This invention was made with Government support under Cooperative Agreement DE- AR0001100 awarded by the Department of Energy. The Government has certain rights in this invention.

BACKGROUND

Passive cooling articles are designed to reduce the temperature of substrates, or objects, to which they are applied without the need for power consumption. For example, passive cooling articles can be designed to reflect light in a solar region of the electromagnetic spectrum and radiate light in an atmospheric window region of the electromagnetic spectrum, both of which may cool the substrate. Non-limiting examples of applications of passive cooling films include commercial building air conditioning, commercial refrigeration (e.g., supermarket refrigerators), data center cooling of heat transfer fluid systems, power generator cooling, vehicle air conditioning or refrigeration (e.g., cars, truck, trains, buses, ships and airplanes), or cooling of electric vehicle batteries. Passive cooling articles can include multi-layer optical films that transmit and/or reflect various wavelengths of light by controlling the number, thickness and refractive index of the individual layers. Industry has recently turned to fluoropolymers for use in one or more layers of these multilayer optical films. Fluoropolymers, and fluorothermoplasts in particular, have a number of desirable properties that make them suitable for such applications, including heat resistance, chemical resistance, weatherability, UV-stability and meltprocessability.

SUMMARY

It is generally known that fluorothermoplasts of moderate melt viscosities can be melt processed by conventional extmsion equipment. However, during melt processing, the fluorothermoplasts are subjected to mechanical and thermal stresses that can produce hydrofluoric acid (HF). Hydrofluoric acid is a strong corrosive that can erode the metal and metal alloy parts typically used to process the fluorothermoplasts into films and fibers. Additionally, the lower melting fluorothermoplasts often contain acidic carboxylic end groups that can further contribute to the erosion of processing equipment. Standard methods to eliminate these carboxylic end- groups (-COOH), such as the post-fluorination techniques used on highly fluorinated polymers, cannot be effectively applied to semi-fluorinated polymers. Thus, processing equipment in which semi-fluorinated polymers are used needs to be protected by special and costly metal alloys having high nickel content, such as Inconel or Hastelloy. This can lead to a substantial cost investment in complex co-extrusion tooling, such as a feed block required for multi-layer film processing. There is a need for semi-fluonnated thermoplastics with the requisite thermal stability and a reduced potential for corrosion hazards.

The present disclosure provides semi-fluorinated thermoplastic copolymers having reduced HF content and nonacidic end groups, and passive cooling articles containing the semifluorinated thermoplastic copolymer.

In one embodiment, the present disclosure provides a copolymer comprising 24 to 47 mole % of tetrafluoroethylene monomers, 5 to 23 mole % of hexafluoropropylene monomers, and 35 to 70 mole % of vinylidene fluoride monomers, wherein at least a portion of the end groups on the copolymer are nonacidic.

In another embodiment, the present disclosure provides an article comprising a reflector having an average reflectance of at least 85% (in some embodiments, at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or even at least 99.5%) in a wavelength range from 0.35 to 2.5 micrometers, wherein the reflector comprises the copolymer.

In a further embodiment, the present disclosure provides a method of making the copolymer comprising combining the tetrafluoroethylene monomers, hexafluoropropylene monomers, and vinylidene fluoride monomers in an aqueous emulsion comprising ammonium 4,8- dioxa-3-H-perfluorononanoate, and polymerizing the monomers in the presence of oxidizing manganese ions and dialkyl ether to create a polymer dispersion, wherein the polymer dispersion contains at least 10 wt.% solids and a particle size distribution of 80 to 150 nm. In preferred embodiments, a source of the oxidizing manganese ion is potassium permanganate and the dialkyl ether is dimethyl ether.

The above summary of the present disclosure is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The description that follows more particularly exemplifies illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an exemplary passive cooling article of the present disclosure and a substrate;

FIG. 2 is a schematic perspective view of the passive cooling article and substrate in FIG. 1 during the day. The passive cooling article reflects sunlight and radiates or emits electromagnetic energy (e.g., infrared light);

FIG. 3 is a schematic perspective view of the passive cooling article and substrate in FIG. 1 during the night. The passive cooling article radiates or emits electromagnetic energy (e.g., infrared light).

Unless otherwise indicated, all figures and drawings in this document are not to scale and are chosen for the purpose of illustrating different embodiments of the invention. In particular, the dimensions of the various components are depicted in illustrative terms only, and no relationship between the dimensions of the various components should be inferred from the drawings, unless so indicated.

DETAILED DESCRIPTION

In the following description of illustrative embodiments, reference is made to the accompanying figures of the drawing which form a part hereof, and in which are shown, by way of illustration, specific embodiments. It is understood that the invention is not limited in its application to the details of use, construction, and the arrangement of components set forth in the following description. The invention is capable of other embodiments and of being practiced or of being carried out in various ways that will become apparent to a person of ordinary skill in the art upon reading the present disclosure.

As used herein:

The term “comprises” and variations thereof do not have a limiting meaning where these terms appear in the description and claims. Such terms will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements. By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of.” Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present. By “consisting essentially of’ is meant including any elements listed after the phrase, and limited to other elements that do not interfere with or contribute to the activity or action specified in the disclosure for the listed elements. Thus, the phrase “consisting essentially of’ indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present depending upon whether or not they materially affect the activity or action of the listed elements.

The terms “a,” “an,” and “the” are used interchangeably with “at least one” to mean one or more of the components being described.

The term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements.

The term “some embodiments” means that a particular feature, configuration, composition, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of such phrases in various places throughout this specification are not necessarily referring to the same embodiment of the disclosure. Furthermore, the particular features, configurations, compositions, or characteristics may be combined in any suitable maimer in one or more embodiments.

The terms “preferred” and “preferably” refer to embodiments of the disclosure that may afford certain benefits, under certain circumstances; however, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the disclosure.

The term “copolymef’ includes binary copolymers, i.e. copolymers of only two different monomers, as well as copolymers that comprise more than two different monomers such as terpolymers and quaterpolymers.

The term “light” refers to electromagnetic energy of any wavelength. In some embodiments, light means electromagnetic energy having a wavelength of at most 16 micrometers, or at most 13 micrometers. In some embodiments, light means radiant energy in a region of the electromagnetic spectmmfrom 0.25 to 16 micrometers.

The term “solar region” refers to a portion of the electromagnetic spectrum that partially or fully includes sunlight or solar energy. The solar spectrum may include at least one of the visible, ultraviolet, or infrared wavelengths of light. The solar spectrum may be defined as wavelengths in a range from 0.35 to 2.5 micrometers or an even greater range.

The term “atmospheric window region” refers to a portion of the electromagnetic spectrum that partially or fully includes wavelengths that can be transmitted through the atmosphere. The atmospheric window region may include at least some infrared wavelengths of light. The atmospheric window region may be defined as wavelengths ranging from 8 to 14 micrometers, or an even greater range.

The terms “transmittance” and “transmission” refer to the ratio of intensity of transmitted light through an object to the intensity of the incident light, which may account for the effects of absorption, scattering, reflection, etc. Transmittance (T) may range from 0 to 1 or be expressed as a percentage (T%).

The term “average transmittance” refers to the arithmetic mean of a sample of transmittance measurements over a range of wavelengths.

The terms “reflectance” and “reflectivity” refer to the to the ratio of intensity of incident light reflected off an object to the intensity of the incident light. Reflectance may range from 0 to 1 or be expressed as a percentage.

The term “average reflectance” refers to the arithmetic mean of a sample of reflectance measurements over a range of wavelength determined by at least one of: a measurement of the reflectance of uniformly unpolarized light (for at least one incidence angle) or the average of reflectance measurements of two or more polarizations of light (for example, s and p polarizations, for at least one angle of incidence).

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:

A = -logio T = 2 - logio T%

The term “minimum absorbance” refers to the lowest absorbance value over a range of wavelengths.

