having a plurality of nanoparticles with an emissive polymer is highly stable chemically and physically at high temperatures. The nanocomposite of the present disclosure provides nanocomposite exhibits a solar reflectance in a range of 82 to 96.3 % and an atmospheric window thermal emissivity in a range of 0.88 to 0.99. Further, the nanocomposite has a cooling power in the range of 90 to 150 Wm'2. The nanocomposite and the coating composition of the present disclosure is an economically cheap cooling material compared to existing cooling paints. Moreover the process of preparing the nanocomposite and the coating composition is prepared by simple industrially scalable process. Therefore, the simple solution- processed, low-cost nanocomposite exhibit remarkable radiative cooling performances, and is a perfect choice for cooling paints for surfaces.

Claims

I / We claim:

1. A nanocomposite compri sing : a. a plurality of nanoparticles having band gap in a range of 5 to 8 eV; and b. an emissive polymer, wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

2. The nanocomposite as claimed in claim 1, wherein the nanoparticle has a refractive index in a range of 1.5 to 2.

3. The nanocomposite as claimed in claim 1 , wherein the plurality of nanoparticles is selected from magnesium oxide (MgO), calcium-barium-zinc oxide (CBZO), titanium dioxide (TiCh), alumina (AI2O3), or combinations thereof.

4. The nanocomposite as claimed in claim 1, wherein the emissive polymer is selected from polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate, or combinations thereof; and the plurality of nanoparticles is selected from magnesium oxide (MgO), calcium-barium-zinc oxide (CBZO), titanium dioxide (TiO2), alumina (AI2O3), or combinations thereof.

5. The nanocomposite as claimed in claim 1 or claim 4, wherein the nanocomposite has a particle size in a range of 30 to 80 nm.

6. The nanocomposite as claimed in claim 1 or claim 4, wherein the nanocomposite exhibits a solar reflectance in a range of 82 to 96.3 % and an atmospheric window thermal emissivity in a range of 0.88 to 0.99.

7. The nanocomposite as claimed in claim 1 or claim 4, wherein the nanocomposite has a cooling power in the range of 90 to 150 Wm'2.

8. A process of preparing the nanocomposite as claimed in claim 1, the process comprising: stirring a predetermined amount of a plurality of nanoparticles in a solution of an emissive polymer, followed by processing to obtain the nanocomposite,wherein the nanoparticle and the polymer are in a weight ratio in a range of 1 : 1 to 6: 1; and the plurality of nanoparticles are dispersed in the emissive polymer.

9. The process as claimed in claim 8, wherein the solution of an emissive polymer is obtained by adding the emissive polymer in a solvent selected from N- methyl-2-pyrrolidone, dimethyl formamide, dimethyl sulfoxide, dimethyl acetamide, or combinations thereof, wherein the emissive polymer is selected from polyvinylidene fluoride, polyvinyl alcohol, polymethyl methacrylate, or combinations thereof; and the plurality of nanoparticles is selected from magnesium oxide (MgO), calcium-barium-zinc oxide (CBZO), titanium dioxide (TiCh), alumina (AI2O3), or combinations thereof.

10. The process as claimed in claim 8, wherein stirring is carried out in a magnetic stirrer for a time period in a range of 30 to 90 minutes.

11. The process as claimed in claim 8, wherein stirring is carried out at a temperature in the range of 30 to 50°C.

12. The process as claimed in claim 8, wherein the processing comprises homogenizing by sonication, stirring, or combinations thereof.

13. The process as claimed in claim 8, wherein the processing is carried out for a time period in a range of 30 to 90 minutes.

14. The process as claimed in claim 8, wherein the nanocomposite is further drop- casted on a substrate followed by heating, to obtain a film of nanocomposite.

15. The process as claimed in claim 14, wherein heating is carried out at a temperature in a range of 30 to 50°C for a time period in a range of 60 to 180 minutes.

16. The process as claimed in claim 14, wherein the film of nanocomposite has a thickness in a range of 50 to 300 pm.

17. The process as claimed in claim 14, wherein the substrate is selected from silica wafer, cement, ceramic, magnesium oxide, glass, wood, or combinations thereof.

18. The process as claimed in claim 14, wherein the thin film of nanocomposite provides a cooling effect to the substrate by reducing temperature in a range of 7 to 14°C with respect to ambient temperature.

19. A coating composition comprising the nanocomposite as claimed in claims 1 to 7.

20. The coating composition as claimed in claim 19, wherein the composition further comprises additives, fillers, adhesives, binders, colorants, or combinations thereof.

21. An article comprising the coating composition as claimed in claim 19.

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