WO2025083491A1

WO2025083491A1 - Synergistic passive cooling fins

Info

Publication number: WO2025083491A1
Application number: PCT/IB2024/059342
Authority: WO
Inventors: Timothy J. Hebrink; William J. Kopecky
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2023-10-19
Filing date: 2024-09-25
Publication date: 2025-04-24
Anticipated expiration: 2026-04-19

Abstract

A cooling film for use in passively cooling modular data centers, electrical power transformers, and other surfaces. The cooling film includes a specular reflective multilayer mirror film attached to the passive cooling heat transfer fins. An antisoiling layer is secured to a first major surface of a specular reflective multi-layer film. The specular reflective multi-layer film can include a metal layer and is specular reflective of electromagnetic radiation over a majority of wavelengths in the range of 400 to 2500 nanometers. The specular reflective multi-layer film can also comprise a multi-layer optical film comprising first and second optical layers that constructively reflect electromagnetic radiation over a majority of wavelengths in the range of 400 to 2500 nanometers while simultaneously absorbing electromagnetic radiation over a majority of wavelengths in the range of 4000 to 20,000 nanometers.

Description

SYNERGISTIC PASSIVE COOLING FINS

BACKGROUND

Passive cooling can use a reflective and emissive film attached to a surface to be cooled. Applying innovative radiative cooling materials in addition to convective cooling materials on data centers, electrical power transformers, heat transfer panels, and other surfaces to be cooled will allow them to operate more efficiently.

SUMMARY

Synergistic passive cooling fins for use with data centers, electrical power transformers, heat transfer panels, and other surfaces to be cooled includes a specular reflective multilayer mirror film attached to passive cooling heat transfer fins. An antisoiling layer is secured to a first major surface of a specular reflective multi-layer film. The specular reflective multi-layer film can include a metal layer and is specular reflective of electromagnetic radiation over a majority of wavelengths in the range of 400 to 2500 nanometers. The specular reflective multi-layer film can also comprise a multi-layer optical film comprising first and second optical layers that constructively reflect electromagnetic radiation over a majority of wavelengths in the range of 400 to 2500 nanometers while simultaneously absorbing electromagnetic radiation over a majority of wavelengths in the range of 4000 to 20,000 nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of an exemplary composite cooling film.

FIG. 2 is a perspective view of synergistic passive cooling fins.

FIG. 3 is a perspective view of a modular data center with heat sink cooling fins.

FIG. 4 is a perspective view of an electrical power transformer with heat sink cooling fins.

FIG. 5 is a cross-sectional side view of a passive cooling fin design.

FIG. 6 is a cross-sectional side view of multiple passive cooling fins tiled together on a surface to be cooled.

FIG. 7 is a cross-sectional side view of a modular data center with passive cooling fins.

DETAILED DESCRIPTION

Embodiments include cooling films on passive radiative cooling fms for cooling modular data centers, electrical power transformers, and other surfaces.

Cooling Film

As used herein:

"fluoropolymer" refers to any organic polymer containing fluorine;

"infrared" (IR) refers to infrared electromagnetic radiation having a wavelength of >700 nm to 1 mm, unless otherwise indicated; "visible" (VIS) refers to visible electromagnetic radiation having a wavelength to from 400 nm to 700 nm, inclusive, unless otherwise indicated;

"ultraviolet" (UV) refers to ultraviolet electromagnetic radiation having a wavelength of at least 250 nm and up to but not including 400 nm, unless otherwise indicated;

"microporous" means having internal porosity (continuous and/or discontinuous) having average pore diameters of 50 to 10,000 nm;

"micro-voided" means having internal discrete voids having an average void diameter of 100 to 3000 nm;

“multi-layered optical film” means a multi-layer film with a plurality of first optical layers having higher refractive indices and a plurality of second optical layers having lower refractive indices that work constructively together to reflect electromagnetic radiation;

"nonfluorinated polymer" refers to any organic polymer not containing fluorine;

"radiation" means electromagnetic radiation unless otherwise specified;

"secured to" means directly or indirectly affixed to (e.g., in direct contact with, or adhesively bonded to by a unitary layer of adhesive)

"average reflectance" means reflectance averaged over a specified wavelength range;

"reflective" and "reflectivity" refer to the property of reflecting light or radiation, especially reflectance as measured independently of the thickness of a material; and

"reflectance" is the measure of the proportion of light or other radiation staking a surface at normal incidence which is reflected off it. Reflectivity typically varies with wavelength and is reported as the percent of incident light that is reflected from a surface (0 percent - no reflected light, 100 - all light reflected. Reflectivity, reflection, and reflectance are used interchangeably herein;

“specular reflectance” means electromagnetic radiation is reflected into a single outgoing direction at the same angle as the incident light;

“diffuse reflectance” means electromagnetic radiation is reflected into multiple outgoing directions.

Absorbance can be measured with methods described in ASTM E903-12 "Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres". Absorbance measurements described herein were made by making transmission measurements as previously described and then calculating absorbance using Equation 1.

As used herein, the term "absorbance" refers to the base 10 logarithm of a ratio of incident radiant power to transmitted radiant power through a material. The ratio may be described as the radiant flux received by the material divided by the radiant flux transmitted by the material. Absorbance (A) may be calculated based on transmittance (T) according to Equation 1 :

A = -log₁₀ T (1)

Emissivity can be measured using infrared imaging radiometers with methods described in ASTM E1933-14 (2018) "Standard Practice for Measuring and Compensating for Emissivity Using Infrared Imaging Radiometers." As shown in FIG. 1, an exemplary composite cooling film comprises a reflective multi-layer film 110 having an antisoiling layer 160 secured thereto. Antisoiling layer 160 is secured to a major surface 112 of specular reflective multi-layer film 110 such that the outwardly facing antisoiling surface 162 is opposite specular reflective multi-layer film 110.

