Elastomeric coating, methods of making, and methods of using for radiative cooling

Abstract

Elastomeric coatings and related methods. Such an elastomeric coating includes a silicone-based elastomeric composition exhibiting siloxane bonding and containing a high band gap pigment that reflects solar radiation with a solar reflectance of greater than 90% and has a thermal emittance of greater than 90%. The elastomeric coating can be made by forming a low-viscosity silicone-based composition, thoroughly mixing a high band gap pigment that reflects solar radiation with a solar reflectance of greater than 90% and has a solar reflectance of greater than 90% into the silicone-based composition, and curing the mixture using a moisture cure polymerization process. The elastomeric coating is particularly well suited for radiative cooling applications.

Description

ELASTOMERIC COATING, METHODS OF MAKING,AND METHODS OF USING FOR RADIATIVE COOLINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of provisional U.S. Patent Application No. 63 / 612,665 filed December 20, 2023, the contents of which are incorporated herein by reference.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under contract number 2102645 awarded by National Science Foundation. The government has certain rights in the invention.BACKGROUND OF THE INVENTION

[0003] The invention relates generally to elastomeric coatings, methods of making, and methods of using, particularly for radiative cooling applications.

[0004] Radiative cooling is a technology that can be used to combat global warming, reduce the carbon footprint associated with cooling, reduce heat island effects, and reduce high cooling costs. Radiative cooling is a spontaneous cooling approach that operates by reflecting sunlight and discharging heat from the surface of a structure (e.g., pipes, walls, roofs, heat exchanger fins, shrouds, etc.) in the form of thermal radiation into the extremely cold deep space. Radiative cooling can be achieved on many different types and shapes of structures by applying specially adapted coatings to the outer surfaces of the structures, preferably on surfaces that are directed toward the sky. Radiative cooling is achieved by reducing the thermal energy absorbed from solar radiation and emitting high thermal energy into the transparent spectral window in the mid infrared (IR) range (8-13 pm wavelength) of solar and / or thermal radiation (also referred to as the "sky window"). The solar radiation requires a high reflectance in the solar region (solar reflectance),while the thermal emission of thermal radiation needs to absorb thermal energy and emit thermal radiation in the sky window to transfer thermal energy to the deep space (T about 3 K). This may be achieved by incorporating high band gap materials to act as scattering particles and / or highly porous structures, where air voids act as scatterers, within a composite material. Typically, such composite materials are applied to the surface of a structure to be cooled in the form of a relatively thin coating, such as a paint or fdm.

[0005] Silicones are a subset of Si-0 based synthetic polymers that are extremely versatile and typically heat-resistant. Silicones are used in a wide variety of applications, including (but not limited to) electronic encapsulations, adhesives, cosmetics, medical devices, and coatings, among others. Silicones typically have Si bonded to H, C, and O, with a basic structure of poly- organosiloxane. The silicon atoms are bonded with oxygen, where the remaining valences are linked with organic groups, such as (but not limited to) vinyl, amine, methyl, and others. Silicone rubber is an elastomeric polymer that is often used as an overcoat due to the ability to form the coating in a relatively viscous state suitable for molding and then curing the material to maintain its final intended shape.

[0006] Silicone polymerization (curing) can occur via various different methods, including ultraviolet (UV) irradiation, heat, addition, or condensation reactions. Addition polymerization typically uses a two-part system including a base and a hardener, where a chain growth polymerization occurs formed by simple linking of monomers (CHs-SiO units), with no byproduct of the curing mechanism. Thus, new units can be added to the chain polymer molecule one by one through single, double, or triple bonds in the monomer. In contrast, condensation polymerization (step growth) utilizes bi-functional monomers (e.g., siloxane, which needs two bonds to complete the last shells of O and Si ), or tri -functional monomers forming dimers, then trimers, then oligomers, then eventually polymer chains. In condensation reactions, any molecule (monomer, oligomer, or polymer) can react with any molecule present (monomer, oligomer, polymer), where polymers are formed at the very end of the reaction, utilizing long oligomers reacting with each other. However, these curing methods can be difficult to implement and / or cause harm to anunderlying substrate if used to cure a thin silicone-based coating onto the surface of the substrate.

