EP4626985A1 - Radiative cooling paint mixtures, single-layer paints, and methods of producing and using the same - Google Patents

Abstract

Radiative cooling paint mixtures containing a subambient radiative pigment, a solvent, and a polysiloxane binder that exhibit very low solar absorptance, and methods of producing and using the same. The paint mixtures reflect incoming solar irradiation and emit thermal radiation through a spectrally transparent window in the atmosphere (sky window) that exists from 8-13 μm. The subambient radiative pigment may be hexagonal boron nitride. The ratio of the volumes of the subambient radiative pigment, the polysiloxane binder, and the solvent in the paint mixtures and single-layer paints formed therefrom may be adjusted to promote hydrophobic, reflective, and/or strength characteristics.

Description

RADIATIVE COOLING PAINT MIXTURES, SINGLE-LAYER PAINTS, AND METHODS OF PRODUCING AND USING THE SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of provisional U.S. Patent Application No. 63/385,542 filed November 30, 2022, the contents of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0002] This invention was made with government support under Contract No. 2102645 awarded by the U.S. National Science Foundation. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0003] The present invention generally relates to radiative cooling paints capable of dissipating thermal energy from surfaces to which they are applied. The invention particularly relates to radiative cooling paints that contain a binder capable of improving and/or enhancing various desirable properties of the paints.

[0004] Air conditioning accounts for a large proportion of total energy costs for residential buildings in the United States. One potential way to reduce the energy costs required to cool residential buildings is to switch from active cooling, which requires energy use to cool down spaces through a heat pump or other mechanism, to passive cooling, or the cooling of a space without energy consumption. One of the most promising passive cooling technologies is the emerging technology of sub-ambient daytime radiative cooling, a feat previously thought impossible.

[0005] One manner in which subambient cooling is achieved is by the application of radiative cooling paints. Radiative cooling works through two distinct methods. First, the material used preferably has extreme reflectance, often above 95% of the incoming irradiation. Second, the material preferably has high emissivity of thermal radiation within the “sky window,” a spectrally transparent window in the atmosphere that exists at wavelengths between 8-13 [im and through which any thermal radiation from earth is able to escape through the earth's atmosphere and be emitted directly into deep space, which can be considered an infinite heat sink. Thus, radiative cooling materials are typically formulated to both reflect as much incoming solar radiation as possible while emitting radiation into the sky window. With these two effects, the paints can expel more heat than they receive so as to be able to cool to temperatures below the surrounding ambient temperature, effectively reaching subambient temperatures.

[0006] Early work in radiative cooling featured a multilayer structure, including an upper layer of highly emissive paint and a base layer of highly reflective metal. This dual-layer technology is not ideal for large-scale implementation due to the difficulty and cost of applying metal layers to non-metal roofing materials. Recent developments have gone beyond the dual-layer system. Rather, air pockets are added to the material matrix to enhance reflectance and achieve subambient temperatures. Current research efforts have explored various pigments for use with single-layer subambient cooling, including hexagonal boron nitride (hBN). Within these works, optimization efforts focused on particle size distribution have further refined the positive effects of radiative cooling paints.

[0007] As interest in wide-scale implementation of this technology grows, so too do questions on how recent formulations can be scaled to a commercial grade. Of particular concern is how these single-layer pigment-based paints will perform after long-term exposure to natural elements and contaminants. Many existing paints do not exhibit hydrophobic properties that promote the ability of the paints to do so.

[0008] In view of the above, it can be appreciated that it would be desirable if single-layer pigment-based paints were available that exhibited hydrophobic properties and were capable of retaining high transmission and low absorption after long-term exposure to contaminants and natural elements.

BRIEF SUMMARY OF THE INVENTION

[0009] 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.

[0010] The present invention provides, but is not limited to, subambient radiative cooling paint mixtures and single-layer paints formed therefrom, as well as methods for their production and use.

[0011] According to one nonlimiting aspect of the invention, a radiative cooling paint mixture is provided that contains subambient radiative pigment with a particle size distribution in the radiative cooling paint mixture, a solvent, and a polysiloxane binder that exhibits hydrophobic properties and very low solar absorptance. A single-layer paint formed with the radiative cooling paint mixture contains the subambient radiative pigment and the polysiloxane binder, reflects incoming solar irradiation, and emits thermal radiation through a spectrally transparent window in the atmosphere that exists from 8-13 pm.