Optional metal layer 150 is secured to reflective microporous layer 110 opposite antisoiling layer 160. Optional adhesive layers 170, 172 may adhere various components together as shown in FIG. 1. Optional adhesive layer 174 may be releasably bonded to optional liner 180. In one embodiment, after removal of optional liner 180, optional adhesive layer 174 may be bonded to a substrate (e g., a cooling fin, a radio frequency antenna surface) to be cooled.

Composite cooling films according to the present disclosure preferably have an average absorbance over the wavelength range 4-20 microns of at least 0.80, preferably at least 0.85, and more preferably at least 0.90, although this is not a requirement.

Reflective Microporous Layer

The reflective microporous layer may comprise a network of interconnected voids and/or discrete voids, which may be spherical, oblate, or some other shape. Primary functions of the reflective microporous layer include reflecting at least a portion of visible and infrared radiation of the solar spectrum and to emit thermal radiation in the atmospheric window (i.e., wavelengths of 8 to 13 microns).

Accordingly, the reflective microporous layer has voids that are of appropriate size that they diffusely reflect light with wavelengths in the 400 nm to 700 nm, preferably 300 nm to 2500 nm, wavelength range. Generally, this means that the void sizes should be in a size range (e.g., 50 to 3000 nm) capable of reflecting light in the 300 nm to 2500 nm wavelength range. Preferably, a range of void sizes corresponding to those dimensions is present so that effective broadband reflection with be achieved.

Reflectivity of the reflective microporous layer is generally a function of the number of polymer film/void interfaces, since reflection (typically diffuse reflection) occurs at those locations. Accordingly, the porosity and thickness of the reflective microporous layer will be important variable. In general, higher porosity and higher thickness correlate with higher reflectivity. However, for cost considerations film thickness is preferably minimized, although this is not a requirement. Accordingly, the thickness of the reflective microporous layer is typically in the range of 10 microns to 500 microns, preferably in the range of 10 microns to 200 microns, although this is not a requirement. Likewise, the porosity of the reflective microporous layer is typically in the range of 10 volume percent to 90 volume percent, preferably in the range of 20 volume percent to 85 volume percent, although this is not a requirement

Exemplary materials that may be useful at least one (preferably only one) of the reflective microporous layer (which contains at least one fluoropolymer) or an auxiliary reflective microporous layer (which does not include a fluoropolymer) are set forth below. Selection of which microporous material to include in which layer(s) will be apparent in view of the preceding discussion. Microporous polymer films suitable for use as the reflective microporous layer are known in the art and are described, for example, in US Patent Nos. 8,962,214; 10,240,013; and 4,874,567. These films may have average pore diameters of at least 0.05 microns.

In certain embodiments, the reflective microporous layer includes at least one Thermally Induced Phase Separation (TIPS) material. The pore size of TIPS materials can be generally controlled due to the ability to select the extent of stretching of the layer. TIPS materials are relatively inexpensive to make, and methods for making them are known to the skilled practitioner. For example, various materials and methods are described in detail in US Patent Nos. 4,726,989; 5,238,623; 5,993,954; and 6,632,850. Reflective microporous layers for use in aspects of the present disclosure also include Solvent Induced Phase Separated (SIPS) materials (e.g., US Patent No. 4,976,859) and other reflective microporous layers made by extrusion, extrusion/stretching and extrusion/ stretching/ extraction processes. Suitable reflective microporous layers that may be formed by SIPS include for example and without limitation polyvinylidene fluoride (PVDF), polyether sulfone (PES), polysulfone (PS), polyacrylonitrile (PAN), nylon (i.e., polyamide), cellulose acetate, cellulose nitrate, regenerated cellulose, and polyimide. Suitable reflective microporous layers that may be formed by stretching techniques (e.g., US Patent No. 6,368,742) include for example and without limitation polytetrafluoroethylene (PTFE) and polypropylene.

In certain embodiments, the reflective microporous layer comprises a thermoplastic polymer, for instance polyethylene, polypropylene, 1 -octene, styrene, polyolefin copolymer, polyamide, poly-1- butene, poly-4-methyl-l -pentene, polyethersulfone, ethylene tetrafluoroethylene, polyvinylidene fluoride, polysulfone, polyacrylonitrile, polyamide, cellulose acetate, cellulose nitrate, regenerated cellulose, polyvinyl chloride, polycarbonate, polyethylene terephthalate, polyimide, polytetrafluoroethylene, ethylene chlorotrifluoroethylene, polytetrafluoroethylene, or combinations thereof.

In some embodiments, the solar reflective microporous polymer layer includes GORE-TEX available form W. L. Gore.

Materials suitable for use as the reflective microporous layer include non-woven fibrous layers.

Polymeric non-woven layers can be made using a melt blowing process. Melt blown non-woven fibrous layers can contain very fine fibers. In melt-blowing, one or more thermoplastic polymer streams are extruded through a die containing closely arranged orifices. These polymer streams are attenuated by convergent streams of hot air at high velocities to form fine fibers, which are then collected on a surface to provide a melt-blown non-woven fibrous layer. Depending on the operating parameters chosen, the collected fibers may be semi-continuous or essentially discontinuous.

