Geopolymer-based sub-environmental daytime radiative cooling coating

Abstract

The present invention provides a sub-environmental daytime radiative cooling (SDRC) coating, a coating formulation comprising the same, and a method of preparation and use thereof, the SDRC coating comprising: alkali-activated metakaolin, BaSO4, and silica nanospheres, wherein the alkali-activated metakaolin is prepared by reacting metakaolin with an alkali activator, the alkali activator comprising water glass and a strong base, the strong base being selected from the group consisting of LiOH, NaOH, KOH, Ca(OH)2, Li2O, Na2O, K2O, CaO, and mixtures thereof.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 369,255, filed July 25, 2022, which is incorporated herein by reference in its entirety. Technical Field

[0003] This disclosure relates to a sub-environmental daytime radiation cooling coating prepared from readily available metakaolin that exhibits good solar reflectivity and infrared emissivity, a coating formulation containing the coating, and methods for its use and preparation. Background Technology

[0004] In recent years, Sub-Ambient Daytime Radiative Cooling (SDRC) technology has garnered worldwide attention for its potential to cool object surfaces to below ambient temperature with zero energy consumption under direct sunlight. SDRC can be achieved by designing a surface with high solar reflectivity and high emissivity within a transparent window in the sky (infrared atmospheric window). Specifically, Earth exists within a dynamic radiative heat exchange system of Earth, space, and the Sun. Assuming both the Sun and Earth are blackbodies, Earth's temperature in a steady state is approximately 279 K. In reality, the average temperature of Earth's surface is much higher than 279 K because Earth's radiation is blocked by the atmosphere in most wavelengths due to its low transmittance. The increase in greenhouse gases in the atmosphere further blocks outward radiation, leading to global warming and extreme weather events. Fortunately, the main transparent band of the atmosphere, the so-called atmospheric window, ranges from 8 μm to 13 μm, coinciding with the blackbody radiation peak of objects around 300 K. Therefore, objects on Earth can radiate heat into outer space through this "window," thus achieving passive cooling.

[0005] A breakthrough has been reported in SDRC technology, which achieves sub-environmental cooling under direct sunlight using a planar photonic emitter. The emitter consists of seven stacked layers of HfO2 and SiO2 of varying thicknesses on a 200nm-thick Ag mirror and a 750μm-thick Si substrate. It achieves high solar reflectivity of 97% and selective emission over atmospheric windows. However, the aforementioned photonic structure-based technology, and other metamaterials developed in subsequent research, may be hampered by complex micro / nanofabrication and the use of metallic back reflectors, leading to high costs that could hinder large-scale applications.

[0006] Subsequently, organic polymer coatings were studied to promote the wider application of DRC technology due to their ease of manufacture, low cost, good spectral tunability, and versatility (e.g., through the use of various functional fillers). Of particular note is that many polymers possess functional group characteristics that can promote sufficient infrared radiation in the atmospheric window and high transparency to the solar spectrum. Several strategies have been employed to enhance the cooling performance of polymer DRC coatings: (1) selecting organic polymer matrices with molecular bonds including CO, CF, C-Cl, etc., to achieve high emissivity in the infrared atmospheric window; (2) further enhancing infrared radiation through phonon resonance of SiO2 particles; (3) improving solar reflectivity by introducing functional fillers (TiO2, Al2O3, CaCO3, BaSO4, ZnO, etc.) or hierarchical air pores; and (4) applying modified organic polymer coatings to high-reflectivity metallic substrates.

[0007] The whitest paint developed in 2021 uses an acrylic-based coating as a matrix, then modifies it with high-capacity BaSO4 nanoparticles, achieving a solar reflectance of 98.1% and a skylight emissivity of 0.95. It is claimed that applying this DRC paint to a roof area of ​​approximately 1000 square feet can generate 10 kilowatts of cooling power, making it an attractive alternative to air-conditioned buildings. Criticisms of organic polymer coatings include their reliance on non-renewable fossil resources and the potential for volatile organic compound (VOC) emissions during manufacturing, which can negatively impact human health and the environment. Furthermore, the aging of organic materials remains a significant concern under various environmental and climatic conditions, such as UV radiation, high temperatures, humidity, and fire.

[0008] Geopolymers are a new type of clinker-free (i.e., significantly reducing CO2 emissions during production) cementing material with a three-dimensional inorganic aluminosilicate network composed of silicon-oxygen tetrahedra [SiO4]. 4- And aluminum-oxygen tetrahedron [AlO4] - It is constructed through silicon-aluminum-oxygen bond bridging. The high bond energy of the Si-O and Al-O bonds in the geopolymer structure makes it difficult to react with most acids at room temperature. Furthermore, the dense nanoscale pores within the network exhibit superior durability, chemical resistance, and impermeability to other materials. Most importantly, the metakaolin-based geopolymer itself has a relatively high solar reflectance (over 70%), making it a good candidate for the preparation of DRCCs, such as... Figure 1 As shown.

[0009] Therefore, there is a need to develop an improved SDRC coating that overcomes at least some of the aforementioned drawbacks. Summary of the Invention

[0010] One object of this disclosure is to develop environmentally friendly inorganic SDRC coatings using geopolymers. The optical properties (including solar reflectance and infrared emissivity) of the geopolymer SDRC coatings were characterized. Furthermore, the durability, abrasion resistance, and water resistance of the developed coatings were tested to meet relevant industry standards for coatings.