[0007] It would be desirable if elastomeric coatings were available that could be utilized for radiative cooling applications and exhibited at least some of the benefits of silicones.BRIEF SUMMARY OF THE INVENTION

[0008] The intent of this section of the specification is to briefly indicate the nature and substance of the invention, as opposed to an exhaustive statement of all subject matter and aspects of the invention. Therefore, while this section identifies subject matter recited in the claims, additional subject matter and aspects relating to the invention are set forth in other sections of the specification, particularly the detailed description, as well as any drawings.

[0009] The present invention provides, but is not limited to, elastomeric coatings, methods of making elastomeric coatings, and methods of cooling a structure.

[0010] According to a nonlimiting aspect, an elastomeric coating for radiative cooling includes a silicone-based elastomeric composition exhibiting siloxane bonding and containing a high band gap pigment that does not absorb UV light, reflects solar radiation with a solar reflectance of greater than 90%, and has a thermal emittance of greater than 90%.

[0011] According to another nonlimiting aspect, a method of making an elastomeric coating for radiative cooling includes forming a low-viscosity silicone-based composition, incorporating into the silicone-based composition a high band gap pigment that reflects solar radiation with a solar reflectance of greater than 90% and has a solar reflectance of greater than 90%, and curing the silicone-based composition and incorporated pigment using a moisture cure polymerization process.

[0012] According to yet another nonlimiting aspect, a method of making an elastomeric coating for radiative cooling is provided. The method includes adjusting the viscosity of a silicone-based matrix with silicone oil to form a viscous silicone composition. A radiative high band gap pigmentis dispersed into the viscous silicone composition to form a silicone-pigment composite. A mixture of at least one crosslinker and a tin-based catalyst with the silicone-pigment composite is formed. The mixture is polymerized by exposing the mixture to moisture in the surrounding atmosphere to form the elastomeric coating.

[0013] According to still another nonlimiting aspect, a method of cooling a structure includes coating a surface of the structure with any of the elastomeric coatings as described above, and cooling the structure by a process of radiative cooling caused by the elastomeric coating.

[0014] Technical aspects of elastomeric coatings and methods as described above preferably include the ability to be utilized for radiative cooling applications while exhibiting at least some of the benefits of silicones.

[0015] These and other aspects, arrangements, features, and / or technical effects will become apparent upon detailed inspection of the figures and the following description.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIGS. l A and IB show a sample of a radiative cooling silicone-based elastomeric coating according to a nonlimiting embodiment of the invention. FIG. 1A shows the sample in a free standing, unflexed condition. FIG. IB shows the sample in a flexed condition illustrating elastic behavior of the radiative cooling BaSO4-based silicone elastomeric coating.

[0017] FIG. 2 illustrates certain steps in a method of making the radiative cooling silicone-based elastomeric coating according to a nonlimiting embodiment of the invention.

[0018] FIG. 3 is a graph illustrating a comparison of spectral reflectance between a BaSC -based silicone elastomeric coating at 60% volume concentration in accordance with certain nonlimiting embodiments of the invention versus a conventional Henry Tropi-cool 887 silicone elastomeric coating.

[0019] FIG. 4 is a graph illustrating spectral reflectance performance for BaSO4-, CaCCh-, andhBN-based silicone elastomeric coatings at 60% volume concentrations.

[0020] FIG. 5 is a graph illustrating a comparison of reflectance versus coating thickness among the BaSCU-, CaCCh-, and hBN-based silicone elastomeric coatings at 60% volume concentrations.

[0021] FIG. 6 is a graph illustrating the thermal emittance of the BaSCU, CaCCL, and hBN variations of silicone elastomer composites.

[0022] FIG. 7 is a graph illustrating comparisons between reflectance and absorption of the BaSO4-based silicone elastomeric coating versus the Henry Tropi-Cool 887 silicone coating relative to radiation wavelength.DETAILED DESCRIPTION OF THE INVENTION

[0023] The intended purpose of the following detailed description of the invention and the phraseology and terminology employed therein is to describe what is shown in the drawings, which include the depiction of and / or relate to one or more nonlimiting embodiments of the invention, and to describe certain but not all aspects of what is depicted in the drawings, including the embodiment(s) to which the drawings relate. The following detailed description also describes certain investigations relating to the embodiment(s), and identifies certain but not all alternatives of the embodiment(s). As nonlimiting examples, the invention encompasses additional or alternative embodiments in which one or more features or aspects shown and / or described as part of a particular embodiment could be eliminated, and also encompasses additional or alternative embodiments that combine two or more features or aspects shown and / or described as part of different embodiments. Therefore, the appended claims, and not the detailed description, are intended to particularly point out subject matter regarded to be aspects of the invention, including certain but not necessarily all of the aspects and alternatives described in the detailed description.