Polymeric non-woven layers can also be made by a process known as melt spinning. In melt spinning, the non-woven fibers are extruded as filaments out of a set of orifices and allowed to cool and solidify to form fibers. The filaments are passed through an air space, which may contain streams of moving air, to assist in cooling the filaments and passing through an attenuation (i.e., drawing) unit to at least partially draw the filaments. Fibers made through a melt spinning process can be "spunbonded," whereby a web comprising a set of melt-spun fibers are collected as a fibrous web and optionally subjected to one or more bonding operations to fuse the fibers to each other. Melt-spun fibers are generally larger in diameter than melt-blown fibers.

Polymers suitable for use in a melt blown or melt spinning process include polyolefins such as polypropylene and polyethylene, polyester, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyurethane, polybutene, polylactic acid, polyphenylene sulfide, polysulfone, liquid crystalline polymer, polyethylene-co-vinyl acetate, polyacrylonitrile, cyclic polyolefin, and copolymers and blends thereof. In some embodiments, the polymer, copolymer, or blend thereof represents at least 35% of the overall weight of the directly formed fibers present in the non-woven fibrous layer.

Non-woven fibers can be made from a thermoplastic semi -crystalline polymer, such as a semicrystalline polyester. Useful polyesters include aliphatic polyesters. Non-woven materials based on aliphatic polyester fibers can be especially advantageous in resisting degradation or shrinkage at high temperature applications. This property can be achieved by making the non-woven fibrous layer using a melt blowing process where the melt blown fibers are subjected to a controlled in-flight heat treatment operation immediately upon exit of the melt blown fibers from the multiplicity of orifices. The controlled in-flight heat treatment operation takes place at a temperature below a melting temperature of the portion of the melt blown fibers for a time sufficient to achieve stress relaxation of at least a portion of the molecules within the portion of the fibers subjected to the controlled in-flight heat treatment operation. Details of the in-flight heat treatment are described in US Patent Application Publication No. 2016/0298266.

Non-woven fibrous layers that may be used for the reflective microporous layer include ones made using an air laid process, in which a wall of air blows fibers onto a perforated collection drum having negative pressure inside the drum. The air is pulled though the drum and the fibers are collected on the outside of the drum where they are removed as a web. Exemplary embodiments of microporous membrane fabricated with non-woven fibers are highly reflective white papers comprising polysaccharides. Micro-porous polysaccharide white papers having greater than 90 % reflectance over visible wavelengths of 400 to 700 nm are available from International Paper, Memphis, Tennessee, under the trade designations IP ACCENT OPAQUE DIGITAL (100 lbs), IP ACCENT OPAQUE DIGITAL (100 lbs), HAMMERMILL PREMIUM COLOR COPY (80 lbs), and HAMMERMILL PREMIUM COLOR COPY (100 lbs). Titania, BaSOq and other white pigments are often added to paper to increase their reflection of visible light (400-700 nm).

Other non-woven fibrous layers that may be used for the reflective microporous layer include those made using a wet laid process. A wet laying or "wetlaid" process comprises (a) forming a dispersion comprising one or more types of fibers, optionally a polymeric binder, and optionally a particle filler(s) in at least one dispersing liquid (preferably water); and (b) removing the dispersing liquid from the dispersion.

Suitable fibers for use in air laid and wet laid processes include those made from natural (animal or vegetable) and/or synthetic polymers, including thermoplastic and solvent-dispersible polymers. Useful polymers include wool; silk; cellulosic polymers (e.g., cellulose and cellulose derivatives); fluorinated polymers (e.g., polyvinyl fluoride, polyvinylidene fluoride, copolymers of vinylidene fluoride such as poly(vinylidene fluoride-co-hexafluoropropylene), and copolymers of chlorotrifluoroethylene such as poly(ethylene-co-chlorotrifluoroethylene)); chlorinated polymers; polyolefins (e g., polyethylene, polypropylene, poly- 1 -butene, copolymers of ethylene and/or propylene, with 1-butene, 1-hexene, 1- octene, and/or 1-decene (e g., poly(ethylene-co- 1-butene), poly(ethylene-co-l-butene-co-l-hexene)); polyisoprenes; polybutadienes; polyamides (e g., nylon 6, nylon 6,6, nylon 6,12, poly(iminoadipoyliminohexamethylene), poly(iminoadipoyliminodecamethylene), or polycaprolactam); polyimides (e.g., poly(pyromellitimide)); polyethers; polyether sulfones (e,g., poly(diphenyl ether sulfone), or poly(diphenyl sulfone-co-diphenylene oxide sulfone)); polysulfones; polyvinyl acetates; copolymers of vinyl acetate (e.g., poly(ethylene-co-vinyl acetate), copolymers in which at least some of the acetate groups have been hydrolyzed to provide various poly(vinyl alcohols) including poly(ethylene- co-vinyl alcohol)); polyphosphazenes; polyvinyl esters; polyvinyl ethers; poly(vinyl alcohols); polyaramids (e.g., para-aramids such as poly(paraphenylene terephthalamide) and fibers sold under the trade designation KEVLAR by DuPont Co., Wilmington, Delaware, pulps of which are commercially available in various grades based on the length of the fibers that make up the pulp such as, e.g., KEVLAR 1F306 and KEVLAR 1F694, both of which include aramid fibers that are at least 4 mm in length); polycarbonates; and combinations thereof. Nonwoven fibrous layers may be calendered to adjust the pore size.