The term “average absorbance” refers to the arithmetic mean of a sample of absorbance measurements over a range of wavelengths. For example, absorbance measurements in a range from 8 to 14 micrometers can be averaged over that range.

The term “reflective band” refers to a wavelength range where the average reflectance is at least 80%, more preferably at least 90%.

All numbers are assumed to be modified by the term “about”. As used herein in connection with a measured quantity, the term “about” refers to that variation in the measured quantity as would be expected by the skilled artisan making the measurement and exercising a level of care commensurate with the objective of the measurement and the precision of the measuring equipment used.

The recitations of numerical ranges by endpoints include all numbers subsumed within that range as well as the endpoints (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). The phrase “up to” a number (e.g., up to 50) includes the number (e.g., 50).

The present disclosure provides passive cooling articles comprising a semi-fluorinated thermoplastic copolymer (also referred to herein as “SFT copolymer”). The SFT copolymer generally comprises repeating units of tetrafluoroethylene (TFE) monomers, hexafluoropropylene (HFP) monomers, and vinylidene fluoride (VDF) monomers, where at least a portion of the end groups on the copolymer are nonacidic.

In some embodiments, the copolymer comprises at least 24 mole %, at least 30 mole %, at least 35 mole %, or at least 40 mole % TFE monomers. In some embodiments, the copolymer comprises up to 47 mole %, up to 40 mole %, up to 35 mole %, or up to 30 mole % TFE monomers. In some embodiments, the copolymer comprises 24 to 47 mole %, more particularly 30 to 47 mole % TFE monomers. In some embodiments, the copolymer comprises 39 mole % TFE monomers.

In some embodiments, the copolymer comprises at least 5 mole %, at least 7 mole %, at least 9 mole %, at least 11 mole %, at least 13 mole %, at least 15 mole %, at least 17 mole %, at least 19 mole %, or at least 21 mole % HFP monomers. In some embodiments, the copolymer comprises up to 23 mole %, up to 21 mole %, up to 19 mole %, up to 17 mole %, up to 15 mole %, up to 13 mole %, up to 11 mole %, up to 9 mole %, or up to 7 mole % HFP monomers. In some embodiments, the copolymer comprises 5 to 23 mol %, more particularly 5 to 13 mole % HFP monomers. In some embodiments, the copolymer comprises 11 mole % HFP monomers.

In some embodiments, the copolymer comprises at least 35 mole %, at least 40 mole %, at least 45 mole %, at least 50 mole %, at least 55 mole %, at least 60 mole %, or at least 65 mole % VDF monomers. In some embodiments, the copolymer comprises up to 70 mole %, up to 65 mole %, up to 60 mole %, up to 55 mole %, up to 50 mole %, up to 45 mole %, or up to 40 mole % VDF monomers. In some embodiments, the copolymer comprises 35 to 70 mole %, more particularly 35 to 60 mole %, even more particularly 35 to 50 mole % VDF monomers. In some embodiments, the copolymer comprises 50 mole % VDF monomers.

The nonacidic end groups derive from the choice of chain transfer agent and initiator used to make the SFT copolymer. The chain transfer agent typically includes a dialkyl ether, such as dimethyl ether or methyl t-butyl ether. In preferred embodiments, the dialkyl ether is dimethyl ether. Although the dialkyl ethers may be expected to react similarly as chain transfer agents, in practice, some dialkyl ethers (e.g., diethyl ether) are less favorable due to handling difficulties. The initiator typically includes oxidizing manganese ions, such as those deriving from potassium permanganate, sodium permanganate, or Mn ³⁺ salts (like manganese triacetate, manganese oxalate, etc.). The preferred metal salt is KMiiOj.

Without being bound by theory, it is believed, for example, that the dimethyl ether reacts with the Mn ³⁺ salts (generated by KMnOj in the presence of TFE) as provided below.

The H ₃ C-O-CH2» radicals are then capable of initiating a polymerization chain reaction with the TFE monomer:

A similar chain reaction can be initiated with the HFP monomer:

Resulting from reactions [2] and [3], the H ₃ C-O-CH2» radical formed by [1] is finally incorporated into the polymer chain as H3C-O-CH2- end-group. These end-groups are discernible at two vibrations in a Fourier-transform infrared (FTIR) spectrum located at 2833 and 2858 wavenumbers (cm ).

Additionally, the H ₃ C-O-CH2» radical may undergo fragmentation to form formaldehyde (analytically detectable in the aqueous phase after the polymerization) and the methyl radical CH ₃ » by: The so-formed methyl radical CH ₃ * is also able to initiate polymerization with TFE and HFP monomers:

Further, dimethyl ether is also able to react in the reaction pattern of a classic chain transfer agent:

The so-formed H ₃ C-O-CH ₂ » radical is again able to reinitiate the polymerization by [2] and [3] or might undergo fragmentation to a H ₃ C* radical by [4] and reinitiate the polymerization by [5] and [6],

In the above reaction scheme, dimethyl ether is acting as both an initiator and chain transfer agent. Thus, the process replaces at least a portion of the highly acidic -COOH end groups with the more neutral end groups -CH ₃ , -CH ₂ -O-CH ₃ , -CF ₂ -H, or combinations thereof. In some embodiments, the copolymer exhibits an absorbance of at least 0.0048 at 2833 wavenumbers (cm’ ) and an absorbance of at least 0.0040 at 2858 wavenumbers (cm’ ¹ ), evidencing the presence of the non-acidic end groups .

It was observed that, unlike TFE and HFP, the H ₃ C-O-CH2» radical or the H ₃ C* radicals were not as effective at initiating polymerization with VDF. Therefore, it was found for purposes of this disclosure, that the SFT copolymers comprise no more than 70 mole % VDF.

The SFT copolymer can be made by combining the TFE monomers, HFP monomers, and VDF monomers in an aqueous emulsion comprising ammonium 4,8-dioxa-3-H- perfluorononanoate, and polymerizing the monomers in the presence of oxidizing manganese ions and dialkyl ether to create a polymer dispersion. In some embodiments, additional monomers may be present, as well. In preferred embodiments, a source of the oxidizing manganese ion is potassium permanganate and the dialkyl ether is dimethyl ether.

A reactor is charged with water and the emulsifier ammonium 4,8-dioxa-3-H- perfluorononanoate (available under the trade designation Dyneon ADONA Emulsifier from 3M Company, Saint Paul, Minnesota). The ammonium 4,8-dioxa-3-H-perfluorononanoate provides colloidal stability and higher solid content in the resultant polymer dispersions. The reactor may optionally be charged with auxiliaries, such as buffers.

The reactor is then charged with the chain transfer agent (e.g., dimethyl ether) and the TFE, HFP and VDF monomers. The amount of chain transfer agent used is generally selected to obtain the desired molecular weight and is typically between 0.1 and 10 parts per thousand. The parts per thousand is based on the total weight of monomers fed to the polymerization reaction. The chain transfer agent concentration, e.g. dialkyl ether, may also be varied throughout polymerization to influence the molecular weight distribution, i.e. to obtain a broad molecular weight distribution or to obtain a bimodal distribution.

The TFE, HFP and VDF monomers may be charged batchwise or in a continuous or semicontinuous manner. By semi-continuous is meant that a plurality of batches of the monomer are charged to the vessel during the course of the polymerization. The independent rate at which the monomers are added to the reactor will depend on the consumption rate of the particular monomer with time. Preferably, the rate of addition of monomer will equal the rate of consumption of monomer, i.e. conversion of monomer into polymer.

