MODIFICATION OF UV ABSORPTION PROFILE OF POLYMER FILM REFLECTORS TO INCREASE SOLAR-WEIGHTED REFLECTANCE

Abstract

Provided are reflective thin film constructions including a reduced number of layers, which provides for increased solar-weighted hemispherical reflectance and durability. Reflective films include those comprising an ultraviolet absorbing abrasion resistant coating over a metal layer. Also provided are ultraviolet absorbing abrasion resistant coatings and methods for optimizing the ultraviolet absorption of an abrasion resistant coating. Reflective films disclosed herein are useful for solar reflecting, solar collecting, and solar concentrating applications, such as for the generation of electrical power.

Claims

1. A multilayer reflective film comprising: a metal layer; a polymeric layer; and an abrasion resistant layer above the polymeric layer; wherein the abrasion resistant layer has a cut-off wavelength less than 385 nm, the cut-off wavelength being an ultraviolet wavelength of the terrestrial solar spectrum at which a transmittance value of the abrasion resistant layer is equal to fifty percent of a maximum transmittance value of the abrasion resistant layer in the visible region of the terrestrial solar spectrum.

2-59. (canceled)

60. The multilayer reflective film of claim 1, further comprising an adhesive layer, wherein: the metal layer is above an adhesive layer; and the polymeric layer is above the metal layer.

61. The multilayer reflective film of claim 1, wherein: the metal layer is above the polymeric layer; and the abrasion resistant layer is above the metal layer.

62. The multilayer reflective film of claim 61, further comprising an adhesive layer beneath the polymeric layer.

63. The multilayer reflective film of claim 1, further comprising a backside metal protective layer below the metal layer.

64. The multilayer reflective film of claim 1, further comprising an adhesion-promoting layer below the abrasion resistant layer.

65. The multilayer reflective film of claim 1, wherein the abrasion resistant layer has a cut-off wavelength selected from the range of 345 nm to 385 nm.

66. The multilayer reflective film of claim 1, wherein the abrasion resistant layer has a thickness selected from the range of 2 m to 10 m.

67. The multilayer reflective film of claim 1, wherein the abrasion resistant layer comprises one or more ultraviolet absorbing compounds having a cut-off wavelength less than 385 nm, wherein the ultraviolet absorbing compound is selected from the group consisting of oxanilide, benzophenone, HP triazine, benzotriazole, formamidine and any derivatives of these.

68. The multilayer reflective film of claim 1, wherein the metal layer comprises a silver layer or a multilayer including a copper backside protective layer and a silver layer.

69. A method of making a multilayer reflective film, the method comprising the steps of: providing a polymer film; providing a metal layer; and providing an abrasion resistant layer above the polymer film; wherein abrasion resistant layer has a cut-off wavelength less than 385 nm, the cut-off wavelength being an ultraviolet wavelength of the terrestrial solar spectrum at which a transmittance value of the abrasion resistant layer is equal to fifty percent of a maximum transmittance value of the abrasion resistant layer in the visible region of the terrestrial solar spectrum.

70. The method of claim 69, wherein the metal layer is provided onto a first side of the polymer film; an adhesive layer is provided onto the metal layer; and the abrasion resistant layer is provided onto a second side of the polymer film.

71. The method of claim 69, wherein the metal layer is provided onto a first side of the polymer film; and the abrasion resistant layer is provided onto the metal layer.

72. The method of claim 71, further comprising the step of providing an adhesive layer onto a second side of the polymer film.

73. The method of claim 69, wherein the reflective film is constructed using a roll-to-roll processing method.

74. A multilayer reflective film comprising: an adhesive layer; a metal layer above the adhesive layer; a polymeric layer; and an abrasion resistant layer above the polymeric layer; wherein: the abrasion resistant layer has a cut-off wavelength less than 400 nm, and the abrasion resistant layer has a transmittance of greater than or equal to 50% for electromagnetic radiation having wavelength ranging from the cut-off wavelength to 2.5 m.