[0011] In a first aspect, this article provides a sub-environmental daytime radiative cooling (SDRC) coating comprising: alkali-activated metakaolin, BaSO4, and silica nanospheres, wherein the alkali-activated metakaolin is prepared by reacting metakaolin with an alkali activator, the alkali activator comprising water glass and a strong alkali, the strong alkali being selected from the group consisting of LiOH, NaOH, KOH, Ca(OH)2, Li2O, Na2O, K2O, CaO, and mixtures thereof.

[0012] In some implementations, SDRC has a solar reflectance of 0.9667 to 0.9758 in the range of 100 nm to 2500 nm.

[0013] In some implementations, SDRC has an infrared emissivity of 0.90 to 0.9491 in the 8 μm to 13 μm range.

[0014] In some embodiments, BaSO4 is present in the coating at 50 wt% to 70 wt%.

[0015] In some embodiments, BaSO4 is present in the coating at 60 wt% to 63 wt%.

[0016] In some embodiments, the silica nanospheres have an average diameter of 10 nm to 100 nm.

[0017] In some embodiments, the silica nanospheres have an average diameter of 20 nm to 50 nm.

[0018] In some embodiments, silica nanospheres are present in the coating at a concentration of 0.5 wt% to 2.0 wt%.

[0019] In some embodiments, silica nanospheres are present in the coating at 1.5 wt% to 2.0 wt%.

[0020] In some embodiments, BaSO4 is present in the coating at 60 wt% to 63 wt%, and silica nanospheres are present in the coating at 1.5 wt% to 2.0 wt%.

[0021] In some embodiments, the alkali activator includes NaOH and water glass in water, wherein the water glass comprises sodium silicate.

[0022] In some implementations, the water glass has a modulus of 1 to 5.

[0023] In some embodiments, metakaolin and alkali activator are combined in a mass ratio of metakaolin to alkali activator of 1:1 to 1.2:1.

[0024] In some embodiments, the SDRC coating further comprises one or more additives selected from the group consisting of: polytetrafluoroethylene, Ca(OH)2, MgF2, BaTiO3, light-weight limestone, and ground limestone.

[0025] In some embodiments, metakaolin is activated by reacting with an alkaline activator, wherein the alkaline activator comprises water glass and NaOH, the water glass comprising sodium silicate having a modulus of 3 to 3.5, and the metakaolin and the alkaline activator are combined in a mass ratio of metakaolin to the solid component of the alkaline activator of 1:1 to 1.1:1.

[0026] BaSO4 is present in the coating at a concentration of 60 wt% to 63 wt%;

[0027] Silica nanospheres have an average diameter of 20 nm to 50 nm;

[0028] Silica nanospheres are present in the coating at 1.5 wt% to 2.0 wt%; and

[0029] SDRC has an infrared emissivity of 0.93 to 0.9491 in the 8 μm to 13 μm range.

[0030] In a second aspect, this document provides a sub-environmental daytime radiative cooling (SDRC) coating formulation comprising the SDRC coating of claim 1 and water.

[0031] In a third aspect, this article provides a method for applying an SDRC coating formulation to a substrate surface, the method comprising: applying the SDRC coating formulation to the substrate surface to form an SDRC coating on the substrate surface; and removing at least a portion of the water from the SDRC coating. Attached Figure Description

[0032] The above and other objects and features of this disclosure will become apparent from the following description of this disclosure when taken in conjunction with the accompanying drawings.

[0033] Figure 1 A schematic cooling mechanism for geopolymer cooling coatings is depicted.

[0034] Figure 2The optical properties of SDRC coating samples with different functional fillers were depicted. (a) Solar reflectance of AAGP matrix and AAGP coatings with different fillers. (b, c) Solar reflectance of AAGP coatings further modified with different addition ratios of BaSO4 (b) and SiO2 (c). (d) Infrared emissivity of optimized AAGP cooling coating.

[0035] Figure 3 XRD patterns were depicted for the raw material (a) and AAGP coatings (b) with different fillers. FTIR patterns were also depicted for the raw material (c), AAGP matrix, and AAGP coatings (d) with different fillers.

[0036] Figure 4 SEM micrographs of metakaolin (a), SiO2 (b), BaSO4 (c), PTFE (d), and AAGP coatings at 2000x magnification (e) and 10000x magnification (f) are depicted.

[0037] Figure 5 SEM images (a), false-color SEM images (b), and elemental distribution maps (c) of the AAGP coating cross-section are depicted.

[0038] Figure 6 The cooling performance of AAGP coated panels and uncoated panels was compared with that of ambient temperature.

[0039] Figure 7 This is a schematic diagram and photograph of the AAGP coating testing device.

[0040] Figure 8 The images show SEM images of the AAGP coating before and after the mechanical linear wear test: thickness reduction (a), solar reflectance (b), and surface morphology of the coating before (c) and after (d).

[0041] Figure 9 The results are shown in (a) of the pull-out adhesion test, (b) of the pad used in the pull-out test, and (c) of the water penetration test.

[0042] Figure 10 The graphs are TG-DTA thermal analysis diagrams of AAGP matrix (a) and AAGP coatings with different fillers at high temperatures (30°C to 1000°C). Detailed Implementation

[0043] definition

[0044] Throughout the application, where a composition is described as having, including, or containing specific components, or where a method is described as having, including, or containing specific method steps, it is contemplated that the compositions of this teaching may also consist substantially of the listed components, or consist of the listed components, and the methods of this teaching may also consist substantially of the listed method steps, or consist of the listed method steps.