After an initial charge of dimethyl ether, TFE, HFP and VDF, the initiator (e.g., KMnOj) is added to the aqueous phase to initiate the polymerization. During the polymerization initiator is typically further added in one or more portions or continuously. The amount of initiator continuously added throughout the polymerization is typically between 0.001 and 0.3% by weight, preferably between 0.005 and 0.1% by weight based on the total amount of polymer dispersion produced. The monomers may be further added as well during the polymerization. The benefit of such an initiator is that only an oxidizing agent (e.g. KMnOj) is added to initiate the polymerization and to continue the polymerization. In certain cases a complexing agent (e.g. oxalic acid, or salts thereof) might be added to avoid precipitation of the active metal complexes, but this is not a necessity.

During the initiation of the polymerization reaction, the sealed reactor and its contents are pre-heated to the reaction temperature. Preferred polymerization temperatures are 10 to 100°C, preferably 30°C to 80°C and the pressure is typically between 2 and 30 bar, more particularly 5 to 20 bar. The reaction temperature may be varied to influence the molecular weight distribution, i.e. to obtain a broad molecular weight distribution or to obtain a bimodal distribution.

The amount of polymer solids that can be obtained at the end of the polymerization is typically at least 10 wt.% (in some embodiments at least 15 wt.%, 20 wt.%, 25 wt.%, 30 wt.%, or event at least 35 wt.%), and the average particle size of the resulting fluoropolymer is typically between 80 nm and 150 nm. In some preferred embodiments, the amount of polymer solids at the of polymerization is at least 35 wt.%. The polymer agglomerate can be isolated from the dispersion and further processed to form melt pellets.

As noted above, the mechanical and thermal stresses attributed to the processing of melt pellets can produce hydrofluoric acid (HF), which is caustic to the metal and metal alloy parts typically used to process the fluorothermoplasts into films and fibers. It has been found that posttreatment processing can reduce or eliminate the amount of HF remaining on the melt pellets. In one embodiment, warm air is blown through the pellets to reduce the HF associated therewith. In another embodiments, the melt pellets are washed with a K2HPO3 solution, subsequently washed with desalted water, and dried.

The SFT copolymers are generally melt processible, i.e. they will typically have a melt flow index between 20 to 5,000 g/10 minutes measured with a support weight of 5 kg and a temperature of 265°C, as set as set forth in the below Examples. In some embodiments, the SFT copolymers have a melt flow index of at least 20 g/10 minutes, or at least 24 g/10 minutes. Due to the melt processible characteristics, the SFT copolymers can be extrusion processed into films and fibers for a variety of applications, including multi-optical films, light transmitters in polymer optical fibers, cladding material, and passive cooling films.

Passive Cooling Articles

An exemplary passive cooling article comprising the SFT copolymer of the present disclosure is illustrated in FIG. 1. The passive cooling article 112 is disposed on a substrate 110 and may be used to reflect light in the solar region and to radiate light in atmospheric window region. Article 112 may include multiple components, which may cooperatively provide reflective and absorptive properties described herein to cool substrate 110. In some embodiments, article 112 is thermally coupled to substrate 110 to transfer heat therebetween. In some embodiments, substrate 110 is coupled to a fluid, liquid or gas, which can transfer heat away from another article (such as a building, battery, refrigerator, freezer, air conditioner, photovoltaic module).

The article 112 includes a reflector 204 that is attached either directly or indirectly to the substrate. In the embodiment illustrated in FIG. 1, the reflector is attached to the substrate by adhesive layer 209. The adhesive layer may comprise thermally conductive particles to aid in heat transfer. Thermally conductive particles include aluminum oxide and alumina nanoparticles. Additional thermally conductive particles for the adhesive layer include those available under the trade designation “3M BORON DINITRIDE” from 3M Company.

An optional outer layer 202 may partially or fully cover the reflector 204. In general, outer layer 202 may be positioned between reflector 204 and at least one source of solar energy (e.g., the sun). Outer layer 202 may be exposed to elements in an outdoor environment and may be formed of material particularly suited for such environments.

In some embodiments, such as that illustrated in FIG. 1, outer layer 202 may exhibit a high transmittance in the solar region to allow light to pass through to the reflector and exhibit a high absorbance in the atmospheric window region to radiate energy in wavelengths of the atmospheric window region away from the article. In some embodiments, outer layer 202 is thermally coupled to reflector 204 to transfer heat therebetween. Heat from substrate 110 that is transferred to reflector 204 may be further transferred to outer layer 202, which may be radiated as light in the atmospheric window region to cool substrate 110 at night and during the day. In some embodiments, outer layer 202 may have a minimum absorbance or average absorbance in the atmospheric window region of at least 0.15 (in some embodiments, at least 0.3, 0.45, 0.6, 0.8, 1 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or even at least 5).

The outer layer 202 typically comprises at least one polymer. In some embodiments, the outer layer comprises at least one of a polymethyl methacrylate, an acrylate copolymer, a polyurethane, or a fluoropolymer. In some embodiments, the outer layer comprises the SFT copolymer. The outer layer may also include inorganic particles or surface structures to further enhance passive radiation cooling.

Reflector 204 may partially or fully cover the substrate 110. In general, reflector 204 may be positioned between substrate 110 and outer layer 202 or at least one source of solar energy. In some preferred embodiments, reflector 204 is partially or fully covered by outer layer 202 to protect the reflector 204 from environmental elements.

In some embodiments, reflector 204 may have an average reflectance in the solar region of at least 85% (in some embodiments, at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or even at least 99.5%).

In some embodiments, reflector 204 may be thin to facilitate heat transfer from substrate 110 to the opposite surface of reflector 204 and, if present, outer layer 202. In general, a thinner reflector 204 provides better heat transfer. In some embodiments, overall thickness 228 of reflector 204 is at most 50 (in some embodiments, at most 40, 30, 25, 20, 15, or even at most 10) micrometers.

Reflector 204 may include one or more multi-layer optical films 206. In some embodiments, as illustrated in FIG. 1, the multi-layer optical film 206 includes a plurality of first optical layers 210 and a plurality of second optical layers 212. Optical layers 210, 212 in film 206 may alternate or be interleaved, and have different refractive indices. Each first optical layer 210 may be adjacent to second optical layer 212, or vice versa. Most of first optical layers 210 may be disposed between adjacent second optical layers 212, or vice versa (e.g., all layers except one).

The reflective band of film 206 may be defined by the number of optical layers, the thicknesses, and the refractive indices of optical layers 210, 212 as known by a person of ordinary skill in the art of making reflective multi-layer optical films.

In some embodiments, film 206 has up to 1000 (in some embodiments, up to 700, 600, 500, 400, 300, 250, 200, 150, or even up to 100) total optical layers 210, 212.

The thicknesses of optical layers 210, 212 in one film 206 may vary. Optical layers 210, 212 may define a maximum thickness 232. Some of optical layers 210, 212 may be thinner than the maximum thickness 232. The maximum thickness 232 of optical layers 210, 212 may be much smaller than a minimum thickness 230 of outer layer 202. In some embodiments, outer layer 202 may provide structural support for film 206, particularly when outer layer 202 is co-extruded with film 206. In some embodiments, minimum thickness 230 of outer layer 202 is at least 5 (in some embodiments, at least 10, or even at least 15) times greater than maximum thickness 232 of optical layers 210, 212.

The refractive indices of optical layers 210, 212 may be different. First optical layer 210 may be described as a low index layer and second optical layer 212 may be described as a high index layer, or vice versa. In some embodiments, a first refractive index (or average refractive index) of the low index layers is less than a second refractive index (or average refractive index) of the high index layers by at least 4% (in some embodiments, at least 5%, 10%, 12.5%, 15%, 20%, or even at least 25%). In some embodiments, the first refractive index of the low index layer may be at most 1.5 (in some embodiments, at most 1.45, 1.4, or even at most 1.35). In some embodiments, the second refractive index of the high index layer may be at least 1.4 (in some embodiments, at least 1.42, 1.44, 1.46, 1.48, 1.5, 1.6, or even at least 1.7).