[0012] According to another nonlimiting aspect of the invention, a method of producing the aforementioned radiative cooling paint mixture includes determining a final dry volume ratio of the subambient radiative pigment to the polysiloxane binder in the single-layer paint, determining a ratio for the solvent to the subambient radiative pigment in the radiative cooling paint mixture, weighing the polysiloxane binder, the subambient radiative pigment, and the solvent to provide the ratios, forming a solution by mixing the polysiloxane binder and the solvent until separation is no longer observed, adding the subambient radiative pigment to the solution to form the radiative cooling paint mixture, and sonicating the radiative cooling paint mixture until particles of the subambient radiative pigment are fully suspended therein.

[0013] According to another nonlimiting aspect of the invention, a method of using the aforementioned radiative cooling paint mixture includes applying the radiative cooling paint mixture to form the single-layer paint on a structure or surface such that the single-layer paint reflects incoming solar radiation, emits thermal radiation, and reduces the temperature of the surface or structure to subambient levels.

[0014] Another nonlimiting aspect of the invention is a single-layer paint formed with the radiative cooling paint mixture.

[0015] Technical aspects of radiative cooling paint mixtures, single-layer paints, and methods as described above preferably include the ability of the polysiloxane binder to provide advantageous hydrophobic, self-cleaning properties while maintaining or enhancing the subambient radiative properties of the pigment-based paint. In some configurations, the paints can provide a self-cleaning and/or dirt-resistant, high-albedo coating.

[0016] Other aspects and advantages will be appreciated from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 contains a graph summarizing spectral optical properties from 0.25-2.5 microns for a sample of a radiative cooling paint containing hexagonal boron nitride (hBN) pigment and a polysiloxane-based binder. The shaded region represents the AM 1.5 condition, which describes the rate at which the sun produces radiation at each shown wavelength. It is used together with the spectral reflectance in order to calculate the total solar reflectance.

[0018] FIGS. 2A through 2D contain graphs representing spectral data for the poly siloxane- based binder of FIG. 1 and also the spectral data for an acrylic binder: (FIG. 2A) Spectral refractive index for the acrylic binder at thicknesses of 90, 155, 215, and 365 pm. (FIG. 2B) Spectral extinction coefficient for the acrylic binder. (FIG. 2C) Spectral refractive index for the polysiloxane-based binder at thicknesses of 80, 163, and 350 pm. (FIG. 2D) Spectral extinction coefficient for the polysiloxane-based binder.

[0019] FIG. 3 contains a graph plotting volume loss as a result of abrasion testing, as measured every 250 cycles for various hBN-containing paint formulations.

[0020] FIG. 4 contains a graph plotting pigment loadings in intervals of 10% pigment loading of radiative cooling paints containing hBN pigment and the polysiloxane-based binder and the effect of the different loadings on reflectance.

[0021] FIGS. 5A and 5B contain images captured with static (FIG. 5A) and dynamic (FIG. 5B) water droplet contact angle tests.

[0022] FIGS. 6 A and 6B contain SEM images showing the radiative cooling paints containing hBN pigment and the poly siloxane-based binder with (FIG. 6A) 55% pigment volume loading and (FIG. 6B) 75% pigment volume loading.

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 one or more nonlimiting embodiments of the invention, and to describe certain but not all aspects of the embodiment(s). 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 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 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 reference to 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] According to a nonlimiting aspect of the invention, a radiative cooling paint is produced from a paint mixture that contains a subambient radiative pigment and a silicone-based binder, such as a polysiloxane-based binder. An example of such a polysiloxane-based binder is the polysiloxane-based thermal cure coating commercially available from SDC Technologies, Inc., under the name MP-101, which in investigations leading to the present invention was determined to provide hydrophobic properties, a very low absorption value for incoming solar radiation, and yield paints having strengths similar to other hBN (hexagonal boron nitridej-containing paints, but at lower pigment loadings than other hBN-containing paints. In this context, MP-101 may be utilized as a hydrophobic binder to provide radiative cooling paint mixtures that contain the subambient radiative pigment and a solvent, and provide single-layer paints that contain the hydrophobic binder and radiative pigment and exhibit advantageous hydrophobic, self-cleaning properties while maintaining or enhancing the subambient radiative properties of the single-layer paint.