[0045] In this application, when an element or component is referred to as being included in and / or selected from the list of listed elements or components, it should be understood that the element or component can be any one of the listed elements or components, or the element or component can be selected from a group consisting of two or more listed elements or components. Furthermore, it should be understood that the elements and / or features of the compositions or methods described herein can be combined in various ways without departing from the spirit and scope of this teaching, whether express or implied herein.

[0046] It should be understood that the order of steps or the sequence of actions is irrelevant as long as this teaching remains operational. Furthermore, two or more steps or actions can be performed simultaneously.

[0047] Unless otherwise expressly stated, the singular as used herein includes the plural (and vice versa). Additionally, if the term “about” is used before a quantity value, this teaching also includes the specific quantity value itself, unless otherwise specified. As used herein, unless otherwise stated or inferred, the term “about” means a variation of ±10%, ±7%, ±5%, ±3%, ±1%, or ±0% of an index value.

[0048] This article provides SDRC coatings that can be used for sub-environmental radiative cooling. SDRC coatings may include: alkali-activated metakaolin, BaSO4, and silica nanospheres, wherein the alkali-activated metakaolin is prepared by reacting metakaolin with SiO2 and M2O, where M is independently Li, Na, or K in each case.

[0049] Metakaolin (Al2Si2O7) is a dehydration product of kaolinite (Al2(OH)4Si2O5) and can be produced by calcining kaolinite.

[0050] The alkaline activation of metakaolin involves the alkaline hydrolysis and dissolution of metakaolin, as well as the formation of complex mixtures of monomers, dimers, and oligomeric oxides, the nucleation of complex mixtures, and the re-precipitation of aluminosilicates.

[0051] Alkaline activation of metakaolin can be accomplished by reacting metakaolin with an alkaline activator, which includes water glass and a strong base, such as LiOH, NaOH, KOH, Ca(OH)₂, Li₂ONa₂O, K₂O, CaO, or mixtures thereof. In some embodiments, alkaline activation of metakaolin is accomplished by reacting metakaolin with an alkaline activator comprising water glass and M₂O or MOH, wherein M is independently Li, Na, or K in each case, and the water glass comprises one or more metal silicates containing cations selected from the group consisting of Li, Na, K, and Ca. In some embodiments, MOH is NaOH. In some embodiments, metakaolin is activated by reacting with an alkaline activator comprising NaOH in water and water glass comprising sodium silicate.

[0052] As used herein, water glass can refer to an aqueous solution comprising one or more metal silicates selected from the group consisting of lithium silicate, sodium silicate, potassium silicate, calcium silicate, and mixtures thereof. In some embodiments, the water glass comprises sodium silicate. Water glass is commercially available or can be prepared directly by combining one or more metal silicates with water.

[0053] Water glass can have a modulus of 1 to 5, 2 to 5, 2.5 to 5, 3 to 5, 3 to 4.5, 3 to 4, 3 to 3.5, 1.5 to 4.5, 1.5 to 4, 1.5 to 3.5, 2 to 3.5, or 2.5 to 3.5. In some embodiments, the modulus of the water glass is about 3.2.

[0054] The solids content of water glass can be 20% to 50%, 20% to 40%, or 30% to 40%. In some embodiments, the water glass has a solids content of about 34.2%.

[0055] The alkali activator may have a solids content of 10% to 70%, 10% to 60%, 10% to 50%, 10% to 40%, 20% to 60%, 20% to 50%, 20% to 40%, 30% to 40%, 35% to 40%, 20% to 60%, 30% to 50%, 35% to 45%, or 40% to 45%. In some embodiments, the alkali activator has a solids content of about 38.5% or about 40.7%.

[0056] The alkali activator may have a modulus of 1 to 4, 1 to 3.5, 1 to 3, 1 to 2.5, 1 to 2, or 1 to 1.15. In some embodiments, the alkali activator has a modulus of about 1.23.

[0057] The mass ratio of the solid component of the alkali activator to metakaolin can be 10:1 to 1:10, 5:1 to 1:5, 4:1 to 1:4, 3:1 to 1:3, 2:1 to 1:2, 3:2 to 1:2, 3:2 to 2:3, 4:5 to 5:4, or 11:10 to 10:11. In some embodiments, the mass ratio of the solid component of the alkali activator to metakaolin is approximately 77 to approximately 80.

[0058] SDRC coatings may contain BaSO4 at concentrations of 50 wt% to 70 wt%, 60 wt% to 70 wt%, 63 wt% to 70 wt%, or 60 wt% to 63 wt%. In some embodiments, SDRC coatings contain BaSO4 at a concentration of about 60 wt%.

[0059] SDRC coatings may contain silica nanospheres at concentrations of 0.5 wt% to 2.5 wt%, 1.0 wt% to 2.5 wt%, 1.5 wt% to 2.5 wt%, 2.0 wt% to 2.5 wt%, 0.5 wt% to 2.0 wt%, 0.5 wt% to 1.5 wt%, 1.0 wt% to 1.5 wt%, or 1.5 wt% to 2.0 wt%. In some embodiments, the SDRC coating contains silica nanospheres at a concentration of about 1.5 wt%.

