Patent Publication Number: US-2021171731-A1

Title: Scalable method of fabricating structured polymers for passive daytime radiative cooling and other applications

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is a national stage filing of International patent Application No. PCT/US2018/064743, filed Dec. 10, 2018, which claims the benefit of U.S. Provisional Application No. 62/596,145, filed Dec. 8, 2017, which are incorporated by reference as if disclosed herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the field of polymer coatings and to the field of solar-reflective coating materials. 
     BACKGROUND 
     For a variety of reasons, including, e.g., cost and environmental impact, there are many ongoing efforts to reduce energy usage through building materials that possess beneficial solar reflectance and thermal emittance characteristics. Such materials, however, may not be aesthetically pleasing, may also be difficult and/or expensive to manufacture, and may not always have suitable solar reflectance or thermal emittance characteristics. Accordingly, there is a long-felt need in the art for materials that possess beneficial solar reflectance characteristics while also having suitable thermal emissivity properties. 
     SUMMARY 
     In meeting the long-felt needs described above, the present disclosure first provides a structured material including a porous structured polymer layer. In some embodiments, the polymer layer includes a plurality of voids disposed therein, and has a hemispherical reflectance of from about 50% to about 99% for radiation having a wavelength of from about 0.35 to about 2.5 micrometers, as well as a room temperature hemispherical thermal emittance of at least 75% for radiation having a wavelength of from about 8 to about 13 micrometers or longer. 
     Additionally disclosed are methods of forming the structured material, including disposing a mixture including a polymer, a solvent, and a non-solvent onto a substrate and sequentially evaporating the solvent and the non-solvent to produce a structured porous polymer coating atop the substrate. 
     Additionally disclosed are methods of forming the structured material, including disposing a mixture including a polymer, a solvent, and a non-solvent onto a substrate and removal of the solvent by dilution with additional non-solvent, and subsequent evaporation of the non-solvent to produce a structured porous polymer coating atop the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings show embodiments of the disclosed subject matter for the purpose of illustrating the invention. However, it should be understood that the present application is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1A  is an image of a structured material according to some embodiments of the present disclosure; 
         FIG. 1B  is an image of a structured material according to some embodiments of the present disclosure; 
         FIG. 1C  is an image of a structured material according to some embodiments of the present disclosure; 
         FIG. 1D  is an image of a structured material according to some embodiments of the present disclosure; 
         FIG. 2  is a graph showing variation of spectral reflectance of a structured material according to some embodiments of the present disclosure; 
         FIG. 3A  is a chart of a method of making a structured material according to some embodiments of the present disclosure; and 
         FIG. 3B  is a chart of a method of making a structured material according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1A , some aspects of the disclosed subject matter include a structured material  100  including a substrate  102  and a polymer layer  104 . In some embodiments, substrate  102  is a building material, glass, plastic, metal, textile, siding, roofing, decking, or combinations thereof. In some embodiments, substrate  102  is light-permeable. In some embodiments, substrate  102  is flexible. In some embodiments, polymer layer  104  is a porous structured polymer layer including a plurality of voids  106 . In some embodiments, polymer layer  104  is freestanding and/or self-supporting. In some embodiments, polymer layer  104  is flexible. Polymer layer  104  has a high hemispherical reflectance of light in the solar wavelengths, i.e., about 0.35 micrometers to about 2.5 micrometers. In some embodiments, the hemispherical reflectance of polymer layer  104  is from about 50% to about 99% in the solar wavelengths. In some embodiments, the hemispherical reflectance of polymer layer  104  is from about 78 to 95%, from about 81% to about 93%, from about 83% to about 90%, from about 85% to about 89%, or about 87% in the solar wavelengths. Polymer layer  104  also has a high hemispherical emittance in the thermal wavelengths, i.e., greater than about 7 micrometers). In some embodiments, the hemispherical thermal emittance of polymer layer  104  is at least about 75% for radiation having a wavelength from about 8 to about 13 micrometers or longer. In some