The use of a reflective micro-voided polymer film as the reflective microporous layer may provide a reflectance that is even greater than that of a silvered mirror. In some embodiments, a reflective micro-voided polymer film reflects a maximum amount of solar energy in a range from 300 to 2500 nanometers (nm). In particular, the use of a fluoropolymer blended into the micro-voided polymer film may provide a reflectance that is greater than other conventional multilayer optical films. Further, inorganic particles including barium sulfate, calcium carbonate, silica, alumina, aluminum silicate, zirconia, and titania may be blended into the micro-voided polymer film for providing high solar reflectance in solar radiation spectra of 0.3 to 2.5 microns and high absorbance in the atmospheric window of 8 to 13 microns, or even 4 to 25 microns. The outer layer may be suitable for protecting the reflective microporous layer, particularly, in outdoor environments. Including the outer layer may also facilitate less soiling of the surface and ease of cleaning the surface.

Exemplary polymers useful for forming the reflective micro-voided polymer film include polyethylene terephthalate (PET) available from 3M Company. Modified PET copolyesters including PETG available, for example, as SPECTAR 14471 and EASTAR GN071 from Eastman Chemical Company, Kingsport, Tennessee, and PCTG available, for example, as TIGLAZE ST and EB0062 also from Eastman Chemical Company are also useful high refractive index polymers. The molecular orientation of PET and PET modified copolyesters may be increased by stretching which increases its inplane refractive indices providing even more reflectivity in the multilayer optical film. In general, an incompatible polymer additive, or inorganic particle additive, is blended into the PET host polymer at levels of at least 1 wt. %, at least 10 wt. %, at least 20 wt. %, at least 40 wt. %, or even at least 49 wt. % during extrusion prior to stretching to nucleate voids during the stretching process. Suitable incompatible polymers additives for PET include: fluoropolymers, polypropylenes, polyethylenes, and other polymers which do not adhere well to PET. Similarly, if polypropylene is the host polymer, then incompatible polymer additives such as PET or fluoropolymers can be added to the polypropylene host polymer at levels of at least 10 wt. %, at least 20 wt. %, at least 30 wt. %, at least 40 wt. %, or even at least 49 wt. % during extrusion prior to stretching to nucleate voids during the stretching process. Exemplary suitable inorganic particle additives for nucleating voids in micro-voided polymer films include titania, silica, alumina, aluminum silicate, zirconia, calcium carbonate, barium sulfate, and glass beads and hollow glass bubbles, although other inorganic particles and combinations of inorganic particles may also be used. Crosslinked polymeric microspheres can also be used instead of inorganic particles. Inorganic particles can be added to the host polymer at levels of at least 10 wt. %, at least 20 wt. %, at least 30 wt. %, at least 40 wt. %, or even at least 49 wt. % during extrusion prior to stretching to nucleate voids during the stretching process. If present, the inorganic particles preferably have a volume average particle diameter of 5 nm to 1 micron, although other particle sizes may also be used. Hard particles including glass beads and/or glass bubbles can be present on the surface layer of UV mirror skin layer or the antisoiling layer to provide scratch resistance. In some embodiments, glass beads and/or glass bubbles may even protrude from the surface as hemispheres or even quarter spheres.

In some embodiments, micro-voided polymer films comprise a fluoropolymer continuous phase. Exemplary suitable polymers include ECTFE, PVDF, PTFE, and copolymers of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride such as, for example, those available under the trade designation THV from 3M Company.

Exemplary micro-voided PET film comprising barium sulfate is available as LUMIRROR XJSA2 from Toray Plastics (America) Inc., North Kingstown, Rhode Island. LUMIRROR XJSA2 comprises CaCO - inorganic additive to increase its reflectivity of visible light (400-700nm). Additional exemplary reflective micro-voided polymer films are available from Mitsubishi Polymer Film, Inc., Greer, South Carolina, as HOSTAPHAN V54B, HOSTAPHAN WDI3, and HOSTAPHAN W270.

Exemplary micro-voided polyolefin sheets are described in, for example, US Patent No. 6,261,994.

The reflective microporous layer is diffusely reflective, for example, of visible radiation over a majority of wavelengths in the range of 400 to 700 nanometers, inclusive. In some embodiments, the reflective microporous layer may have an average reflectance of at least 85 % (in some embodiments, at least 90 %, at least 91 %, at least 92 %, at least 93 %, at least 94 %, at least 95 %, at least 96 %, at least 97 %, at least 98 %, at least 99 %, or even at least 99.5 %) over a wavelength range of at least 400 nm up to 700 nm.

The reflectivity of the reflective microporous layer may be reflective over a broader wavelength range. Accordingly, in some embodiments, the reflectivity of the microporous polymer layer may have an average reflectivity of at least 85 % (in some embodiments, at least 90 %, at least 91 %, at least 92 %, at least 93 %, at least 94 %, at least 95 %, at least 96 %, at least 97 %, at least 98 %, at least 99 %, or even at least 99.5 %) over a wavelength range of at least 400 nm up to 2.5 micrometers, preferably at least 300 nm to 3.0 micrometers, although this is not a requirement.

Antisoiling Layer

The antisoiling layer provides a degree of protection from soil accumulation on the surface that could impede the function of the composite cooling film (e g., by absorbing solar radiation).

Typically, the antisoiling layer is a polymer film, preferably comprising one or more repellent polymers such as, for example, fluoropolymers. Examples of comonomers for making fluoropolymers that may be used include TFE, HFP, THV, PPVE. Exemplary fluoropolymers for use as the antisoiling layer include PVDF, ECTFE, ETFE, PFA, FEP, PTFE, HTE, and combinations thereof. In some embodiments, the fluoropolymer includes FEP. In some embodiments, the fluoropolymer includes PFA.

In some embodiments, the antisoiling layer is applied as a coating onto the reflective microporous layer. Numerous applied antisoiling compositions are known in the art including, for example, those described in US Patent Applications Publication Nos. 2015/0175479 and 2005/0233070, US Patent No. 6,277,485, and PCT Publication WO 02/12404.