[0024] As used herein the terms "a" and "an" to introduce a feature are used as open-ended, inclusive terms to refer to at least one, or one or more of the features, and are not limited to only one such feature unless otherwise expressly indicated. Similarly, use of the term "the" in referenceto a feature previously introduced using the term "a" or "an" does not thereafter limit the feature to only a single instance of such feature unless otherwise expressly indicated.

[0025] The present application discloses an elastic radiative cooling silicone-based elastomeric coating that is capable of achieving high reflectance in the solar region (solar reflectance) and high thermal emittance in the mid-IR region (8-13 pm wavelength). The elastomeric coating is made of a silicone matrix-pigment particle composite material. The elastomeric coatings incorporate BaSC (barium sulfate), CaCCh (calcium carbonate), and / or hBN (hexagonal boron nitride) pigment particles at high volume concentrations into silicone-based coatings to achieve high solar reflectances of 95.9%, 96.5%, and 94.5%, respectively. The elastomeric coatings are based on room temperature vulcanized silicone rubbers and fabricated using a condensation cure approach. The result is a one-part system that can in various configurations spontaneously react with moisture in the surrounding air, conform to the surface contours of various structures, and / or provide a highly durable, elastic, and reflective coating. The elastomeric coating is preferably capable of providing a coating with improved durability, providing extreme protection to surfaces, and / or deliver high radiative cooling performance, for example, for industrial and commercial applications.

[0026] In some embodiments, the elastomeric coating is made from a formulation including hydroxyl-terminated silicone oil (PDMS-OH) used as a binder, where hydroxyl groups can condense to form siloxane links (O-Si-O type bonds) and produce water as a byproduct. Reactive alkoxysilanes can be used to enhance performance, and / or facilitate crosslinking. Tetraethoxy silane (TEOS; also known as tetraethyl orthosilicate) can be used as a crosslinking agent, coupled with 3 -aminopropyltrimethoxy silane (APTMS) to enhance crosslinking and promote adhesion. Tin catalyst can be used to initiate the curing reaction, such that the coating can spontaneously cure when exposed to moisture in the atmosphere. In this way, the mixture can be applied to a substrate surface in an uncured relatively viscous condition, for example with an applicator or sprayer, and subsequently be cured in a shape that conforms and adheres to the contours of the substrate. Upon complete curing, the surface is protected by a highly stableelastomeric film coating that conforms to the shape and contours of the structure and provides good water proofing. High band gap materials, such as BaSC , hBN, and CaCCh, can be used as pigment particles to help promote high solar reflectance and impart radiative cooling.

[0027] Turning now to the drawings, FIGS. 1A and IB show an elastomeric coating 10 that, when coated onto the surface of a structure, can cause passive radiative cooling of the structure. The elastomeric coating 10 is a silicone-based elastomeric composition comprising a silicone elastomeric matrix that exhibits siloxane bonding. The elastomeric coating 10 contains a pigment that does not absorb UV light (e g., radiation having a wavelength of 100-400 nm) and reflects solar radiation with a solar reflectance of greater than 90%. The pigment also causes the elastomeric coating 10 to have a thermal emittance of greater than 90%. In some embodiments, the pigment includes at least one of BaSC , CaCCh, and hBN. The example elastomeric coating 10 in FIGS. 1A and IB is a BaSO4-based silicone elastomeric coating. The silicone elastomeric matrix may include a plasticizer to provide a desired elastomeric elasticity and / or a desired adhesive quality when coated to a surface. The elastomeric coating 10 has an at rest shape when not acted upon by an outside force, such as the flat shape shown in FIG. 1A, that is defined by the shape of the coating 10 when the silicone elastomeric matrix is cured. Upon the application of an outside force, however, the elastomeric coating 10 can deform, for example by bending or flexing as shown in FIG. IB. When the outside force is removed, the elastomeric coating 10 elastically returns to its original at rest shape again, for example as shown in FIG. 1A.