embodiments, the hemispherical thermal emittance of polymer layer  104  is at least about 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or even about 99% for radiation having a wavelength from about 8 to about 13 micrometers or longer. In some embodiments, polymer layer  104  has a hemispherical reflectance from about 50% to about 99% and a hemispherical thermal emittance of at least about 75%. In some embodiments, polymer layer  104  has a hemispherical reflectance from about 75% to about 99% and a hemispherical thermal emittance of at least about 75%. Referring now to  FIG. 1B , in some embodiments, polymer layer  104  has a bright white appearance under typical lighting conditions. Polymer layer  104  effectively scatters light ranging from the solar to thermal wavelengths. For example, in the solar wavelengths (e.g., 0.35-2.5 micrometers), where the polymer has negligible intrinsic absorption, this may result in back-scattering and high reflectivity, which can result in a white appearance of the film. At the same time, in the thermal wavelengths (e.g., 2.5-40 micrometers) where the polymer is intrinsically absorptive or emissive, this can lead to a high absorbance (black appearance) or emittance (glowing appearance) for the film. The structure can also introduce a refractive index gradient and lower the effective refractive index of the layer for longer thermal wavelengths, reducing the reflection of light and further enhancing the absorbance or emittance. In some embodiments, polymer layer  104  includes a colorant, such as a dye or pigment. In some embodiments, the colorant is a visible dye, infrared reflective dye, visible pigment, infrared reflective pigment, or combinations thereof. In some embodiments, the colorant is one or a combination of sudan blue, brilliant blue FCF, unisol blue, indigo carmine, sudan yellow, yellow 5, hansa yellow, titanium dioxide white, zinc oxide white, paliogen black, and chromium iron oxide black. In some embodiments, the colorant absorbs only a desired wavelength of light, e.g., blue and green if red color is desired, or green and red if blue is desired, and not others. In some embodiments, the colorant does not absorb the infrared component of the solar wavelengths, i.e., 0.7 to 2.5 micron. In some embodiments, polymer layer  104  includes additives to enhance mechanical strength. Referring now to  FIG. 1C , in some embodiments, polymer layer  104  has an additional top-coating  108 . In some embodiments, top-coating  108  includes one or more colorants. In some embodiments, top-coating  108  includes a material that provides protection from weather or moisture. Referring now to  FIG. 1D , in some embodiments, polymer layer  104  is deployed as a coating  104 A on substrates, e.g., copper, as a freestanding sheet  104 B suitable for use as fabrics or tarpaulins, or combinations thereof. 
     In some embodiments, polymer layer  104  includes poly(vinylidene difluoride), poly(vinylidene difluoride-co-hexafluoropropene), poly(methyl methacrylate), poly(vinyl chloride), poly(vinyl fluoride), poly(styrene), poly(dimethyl siloxane), poly(vinyl alcohol), ethyl-cellulose, methyl-cellulose, cellulose acetate, or combinations thereof. In some embodiments, polymer layer  104  has a thickness between 50 micrometers to 2 mm. In some embodiments, polymer layer  104  has a thickness between 200 micrometers to 400 micrometers. In some embodiments, voids  106  have cross-sectional dimensions of less than 2 micrometers. In some embodiments, voids  106  have cross-sectional dimensions between 20 nm to 700 nm. In some embodiments, voids  106  have cross-sectional dimensions between about 20 to about 700 nm, between about 30 to about 670 nm, between about 40 to about 630 nm, between about 60 to about 600 nm, between about 80 to about 580 nm, between about 100 to about 550 nm, between about 150 to about 520 nm, between about 180 to about 490 nm, between about 200 to about 470 nm, between about 220 to about 440 nm, between about 250 to about 410 nm, between about 270 to about 380 nm, between about 290 to about 350 nm, between about 310 to about 340 nm, or about 330 nm. In some embodiments, voids  106  have cross-sectional dimensions between 3 micrometers to 20 micrometers. In some embodiments, voids  106  have cross-sectional dimensions between 5 micrometers to about 10 micrometers. In some embodiments, voids  106  have cross-sectional dimensions of about 5 about 6 about 7 about 8 about 9 or about 10 In some embodiments, voids  106  have cross-sectional dimensions of less than 2 micrometers and between 3 micrometers to 20 micrometers. Without wishing to be bound by theory, relatively smaller voids, e.g., less than about 2 micrometers in cross-section) more efficiently scatter smaller wavelengths of radiation while larger voids, e.g., from about 3 to about 20 micrometers more efficiently scatter larger wavelengths, so a polymer layer  104  including both the smaller and larger voids  106  should lead to more efficient radiative cooling performance. 