In some embodiments, suitable antisoiling layers include a cross-linked siloxane coating available from Momentive under the trade name SilFORT AS4700 or a cross-linked siloxane coating available from California HardCoating Company under the tradename Perma-New 6000.

In some embodiments, the outward facing surface of the antisoiling layer (i.e., the antisoiling surface) may be micro-structured and/or nano-structured over some or all of its surface; for example, as described in PCT International Application No. PCT/IB2018/060527, filed December 21, 2018 and entitled "ANTISOILING SURFACE STRUCTURES".

An exemplary antisoiling is THV815 which can be coextruded with THV221 to create a bi -layer film having THV815 with a high melting point and THV221 with a low melting point. The THV221 layer can be used as a hot melt adhesive either by coextrusion coating the THV815/THV221 bi-layer film onto micro-porous solar reflective layer or by hot lamination of the THV 815/THV221 bi-layer film onto the micro-porous solar reflective layer. Alternate fluoropolymers to THV815 having melting points greater than 150C can also be used as the antisoiling layer. Alternate fluoropolymers to THV221 having melting points less than 150C can be used as the hot melt adhesive.

In some embodiments, the nano-structure may be superimposed on the micro-structure on the surface of the antisoiling layer.

The antisoiling layer has a major surface (i.e., an antisoiling surface) that can include microstructures and/or nano -structures. The micro-structures may be arranged as a series of alternating micropeaks and micro-spaces. The size and shape of the micro-spaces between micro-peaks may mitigate the adhesion of dirt particles to the micro-peaks. The nano-structures may be arranged as at least one series of nano-peaks disposed on at least the micro-spaces. The micro-peaks may be more durable to environmental effects than the nano-peaks. Because the micro-peaks are spaced only by a micro-space, and the micro-spaces are significantly taller than the nano-peaks, the micro-peaks may serve to protect the nano-peaks on the surface of the micro-spaces from abrasion.

In reference to the antisoiling layer, the term or prefix "micro" refers to at least one dimension defining a structure or shape being in a range from 1 micrometer to 1 millimeter. For example, a microstructure may have a height or a width that is in a range from 1 micrometer to 1 millimeter.

As used herein, the term or prefix "nano" refers to at least one dimension defining a structure or a shape being less than 1 micrometer. For example, a nano-structure may have at least one of a height or a width that is less than 1 micrometer.

Composite cooling films according to the present disclosure preferably have an average absorbance over the wavelength range 8-13 microns of at least 0.85, preferably at least 0.9, and more preferably at least 0.95, although this is not a requirement.

Exemplary anti-soiling layer comprise a cross-linked hard coat comprising UV absorbing additives. Suitable materials for the cross-linked hard coat include acrylates, siloxanes, and urethanes, or combinations thereof. An exemplary anti-soiling layer comprising both acrylate and siloxane comonomers is described in U.S. Patent No. 10,072,173.

Specular Reflective Multilayer Optical Film

Specular reflective multilayer optical films as described in US Patent No. 9,523,516 and PCT Publication WO 2019/130199 may be applied to an antenna surface to minimize solar absorption and thus increase its ability to cool the electronics within the antenna. Such multilayer optical films can also be applied to cooling fins for use in cooling a surface.

The specular reflective multilayer film may be composed of materials that provide an average reflectance of at least 90 percent over at least the wavelength range of 400 to 1000 nm, and preferably 400 to 2000 nm, and more preferably 350 to 2500 nm.

The number of layers in the specular reflective multilayer optical film is selected to achieve the desired optical properties using the minimum number of layers for reasons of film thickness, flexibility and economy. In the case of reflective films such as mirrors, the number of layers is preferably less than about 2,000, more preferably less than about 1,000, and even more preferably less than about 750. In some embodiments, the number of layers is at least 150 or 200. In other embodiments, the number of layers is at least 250.

Specular reflective multilayer optical film comprises multiple low/high index pairs of film layers, wherein each low/high index pair of optical layers having a combined optical thickness of 1/2 the center wavelength of the band it is designed to reflect. Stacks of such films are commonly referred to as quarterwave stacks. In some embodiments, different low/high index pairs of layers may have different combined optical thicknesses, such as where a broadband reflective optical film is desired.

The optical layers may comprise fluorinated polymers (i.e., fluoropolymers), non-fluorinated polymers, and blends thereof. Examples of fluoropolymers that may be used include copolymers of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and vinylidene fluoride (e.g., available from 3M Company under the trade designation 3M DYNEON THV); a copolymer of TFE, HFP, vinylidene fluoride, and perfluoropropyl vinyl ether (PPVE) (e g., available from 3M Company under the trade designation 3M DYNEON THVP); a polyvinylidene fluoride (PVDF) (e.g., 3M DYNEON PVDF 6008 from 3M Company); ethylene chlorotrifluoroethylene polymer (ECTFE) (e g., available as HALAR 350LC ECTFE from Solvay, Brussels, Belgium); an ethylene tetrafluoroethylene copolymer (ETFE) (e.g., available as 3M DYNEON ETFE 6235 from 3M Company); perfluoroalkoxyalkane polymers (PF A); fluorinated ethylene propylene copolymer (FEP); a polytetrafluoroethylene (PTFE); copolymers of TFE, HFP, and ethylene (HTE) (e.g., available as 3M DYNEON HTE1705 from 3M Company). Combinations of fluoropolymers can also be used. In some embodiments, the fluoropolymer includes FEP. In some embodiments, the fluoropolymer includes PFA.