[0060] The average diameter of the silica nanospheres can be 10 nm to 300 nm, 10 nm to 250 nm, 10 nm to 200 nm, 10 nm to 150 nm, 10 nm to 100 nm, 10 nm to 90 nm, 10 nm to 80 nm, 10 nm to 70 nm, 10 nm to 60 nm, 10 nm to 50 nm, 20 nm to 50 nm, 20 nm to 40 nm, or 25 nm to 35 nm. In some embodiments, the silica nanospheres have an average diameter of about 30 nm. In some embodiments, the silica nanospheres are monodisperse.

[0061] SDRC coatings may optionally contain one or more additives selected from the group consisting of: polytetrafluoroethylene, Ca(OH)2, MgF2, BaTiO3, light limestone and heavy limestone. One or more additives may be present in SDRC coatings at concentrations of 1 wt% to 10 wt%, 1 wt% to 9 wt%, 1 wt% to 8 wt%, 1 wt% to 7 wt%, 1 wt% to 6 wt%, 1 wt% to 5 wt%, 1 wt% to 4 wt%, 1 wt% to 3 wt%, 1 wt% to 2 wt%, 2 wt% to 10 wt%, 3 wt% to 10 wt%, 4 wt% to 10 wt%, 5 wt% to 10 wt%, 6 wt% to 10 wt%, 5 wt% to 10 wt%, 6 wt% to 10 wt%, 7 wt% to 10 wt%, 8 wt% to 10 wt%, 9 wt% to 10 wt%, 2 wt% to 9 wt%, 3 wt% to 8 wt%, 4 wt% to 6 wt%, 4 wt% to 5 wt%, or 5 wt% to 6 wt%.

[0062] SDRC coatings can be applied to the surface of a substrate by depositing an SDRC coating formulation comprising SDRC coating and water. The SDRC coating formulation can be a suspension, emulsion, or mixture. The SDRC coating formulation may contain up to 40%, 35%, 30%, 25%, 20%, 15%, 10%, or 5% water by weight. In some embodiments, the SDRC coating formulation contains 5% to 40%, 10% to 40%, 15% to 40%, 20% to 40%, 20% to 35%, 25% to 35%, or 25% to 30% water by weight.

[0063] SDRC coatings can exhibit solar reflectance values ​​of 0.9667 to 0.9758, 0.9633 to 0.9758, 0.9694 to 0.9758, 0.9719 to 0.9758, or 0.9722 to 0.9758 in the range of 100 nm to 2500 nm. In some embodiments, SDRC coatings exhibit a solar reflectance of approximately 0.9758.

[0064] The infrared emissivity of SDRC coatings is 0.90 to 0.9491, 0.91 to 0.9491, 0.92 to 0.9491, 0.93 to 0.9491, or 0.95 to 0.9491 in the 8 μm to 13 μm range. In some embodiments, SDRC exhibits an infrared emissivity of approximately 0.9491.

[0065] This disclosure also provides a method for applying an SDRC coating formulation to a substrate surface, the method comprising: applying the SDRC coating formulation to the substrate surface to form an SDRC coating on the substrate surface; and removing at least a portion of the water from the SDRC coating to cause the SDRC coating to harden.

[0066] SDRC coating formulations can be applied to a variety of surfaces, such as those made of paper, wood, concrete, cement, asphalt, metals (e.g., steel, aluminum alloys), glass, gypsum, ceramics, tiles, plastics, plaster, masonry, resins, and roofing substrates (e.g., asphalt coatings, roofing felt, foamed polyurethane insulation); or to previously painted, primed, undercoated, worn, or weathered substrates. In some embodiments, SDRC coating formulations are applied to the exterior walls and / or roofs of commercial, industrial, or residential buildings.

[0067] The SDRC coating may further include a waterproof layer disposed on the surface of the SDRC coating, which may enhance the waterproofness of the applied SDRC coating. In some embodiments, the waterproof layer comprises a silane coupling agent, such as γ-aminopropyltriethoxysilane, γ-epoxypropyltriethoxysilane, γ-glycidoxybutyltrimethoxysilane, glycidoxymethyltriethoxysilane, and mixtures thereof.

[0068] experiment

[0069] Material

[0070] The inorganic geopolymer coating is made from metakaolin (i.e., the precursor material) and an alkali activator solution (AA), which is prepared by mixing water, sodium hydroxide, and water glass (with a modulus of 3.2 and a solids content of 34.2%). Venator BLANC FIXE N-type nano-precipitated barium sulfate (BaSO4, 1 μm) and monodisperse silica nanospheres (nano-SiO2, 99.5%, 30 ± 5 nm) are used to improve reflectivity and infrared emission. Polytetrafluoroethylene (PTFE DISP33, a fluoropolymer resin) is used as a lubricant additive to improve film formation, and analytical grade calcium hydroxide powder (Ca(OH)2) is used to improve early strength under environmental curing. Magnesium fluoride (MgF2, AR, 3μm, Sinopharm Chemical Reagent Co., Ltd.); barium titanate (BaTiO3, AR, 200nm, RHAWN Chemical Co., Ltd.); light limestone (light calcium carbonate, CaCO3, AR, 1μm, Xilong Scientific Co., Ltd.); and heavy limestone (heavy calcium carbonate, CaCO3, AR, 2μm, Tianjin Kemiou Chemical Reagent Co., Ltd.) were selected for optical property comparison. The developed coatings were hydrophobically treated with a modified silane coupling agent (KH-800, Hangzhou Jessica Chemical Co., Ltd.) to improve their waterproof properties and durability.