In some embodiments, first optical layer 210 is formed of different material than second optical layer 212. One of first and second optical layers 210, 212 may include the SFT copolymer. The other of first and second optical layers 210, 212 may include a different fluoropolymer or include a non-fluorinated polymer. In some embodiments, the first optical layer includes a fluoropolymer (e.g., SFT copolymer) and the second optical layer includes a non-fluorinated polymer. Generally, the reflective power of the interface between the first optical layers and the second optical layers is proportional to the square of the difference in in-plane refractive indices of the first optical layer and the second optical layer, e.g., (m-m) ² where is the in-plane refractive index of the first optical layer and n ₂ is the in-plane refractive index of the second optical layer.

Exemplary non-SFT fluoropolymers include a polymer of tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and vinylidene fluoride (e.g., available under the trade designation “3M DYNEON THV” from 3M Company), a polymer of TFE, HFP, vinylidene fluoride, and perfluoropropyl vinyl ether (PPVE) (e.g., available under the trade designation “3M DYNEON THVP” from 3M Company), a polyvinylidene fluoride (PVDF) (e.g., “3M DYNEON PVDF 6008” available from 3M Company), an ethylene chlorotrifluoroethylene (ECTFE) polymer (e.g., available under the trade designation “HALAR 350LC ECTFE” from Solvay, Brussels, Belgium), an ethylene tetrafluoroethylene (ETFE) (e.g., available under the trade designation “3M DYNEON ETFE 6235” from 3M Company), a perfluoroalkoxy alkane (PF A) polymer, a fluorinated ethylene propylene (FEP) polymer, a polytetrafluoroethylene (PTFE), a polymer of TFE, HFP, and ethylene (e.g., available under the trade designation “3M DYNEON HTE1705” from 3M Company), or various combinations thereof. In general, various combinations of fluoropolymers can be used. In some embodiments, the fluoropolymer includes FEP. In some embodiments, fluoropolymer Additional examples of fluoropolymers include those available, for example, from 3M Company under the trade designations “3M DYNEON THV221GZ”, “3M DYNEON THV2030GZ”, “3M DYNEON THV610GZ”, and “3M DYNEON THV815GZ”. Examples of fluoropolymers also include PVDF available, for example, under the trade designations “3M DYNEON PVDF 6008” and “3M DYNEON PVDF 11010” from 3M Company; FEP available, for example, under the trade designation “3M DYNEON FLUOROPLASTIC FEP 6303Z” from 3M Company; and ECTFE available, for example, under the trade designation “HALAR 350LC ECTFE” from Solvay.

Examples of non-fluorinated polymers include at least one of a polyethylene terephthalate (PET), a co-polymer of ethyl acrylate and methyl methacrylate (CoPMMA), a polypropylene (PP), a polyethylene (PE), a polyethylene copolymer, a polymethyl methacrylate (PMMA), an acrylate copolymer, a polyurethane, or various combinations thereof. In general, various combinations of non-fluorinated polymers can be used.

Exemplary isotropic optical polymers, especially for use in the low refractive index optical layers, may include homopolymers of polymethylmethacrylate (PMMA), such as those available from Ineos Acrylics, Inc., Wilmington, DE, under the trade designations “CP71” and “CP80;” and polyethyl methacrylate (PEMA), which has a lower glass transition temperature than PMMA. Additional useful polymers include copolymers of PMMA (CoPMMA), such as a CoPMMA made from 75 wt.% methylmethacrylate (MMA) monomers and 25 wt.% ethyl acrylate (EA) monomers, (e.g., available under the trade designation “PERSPEX CP63” from Ineos Acrylics, Inc., or available under the trade designation “ATOGLAS 510” from Arkema, Philadelphia, PA), a CoPMMA formed with MMA comonomer units and n-butyl methacrylate (nBMA) comonomer units, or a blend of PMMA and poly(vinylidene fluoride) (PVDF). Additional exemplary optical polymers include acrylate triblock copolymers, where each endblock of at least one of the first block copolymer, the second block copolymer, or the at least one additional block copolymer is comprised of poly(methyl methacrylate), and further wherein each midblock of at least one of the first block copolymer or the second block copolymer is comprised of poly(butyl acrylate). In some exemplary embodiments, at least one of the first block copolymer, the second block copolymer, or the at least one additional block copolymer is comprised of from 30 wt.% to 80 wt.% endblocks, and from 20 wt.% to 70 wt.% midblocks, based on a total weight of the respective block copolymer. In certain particular exemplary embodiments, at least one of the first block copolymer, the second block copolymer, or the at least one additional block copolymer is comprised of from 50 wt.% to 70 wt.% endblocks, and from 30 wt.% to 50 wt.% midblocks, based on the total weight of the respective block copolymer. In any of the foregoing exemplary embodiments, the first block copolymer may be selected to be the same as the second block copolymer. Triblock acrylate copolymers are available, for example, under the trade designation KURARITY LA4285 from Kuraray America, Inc., Houston, TX.

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) (PE-PO) (e.g., available under the trade designation “ENGAGE 8200” from Dow Elastomers, Midland, MI), poly (propylene-co-ethylene) (PPPE) (e.g., available under the trade designation “Z9470” from Atofina Petrochemicals, Inc., Houston, TX), and a copolymer of atactic polypropylene (aPP) and isotactic polypropylene (iPP). The multilayer optical films can also include, for example, a functionalized polyolefin, such as linear low-density polyethylene-graft- maleic anhydride (LLDPE-g-MA) (e.g., available under the trade designation “BYNEL 4105” from E.I. du Pont de Nemours & Co., Inc., Wilmington, DE).

Non-limiting examples of polyethylene copolymers include copolymers, for example, available under the trade designation “ENGAGE” from Atofina Petrochemicals, Inc. Houston, TX, copolymers available under the trade designation “TPX” from Mitsui Chemicals, Osaka, Japan, or cyclic olefin copolymers available under the trade designation “COC” from Zeon Chemicals, Louisville, KY. In general, various combinations of polyethylene copolymers can be used.

Exemplary polymers useful for forming the high refractive index optical layers include polyethylene terephthalate (PET), available from 3M Company, and also available from Nan Ya Plastics Corporation, Wharton, TX. Copolymers of PET including PETG and PCTG under the trade designation “SPECTAR 14471” and “EASTAR GN071” from Eastman Chemical Company, Kingsport, TN, are also useful high refractive index layers. The molecular orientation of PET and CoPET may be increased by stretching which increases its in-plane refractive indices providing even more reflectivity in the multilayer optical film.

Material may be selected based on absorbance or transmittance properties, as well as on refractive index. In general, the greater the refractive index between two materials in film 206, the thinner the film can be, which may be desirable for efficient heat transfer.

Additives can also be added to the polymer layers to impart desired properties. For example, UV stabilization with UV-absorbers (UVAs) and Hindered Amine Light Stabilizers (HALs) can intervene in the prevention of photo-oxidation degradation of PETs, PMMAs, and CoPMMAs. UVAs for incorporation into PET, PMMA, or CoPMMA optical layers include benzophones, benzotriazoles, and benzotriazines. Exemplary UVAs for incorporation into PET, PMMA, or CoPMMA optical layers include those available under the trade designations “TINUVIN 1577” and “TINUVIN 1600,” from BASF Corporation, Florham Park, NJ. Typically, UVAs are incorporated in the polymer at a concentration of 1-10 wt.%. Exemplary HALs for incorporation into PET, PMMA, or CoPMMA optical layers include those available under the trade designations “CHIMMASORB 944” and “TINUVIN 123,” from BASF Corporation. Typically, the HALs are incorporated into the polymer at are 0.1-1.0 wt%. A 10: 1 ratio of UVA to HALs can be optimum.