[0028] High band gap materials, such as BaSO4 and CaCOs, have been studied in reflective paints. Furthermore, hBN coatings have been shown to have good scattering performance at low thicknesses, attributed to the high refractive index and bang gap of hBN. High band gap materials don’t absorb thermal radiation within the UV band, due to having a band gap larger than the UV cut-off energy. Coupled with a contrast within the refractive indices between the matrix and high band gap materials, the high band gap pigment-silicone elastomer matrix composite material allows for a high scattering performance. The scattering mechanism can reduce solar heating, facilitate a reduction of cooling costs, reduce the urban heat island effect, and provide a highprotection of surfaces.

[0029] As illustrated in FIG. 2, the elastomeric coating 10 is made using a process 20 that includes moisture cure polymerization of a composite material of a silicone matrix and a high band gap pigment particle. At 22, a low-viscosity silicone-based elastomeric composition is formed. For example, one or more silicone oils, such as PDMS silicone oils, may be blended with one or more silicone base matrix materials, such as hydroxyl-terminated polydimethylsiloxane (PDMS-OH), to form the low-viscosity silicone-based composition. Optionally, a plasticizer may also be blended with the silicone-based composition to adjust the composition to have a desired viscosity, for example, to promote mixing and / or provide a desired viscosity for applying to a substrate. Some suitable plasticizers include dimethyl phthalate, dibutyl phthalate, and / or dioctyl phthalate. At 24, a pigment having high spectral reflectance and high thermal emittance is incorporated into the silicone-based elastomeric composition, for example by thoroughly mixing pigment particles into the silicone-based elastomeric composition. In some embodiments, the pigment may be or include one or more of BaSC , CaCCh, and / or hBN. The pigment may be provided at a concentration suitable to provide a desired amount of reflectivity and emittance of the final elastomeric coating 10. In some embodiments, the pigment is provided at a concentration of about 30%-60% by volume; however, other concentrations could be used. At 26, the mixture of the silicone-based composition and pigment is then cured using a moisture cure polymerization process to form the elastomeric coating 10. In some embodiments, the moisture cure polymerization process includes reacting silicone with a hydroxyl group (OH) in the presence of a tin-based catalyst and crosslinking the silicone using tetraethoxysilane (TEOS) and 3 -aminopropyltrimethoxy silane (APTMS).

[0030] The elastomeric coating 10 may be formed on or otherwise applied to a surface of a structure to provide or promote radiative cooling of the structure. In some nonlimiting embodiments, the structure may be coated with the uncured (or at most partially cured) mixture of pigment and silicone-based composition while in a desired or otherwise suitable viscous condition, for example for application by a brush, roller, or sprayer. After being applied to the surface, themixture can then be fully cured to set into the shape of the surface to which it is applied. In other embodiments, the elastomeric coating 10 may be fully formed and cured and the coating 10 may be applied to the surface of the structure after the having been fully cured. Due to its elasticity, the coating 10 may still be able to conform closely to the contours of the surface to which it is applied. When the structure is coated with the elastomeric coating 10, the coating 10 causes the structure to be cooled by a process of radiative cooling from both the solar radiation reflective properties and the thermal radiation emittance properties of the elastomeric coating 10 in the sky window.