[0071] Example 1 - Synthesis of Geopolymers

[0072] An alkali activator (AA) with a molar ratio (i.e., SiO2 / Na2O) of 1.2 and a solid content of 38.5% was prepared. First, 50g of AA was added to a beaker, followed by 20g of MK and approximately 25g of zirconium grinding beads, and the mixture was stirred. During stirring, 20g of water, 3g of SiO2, 70g of BaSO4, and 10g of PTFE emulsion were added sequentially. The mixture was then stirred at 1200 rpm for 20 minutes. Next, 1.5g of Ca(OH)2 was added to the beaker, and the mixture was stirred for 2 minutes. The zirconium grinding beads were then removed by filtration to obtain the final product, an alkali-activated geopolymer (AAGP) slurry. The coating was then sprayed onto a standard fiber cement board substrate (GB / T9271-2008) with dimensions of 300mm × 300mm × 4mm using a spray gun at a pressure of 5MPa for approximately 50 seconds, and the process was repeated over approximately 3 minutes. The thickness of the dried coating was measured to be 500±50μm using a micrometer. Finally, the sample surface was treated with a silane coupling agent KH-800 solution (diluted to 20% in ethanol) for hydrophobic treatment prior to field and durability testing.

[0073] Example 2 - Optical Measurement

[0074] The solar absorptivity, reflectivity, and transmittance of the developed AAGP coating were measured according to ASTM E903-12, using a Perkin Elmer Lambda1050+ UV / VIS / NIR broadband spectrometer equipped with an integrating sphere. The infrared emissivity of the AAGP coating was measured using an FTIR spectrometer (Vertex 70, Bruker).

[0075] Example 3 - Material Characterization

[0076] To analyze the physicochemical properties of the raw materials (i.e., metakaolin, BaSO4, nano-silica, PTFE, and Ca(OH)2) and the resulting AAGP coating (i.e., with different fillers), XRD and FTIR analyses were performed. XRD tests were conducted on a Bruker D8 ADVANCE A25X X-ray diffractometer (Bruker AXS Ltd., Germany) to determine the crystal structure of the materials; FTIR spectra were measured on a Thermo Fisher Nicolet iS10 FTIR spectrophotometer (Thermo Fisher, Germany) to analyze functional groups and corresponding characteristic peaks. To further confirm the microstructure and elemental distribution of the AAGP cooling coating, SEM and EDS were examined using a TESCAN VEGA3 (TESCAN ORSAY HOLDING, Kohoutovice, Brno). It should be noted that optical tests were performed only on all formed AAGP coatings, while the other material characterization tests mentioned above were performed only on AAGP coatings with optimal BaSO4 and SiO2 contents (which were determined to be 60 wt% and 1.5 wt% respectively based on previous optical tests, as discussed later).

[0077] Example 4 - Field Test

[0078] To evaluate cooling performance, an AAGP cooling coating was prepared on a standard fiber-reinforced cement board, followed by hydrophobic treatment with silane coupling agent KH-800 to maintain optimal performance over time. Real-time surface temperatures of both uncoated and AAGP-coated boards were recorded using a multi-channel temperature acquisition unit (MEMORYHiLOGGER LR8431-30, HIOKI, EECo. Ltd.). Thermocouple wire KPS-TT-K-24-SLE-100 was purchased from Tianjin KAIPUSEN Heating & Cooling Equipment Co., Ltd. A miniature weather station (Beijing Top Flag Technology Co., Ltd.) was installed for temperature and humidity data acquisition.

[0079] Example 5 - Durability Test

[0080] A linear abrasion test with a 500g abrasion load was performed on standard sandpaper (320 grit) to compare the solar reflectance and microstructure of the coating surface before and after abrasion. The water permeability of the AAGP coating was tested according to Japan Society of Civil Engineers JSCE-K571-2004. Pull-out tests of the AAGP coating were performed according to ASTM D4541 using specially designed equipment. The adhesion was tested using an AT-M adhesion tester. The stage used was 20 mm in diameter. All tests were performed at room temperature. To determine the thermal stability and composition of AAGP coatings containing different fillers, TGA tests were performed on a Rigaku Thermo Plus EVO2 thermogravimetric analyzer.

[0081] Example 6 - Optical Performance

[0082] To achieve high radiative cooling performance and daytime sub-environmental cooling, the coating needs to meet two conditions, as follows:

[0083] (1) Minimize the absorption of the solar spectrum (high solar reflectance).

[0084] (2) Maximize the emissivity at the atmospheric window (high emissivity in the wavelength range of 8μm to 13μm).

[0085] Similar to previously reported strategies for improving the solar reflectance and infrared emissivity of polymer coatings, nanoscale functional fillers were used to modify the optical properties of AAGP coatings. Nano-sized SiO2 was chosen to enhance infrared emissivity. For the solar spectrum, hierarchical pores were formed in the geopolymer matrix to introduce mild light scattering. Furthermore, other types of powders were added to the AAGP matrix to further promote multiple Mie scattering of sunlight. Semiconductors with large band gaps and high refractive indices were advantageous for effective scattering.