UVAs and HALs can also be incorporated into fluoropolymers. U.S. Pat. No. 9,670,300 (Olson et al.) and U.S. Pat. App. Pub. No. 2017/0198129 (Olson et al.) describe exemplary UVA oligomers that are compatible with PVDF fluoropolymers.

Other UV blocking additives may be included in the fluoropolymer. Small particle non- pigmentary zinc oxide and titanium oxide can also be used as UV blocking additives in the fluoropolymer surface layer. Nanoscale particles of zinc oxide and titanium oxide will reflect, or scatter, UV light while being transparent to visible and near infrared light. These small zinc oxide and titanium oxide particles that can reflect UV light are available, for example, in the size range of 10-100 nanometers from Kobo Products, Inc., South Plainfield, NJ.

Resistance to photo-oxidation can be measured by changes in reflectivity or changes in color. In some embodiments, passive radiation cooling films described herein may not have a change in reflectivity of greater than 5% over at least 5 years. In some embodiments, passive radiation cooling films described herein may not have a change in color, described as b* per ASTM G-155-13 (2013), of greater than 5 after exposure to 18,700 kJ/m ² at 340 nanometers. One mechanism for detecting the change in physical characteristics is the use of the weathering cycle described in ASTM G155-05a (October 2005) using a D65 light source in the reflected mode. Under the noted test, the article should withstand an exposure of at least 18,700 kJ/m ² at 340 nanometers without change in reflectivity, color, onset of cracking, or surface pitting.

Anti-stat additives may also be useful to reduce unwanted attraction of dust, dirt, and debris. Ionic salt anti-stats available from 3M Company may be incorporated into PVDF fluoropolymer layers to provide static dissipation. Anti-stat additives for PMMA and CoPMMA (e.g., available under the trade designation “STAT-RITE” from Lubrizol Engineered Polymers, Brecksville, OH, or under the trade designation “PELESTAT” from Sanyo Chemical Industries, Tokyo, Japan).

It has been found that the SFT copolymers provides good interlayer adhesion with adjacent optical layers in the multi-layer optical films of the present disclosure. In some embodiments, the interlayer adhesion between the SFT copolymer and an adjacent fluorinated or non-fluorinated optical layer is at least 50 grams per inch, more preferably at least 75 grams/inch.

The multilayer optical fdms described herein can be made using general processing techniques, such as those described in U.S. Pat. No. 6,783,349 (Neavin et al.).

Desirable techniques for providing a multilayer optical film with a controlled spectrum may include, for example: (1) the use of an axial rod heater control of the layer thickness values of coextruded polymer layers as described, for example, in U.S. Pat. No. 6,783,349 (Neavin et al.); (2) timely layer thickness profile feedback during production from a layer thickness measurement tool such as, for example, an atomic force microscope (AFM), a transmission electron microscope, or a scanning electron microscope; (3) optical modeling to generate the desired layer thickness profile; and (4) repeating axial rod adjustments based on the difference between the measured layer profile and the desired layer profile.

In some embodiments, the basic process for layer thickness profile control may involve adjustment of axial rod zone power settings based on the difference of the target layer thickness profile and the measured layer profile. The axial rod power increase needed to adjust the layer thickness values in a given feedblock zone may first be calibrated in terms of watts of heat input per nanometer of resulting thickness change of the layers generated in that heater zone. For example, fine control of the spectrum is possible using 24 axial rod zones for 275 layers. Once calibrated, the necessary power adjustments can be calculated once given a target profile and a measured profile. The procedure is repeated until the two profiles converge.

In some embodiments, the optical layer thickness profile of multilayer optical film reflecting greater than 85% of visible light over a wavelength range of 400 nm to 800 nm can be adjusted to be approximately a linear profile with the thinnest optical layers to being a ! wave optical thickness (refractive index times physical thickness) for reflecting 400 nm light and progressing in thickness to the thickest optical layers which would be adjusted to be a ! wave optical thickness of 800 nm light.

Including the outer layer may also facilitate ease of manufacturing, particularly during coextrusion with the reflector.

Although FIG. 1 illustrates a single multi-layer optical film 206, it should be understood that the article 112 can be made up of two or more multi-layer optical films, each comprising one or more low index layers and one or more high index layers, as described above. The individual multi-layer optical films may have complementary reflectance spectra that together may provide the article with a broader reflective band than would be possible with a single multi-layer film alone.

For example, in one embodiment, the reflector may comprise a visible reflector having an average reflectance of at least 85% (in some embodiments, at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or even at least 99.5%) in a wavelength range from 0.4 to 0.8 micrometers, and a UV reflector at least partially covering the visible reflector having an average reflectance of at least 85% (in some embodiments, at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or even at least 99.5%) in a wavelength range from 0.3 to less than 0.4 micrometers, wherein at least one of the visible reflector, UV reflector, or combination thereof comprises the SFT copolymer. In some embodiments, the UV reflector has an average transmittance of at least 50% (in some embodiments, at least 60%, 70%, 80%, 90%, or at least 95%) in the wavelength range from 0.4 to 0.8 micrometers. The visible reflector can be a multi-layer optical film comprising a plurality of first optical layers and a plurality of second optical layers. The first optical layer is either a low index layer or a high index layer; the second optical layer is the other of the low index layer or high index layer. The layers can be made from the polymers list above. In some embodiments, the first optical layer comprises the SFT copolymer. In some preferred embodiments, the second optical layer comprises polyethylene terephthalate (PET). In some embodiments, a first optical layer and adjacent second optical layer have an interlayer adhesion of at least 50 grams/inch, more preferably at least 75 grams/inch.

The UV reflector can be a multi-layer optical film comprising a plurality of third optical layers and a plurality of fourth optical layers. The third optical layer is either a low index layer or a high index layer; the fourth optical layer is the other of the low index layer or high index layer. The layers can be made from the polymers list above. In some embodiments, the third optical layer comprises the SFT copolymer. In some preferred embodiments, the second optical layer comprises copolymers of polymethylmethacrylate (CoPMMA). In some embodiments, a third optical layer and adjacent fourth optical layer have an interlayer adhesion of at least 50 grams/inch, more preferably at least 75 grams/inch.

With reference to FIG. 1, the reflector 204 may optionally include an infrared (IR) reflective metallic layer having an average reflectance of at least 85% (in some embodiments, at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or even at least 99.5%) in a wavelength range from 0.8 to 14 micrometers.

Metallic layer 208 may be disposed on substrate layer 110 or on the bottom of film 206. In some embodiments, metallic layer 208 is coated onto substrate layer 110 or under the film 206. Metallic layer 208 may be disposed between substrate layer 110 and film 206. Metallic layer 208 may reflect light for at least part of the reflective band. In some embodiments, metallic layer 208 reflects light over the wavelength range of 0.5 to 3 micrometers (in some embodiments, 0.6 to 3 micrometers, or even 0.7 to 3 micrometers).

In some embodiments, optical film 206 or metallic layer 208 alone may not provide sufficient reflectance across a desired wavelength range. However, by using a metallic layer 208 and a film 206 having complementary reflectance spectra, it is possible to provide reflectance across a broader wavelength range, thus increasing the reflective band of the passive cooling article. For example, film 206 may be highly reflective in one range of the reflective band and metallic layer 208 may be highly reflective in another range of the reflective band where the film is not highly reflective. In some embodiments, a high reflectance may be described as an average reflectance of at least 90%.

In some embodiments, film 206 is highly reflective in a lower wavelength range and metallic layer 208 is highly reflective in a higher wavelength range, which is adjacent to the lower wavelength range. In one example, film 206 is highly reflective in a range from 0.35 to 0.8 micrometer, and metallic layer 208 is highly reflective in a complementary range from 0.8 to 2.5 micrometers. In other words, the highly reflective range of metallic layer 208 begins near where highly reflective range of film 206 ends. Together, film 206 and the metallic layer 208 may provide high reflectivity in a range from 0.35 to 2.5 micrometers.