[0031] In coating applications, a one-part / moisture cure system (polymerization) is more favorable than other curing methods (e.g., two-part addition curing, UV curing, or heat curing) because one component, which undergoes polymerization through a spontaneous reaction with the presence of moisture, can provide extreme protection and / or functionalize surfaces for commercial and industrial applications. To carry out the moisture cure polymerization reaction, a series of hydrolysis and polycondensation reactions are carried out. Condensation reactions are typically characterized by the byproduct of the polymerization reaction, which are based on oxime, acetone, or alcohol moieties / byproducts. To initiate the reaction, cross linkers are hydrolyzed, where reaction factors such as heat addition, or pH at acidic conditions are favorable to induce hydrolysis. However, basic conditions favor high condensation levels. And, while low pH may induce hydrolysis, it also may react with a metal substrate, etching or corroding metal surfaces in an undesired manner. Therefore, different processes could be used to promote hydrolysis to initiate the reaction. Tin-based catalysts can be used to promote hydrolysis in moisture cure reactions, where tin provides reasonable curing times for a complete condensation reaction. Crosslinkers are then added to a mixture of the silicone oil base and the tin-based catalyst, by which hydrolysis and condensation processes take place. Tetraethoxy silane (TEOS) is a common alkoxysilane crosslinker, that when hydrolyzed, releases byproducts of water and ethanol. In order for the crosslinked polysiloxane coatings to adhere well to different surfaces, a co-crosslinker, such as 3-aminopropyltrimethoxysilane (APTMS), may be used to promote adhesion and help bond the coating to a substrate surface, such as glass, metal (e g., aluminum), polyethylene film, and other substrate materials. Without wishing to be bound by theory, it is believed that APTMSsignificantly improves bonding and helps accelerate the condensation reaction, due to having highly reactive methoxy groups (CHa-OSiRj) which are easier to hydrolyze, unlike TEOS which contains ethoxy groups (CHa-CEE-OSiRs). When hydrolyzed, the methoxy groups release methanol and water, whereas the ethoxy groups release ethanol and water. Thus, upon hydrolysis of both the TEOS and APTMS, the cross-linkers form silanol (Rs-Si-OH) groups, which condense to form siloxane bonds and complete the reaction with emission of water, ethanol, and methanol.

[0032] To promote softness and elongation properties of the elastomeric coating 10, non-reactive silicone oil can be incorporated and blended into the composition. Silicone oils with different molecular weights and viscosities may be used in varying amounts as desired to impart a desired mechanical flexibility and softness, which can disturb the molecular structure of the silicone composite. Additionally, lowering the crosslinking density can result in a softer silicone composite. Silicone oils are linear polysiloxane compounds that orient in a spiral chain, which can easily glide / slip over one another. Silicone oils can be reactive to be used in sealants / adhesives, or non-reactive for use as lubricants. Crosslinking in polymers occurs due to the process of breaking bonds between molecules of a polymer, which then reform into a beneficial structure, and / or add functionality. Reactive Silicones can undergo crosslinking through hydrolysis and condensation reaction for moisture cure silicones. High mechanical performing polysiloxanes are believed to be attributable to the stability of the siloxane chain, which demonstrates a higher bonding force, than polymers with C-C backbone. Thus, polysiloxanes can offer great protection against wear, scratches, and thermal stresses, especially in harsh conditions. The Si-0 bond exhibits a larger atomic radius and larger bonding length, which translates to a high bonding energy. Moreover, the (Si-O) bond has a length of 1.64 ± 0.3 A, which is shorter than the theoretical length calculated from atomic radii additivity (1.83 A), which indicates a robust bond. Additionally, the (Si-O) bond's substantial bond dissociation energy provides the material with superior thermal resistance, surpassing that of (C-C) and (C-O) bonds typically found in other binders.

[0033] Plasticizers such as dimethyl phthalate, dibutyl phthalate, or dioctyl phthalate can be incorporated into the silicone matrix-pigment mixture to promote high material elongation.

[0034] Water proofing of the underlying substrate can be achieved due to the presence of stable non-polar methyl groups along the polymer chain at the solid vapor interface. Silicones have one of the lowest surface energies of materials, which prevents the ponding of water to interact with the surface. Moreover, due to hydrophobic based polymers, the incorporation of hydrophobic fumed silica is possible, and easier than incorporating in other coating systems (e.g., water based systems).

[0035] Investigations leading to the development of the present invention have shown that the elastomeric coating 10 and the method 20 of making the coating 10 resulted in the coating 10 exhibiting desirable radiative cooling of an underlying structure onto which the coating 10 is applied.