[0086] Figure 2 (a) shows the measured solar reflectance of AAGP geopolymer coatings modified with commonly used white wide-bandgap materials (including BaSO4; MgF2; BaTiO3; light limestone and heavy limestone). To avoid agglomeration of BaTiO3 powder and agglomeration of light limestone, the coatings were pretreated with an ethanol-dissolved KH-800 silane coupling agent (the powders were immersed in the KH-800 solution and then dried at 80°C for one hour). Table 1 summarizes the integrated reflectance of different coatings in different spectral regions (i.e., UV, VIS, and NIR regions) and the resulting total reflectance. It can be seen that, compared with other white materials, BaSO4 gives the AAGP coatings the highest reflectance in the UV range (0.8546) and a significant total solar reflectance of 0.9690. Therefore, despite the differences between the polymer matrix and the inorganic matrix, the dielectric contrast near the interfaces of geopolymer-air, geopolymer-BaSO4, and air-BaSO4 can effectively introduce multiple Mie scattering, thereby improving the overall solar reflectance.

[0087] Table 1. Solar reflectance of AAGP coatings with different additives in different spectral regions.

[0088]

[0089] The effect of the BaSO4 addition ratio in the AAGP matrix on the solar reflectance of the coating was further investigated. Four different BaSO4 addition ratios, ranging from 50 w% to 70 w%, were selected. The results showed that with the increase of the BaSO4 addition ratio, the solar reflectance of the AAGP coating first increased and then decreased (see...). Figure 2 (b) The total reflectance reaches its maximum at 60 wt% (i.e., 0.969). At the optimized BaSO4 addition ratio (i.e., 60 wt%), different amounts of nano-SiO2 particles with high infrared emissivity (0% to 2.5% of the matrix weight) were further added to the matrix to assist scattering. Figure 2 (c) shows that the addition of 1.5 wt% nano SiO2 particles yields the best overall reflectivity performance (i.e., 0.9758).

[0090] Figure 2 (d) The spectral response of the optimized AAGP cooling coating (i.e., containing 60% BaSO4 and 1.5% SiO2) is given by the measured thermal infrared (6 μm to 20 μm) emissivity. This shows that the infrared emissivity is very high (0.9491) both inside and outside the sky window (8 μm to 13 μm). This high emissivity is attributed to the aluminosilicate network (-Si-O-Al(Si)-O-) of the geopolymer.

[0091] Example 7 - Material Characterization

[0092] Example 8 - XRD and FT-IR spectra

[0093] like Figure 3 (a) and Figure 3 As shown in (b), the characteristic peaks of the coating raw materials and the changes in the crystal structure after coating formation were analyzed by XRD. The characteristic peaks of metakaolin included broadly dispersed low-intensity peaks between 20° and 28.4°, indicating a highly reactive semi-crystalline and amorphous mixed structure, which is conducive to the formation of geopolymers. The results showed that the tetrahedral structures of siloxanes and aluminoxanes in metakaolin disappeared, forming three possible composite structural morphologies (-Al-O-Al-, -Si-O-Si-, or -Si-O-Al- chains). Figure 3 (b) is presented in the form of a phase shift.

[0094] The relatively sharp diffraction peaks of BaSO4 indicate good crystallinity, high purity, and a hexagonal crystal structure (JCPDF file number 24-1035). The characteristic peaks of BaSO4 can also be clearly observed after its addition to the AAGP matrix, such as... Figure 3 (b) (blue line) shows the main functional component of the cooling coating. A small amount of PTFE in the AAGP coating provides a small, sharp characteristic peak at 18.5° (JCPDF document number 54-1595). Figure 3 (The pink line in (b)). The addition of Ca(OH)2 hardly changed the crystal structure of the AAGP coating.

[0095] The functional groups of the raw materials and AAGP coatings with different fillers were studied by FTIR analysis, such as... Figure 3 As shown in (c) and 3(d), the main characteristic peak of metakaolinite is located at 1068 cm⁻¹. -1 At this location, it is related to Si-O asymmetric vibrations. However, it migrates to 1060 cm⁻¹ in the geopolymer matrix. -1 This can be attributed to the partial substitution of [SiO4] tetrahedra by [AlO4] tetrahedra, altering the local chemical environment of the Si-O bonds. The metakaolinite spectrum shows a peak at 798 cm⁻¹. -1 and 777cm -1The characteristic band centered on this point is attributed to the tensile and bending vibrations of six-coordinated Al-O (which is a highly reactive component of metakaolinite). After the geological polymerization reaction, these two peaks merge into one at 784 cm⁻¹. -1 The intermediate peaks at 1060 cm⁻¹ are vibrations of Si-O-Al or Si-O-Si units in the geopolymer structure. These strong peaks (9.434 μm and 12.76 μm) in the infrared atmospheric window contribute to the high emissivity of the geopolymer matrix. The characteristic bands of BaSO₄ nanoparticles (with peaks at 1060 cm⁻¹) are also present. -1 Centered on the shoulder at 984cm -1 (The part) belongs to SO4 2- Symmetric stretching vibrations of the functional group. At 637 and 603 cm⁻¹ -1 The peak obtained at that location is attributed to SO4. 2- Out-of-plane bending vibrations. Since the BaSO4 content exceeds half of the total content, most peaks in the AAGP refrigeration coating exhibit its characteristic peaks.

[0096] Example 9 - SEM and Elemental Distribution Results

[0097] Microscopic surface morphology of raw materials and AAGP coatings, such as Figure 4 As shown, the three-dimensional geopolymer matrix forms a scaffold that binds BaSO4 flakes and silica nanospheres together, resulting in a surface morphology that combines micro / nano protrusions and pores. Interestingly, hierarchical pores are observed in the AAGP matrix, which is beneficial for enhancing solar reflectivity.