Examples of non-fluorinated polymers that may be used in at least one layer of the specular reflective multilayer optical film include at least one of: polyethylene terephthalate, polypropylene, polyethylene, polyethylene copolymers, polymethyl methacrylate, methyl methacrylate copolymers (e.g., copolymers of ethyl acrylate and methyl methacrylate), polyurethanes, extended chain polyethylene polymers (ECPEs), or a combinations thereof. In general, combinations of non-fluorinated polymers can be used. Exemplary non-fluorinated polymers, especially for use in low refractive index optical layers, may include homopolymers of polymethyl methacrylate (PMMA), such as those available as CP71 and CP80 from Ineos Acrylics, Inc., Wilmington, Delaware; and polyethyl methacrylate (PEMA), which has a lower glass transition temperature than PMMA. Additional useful polymers include: copolymers of methyl methacrylate such as, for example, a copolymer made from 75 wt.% methyl methacrylate and 25 wt.% ethyl acrylate, for example, as available from Ineos Acrylics, Inc. as PERSPEX CP63, or as available from Arkema, Philadelphia, Pennsylvania as ALTUGLAS 510, and copolymers of methyl methacrylate monomer units and n-butyl methacrylate monomer units.

Blends of PMMA and PVDF may also be used.

Suitable triblock acrylic copolymers are available, for example, as KURARITY LA4285 from Kuraray America Inc., Houston, Texas. Additional suitable polymers for the optical layers, especially for use in the low refractive index optical layers, may include at least one of: polyolefin copolymers such as poly(ethylene-co-octene) (e.g., available as ENGAGE 8200 from Dow Elastomers, Midland, Michigan), polyethylene methacrylate (e.g., available as ELVALOY from Dow Elastomers), poly (propylene-co- ethylene) (e g., available as Z9470 from Atofina Petrochemicals, Inc., Houston, Texas); and a copolymer of atactic polypropylene and isotactic polypropylene. Materials may be selected based on absorbance or transmittance properties described herein, as well as on refractive index. In general, the greater the refractive index between two materials, the thinner the film can be, which may be desirable for efficient heat transfer. For solar reflective multilayer optical films, a quarterwave stack design preferably results in each of the layers in the multilayer stack having an average thickness of not more than about 0.7 micrometers, although this is not a requirement.

Multilayer optical films (including reflective polarizers and mirrors) can be made by coextrusion of alternating polymer layers having different refractive indices, for example, as described in US Patent Nos. 6,045,894; 6,368,699; 6,531,230; 6,667,095; 6,783,349; 7,271,951; 7,632,568; and 7,952,805; and PCT Publications WO 95/17303 and WO 99/39224.

Optional IR-reflective layers that can be vapor coated under the solar reflective multilayer optical film also include: layers of a metal such as, for example, aluminum, copper, gold, or silver; and layers of metal oxide or metal sulfide such as, for example, cerium oxide, aluminum oxide, magnesium oxide, titanium dioxide, and indium tin oxide.

Exemplary materials used as the first optical layers and second optical layers in the multilayer optical film absorb greater than 50% of electromagnetic radiation over a majority of wavelengths in the range of 4000 to 20,000 nanometers. Exemplary first optical layers include PET (polyethylene terephthalate) and copolymers thereof.

Optional Adhesive Layers

The optional adhesive layers may comprise any adhesive (e.g., thermosetting adhesive, hot melt adhesive, and/or pressure-sensitive adhesive). If present, optional adhesive layer preferably comprises a pressure-sensitive adhesive. In some embodiments, the adhesive may be resistant to ultraviolet radiation damage. Exemplary adhesives which are typically resistant to ultraviolet radiation damage include silicone adhesives and acrylic adhesives containing UV-stabilizing/blocking additive(s), for example, as discussed hereinabove. An exemplary optional adhesive is polyisobutylene which can minimize moisture transmission into microporous and micro-voided solar reflective layer.

The optional adhesive layer may be a hot melt adhesive. An exemplary hot melt adhesive is THV221.

The optional adhesive layers may comprise thermally-conductive particles to aid in heat transfer. Exemplary thermally-conductive particles include aluminum oxide particles, alumina nanoparticles, hexagonal boron nitride particles and agglomerates (e.g., available as 3M BORON NITRIDE from 3M Company), graphene particles, graphene oxide particles, metal particles, and combinations thereof.

Optional releasable liners may comprise, for example, a polyolefin film, a fluoropolymer film, a coated PET film, or a siliconized film or paper.

UV-Stabilizing Additives

UV-stabilizing additives may be added to any component of the composite cooling film (e.g., the UV-reflective multilayer optical film, the optional antisoiling layer, optional adhesive layers, the reflective microporous layer, and/or the IR-reflective layer). UV stabilization with UV-absorbers (UVAs) and/or Hindered Amine Light Stabilizers (HALS) can intervene in the prevention of photo-oxidation degradation of PET, PMMA, and CoPMMAs. Exemplary UVAs for incorporation into PET, PMMA, or CoPMMA polymer layers include benzophenones, benzotriazoles, and benzotriazines. Commercially available UVAs for incorporation into PET, PMMA, or CoPMMA optical layers include those available as TINUVIN 1577 and TINUVIN 1600 from BASF Corporation, Florham Park, New Jersey. Typically, UVAs are incorporated in polymers at a concentration of 1 to 10 weight percent (wt. %).

Exemplary HALS compounds for incorporation into PET, PMMA, or CoPMMA optical layers include those available as CHIMMASORB 944 and TINUVIN 123 from BASF Corporation. Typically, HALS compounds are incorporated into the polymer at a 0. 1- 1.0 wt. %. A 10: 1 ratio of UVA to HALS may be preferred.