[0036] In these investigations, elastomeric coatings 10 were created from three different pigment particles: BaSO4-based elastomer coatings having concentrations of 30%-60% vol. BaSC in the silicone elastomeric matrix; CaCCh-based elastomer coatings having a concentration of 30%-60% vol. CaCCh in the silicone elastomeric matrix; and hBN-based elastomer coatings having a concentration of 30%-60% vol. hBN in the silicone elastomeric matrix. A silicone base matrix was used that had a reactive hydroxyl group (OH) at the chain's end. Silicone oils were incorporated to reduce the viscosity and impart mechanical durability and elasticity. Light scattering pigment particles were then dispersed in the base matrix via ultrasonication or magnetic stirring for 15-30 minutes. To adjust the viscosity, octamethylcyclotetrasiloxane (D4) was used as an exempt solvent, and ethyl alcohol to completely wet the pigment particles (e.g., BaSO4 pigment particles). 3-aminopropyltrimethoxysilane (APTMS) was used to crosslink the matrix and promote adhesion to a surface. Tetraethoxy silane (TEOS) was used as an additional cross-linker to produce O-Si-O bonds. Dibutyltin dilaurate (DBTDL) was used as a catalyst to initiate the reaction of the silicone composite with moisture. Upon reacting with the matrix, the TEOS and APTMS are hydrolyzed, thereby forming silanol (Si-OH) groups, which then condense to form siloxane bridges (O-Si-O bonds), and producing H2O, methanol, and ethanol as byproducts. Upon condensation and evaporation of the byproducts, an elastic, rubbery, water-proof elastomeric coating was obtainedthat exhibited high solar reflectance and high thermal emission.

[0037] In the fabrication process, first, the silicone base matrix of hydroxyl terminated PDMS (PDMS-OH) was mixed with a non-reactive silicone oil, PSF-5cSt, a 100% PDMS silicone oil. The incorporation of the silicone oil introduced higher durability and plasticizes the PDMS-OH. Then, pigment particles (e.g., BaSO4, CaCO3, or hBN) were added, such that 30%-60% volume concentration was achieved following the relation:where Vmatrix is the combined volume of the base and any silicone oils. To further reduce the viscosity, and facilitate pigment dispersion, a small amount of ethyl alcohol, and volatile silicone (D4) octamethylcyclotetrasiloxane were added. Volatile silicones are exempt compounds, and not classified as VOC, which helps in formulating low VOC systems. Once the pigment particles were well dispersed in the silicone matrix, cross linkers were incorporated into the mix. The cross linkers were dispersed and underwent a hydrolysis / condensation reaction. To initiate the reaction, a small amount of DBTDL (condensation catalyst) was added to the mixture and stirred for 30 minutes. DBTDL catalyzes the methoxy and ethoxy groups, forming silanol groups. The silanol groups then condense with the presence of moisture in air, forming siloxane bonds, forming an elastic, soft, and rubber like elastomeric silicone composite material having high solar reflectance and high thermal emittance. This elastomeric silicone composite material can be used to form radiative cooling silicone-based elastomeric coating systems. Similar formulations were implemented for each of BaSC -, hBN-, and CaCCh-based elastomeric coatings. Table 1 below demonstrates preliminary results of these silicone elastomeric coatings suitable for radiative cooling applications.Table 1

[0038] FIG. 3 shows the spectral reflectance of two BaSCh-based silicone elastomeric coatings loaded with a concentration of 60% vol. BaSO4 pigment particles against two Henry silicone coating samples. The result shows a superior performance of the BaSCh silicone elastomeric coating at both high volume concentration with low and high coating thicknesses (about 88 and 675 pm, respectively). Significant absorption of the UV band (e.g., 100-400 nm wavelength) can be seen for the Henry silicone-based coating, which is believed to be caused by the use of titanium oxide (TiCh). This absorption is expected due to the low band gap of the TiCh material. Moreover, at a low thickness of 86 pm, an inefficient reflection of the near infrared (near-IR) region (e.g., 0.7-3 pm wavelength) occurs, where a high amount of solar energy lies. Thus, the reflectance of Henry silicone-based coating was saturated at 86%, while the BaSO4-based silicone elastomeric coating at 60% vol. concentration exceeded its counterpart at both low and high thicknesses.