[0098] The elemental distribution on the coating surface was obtained through EDS analysis, and the different colors vividly showed the different chemical elements uniformly distributed in the analyzed sample. Figure 5 ).

[0099] Example 10 - On-site cooling performance test

[0100] This device (see Field Test Equipment and Details) Figure 7 The device was placed on the roof of a flat building under direct sunlight in Hong Kong on November 13, 2021. The relative humidity remained stable at around 40%. The average wind speed was 3 m / s. –1 . Figure 6 (a) shows the changes in ambient temperature and surface temperature of coated and uncoated cement boards over 24 hours. Figure 6 (b) shows a magnified view at midday (i.e., from 11:00 AM to 1:00 PM). This indicates that the surface temperature of the AAGP coating is consistently lower than the ambient temperature during the day.

[0101] Uncoated board (T) b ), AAGP coating (T) and surrounding environment (T) aThe specific average temperature data are shown in Table 2. The results show that, compared to the ambient temperature, the AAGP coating can achieve an average temperature reduction of 4.09℃ and a maximum temperature reduction of 8.9℃ (i.e., T). a -T), while the average and maximum temperature difference between AAGP coated and uncoated panels (i.e., T) b The -T values ​​were 5.54℃ and 24.30℃, respectively.

[0102] Table 2 shows the average temperature of uncoated and AAGP coated panels.

[0103] Average temperature (°C) (°C) <![CDATA[T b ]]> T <![CDATA[T a ]]> <![CDATA[T a -T]]> <![CDATA[T b -T]]> 11:00 AM to 1:00 PM 32.79 22.07 25.77 3.70 10.72 12:00 AM to 12:00 PM 25.25 19.71 23.80 4.09 5.54

[0104] Furthermore, our AAGP coatings exhibit excellent durability in abrasion, permeability, pull-out, and TGA tests (see Examples 12-14), demonstrating that they are an excellent alternative for long-term, large-scale applications of radiation-cooled coatings.

[0105] Example 11 - Field Testing Equipment

[0106] like Figure 7 The apparatus shown is used to compare the surface temperature difference between uncoated and coated cement boards. A prepared AAGP cooling coating (with an optimized mixing ratio) is sprayed onto the cement board surface and exposed to sunlight after a hydrophobic treatment as described above. A polystyrene foam frame covered with aluminum foil is prepared to support and insulate both the coated and uncoated boards. Thermocouples are mounted beneath the boards (substrate), and temperature data is collected using a data logger. Ambient temperature and humidity are recorded by a weather station.

[0107] Example 12 - Results of Wear Test

[0108] Figure 8 (a) The thickness reduction of AAGP coatings under different linear abrasion distances with an abrasion load of 500g on standard sandpaper (340 mesh) was compared. The results showed that after 50 cm of abrasion, the thickness reduction of the AAGP coating was approximately 16 μm, while for typical commercial polymer coatings, this value is greater than 40 μm. Therefore, the AAGP coating exhibits abrasion resistance that is roughly the same as or even better than that of commercial coatings, indicating superior long-term abrasion resistance. Figure 8 (b) shows the solar reflectance of the AAGP coating before and after wear. The results show that the overall reflectance changed slightly after wear (from 0.9758 to 0.9726, see Table 3). Figure 8 (c) and (d) show the microscopic surface morphology of the AAGP coating before and after wear, indicating that the three-dimensional structure of the combination of protrusions and pores was well maintained after the wear test, that is, the excellent optical properties of the AAGP coating were maintained.

[0109] Example 13 - Results of pull-out adhesion test and water permeability test

[0110] The adhesion properties of AAGP coatings to substrates are very important for their engineering applications. Figure 9 (a) shows the adhesion test results. The average bond strength of the three tests was approximately 2.48 MPa, although the deviation was quite large (mean absolute deviation of 0.68), and the failure mode was substrate failure. Figure 9 (b) indicates good adhesion properties. The waterproofing performance of the AAGP coating was evaluated through a permeability test according to the Japan Society of Civil Engineers JSCE-K571-2004 standard. AAGP coated and uncoated substrates were tested under three funnels, surface-down and filled with water. Figure 9 (c)). The water reduction in each funnel was recorded after 7 days. Test results showed that the AAGP refrigeration coating exhibited a reduction of 2.12 mg / day / cm³. 2 The permeability of this coating is significantly higher than that of typical commercial coatings, which report a permeability of 3.9 mg / day / cm³. 2 The water in the funnel-shaped tube sealed on the uncoated substrate completely disappeared. Therefore, the AAGP coating exhibits excellent water resistance, which is crucial for outdoor applications.