UVAs and HALS compounds can also be incorporated into the fluoropolymer layers. US Patent No. 9,670,300 and US Patent Application Publication No. 2017/0198129 describe exemplary UVA oligomers that are compatible with PVDF fluoropolymers.

Other UV-blocking additives may be included in the fluoropolymer layers. For example, small particle non-pigmentary zinc oxide and titanium oxide can be used. Nanoscale particles of zinc oxide, calcium carbonate, and barium sulfate reflect, or scatter, UV-light while being transparent to visible and near infrared light. Small zinc oxide and barium sulfate particles in the size range of 10-100 nanometers can reflect UV-radiation are available, for example, from Kobo Products Inc., South Plainfield, New Jersey.

Antistatic additives may also be incorporated into any of the polymer films/layers to reduce unwanted attraction of dust, dirt, and debris. Ionic salt antistatic additives available from 3M Company may be incorporated into PVDF fluoropolymer layers to provide static dissipation. Exemplary antistatic additives for PMMA and CoPMMA are commercially available as STAT-RITE from Lubrizol Engineered Polymers, Brecksville, Ohio, or as PELESTAT from Sanyo Chemical Industries, Tokyo, Japan.

Examples of cooling films are described in U.S. Provisional Patent Application Serial No. 63/333152, entitled “Passive Radiative Cooling Film for Antennas,” and filed April 21, 2022, which is incorporated herein by reference as if fully set forth.

Other examples of cooling films are described in PCT Publications WO 2020/240447, WO 2020/240366, and WO 2019/130199.

Passive Cooling Fins

Synergistic passive cooling fins as described below can be used to passively cool modular data centers, electrical power transformers, and other surfaces. FIG. 2 is a perspective view of synergistic passive cooling fins having cooling fins 200 with a passive radiative cooling film 201 laminated to a major surface of fins 200, a heat sink 202 attached to and supporting cooling fins 200, and a fluid heat transfer panel 204 attached to heat sink 202 on a side opposite cooling fins 200. Fluid heat transfer panel 204 includes a fluid inlet 206 to panel 204 and a fluid outlet 208 from panel 204 configured for circulating a fluid through panel 204. Cooling fins 200 can be implemented with metal fins such as aluminum. Heat sink 202 and panel 204 can be implemented with, for example, a metal material such as aluminum. The cooling film 201 laminated to the cooling fins can be implemented with, for example, the composite cooling film described above with respect to FIG. 1. Cooling film 201 preferably covers all or a substantial portion of the major surfaces of each cooling fin. Cooling film 201 is preferably laminated or otherwise attached to each of the cooling fins; alternatively cooling film 201 could be laminated to fewer than all the cooling fins.

FIG. 3 is a perspective view of a modular data center 214 with heat sink cooling fins 210. A heat sink 212 is attached to cooling fins 210 and modular data center 214. Heat sink cooling fins 210 include a passive radiative cooling film laminated to the fins and can be implemented as described with respect to FIG. 2. The fluid transfer panel 204 shown in FIG. 2 can optionally be implemented between heat sink 212 and modular data center 214.

FIG. 4 is a perspective view of an electrical power transformer 220 with heat sink cooling fins 216. A heat sink 218 is attached to cooling fins 216 and electrical power transformer 220. Heat sink cooling fins 216 include a passive radiative cooling film laminated to the fins, and can be implemented as described with respect to FIG. 2. The fluid transfer panel 204 shown in FIG. 2 can optionally be implemented between heat sink 218 and electrical power transformer 220.

FIG. 5 is a cross-sectional side view of a passive cooling fin design. This design includes a passive radiative cooling film 222 attached to an aluminum (or other metal) sheet shaped into a cooling fin 224. A thermally conductive adhesive 226 is attached between cooling fin 224 and a substrate to be cooled 228. The passive cooling fin shown in FIG. 5 could be used for the cooling fins shown in FIGS. 2-4.

FIG. 6 is a cross-sectional side view of multiple passive cooling fins tiled together on a surface to be cooled. A cooling fin 230 includes a passive radiative cooling film 232 attached to an aluminum (or other metal) sheet shaped into a cooling fin 234. A cooling fin 240 having a passive radiative cooling film 233 is attached to and tiled with cooling fin 230 at interface 241. A cooling fin 242 having a passive radiative cooling film 235 is attached to and tiled with cooling fin 240 at interface 243. A thermally conductive adhesive 236 is attached between the cooling fins (230, 240, 242) and a substrate 238 to be cooled. Additional passive cooling fins could be tiled together in the same manner.

FIG. 7 is a cross-sectional side view of a modular data center with passive cooling fins. A cooling fin 244 includes a passive radiative cooling film 246 attached to an aluminum (or other metal) sheet shaped into a cooling fin 248. A cooling fin 252 having a passive radiative cooling film 247 is attached to and tiled with cooling fin 244 at interface 253. A cooling fin 254 having a passive radiative cooling film 249 is attached to and tiled with cooling fin 252 at interface 255. A thermally conductive adhesive 250 is attached between the cooling fins (244, 252, 254) and a modular data center 256 to be cooled. Additional passive cooling fins could be tiled together in the same manner on modular data center 256. The cooling films described in FIGS. 5-7 can be implemented with, for example, the composite cooling film described above with respect to FIG. 1. The cooling films described in FIGS. 5-7 are laminated or otherwise attached to major surfaces of the respective cooling fins. The cooling films described in FIGS. 5-7 preferably cover all or a substantial portion of the major surfaces of each cooling fin. Alternatively, the cooling films can be attached to fewer than all of the cooling fins.