75. The multilayer reflective film of claim 74, wherein the abrasion resistant layer has a cut-off wavelength less than 385 nm.

76. The multilayer reflective film of claim 75, wherein: the metal layer is above an adhesive layer; and the polymeric layer is above the metal layer.

77. The multilayer reflective film of claim 75, wherein: the metal layer is above the polymeric layer; and the abrasion resistant layer is above the metal layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 depicts a multilayer reflective film embodiment.

[0032] FIG. 2 depicts a roll-to-roll processing method for making a reflective film.

[0033] FIGS. 3A-3C depict multilayer reflective film embodiments.

[0034] FIG. 4 depicts a multilayer reflective film embodiment.

[0035] FIGS. 5A-5C depict multilayer reflective film embodiments.

[0036] FIG. 6 depicts a multilayer reflective film embodiment.

[0037] FIGS. 7A-7C depict multilayer reflective film embodiments.

[0038] FIG. 8 provides data showing hemispherical absorptance obtained by two reflective film embodiments.

[0039] FIG. 9 provides data showing spectral hemispherical transmittance of an abrasion resistant layer coated onto a quartz plate substrate.

[0040] FIG. 10 illustrates bond strengths and wavelengths required for breaking various organic bonds.

[0041] FIG. 11 provides data showing the spectral transmittance properties of various ultraviolet absorber packages.

[0042] FIGS. 12, a and b provide data showing transmittance of various ultraviolet absorber packages at two different concentrations/loadings.

[0043] FIG. 13 illustrates the percentage of solar spectrum regainable and the percentage increase in solar-weighted hemispherical reflectance as a function of cut-off wavelength of an ultraviolet absorbing abrasion resistant layer.

[0044] FIG. 14 provides an overlay plot showing: (i) the solar irradiance (W/m.sup.2/nm) as a function of wavelength and (ii) spectral hemispherical reflectance (%) as a function of wavelength for a conventional reflective film incorporating a broadband ultraviolet absorber (UVA) package to provide UV resistance to the reflective film.

DETAILED DESCRIPTION

[0045] In general the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention.

[0046] Cut-off wavelength refers to a wavelength of electromagnetic radiation at which a composition or structure, such as the screening layers of a reflective film, layer or coating, exhibits a transmittance value of 50% of the maximum transmittance for a first spectral region including wavelengths greater than the cut-off wavelength, and for which transmittance values for a second spectral region including wavelengths less than the cut-off wavelength are less than 50% of the maximum transmittance of the spectral region. In embodiments, cut-off wavelength refers to the wavelength at which an abrasion resistant layer has a transmittance value that is 50% of the maximum transmittance of the abrasion resistant layer over the terrestrial solar spectrum or portion thereof, such as the visible region of the terrestrial solar spectrum, and wherein the abrasion resistant layer exhibits a transmittance less than 50% for wavelengths of light of the terrestrial solar spectrum, or portion thereof, less than the cutoff wavelength, for example, by exhibiting a transmittance less than 50% for wavelength of light below the cut-off wavelength in the ultraviolet region of the terrestrial solar spectrum. Some abrasion resistant layers, for example, exhibit a change in transmittance characterized by high transmittance values (e.g., greater than 80% or greater than 90%) in the visible region of the terrestrial solar spectrum and a rapid fall-off in transmittance in the near ultraviolet region, for example exhibiting a change in transmittance approximating a step function, wherein the cut-off wavelength corresponds to a point on the fall-off for which the transmittance is 50% of the maximum value in the visible region of the terrestrial solar spectrum. For some abrasion resistant layers, for example, the transmittance values for wavelengths of the terrestrial solar spectrum 5 nanometers, or optionally 10 nanometers, below the cut-off wavelength are significantly less than 50%, for example less than 10%, and optionally less than 1% and optionally for some embodiments less than 0.1%. For some abrasion resistant layers, for example, the transmittance values for wavelengths of the terrestrial solar spectrum 5 nanometers, or optionally 10 nanometers, above the cut-off wavelength are significantly greater than 50%, for example greater than 80%, optionally greater than 90%, and optionally greater than 95%.