[0111] Example 14 - Results of TGA Test

[0112] TGA tests were performed on AAGP coatings with different additives, and the results are as follows: Figure 10 As shown, the AAGP matrix lost 12.5% ​​of its weight at 200°C and 17.5% at 600°C; it possesses BaSO₄... 4 The AAGP matrix with BaSO4 (60wt%) and SiO2 (1.5wt%) lost 2.5% of its weight at 200°C and 5% of its weight at 400°C; the PTFE-modified AAGP matrix with BaSO4 (60wt%) and SiO2 (1.5wt%) lost 3% of its weight at 200°C and 5% of its weight at 400°C; the PTFE-modified AAGP matrix with BaSO4 (60wt%) and SiO2 (1.5wt%) lost 3% of its weight at 200°C and 5% of its weight at 400°C; 4 The PTFE-modified AAGP matrix (i.e., AAGP cooling coating) with BaSO4 (60wt%) and SiO2 (1.5wt%) and Ca(OH)2 showed a 3% loss at 200°C and a 5% loss at 400°C. It can be concluded that the AAGP matrix with BaSO4 (60wt%) and SiO2 (1.5wt%) exhibits superior high-temperature resistance compared to the matrix without any functional fillers. The addition of relatively small amounts of PTFE emulsion and Ca(OH)2 has little effect on the high-temperature resistance, but they greatly contribute to achieving good processability of the coating.

[0113] Table 3. Solar reflectance of AAGP coating before and after wear test

[0114] AAGP coating UV Vis NIR overall Before wear 0.9067 0.9863 0.9708 0.9758 After wear 0.8735 0.9813 0.9718 0.9726

[0115] This study developed a daytime radiation cooling coating based on an inorganic geopolymer. This environmentally friendly coating can be molded and cured at room temperature. Based on the comprehensive experimental research plan, the following conclusions were drawn:

[0116] (1) The AAGP coating achieved a high sky window emissivity of 0.9491 and a solar reflectivity of 97.6%, which is mainly attributed to the addition of BaSO4 and nano SiO2 particles and the hierarchical pores formed in the AAGP matrix.

[0117] (2) The optimized AAGP coating can achieve a sub-environmental cooling effect of up to 8.9°C under direct sunlight.

[0118] (3) The developed AAGP coating can maintain good performance after mechanical wear and high temperature exposure, and exhibits good waterproof performance.

[0119] (4) Due to the inherent advantages of inorganic coatings over organic coatings, the developed AAGP coating is expected to broaden the applicability of DRC technology for efficient thermal management and energy saving purposes.

Claims

1. A sub-environmental daytime radiation cooling coating comprising: alkali-activated metakaolin, BaSO4, and silica nanospheres, wherein the alkali-activated metakaolin is prepared by reacting metakaolin with an alkali activator, wherein the alkali activator comprises water glass and a strong alkali, wherein the strong alkali is selected from the group consisting of LiOH, NaOH, KOH, Ca(OH)2, Li2O, Na2O, K2O, CaO, and mixtures thereof, wherein the BaSO4 is present in the coating at 50 wt% to 70 wt%, the silica nanospheres are present in the coating at 0.5 wt% to 2.0 wt%, the silica nanospheres have an average diameter of 10 nm to 100 nm, and the water glass has a modulus of 1 to 5.

2. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, The sub-environmental daytime radiation cooling coating has a daylight reflectance of 0.9667 to 0.9758 in the range of 100 nm to 2500 nm.

3. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, The sub-environmental daytime radiation cooling coating has an infrared emissivity of 0.90 to 0.9491 in the 8 μm to 13 μm range.

4. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, The BaSO4 is present in the coating at a concentration of 60 wt% to 63 wt%.

5. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, The average diameter of the silica nanospheres is 10 nm to 50 nm.

6. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, The average diameter of the silica nanospheres is 20 nm to 50 nm.

7. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, The silica nanospheres are present in the coating at a concentration of 1.5 wt% to 2.0 wt%.

8. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, The BaSO4 is present in the coating at 60 wt% to 63 wt%, and the silica nanospheres are present in the coating at 1.5 wt% to 2.0 wt%.

9. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, The alkaline activator includes water glass and NaOH, wherein the water glass includes sodium silicate.

10. The sub-environmental daytime radiation cooling coating according to claim 9, characterized in that, The modulus of the water glass is between 1.5 and 4.

5.

11. The sub-environmental daytime radiation cooling coating according to claim 10, characterized in that, The metakaolin and the alkali activator are combined in a mass ratio of metakaolin to the solid component of the alkali activator of 1:1 to 1.2:

1.

12. The sub-environmental daytime radiation cooling coating according to claim 1, characterized in that, It also contains one or more additives selected from the group consisting of: polytetrafluoroethylene, Ca(OH)2, MgF2, BaTiO3, light limestone and heavy limestone.

13. The sub-environmental daytime radiation cooling coating according to claim 2, characterized in that, The alkaline activator includes water glass and NaOH, the water glass includes sodium silicate, the modulus of the water glass is 3 to 3.5, and the metakaolin and the alkaline activator are combined in a mass ratio of metakaolin to the solid component of the alkaline activator of 1:1 to 1.1:

1. The BaSO4 is present in the coating at a concentration of 60 wt% to 63 wt%. The average diameter of the silica nanospheres is 20 nm to 50 nm; The silica nanospheres are present in the coating at a concentration of 1.5 wt% to 2.0 wt%; and The sub-environmental daytime radiation cooling coating has an infrared emissivity of 0.93 to 0.9491 in the 8 μm to 13 μm range.

14. A sub-environmental daytime radiation cooling coating formulation comprising the sub-environmental daytime radiation cooling coating according to claim 1 and water.

15. A method for applying a sub-environmental daytime radiation cooling coating formulation to the surface of a substrate, the method comprising: The sub-environmental daytime radiation cooling coating formulation according to claim 14 is applied to the surface of the substrate to form a sub-environmental daytime radiation cooling coating on the surface of the substrate. And to remove at least a portion of the water from the sub-environmental daytime radiation cooling coating.

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