In some embodiments, metallic layer 208 may have an average reflectance of at least 90% in a wavelength range of at least 0.8 to 2.5 micrometers (in some embodiments, at least 0.7 to 3.0 micrometers).

Non-limiting examples of metals used in the metallic layer 208 include silver (Ag), copper (Cu), aluminum (Al), gold (Au), Inconel, stainless steel, or various combinations thereof. In some embodiments, metallic layer 208 comprises a layer of silver and a layer of copper to protect the silver from corrosion, where the silver layer is between the reflector and the copper layer.

In one aspect, the article of the present disclosure includes a UV reflector comprising a multi-layer optical film reflecting a wavelength range from 350 to 450 nanometers made with 150 high refractive index layers comprising a CoPMMA (e.g., available under the trade designation “PERSPEX CP63” from Lucite International, Cordova, TN) alternating with 150 low refractive index layers comprising the SFT copolymer, and a visible light reflector comprising a multi-layer optical film reflecting a wavelength range from 450 to 750 nanometers made with 150 high refractive index layers comprising a PET (e.g., available under the trade designation “EASTAPAK 7452” from Eastman Chemical Company, Kingsport, TN), alternating with 150 low refractive index layers comprising the SFT copolymer. The surface of the visible light reflective multi-layer optical film opposite the UV reflector is coated with a 100-nanometer thick layer of silver and a 20-nanometer thick layer of copper. The surface of the UV light reflective multilayer optical film opposite the visible light reflector is a layer having fluoropolymer (e.g., available under the trade designation “3M DYNEON THV815” from 3M Company).

In a preferred embodiment, an article is made by coextruding two multi-layer optical films simultaneously. The first multi-layer optical film is made by coextruding 163 alternating layers of PET (Eastman 7352) high refractive index layers with 162 alternating layers of SFT copolymer low refractive index layers to form a 325 layer multi-layer optical film having a thickness of 796 microns. The second multi-layer optical film is made by coextruding 163 alternating layers of CoPMMA (PEERSPEX CP63) high refractive index layers with 162 alternating layers of SFT copolymer to form a 325 layer multi-layer optical film having a thickness of 373 microns. In addition, 2 skin layers comprising PET are coextruded on the top and bottom sides of the optical fdm stack that are approximately 100 microns each creating a total cast film thickness of 1396 microns. The cast film is then biaxially oriented by first preheating to 100°C for 30 seconds and stretching to a draw ratio of 3.5:1 x 3.5:1 to create a 114 microns thick film. After the biaxial orientation step, the film is also annealed at 125 C for 10 seconds. The resulting article reflects 98% of UV and visible light from 350 nm to 850 nm when measured with a Lambda 1050 spectrophotometer.

The passive cooling articles of the present disclosure may be particularly effective in facilitating cooling during the day when subjected to sunlight by reflecting sunlight that would otherwise be absorbed by the object. Absorption in the atmospheric window region may be particularly effective in facilitating cooling at night by radiating or emitting infrared light. Energy may also be radiated or emitted during the day to some degree.

FIGS. 2 and 3 show examples of environments 100, 102 in which passive cooling article 112 may be used to cool substrate 110 (for example, a panel coupled to a building or a heat transfer system). Article 112 may be disposed or applied to an exterior surface of substrate 110, particularly an exterior surface exposed to sunlight (e.g., an upper surface). In particular, article 112 may be thermally coupled to substrate 110, which may allow for heat transfer therebetween. Article 112 may be suitable for outdoor environments and have, for example, a suitable operating temperature range, water resistance, and ultraviolet (UV) stability.

Article 112 may be used to cover part or all of substrate 110. Article 112 may be generally planar in shape. Article 112, however, does not need to be planar and may be flexible to conform to substrate 110.

Article 112 may reflect light 104 in the solar region of the electromagnetic spectrum to cool substrate 110, which may be particularly effective in daytime environment 100. Without article 112, light 104 may have otherwise been absorbed by the substrate 110 and converted into heat. Reflected sunlight 104 may be directed into atmosphere 108.

Article 112 may radiate light 106 in the atmospheric window region of the electromagnetic spectrum into atmosphere 108 to cool substrate 110, which may be particularly effective in the nighttime environment 102. Article 112 may allow heat to be converted into light 106 (e.g., infrared light) capable of escaping atmosphere 108 through the atmospheric window. The radiation of light 106 may be a property of article 112 that does not require additional energy and may be described as passive radiation, which may cool article 112 and substrate 110 thermally coupled to article 112. During the day, the reflective properties allow the article 112 to emit more energy than is absorbed. The radiative properties in combination with the reflective properties may provide more cooling than an article that only radiates energy through the atmosphere and into space.

Among other parameters, the amount of cooling and temperature reduction may depend on the reflective and absorptive properties of article 112. The cooling effect of article 112 may be described with reference to a first temperature of the ambient air proximate or adjacent to the substrate and a second temperature of the portion of substrate 110 proximate or adjacent to article 112. In some embodiments, the first temperature is greater than the second temperature by at least 2.7 (in some embodiments, at least 5.5, 8.3, or even at least 11.1) degrees Celsius (e.g., at least 5, 10, 15, or even at least 20 degrees Fahrenheit).

In some embodiments, the passive cooling article may be capable of cooling up to 11.1 °C (e.g., 20°F). In some embodiments, the article may form part of a cooling panel that may be disposed on the exterior of at least part of a building or a heat transfer system. The heat transfer system can cool a fluid, liquid or gas, which can then be used to remove heat from a building or vehicle, including an electric vehicle battery.

EXAMPLES

These examples are merely for illustrative purposes only and are not meant to be limiting on the scope of the appended claims. Unless otherwise noted or readily apparent from the context, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.

The following abbreviations are used in this section: mm=millimeters; g=grams; kg=kilograms; °C=degrees Celsius; mL=milliliter; L=liter; min=minutes; hrs=hours; wt%=weight percent; lbs=pounds; rpm=revolutions per minute; and mol=moles. Abbreviations for materials used in this section, as well as descriptions of the materials, are provided in Table 1.

Table 1. Materials Used in the Examples

Test Methods

Melt flow Index

The melt flow index (MFI), reported in g/10 min, was measured according to DIN 53735, ISO 12086 or ASTM D- 1238 at a support weight of 5.0 kg. The MFI was obtained with a standardized extrusion die of 2.1 mm diameter and a length of 8.0 mm. Unless otherwise noted, a temperature of 265°C was applied.

Melting Peaks

Melting peaks of the semi-fluorinated resins were determined according to ASTM 4591 by means of Perkin-Elmer DSC 7.0 under nitrogen flow and a heating rate of 10°C/min. The indicated melting points relate to the melting peak maximum.

Latex Particle Size Determination

The latex particle size determination was conducted by means of dynamic light scattering with a Malvern Zetasizer 1000 HSA in accordance with ISO 13321 (21 CFR Part 11). The reported average particle size is equivalent to the z-average of the particle size distribution. Prior to the measurements, the polymer latexes as yielded from the polymerizations were diluted with 0.001 mol/L KCl-solution, the measurement temperature was 20°C in all cases.

Polymer End Group Determination

Polymer end group detection was conducted in analogy to the method described in U.S. Pat. No. 4,743,658 (Imbalzano et al.). Thin polymer films with a thickness of 0.38 mm were measured on the same Nicolet Model 510 Fourier-transform infrared (FTIR) spectrometer, wherein 32 scans were collected. All other operational settings used were those provided as default settings in the Nicolet control software. Similarly, a film of a reference material known to have none of the end groups to be analyzed was molded and scanned. The reference absorbance spectrum was subtracted from the sample absorbance, using the interactive subtraction mode of the software. The CF ₂ overtone band at 2365 wavenumbers was used to compensate for slight thickness differences between sample and reference during the interactive subtraction of the software. The so-obtained subtraction FTIR spectrum represents the absorbances due to non-fluorinated polymer end groups. The >peak absorbance< E of the two analytical vibrations located at 2833 and 2858 wavenumbers (reported in physical units of 1/cm) were extracted from the baseline of the subtraction spectrum using the deconvolution software >PeakFit< from AINS Software Inc. (version 4.06); these extracted values are reported.