[0026] The subambient radiative pigment may be, but is not limited to, hBN nanoplatelet pigment (also referred to herein simply as hBN). A suitable but nonlimiting particle size distribution for the pigment is about 139 to about 525 nanometers to provide desirable radiative properties. The subambient radiative pigment may have a final dry weight volume in the singlelayer paint of about 25% to about 70%, for example, about 70%, or about 60%, or about 25%, with the balance being the polysiloxane binder. The final dry weight volume of the subambient radiative pigment may be tailored in the single-layer paint to provide a balance of reflective, hydrophobic, and strength properties. [0027] The solvent may be, but is not limited to, dimethylformamide (DMF). The solvent may be present in the paint mixture at a ratio of approximately ten times the amount of the poly siloxane binder, and this amount may be varied depending on pigment loading, wherein a greater amount of the subambient radiative pigment utilizes more solvent, or a final desired viscosity for the radiative cooling paint mixture utilizes relatively more solvent to achieve a lower viscosity.

[0028] In investigations leading to radiative cooling paint mixtures within the scope of the present invention, paint mixtures as described above were fabricated to closely resemble that of commercial processes to facilitate scaling to a manufactured level. Paint mixtures were prepared that contained a subambient radiative pigment, the polysiloxane-based binder, and a solvent. The subambient radiative pigment was hBN, the polysiloxane-based binder was the aforementioned poly siloxane-based thermal cure coating MP-101, and the solvent was DMF. The hBN pigment had a particle distribution size of 332 ± 193 nm. The volume ratios of these ingredients in the paint mixture were varied depending on pigment loading: for instance, if the pigment loading of a sample was 60% by volume, then the pigment content (hBN) of the final dry volume of the resulting paint would be 60% by volume and the polysiloxane binder (MP-101) content would be the remaining 40% by volume. The amount of solvent (DMF) used for this formulation was approximately ten times the amount of MP- 101, but this amount can vary based on pigment loading (higher solvent additions with increasing pigment loadings) and final desired viscosity (more solvent leading to lower viscosity of the paint mixture). Multiple pigment loadings were prepared and studied to optimize the paint’s properties.

[0029] Once the ingredients were weighed to achieve different experimental formulations as outlined above, the polysiloxane-based binder, MP-101, and DMF were mixed together until separation was no longer observed. The hBN pigment was then added to the mixture and the paint mixture was sonicated until the pigment particles were fully suspended in the mixture of the radiative cooling paint mixture. After the radiative cooling paint mixture was completed, it was plated onto glass slides and allowed to dry to form a coating of radiative cooling paint for observation.

[0030] Optical properties were characterized to understand the overall impact of the ingredients of the experimental paint mixtures on the cooling effect of the resulting experimental paints. Characterization was performed with a Lambda 950 UV/VIS spectrometer, with which the transmission, reflection, and absorption were characterized at wavelengths from about 250 to about 2500 nm. The acquired data were then processed to correct for the reflection standard. Following this, the data were weighted according to the AM 1.5 Spectra to better proportion how certain wavelengths would have a higher occurrence within the sun’s incoming radiation. These results evidenced the spectral reflection, transmission, and absorption data for the experimental paints, which could then be compared to formulations known in the past.

[0031] The refractive index (n) and thermal conductivity (K) for the MP-101 binder were calculated by using spectrometry to measure the absorptivity, reflectivity, and transmissivity from 0.25 to 20.00 /m. K was then calculated using the following formulas: where d is the thickness in nm. n was calculated using the following formulas: with

[0032] To compare the experimental radiative cooling paint’s strength to other formulations, cohesion was tested with a Taber Abraser. Specimens of the experimental paint were placed on a circular substrate and abraded with abrasion pads. The weight of the remaining paint on the substrate was measured every 250 cycles, and the pads are sanded down every 500 cycles to ensure abrasion on the paint.

[0033] To assess the self-cleaning capabilities of the experimental radiative cooling paints, water droplet contact angle tests were performed to characterize their hydrophobicity. Hydrophobic materials are often described as self-cleaning, as contaminants (including water) easily flow off of their surfaces. Hydrophobicity was statically and dynamically assessed by determining whether a water droplet contact angle of over 90 degrees was achieved (as measured from the inside of the droplets). Static testing was performed by placing a sample of paint on a glass substrate. In this position, a camera is aligned to view the surface of the sample, where a water drop is placed carefully on top using an automated dispensing system. A photograph was taken and the angle that the water droplet created with the sample was measured with a Rame-hart 290-F1 goniometer. A similar process was employed for the dynamic testing, but with the water droplet being dropped onto a glass substrate from a height to better understand how the surface’s hydrophobicity is changed when wetted.

[0034] The hydrophobicity of the radiative cooling paint was further characterized by SEM images and porosity calculations, to better understand the structure at a microscale level. The SEM images were taken on a FEI Nova NanoSEM at varying magnifications. The porosity dried paint layer was calculated using the following equation: .