[0047] In certain embodiments, a cut-off wavelength refers to an ultraviolet wavelength of the terrestrial solar spectrum at which the transmittance of a composition or a structure, such as the abrasion resistant layer of a reflective film, layer or coating, is equal to 50% of the maximum transmittance value for the composition or structure in the visible region of the terrestrial solar spectrum. FIG. 9 illustrates an exemplary embodiment identifying the cut-off wavelength (.sub.CutOff) of an abrasion resistant layer as having a transmittance value equal to of the maximum transmittance value (.sub.max) of the abrasion resistant layer in the visible region of the spectrum.

[0048] In embodiments, 50% or more of incident radiation having wavelengths above the cut-off wavelength are transmitted through a composition or a structure, such as a film, layer or coating, having the cut-off wavelength. For example, for some embodiments of the present invention an abrasion resistant layer has a transmittance of greater than or equal to 60%, greater than or equal to 70%, greater than or equal to 80%, greater than or equal to 90% or greater than or equal to 95% for electromagnetic radiation having a wavelength greater than the cut-off wavelength or selected between the cut-off wavelength and 2.5 m.

[0049] Ultraviolet absorbing compound refers to a composition which absorbs ultraviolet electromagnetic radiation. In embodiments, ultraviolet absorbing compounds are added to mixtures, films or optically transparent materials to provide absorptance (and thereby prevent or reduce the transmittance) of at least a portion of ultraviolet electromagnetic radiation through the mixtures, films or optically transparent materials. Useful ultraviolet absorbing compounds include those exhibiting a cut-off wavelength selected from the range of 345 nm to 385 nm.

[0050] Abrasion resistant refers to a property of a layer, coating or material to withstand damage sustained through friction or wear, such as damaged sustained by scratching or scuffing. In embodiments, useful abrasion resistant materials include acrylics, acrylic mixtures, polyolefins, cyclic olefin polymers, cyclic olefin copolymers, thermoplastics, polyesters, PETs. Abrasion resistance can be assessed using standardized methods, such as ASTM Standard D4060 or variations thereof. In the context of reflective films having an abrasion resistant coating, useful abrasion resistant coatings include, but are not limited to, those abrasion coatings, which when damaged through friction or wear, that do not impact or have a minimal impact on a specular reflectance of the reflective film, such as a change in specular reflectance less than 2%.

[0051] Absorptance refers to a property of an object or material that absorbs light. In general, the term absorptance refers to the percentage of light absorbed by the object or material. In embodiments, the term absorptance refers to the percentage of light absorbed by the object or material at a specified wavelength or within a specified wavelength range.

[0052] In embodiments, the present invention provides reflective films in which an abrasion resistant layer has an absorptance of greater than or equal to 80%, greater than or equal to 90% or greater than or equal to 95% for at least a portion of electromagnetic radiation having a wavelength less than the abrasion resistant layer's cut-off wavelength. Optionally, an absorptance profile of an abrasion resistant layer is selected such that the absorptance of the abrasion resistant layer is greater than 90% for electromagnetic radiation, for example terrestrial solar electromagnetic radiation, having wavelengths less than 5 or 10 nm below the abrasion resistant layer's cut-off wavelength. Such an absorptance profile can be selected, for example, by adjusting a concentration of one or more ultraviolet absorbing compounds present in the abrasion resistant layer.

[0053] Reflectance and percent reflective refer to a property of an object, material, layer, film or surface. In general, the term reflectance refers to the percentage of light reflected by the object or material. In embodiments, the term reflectance refers to the percentage of light reflected by the object or material at a specified wavelength or within a specified wavelength range.

[0054] For example, for certain embodiments of the present invention a reflective film is more than 90% reflective within the wavelength range between a cut-off wavelength of an abrasion resistant layer and 2.5 m, such as within the range of 285 nm to 2.5 m. Optionally, a reflective film is more than 95% reflective within the wavelength range between a cut-off wavelength of an abrasion resistant layer and 2.5 m.