     Referring now to  FIG. 2 , high reflectance and thermal emittance was demonstrated for a poly(vinylidene difluoride-co-hexafluoropropene) foam film according to some embodiments of the present disclosure. The graph portrays the variation in spectral reflectance (i.e., [1−spectral emittance]) as a function of wavelength. As shown, the reflectance is high (&gt;90%) in the solar wavelengths, i.e., about 0.35 micrometers to about 2.5 micrometers, and the emittance is high in the thermal wavelengths (i.e., greater than about 7 micrometers. This leads to an efficient radiative cooling performance for the film, as the film is reflective of solar radiation and also emits heat efficiently in the thermal wavelengths. 
     Referring now to  FIG. 3A , some embodiments of the present disclosure are directed to a method of forming a structured material. In some embodiments, at  302 , a mixture including polymers, a solvent, and a non-solvent is, e.g., made into a layer. At  304 A, the solvent is evaporated. Referring now to  FIG. 3B , in some embodiments, at  304 B, the solvent is diluted and/or diffused out by immersion in a reservoir containing the non-solvent, followed by removal of the mixture from the reservoir at  304 B′. Referring again to  FIG. 3A , at  306 , the non-solvent is evaporated. In some embodiments, the non-solvent is evaporated  306  after the solvent is evaporated  304 . In some embodiments, after completion of the above-identified steps, a structured polymer layer including a plurality of voids disposed therein is produced. At  308 , the structured polymer layer is disposed on a substrate. In some embodiments, the mixture is disposed on the substrate, e.g., via spraying, painting, or dipping, and the steps  302 - 308  are each allowed to proceed in situ to apply the structured polymer layer to the substrate. 
     Without being bound to any particular theory, during the drying process, the solvent evaporates first, causing the polymer and the non-solvent to form separate phases. Eventually, the non-solvent also evaporates, leaving air in its place. The result is a layer of structured polymer, with the morphology and porosity of the layer being tunable by, e.g., the choices of polymer, molecular weight of the polymer, ambient temperature, solvent, and non-solvent and their mass ratios. 
     In some embodiments, the solvent includes acetone, tetrahydrofuran, methanol, ethanol, propanol, isopropanol, hexane, benzene, toluene, or combinations thereof. In some embodiments, the non-solvent includes water, methanol, ethanol, propanol, isopropanol, or combinations thereof. In some embodiments, the mass ratio of the solvent (X) to polymer (Y) to non-solvent (Z) in the mixture is X&gt;0.5, 0.5&lt;Y&lt;1.25, and 0.5&lt;Z&lt;1.25. In some embodiments, the mass ratio of the solvent (X) to polymer (Y) to non-solvent (Z) in the mixture is X&gt;4, 0.5&lt;Y&lt;1.25, and 0.5&lt;Z&lt;1.25. In some embodiments, the mass ratio of the solvent (X) to polymer (Y) to non-solvent (Z) in the mixture is X&gt;6, 0.5&lt;Y&lt;1.25, and 0.5&lt;Z&lt;1.25. 
     Methods of the present disclosure advantageously provide a porous polymer layer that can be deposited on wide variety of substrates and has a large scattering of light, resulting in a reflective film. As the optical properties provided by the composition and the voids of the polymer layer provide high reflectance and emittance, the layers are particularly advantageous for cool-roof coatings, enabling surfaces coated by the film to stay cool, even under strong sunlight. The performance of the layers is also tunable via addition of visible and infrared dyes to change the spectral absorbance. 
     Although the disclosed subject matter has been described and illustrated with respect to embodiments thereof, it should be understood by those skilled in the art that features of the disclosed embodiments can be combined, rearranged, etc., to produce additional embodiments within the scope of the invention, and that various other changes, omissions, and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.