O.b * A J.Y + 0.4 * A/P - li'H

Where ^sis the solid fraction, and 0.6 and 0.4 are the respective solid volume fractions of the hBN pigment and MP-101 binder.

[0035] The spectral optical properties for an experimental paint with a 70% pigment loading were measured from 250 to 2500 nm, as shown in FIG. 1. The total reflectance, transmittance, and absorptance under the AM 1.5 points were obtained for a 385-785 micron thick sample the experimental paint. The results demonstrated a total average reflectance of 97.52%, transmittance of 0.30%, and an absorptance of 2.14%. This was deemed to be comparable to a 97.9% reflectance that has been reported for an existing hBN-acrylic paint. The ability of the experimental paint to maintain a similar reflectance value was likely due to its similar absorption in UV wavelengths compared to conventional acrylic binders used in prior single-layer radiative cooling paints.

[0036] To further characterize the optical properties of the poly siloxane binder, the refractive index (n) and extinction coefficient (K) of MP-101 were measured and compared to known values for acrylic binders, as shown in FIGS. 2A through 2D. The results showed that the refractive index and extinction coefficient for MP-101 have very similar averages to those of acrylics.

[0037] Abrasion tests were performed at different pigment loadings to collect data on the strengths of the experimental paints. It was expected that as pigment loading decreases, the strength of the paint would increase, as there is more binder to hold the paint together. FIG. 3 outlines the results of these tests. As expected, the paints lost less volume as their pigment loadings decreased, confirming that in general the paint strength increases as the pigment loading decreases. Additionally, cohesion strengths similar to an acrylic paint with a 60% pigment loading were achieved with a pigment loading around 25% for the experimental paints. Therefore, the experimental paints benefit from more of the poly siloxane binder to function at the same strength. At this pigment level, the optical properties slightly suffered, but the reflectance of the formulation was still high, with a measured reflectance of 96.7% as shown in FIG. 4.

[0038] Finally, the hydrophobicity of the experimental paint was characterized to understand its self-cleaning ability. The images in FIGS. 5 A and 5B were captured of an experimental paint under both static (FIG. 5A) and dynamic (FIG. 5B) testing. From these photos, the final contact angles for the lefthand and righthand sides of the static droplet were calculated as 119.2 and 122.4 degrees, respectively, and the final contact angles for the lefthand and righthand sides of the dynamic drop were 96.8 and 111.5 degrees, respectively. Because a material is classified as hydrophobic if a droplet contacting the material exhibits a contact angle of greater than 90 degrees, the experimental hBN-MP-101 paints were concluded to be hydrophobic and could be classified as a self-cleaning paint.

[0039] In FIGS. 6A and 6B, SEM images taken of an experimental paint show a very rough and textured surface. This was further supported by a paint porosity of 74%, which was found using a volume displacement method to calculate the dry density of MP-101. Both of these data follow the lotus effect hypothesis, which states that surface roughness at a micro- and nano-scale leads to hydrophobicity as the water droplets are not able to form flush against the material’s surface, and are instead pushed away by the combination of the paint’s texture and the water’s surface tension. This characteristic is therefore what allowed the experimental paint to achieve its uncharacteristic levels of hydrophobicity.

[0040] Additional tests of radiative cooling paints made in accordance with the procedure discussed herein were also conducted using different pigment loadings. The tests showed that all tested formulations had a high reflectance (>95%) for a majority of the solar spectrum, with some of the highest values across the visible spectrum, and the lowest values around 2-2.5 pm, where there is a small amount of solar irradiation (as shown by the AMI.5 data). Additionally, their emission within the sky window (8-13 pm) was high (avg = 0.81), although not as high as the 60% loaded acrylic formulation’s emission of 0.83, despite plain MP-lOl’s high emittance of 0.92 The spectral optical properties of the radiative cooling paint mixtures of MP-101 and hBN pigment particles with a 91%, 80%, and 63% pigment loading were tested. It was found that the 91% pigment loaded mixture had an average UV-VIS reflectance of 97.57% and a sky window emission of 0.7884, the 80% pigment loaded mixture had an average UV-VIS reflectance of 97.76% and a sky window emission of 0.8074, and the 63% pigment loaded mixture had an average UV-VIS reflectance of 96.81% and an average sky window emission of 0.8227. In contrast, a comparative sample of "plain" MP-101 having a 0% pigment loading had insignificant UV-VIS reflectance and an average sky window emission of 0.9171. These reflectance values were comparable to the reflectance values of 97.9% found for previous hBN-acrylic paints.