Interlayer Adhesion Test Method

The Interlayer Adhesion Test method was determined using ASTM D-1876 as a guide. More specifically, the test method used to measure interlayer adhesion was as follows. The multilayer film to be tested was cut into 25 cm long by 2.5 cm wide pieces. Each piece was laminated to the center of a 25 cm long by 7.5 cm wide glass plate using 2.5 cm wide double stick tape (obtained from 3M Company, St. Paul, MN, under the trade designation 3M Scotch 665 Double-Sided Tape). One end of the taped film assembly was cut back 1 cm from one end with a razor blade. To each laminate a 2.5 cm wide single-sided tape (obtained from 3M Company under the trade designation 3M Super Bond Film Tape 396) was applied. Then the single-sided tape was snapped back over the scored film to initiate delamination of multilayer film and create an attachment tab. The film-glass plate assembly was installed into the plate holder on a slip/peel tester (obtained from IMASS Inc., Accord, MA, under the trade designation “MODEL SP-2000”). The slip/peel tester speed was set at 150 cm/min. The film/tape attachment tab was attached to the transducer clamp of the slip/peel tester. The average force to delaminate the film over a 24 cm length was recorded. The reported interlayer adhesion value was the average of five (5) samples.

Example 1

A semi-crystalline copolymer of the molar composition poly[TFE ₃ 73-co-HFPn 8-co- VDF50.9] was produced. Herein, a 990 L stainless steel reactor equipped with an anchor-type agitation system was employed for the polymerization process. The oxygen free reactor was charged with 595 L deionized water, 274 g oxalic acid dihydrate, 250 g of a 25 wt.% aqueous ammonia solution, and 5800 g of ADONA Emulsifier. The reactor was then heated to 60°C and the agitation system was set to 100 rpm. The reactor was charged with 469 g of dimethyl ether, hexafluoro propylene (HFP) to a pressure of 8.1 bar absolute, vinylidene fluoride (VDF) to 13.9 bar absolute, and tetrafluoro ethylene (TFE) to 17.0 bar absolute reaction pressure. The polymerization was initiated by the addition of 100 ml 2.62 wt.% aqueous potassium permanganate (KM11O4) solution and a continuous feed of KMnOj-sohition was maintained with a feed rate of 4000 ml/hrs. After the reaction was started, the reaction temperature of 60°C and the reaction pressure of 17.0 bar absolute were maintained by feeding TFE, VDF, and HFP into the gas phase with a HFP/TFE (kg/kg) feeding ratio of 0.474 and a VDF/TFE (kg/kg) feeding ratio of 0.872. When the total feed of 168.7 kg TFE was accomplished after 329 min reaction time, the monomer feed was interrupted by closing the monomer valves and the residual monomers were reacted down to 11.0 bar within 10 minutes. The reactor was then vented and flushed with nitrogen in three cycles. The resultant polymer dispersion had a solid content of 40.0% and an average latex particle diameter of 116 nm (as determined by dynamic light scattering). Wet coagulum (1200 g) was removed from the dispersion by filtration. The agglomerated polymer was worked up as described for Comparative Example 4 in WO 02/088207. The polymer agglomerate was compacted and subsequently melt-pelletized. The melt pellets were placed into a steel tank equipped with a thermal control of the jacket wall, and they were purged with hot air for 4 hours at 65°C. The physical properties of the melt pellets are summarized in Table 2. melting point maximum 115 C

The thus-obtained polymer melt pellets showed a 1.7 factor reduction in acidic end groups (e.g. -COOH) when compared to a reference polymer of the same molar composition poly [TFE37.3- co-HFPn 8-CO-VDF509] and the same melt viscosity, where the reference polymer was made by the same process except that dimethyl ether was replaced with ethane.

Example 2 and 3

Two other semi-crystalline copolymers of the molar composition poly[TFE373-co-HFPn 8- CO-VDF509] were produced according to Example 1, except that the reactor was charged with different amounts of dimethyl ether (provided in Table 4) prior to the start of polymerization. After polymerization, the resultant polymer dispersions also showed a solid content of 40.0%. The agglomerated polymer was worked up in accordance with the procedure for Example 1. The agglomerates were blended, compacted and subsequently melt-pelletized. The melt pellets were placed into a steel tank equipped with a thermal control of the jacket wall and purged with hot air for 4 hours at 65°C. The physical properties of the melt pellets are summarized in Table 3.

Table 3

Examples 4 to 9 and Comparative Examples 1 & 2

A variety of semi-crystalline copolymers of the molar composition poly[TFE ₃ 73-co- HFPn 8-CO-VDF509] covering a bandwidth of MFIs ranging from 2.3 to about 4000 g/10’ were produced in a 48 L stainless steel reactor equipped with an anchor-type agitation system. The oxygen free reactor was charged with 30 L deionized water, 2.1 g oxalic acid dihydrate, 13.2 g of ammonium oxalate hydrate, and 293 g of ADONA Emulsifier. The reactor was then heated to 60°C, and the agitation system was set to 240 rpm. The reactor was charged with dimethyl ether (see the amounts given in Table 4), hexafluoro propylene (HFP) to a pressure of 8.1 bar absolute, vinylidene fluoride (VDF) to 13.9 bar absolute, and tetrafluoro ethylene (TFE) to 17.0 bar absolute reaction pressure. The polymerization was initiated by the addition of 150 ml 1.0% aqueous potassium permanganate solution and a continuous feed of KMnOj-sohition was maintained with a feed rate of 100 ml/hrs. After the reaction was started, the reaction temperature of 60°C and the reaction pressure of 17.0 bar absolute was maintained by feeding TFE, VDF, and HFP into the gas phase with a HFP/TFE (kg/kg) feeding ratio of 0.474 and a VDF/TFE (kg/kg) feeding ratio of 0.872. When the total feed of 7200 g TFE was accomplished, the monomer feed was interrupted by closing the monomer valves and the residual monomers were reacted down to 11.0 bar within 10 minutes. The reactor was then vented and flushed with nitrogen in three cycles. The resultant polymer dispersion, typically having a solid content of 35.5% and an average latex particle diameter of typically 85 nm (as determined by dynamic light scattering), was removed from the bottom of the reactor and passed through a glass column containing DOWEX 650C cation exchange resin. The dispersion (8.8 L) was transferred into a 20 L agglomeration vessel. To the polymer dispersion was added 5 L deionized water, 0.12 L concentrated hydrochloric acid, and 1.8 L PF 5070. The reactor was agitated vigorously until the agglomerated solid had fully separated from the aqueous phase. The agglomerate was washed three times with deionized water, the PF 5070 was distilled off, and the polymer was dried in an oven at 80°C for 12 hours. The physical properties of the polymer agglomerates are summarized in Table 4.

Table 4

* The end group vibrations are very weak and tend to vanish from the subtraction FTIR spectra.