Claims

The invention claimed is:

1. A passive cooling fin, comprising: a cooling fin; a passive radiative cooling film attached to a major surface of the cooling fin; and a heat sink attached to the cooling fin, wherein the passive radiative cooling film comprises: an antisoiling layer secured to a first major surface of a specular reflective multi-layer optical film, wherein the specular reflective multi-layer optical film comprises a plurality of first optical layers, a plurality of second optical layers, and is specular reflective of electromagnetic radiation over a majority of wavelengths in the range of 400 to 2500 nanometers, and is absorptive of electromagnetic radiation over a majority of wavelengths in the range of 4000 to 20,000 nanometers..

2. The passive cooling fin of claim 1, further comprising: a fluid heat transfer panel attached to the heat sink on a side opposite the cooling fin; a fluid inlet attached to the fluid heat transfer panel; and a fluid outlet attached to the fluid heat transfer panel, wherein the fluid inlet and the fluid outlet are configured for circulating a fluid through the fluid heat transfer panel.

3. A modular data center having the cooling fin of claim 1.

4. An electrical power transformer having the cooling fin of claim 1.

5. The cooling fin of claim 1, wherein the specular reflective multi-layer has an average absorbance of at least 80 percent over a wavelength range of 8 microns to 13 microns.

6. The cooling fin of claim 1, further comprising a thermally conductive adhesive layer secured to a second major surface of the film opposite the first major surface.

7. The cooling fin of claim 1, wherein a thermally conductive adhesive layer is secured to the cooling fin opposite the reflective multilayer optical film.

8. The cooling fin of claim 1, further comprising an auxiliary reflective metal-layer secured to the reflective multi-layer optical film opposite the antisoiling layer.

9. The cooling fin of claim 1, wherein the reflective metal layer comprises one of silver, aluminum, or copper.

10. The cooling fin of claim 1, wherein an outwardly facing surface of the antisoiling layer comprises a nano-structured surface superimposed on a micro-structured surface.

11. The cooling fin of claim 1, wherein the antisoiling layer comprises a UV absorbing cross-linked hard coat.

12. The cooling fin of claim 1, wherein the reflective multi-layer optical film comprises a multi-layer optical film.

13. The cooling fin of claim 12, wherein the micro-voided polymer film further comprises siloxane.

14. A passive cooling fin, comprising: a cooling fin; a passive radiative cooling film attached to a major surface of the cooling fin; and a heat sink attached to the cooling fin, wherein the passive radiative cooling film comprises: an antisoiling layer secured to a first major surface of a specular reflective multilayer film, wherein the reflective multilayer film compnses an organic polymer, a metal layer, and is specularly reflective of electromagnetic radiation over a majority of wavelengths in the range of 400 to 2000 nanometers, the antisoilmg layer has an outwardly facing antisoiling surface opposite the second major surface, and the metallized layer is secured to a second major surface of the film opposite the first major surface.

15. The passive cooling fin of claim 14, further comprising: a fluid heat transfer panel attached to the heat sink on a side opposite the cooling fin; a fluid inlet attached to the fluid heat transfer panel; and a fluid outlet attached to the fluid heat transfer panel, wherein the fluid inlet and the fluid outlet are configured for circulating a fluid through the fluid heat transfer panel.

16. A modular data center having the cooling fin of claim 14.

17. An electrical power transformer having the cooling fin of claim 14.

18. The cooling fin of claim 14, wherein the solar reflective multilayer film has an average absorbance of at least 50 percent over the wavelength range of 8 to 13 microns.

19. The cooling fin of claim 14, further comprising an infrared-absorptive layer secured to a second major surface of the film opposite the first major surface.

20. The cooling fin of claim 19, wherein the infrared-absorptive layer has an average absorbance of at least 50 percent over the wavelength range of 4 to 20 microns.

21. The cooling fin of claim 19, wherein the infrared-absorptive layer is secured to the solar reflective multilayer film opposite the antisoiling layer.

22. A passive cooling fin, comprising: a cooling fin; a passive radiative cooling film attached to a major surface of the cooling fin; a thermally conductive adhesive attached to the cooling fin on a side opposite the passive radiative cooling film; and a substrate to be cooled attached to the thermally conductive adhesive on a side opposite the cooling fin, wherein the passive radiative cooling film comprises: an antisoiling layer secured to a first major surface of a reflective microporous layer, wherein the reflective microporous layer is diffusely reflective of electromagnetic radiation over a majority of wavelengths in the range of 400 to 2500 nanometers, and the antisoiling layer has an outwardly facing antisoiling surface.

23. A passive cooling fin, comprising: a cooling fin; a passive radiative cooling film attached to a major surface of the cooling fin; a thermally conductive adhesive attached to the cooling fin on a side opposite the passive radiative cooling film; and a substrate to be cooled attached to the thermally conductive adhesive on a side opposite the cooling fin, wherein the passive radiative cooling film comprises: an antisoiling layer secured to a first major surface of a specular reflective multilayer film, wherein the specular reflective multilayer comprises a metal layer and is specular reflective of electromagnetic radiation over a majority of wavelengths in the range of 400 to 2000 nanometers, the antisoiling layer has an outwardly facing antisoiling surface opposite the second major surface, and an optional thermally conductive adhesive is secured to a second major surface of the film opposite the first major surface.

24. The passive cooling fin of claims 22 or 23, further comprising another passive cooling fin tiled with the cooling fin.

25. A modular data center having the passive cooling fin of any of claims 22-24.