[0039] FIG. 4 shows that the reflectance results in 95.5%, 94.3%, and 96.5% for the BaSCh-, CaCCh-, and hBN-based elastomeric coating configurations, respectively. The samples of hBN- and CaCOs-based silicone elastomeric coatings at 60% vol. developed in a similar matter achieved high reflectance. Both the 60% CaCCh and hBN silicone-based coatings achieved high reflectance performance, obtaining 94.5% and 96.4% at about 700 pm and about 425 pm thicknesses forCaCOs- and hBN-based coatings, respectively. FIG. 5 illustrates the reflectance data for the 60% vol. BaSCh-, CaCCh-, and hBN-based silicone elastomeric coatings at different thicknesses ranging from about 290 pm to about 760 pm thicknesses.

[0040] FIG. 6 shows thermal emissivity test data for the 60% BaSO4 silicone elastomeric coating, 20% hBN elastomeric coating with added silicone oil, and 50% CaCCh silicone elastomeric coating, as well as atmospheric transmittance relative to wavelength of the radiation. The BaSCh, CaCCh, and hBN silicone compositions were coated on about0.66 mm aluminum plate, and went through FTIR spectroscopy measurements. FIG. 6 shows that high thermal emissions of about 0.942, about 0.939, and about 0.927 (94.2%, 93.9%, and 92.7%) was obtained for the BaSCh, CaCCh, and hBN based silicone elastomeric coatings configurations, respectively, which indicates a promising high performing coating for radiative cooling applications. As seen from the thermal emittance data in FIG. 6, a boost in thermal performance is achieved utilizing silicone’s active phonon modes in the sky window.

[0041] The effectiveness of radiative cooling substances can be more precisely evaluated by utilizing the established Radiative Cooling Figure of Merit (RC), which is formulated as follows:In this expression,skydenotes the total emissivity within the atmospheric window, and r represents the ratio of incident solar power to a blackbody's emission within that same window, typically estimated to be around 7.14. This ratio is derived from a standard solar irradiance of 1000 W / m2against a blackbody emission of 140 W / m2in the atmospheric window. Rsoiar refers to the overall solar reflectance. The rationale for the Radiative Cooling (RC) index is to provide a simple multiplier that, when applied to the blackbody emission in the atmospheric window, yields the net radiative cooling power. According to this model, a 0.01 increase in solar reflectance corresponds to a 0.0714 increase in atmospheric window emissivity, in terms of its effect on cooling efficiency. This metric indicates that cooling to temperatures below ambient is theoretically achievable whenever the RC value surpasses zero. In this regard, the BaSO4, CaCCh, and hBN silicone-basedformulations achieved RC values of about 0.608, about 0.66, and about 0.51, respectively.

[0042] To validate this work, a comparison between a commercially available silicone-based product and the present elastomeric coatings 10 was conducted based on radiative cooling performance. FIG. 7 displays the high absorption of both UV and near-infrared regions for a conventional Henry Tropi-Cool 887 silicone coating. In contrast, the BaSO4 silicone elastomeric coating displayed a greater solar reflectance performance by mitigating UV and near infrared absorption. Curve “a” shows the solar absorption of the Henry Tropi-Cool 887 silicone coating (approximately 1040 pm). Curve “b” shows the solar reflection of the Henry Tropi-Cool 887 silicone coating (approximately 1040 pm). Curve “c” shows the solar absorption of the 60% BaSC silicone elastomeric coating (approximately 650 pm). Curve “d” shows the solar reflection of the 60% BaSO4 silicone elastomeric coating (approximately 650 pm).

[0043] As illustrated herein, the elastomeric coatings 10 can utilize high particle loading to scatter incoming solar radiation and emit thermal energy into deep space using a silicone matrix: pigment particle composite. The silicone matrix: pigment particle composite exhibits high radiative cooling performance that is capable of promoting / increasing radiative cooling relative to conventional elastomeric coatings. The elastomeric coatings 10 can be used for a wide variety of commercial and / or industrial applications. Radiative cooling is achieved by using high band gap pigment materials with difference in refractive indices within the silicone-based matrix, to scatter solar radiation and emit thermal energy to deep space. The presence of functional silane cross linkers such as (but not limited to) tetraethoxy silane (TEOS) and 3 -aminopropyltrimethoxy silane (APTMS), can provide a stable crosslinked structure with excellent adhesion properties to many different substrates. Dibutyltin dilaurate (DBTDL) can be used as a catalyst to initiate the condensation reaction with the atmosphere and obtain an acceptable curing time. The resulting elastomeric coatings 10 can reflect solar radiation and emit thermal energy, with the potential to achieve below ambient temperature cooling. Additionally, the elastomeric coatings 10 exhibited excellent adhesion to different substrates and high flexibility, demonstrating elastomeric characteristics with radiative cooling capabilities.