Comparative Examples 3

A semi-crystalline copolymer of the molar composition poly[TFE292-co-VDF ₇ 08] was produced in a 48 L stainless steel reactor equipped with an anchor-type agitation system. The oxygen free reactor was charged with 29 L deionized water, 10.1 g oxalic acid dihydrate, 50.3 g of ammonium oxalate hydrate, and 295 g of ADONA Emulsifier. The reactor was then heated to 60°C, and the agitation system was set to 240 rpm. The reactor was charged with 38.5 g dimethyl ether, vinylidene fluoride (VDF) to 10.5 bar absolute, and tetrafluoro ethylene (TFE) to 15.6 bar absolute reaction pressure. The polymerization was initiated by the addition of 50 ml 0.66% aqueous potassium permanganate solution and a continuous feed of KMiiOj-sohition was maintained with a feed rate of 114 ml/hrs. After the reaction was started, the reaction temperature of 60°C and the reaction pressure of 15.6 bar absolute were maintained by feeding TFE and VDF into the gas phase with a VDF/TFE (kg/kg) feeding ratio of 1.242. When the total feed of 4740 g TFE was accomplished within 214 minutes, the monomer feed was interrupted by closing the monomer valves and the residual monomers were reacted down to 11.0 bar within 10 minutes. The reactor was then vented and flushed with nitrogen in three cycles. The resultant polymer dispersion having a solid content of 17.4% was removed from the bottom of the reactor and worked up in the same fashion as described in Example 4. The physical properties of the polymer agglomerate is summarized in Table 5.

Table 5

*The end group vibration is very weak and has the trend to vanish from the subtraction FTIR spectmm.

Example 10

A semi-crystalline copolymer was made under the same reaction conditions as Example 2 and the polymer agglomerate had the same physical characteristics. Deviating from the work-up procedure of Example 2, the melt pellets were not purged with hot air right after melt-pelletization, but they were placed back into the into the stainless steel reactor that was used for the polymerization. Then, 400 L demineralized water and 1000 g of di-potassium hydrogen phosphate (K2HPO4) were added, the temperature was set to 70°C, and mild agitation conditions were applied at 20 rpm for 8 hours. The reactor was cooled down to 25°C and the content was emptied on a sieve screen. The melt pellets were washed three times with demineralized water and finally dried for 4 hours by purging with hot air (65°C).

Example 11

The polymer melt pellets from Example 2 were dusted with 0.1wt.% calcium carbonate. After dusting with calcium carbonate, the polymer melt pellets were extruded through a large polymer melt feed block and extrusion die at 80 Ibs/hrs. Polymer melt residence time was measured to be 35 minutes by adding a colorant to the feed throat of the extruder and measuring the time for peak color to extrude out the die. After 24 hrs of continuous polymer extrusion, the polymer melt feed block was disassembled and examined for steel corrosion caused by hydrogen fluoride. No black spots were found indicating that no corrosion had occurred to the stainless steel.

Example 12

A multilayer optical cast film was made by coextruding two multi-layer optical films simultaneously. The first multi-layer optical film was made by coextruding 163 alternating layers of PET (Eastman 7352) high refractive index layers with 162 alternating low refractive index layers of the SFT copolymer in Example 1 to form a 325 layer multi-layer optical film having a thickness of 796 microns. The second multi-layer optical film was made by coextruding 163 alternating layers of CoPMMA (Optix 63) high refractive index layers with 162 alternating layers of the SFT copolymer in Example 1 to form a 325 layer multi-layer optical film having a thickness of 373 microns. In addition, 2 skin layers comprising PET were coextruded on the top and bottom sides of the optical film stack that were approximately 100 microns each creating a total cast film thickness of 1396 microns. The cast film was then biaxially oriented by first preheating to 100°C for 30 seconds and then stretching to a draw ratio of 3.5:1 x 3.5:1 to create a 114 microns thick fdm. After the biaxial orientation step the film was also annealed at 125°C for 10 seconds. The resulting multilayer optical film reflected 98% of UV and visible light from 350 nm to 850 nm as measured with a Lambda 1050 spectrophotometer. Interlayer adhesion of the multilayer optical cast film was measured to be 86.9 grams per inch using modified ASTM D-1876 as described above.

Example 13

The multi-layer optical cast film in Example 12 was then vapor coated with 100 nm of Ag to reflect near infrared light from 850 nm to 2500 nm. An additional 20 nm layer of Cu was vapor coated onto the 100 nm Ag layer to protect it from corrosion. The resulting film was measured to reflect 97% of light between 350 nm and 2500 nm using a Lambda 1050 spectrophotometer.

Comparative Example 1

A multilayer optical cast film was made by coextruding 76 alternating layers of PET (Eastman 7352) high refractive index layers with 75 alternating layers of fluoropolymer (3M THV221) low refractive index layers to form a 151 layer optical layer stack having a thickness of 250 microns. In addition, 2 skin layers comprising PET were coextruded on the top and bottom sides of the optical stack that were approximately 100 microns each creating a total cast film thickness of 450 microns. The cast film was then biaxially oriented by first preheating to 100°C for 30 seconds and then stretching to a draw ratio of 2.75:1 x 2.75:1 to create a 60 microns thick film. After the biaxial orientation step the film was also annealed at 225°C for 15 seconds. The resulting multilayer optical film reflected 80% of near infrared light from 750 nm to 850 nm as measured with a Lambda 1050 spectrophotometer. Interlayer adhesion of the multilayer optical cast film was measured to be 158.9 grams per inch using modified ASTM D-1876 as described above.

Comparative Example 2

A multilayer optical cast film was made by coextruding 76 alternating layers of PET (Eastman 7352) high refractive index layers with 75 alternating layers of fluoropolymer (3M THV2030) low refractive index layers to form a 151 layer optical layer stack having a thickness of 250 microns. In addition, 2 skin layers compnsing PET were coextruded on the top and bottom sides of the optical stack that were approximately 100 microns each creating a total cast film thickness of 450 microns. The cast film was then biaxially oriented by first preheating to 100°C for 30 seconds and then stretching to a draw ratio of 2.75:1 x 2.75:1 to create a 60 microns thick film. After the biaxial orientation step the film was also annealed at 225°C for 15 seconds. The resulting multilayer optical film reflected 80% of near infrared light from 750 nm to 850 nm as measured with a Lambda 1050 spectrophotometer. Interlayer adhesion of the multilayer optical cast film was measured to be 63 grams per inch using modified ASTM D-1876 as described above.

Comparative Example 3

A multilayer optical cast film was made by coextruding 76 alternating layers of PET (Eastman 7352) high refractive index layers with 75 alternating layers of fluoropolymer (3M THV415) low refractive index layers to form a 151 layer optical layer stack having a thickness of 250 microns. In addition, 2 skin layers comprising PET were coextruded on the top and bottom sides of the optical stack that were approximately 100 microns each creating a total cast film thickness of 450 microns. The cast film was then biaxially oriented by first preheating to 100°C for 30 seconds and then stretching to a draw ratio of 2.75:1 x 2.75:1 to create a 60 microns thick film. After the biaxial orientation step the film was also annealed at 225°C for 15 seconds. The resulting multilayer optical film reflected 80% of near infrared light from 750 nm to 850 nm as measured with a Lambda 1050 spectrophotometer. Interlayer adhesion of the multilayer optical cast film was measured to be 51 grams per inch using modified ASTM D-1876 as described above.

Comparative Example 4

A multilayer optical cast film was made by coextruding 76 alternating layers of PET (Eastman 7352) high refractive index layers with 75 alternating layers of fluoropolymer (3M THV500) low refractive index layers to form a 151 layer optical layer stack having a thickness of 250 microns. In addition, 2 skin layers comprising PET were coextruded on the top and bottom sides of the optical stack that were approximately 100 microns each creating a total cast film thickness of 450 microns. The cast film was then biaxially oriented by first preheating to 100°C for 30 seconds and then stretching to a draw ratio of 2.75:1 x 2.75:1 to create a 60 microns thick film. After the biaxial orientation step the fdm was also annealed at 225°C for 15 seconds. The resulting multilayer optical film reflected 80% of near infrared light from 750 nm to 850 nm as measured with a Lambda 1050 spectrophotometer. Interlayer adhesion of the multilayer optical cast film was measured to be 43 grams per inch using modified ASTM D-1876 as described above.