[0041] The emergence of radiative cooling paints as a promising deterrent to climate change has made the study and optimization of the above-described experimental paints critical to their commercialization. The optical properties of a poly siloxane (MP-101) binder alone performed similarly to that of commonly-used acrylic binders as the absorption is around 1% more (2.14% compared to 1.02%). Such a very low absorption value (about 2% or less) allows for incoming radiation to interact with the pigment and subsequently be reflected away from the substrate, creating a more effective radiative cooling paint. The experimental paints exhibited a total reflectance of 97.52%, which was slightly less than a 97.9% reflectance that has been reported for an acrylic-hBN formulation.

[0042] The experimental paints allow for an hBN-based paint capable of surviving outdoor conditions while still maintaining desirable optical properties. The experimental radiative cooling paints of the present invention were able to achieve similar strengths as other hBN-containing paints at lower pigment loadings. Additionally, the paints are expected to be self-cleaning and anti-soiling due to their hydrophobic nature. Tests showed that in the radiative cooling paint of the present invention, use of a silicone-based binder, and in particular the polysiloxane-based binder MP-101, had minimal impact on optical properties that are required for radiative cooling, while also creating a hydrophobic formulation that allows for a self-cleaning paint.

[0043] As previously noted above, though the foregoing brief 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 loading of the subambient radiative pigment may be a predetermined ratio such that certain advantageous properties relating to hydrophobicity, strength, reflectivity, or application are optimized. A preferred method of producing such a radiative cooling paint mixture such that one or more of the aforementioned advantageous properties are optimized may be provided. 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.

Claims

CLAIMS:

1. A radiative cooling paint mixture comprising: a subambient radiative pigment with a particle size distribution in the radiative cooling paint mixture; a solvent; and a polysiloxane binder that exhibits hydrophobic properties and very low solar absorptance; wherein a single-layer paint formed with the radiative cooling paint mixture contains the subambient radiative pigment and the polysiloxane binder, reflects incoming solar irradiation, and emits thermal radiation through a spectrally transparent window in the atmosphere that exists from 8-13 pm.

2. The radiative cooling paint mixture of claim 1, wherein the subambient radiative pigment is hexagonal boron nitride.

3. The radiative cooling paint mixture of claim 1, wherein the solvent is dimethylformamide.

4. The radiative cooling paint mixture of claim 1, wherein the particle size distribution of the subambient radiative pigment is between about 139 and about 525 nanometers.

5. The radiative cooling paint mixture of claim 1, wherein the subambient radiative pigment has a final dry weight volume in the single-layer paint of about 25% to about 80%.

6. The radiative cooling paint mixture of claim 1, wherein the subambient radiative pigment has a final dry weight volume in the single-layer paint of about 70%.

7. The radiative cooling paint mixture of claim 1, wherein the subambient radiative pigment has a final dry weight volume in the single-layer paint of about 25%.

8. The radiative cooling paint mixture of claim 1, wherein the subambient radiative pigment has a final dry weight volume in the single-layer paint of between about 51% and about 55%.

9. The radiative cooling paint mixture of claim 1, wherein the subambient radiative pigment has a final dry weight volume in the single-layer paint of about 52%.

10. The radiative cooling paint mixture of claim 1, wherein the subambient radiative pigment has a final dry weight volume in the single-layer paint that provides a balance of hydrophobic, radiative, and strength properties.

11. A method of producing the radiative cooling paint mixture of claim 1, the method comprising: determining a first ratio for a final dry volume of the subambient radiative pigment to the polysiloxane binder in the single-layer paint; determining a second ratio for the solvent to the subambient radiative pigment in the radiative cooling paint mixture; providing a first amount of the polysiloxane binder, a second amount of the subambient radiative pigment, and a third amount of the solvent to provide the first and second ratios; forming a solution by mixing the first amount of poly siloxane binder and the third amount of solvent until separation is no longer observed; adding the second amount of subambient radiative pigment to the solution to form the radiative cooling paint mixture; and sonicating the radiative cooling paint mixture until substantially all particles of the subambient radiative pigment are fully suspended therein.

12. A method of using the radiative cooling paint mixture of claim 1, the method comprising: applying the radiative cooling paint mixture to form the single-layer paint on a surface such that the single-layer paint reflects incoming solar radiation, emits thermal radiation, and reduces the temperature of the surface or structure to subambient levels.

13. The single-layer paint formed with the radiative cooling paint mixture of claim 1.

14. The single-layer paint of claim 13, wherein the single-layer paint consists of the subambient radiative pigment and the polysiloxane binder