[0044] As previously noted above, though the foregoing detailed description describes certain aspects of one or more particular embodiments of the invention, alternatives could be adopted by one skilled in the art. For example, the elastomeric coatings 10 and their components could differ in appearance and construction from the embodiments described herein and shown in the drawings, functions of certain components of the elastomeric coatings 10 could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, and various materials could be used in the fabrication of the elastomeric coatings 10 and / or their components. As such, and again as was previously noted, it should be understood that the invention is not necessarily limited to any particular embodiment described herein or illustrated in the drawings.

Claims

CLAIMS:

1. An elastomeric coating comprising a silicone-based elastomeric composition exhibiting siloxane bonding and containing a high band gap pigment that does not absorb UV light, reflects solar radiation with a solar reflectance of greater than 90%, and has a thermal emittance of greater than 90%.

2. The elastomeric coating of claim 1, wherein the pigment comprises at least one of BaSO4, CaCO3, and hBN.

3. The elastomeric coating of claim 1, wherein the silicone-based elastomeric composition further comprises a plasticizer.

4. The elastomeric coating of claim 1, wherein the silicone-based elastomeric composition is made using a process of moisture cure polymerization.

5. The elastomeric coating of claim 4, wherein the process of moisture cure polymerization comprises: reacting silicone with a hydroxyl group (OH) in the presence of a tin-based catalyst; and crosslinking the silicone using tetraethoxysilane (TEOS) and 3 -aminopropyltrimethoxy silane (APTMS).

6. A method of making an elastomeric coating, the method comprising: forming a low-viscosity silicone-based composition; incorporating into the silicone-based composition a high band gap pigment that reflects solar radiation with a solar reflectance of greater than 90% and has a solar reflectance of greater than 90%; and curing the silicone-based composition and incorporated pigment using a moisture cure polymerization process.

7. The method of claim 6, wherein the curing comprises: reacting silicone with a hydroxyl group (OH) in the presence of a tin-based catalyst; and crosslinking the silicone.

8. The method of claim 7, wherein the crosslinking comprises incorporating tetraethoxysilane (TEOS) and 3-aminopropyltrimethoxysilane (APTMS) into the silicone to facilitate siloxane bonding.

9. The method of claim 6, wherein the forming comprises blending a plasticizer with the silicone-based composition to adjust the viscosity to a selected viscosity.

10. The method of claim 9, wherein the plasticizer comprises at least one of dimethyl phthalate, dibutyl phthalate, and dioctyl phthalate.

11. A method of making an elastomeric coating, the method comprising: adjusting the viscosity of a silicone-based matrix with silicone oil to form a viscous silicone composition; dispersing a radiative high band gap pigment into the viscous silicone composition to form a silicone-pigment composite; forming a mixture of at least one crosslinker and a tin-based catalyst with the silicone- pigment composite; and polymerizing the mixture by exposing the mixture to moisture in the surrounding atmosphere to form the elastomeric coating.

12. The method of claim 11, wherein the adjusting of the viscosity comprises mixing a plasticizer with the silicone oil composition.

13. The method of claim 12, wherein the radiative pigment comprises at least one of BaSO4, CaCO3, and hBN.

14. The method of claim 13, wherein the crosslinker comprises at least one of tetraethoxysilane (TEOS) and 3-aminopropyltrimethoxysilane (APTMS).

15. The method of claim 11, wherein the crosslinking comprises mixing the tin-based catalyst with the crosslinked pigmented solution.

16. A method of cooling a structure having a surface, the method comprising; coating the surface with the elastomeric coating of claim 1; and cooling the structure by a process of radiative cooling caused by the elastomeric coating.

17. The method of claim 16, wherein the step of coating comprises: applying the silicone-based elastomeric composition containing the high band gap pigment onto the surface in an uncured condition; and curing the silicone-based elastomeric composition containing the high band gap pigment a moisture cure polymerization process on the surface.

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