[0055] Solar-weighted hemispherical reflectance refers to a standardized measure characterizing the quality and performance of solar reflectors. In certain embodiments, specific wavelength regions contribute to and comprise a portion of the solar-weighted hemispherical reflectance of a solar reflector. In certain cases, specific wavelength regions do not contribute to the solar-weighted hemispherical reflectance of a solar reflector, for example if the electromagnetic radiation in the specific wavelength region is absorbed by the solar reflector.

[0056] Solar radiation refers to electromagnetic radiation from the sun. Terrestrial solar radiation refers to solar radiation that is transmitted through the atmosphere of the Earth. Incident solar radiation refers to solar radiation received by a film, mirror or device.

[0057] Polymer refers to a macromolecule composed of repeating structural units connected by covalent chemical bonds or the polymerization product of one or more monomers, often characterized by a high molecular weight. The term polymer includes homopolymers, or polymers consisting essentially of a single repeating monomer subunit. The term polymer also includes copolymers, or polymers consisting essentially of two or more monomer subunits, such as random, block, alternating, segmented, graft, tapered and other copolymers. Useful polymers include organic polymers, inorganic polymer and/or hybrid polymers and may be in amorphous, semi-amorphous, crystalline or partially crystalline states. Cross linked polymers having linked monomer chains are particularly useful for some applications, such as for abrasion resistant coating (ARCs). Polymers useful in the present methods, devices and device components include, but are not limited to, plastics, thermoplastics and acrylates. Exemplary polymers include, but are not limited to, acetal polymers, cellulosic polymers, fluoropolymers, nylons, polyacrylonitrile polymers, polyamide-imide polymers, polyimides, polyarylates, polybenzimidazole, polybutylene, polycarbonate, polyesters, polyetherimide, polyethylene, polyethylene copolymers and modified polyethylenes, polyketones, poly(methyl methacrylate, polymethylpentene, polyphenylene oxides and polyphenylene sulfides, polyphthalamide, polypropylene, polyurethanes, styrenic resins, sulfone based resins, vinyl-based resins, rubber (including natural rubber, styrene-butadiene, polybutadiene, neoprene, ethylene-propylene, butyl, nitrile, silicones), acrylic, nylon, polycarbonate, polyester, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyolefin, polyolefins, cyclic olefin polymers, cyclic olefin copolymers, thermoplastics, polyesters, PETs or any combinations of these. A polymeric layer refers to a layer of a reflective film comprising a polymer or consisting essentially of a polymer. Useful polymeric layers include, but are not limited to, polymer films and polymer coatings.

[0058] Optical communication refers to a relative positioning of two objects such that electromagnetic radiation can be directed directly between the objects or indirectly between the objects, such as through one or more intervening optical elements, for example a lens, a reflector, a filter, a grating, etc.

[0059] Roll-to-roll processing method refers to a method for forming multilayer films where a roll of film is processed by unwinding the film from a first roll, applying coating layers, joining with additional rolls of film or otherwise processing the film and winding the processed multilayer film onto a second roll.

[0060] FIG. 1 shows a cross section of an exemplary multilayer reflective film 100. In this embodiment, the lowest layer 101 comprises an adhesive, the layer immediately above the adhesive layer 101 comprises a metal layer 102, the layer immediately above the metal layer 102 comprises a polymeric layer 103 and the topmost layer comprises an abrasion resistant layer 104. Optionally, a release liner is applied beneath the adhesive, for example to facilitate winding multilayer reflective film 100 onto a roll. In an embodiment, for example, adhesive layer 101 is in physical contact, and optionally directly bonded to (e.g., via covalent bonding, intermolecular forces, Van Der Waals forces, etc.) to metal layer 102. In an embodiment, for example, metal layer 102 is in physical contact, and optionally directly bonded to (e.g., via covalent bonding, intermolecular forces, Van Der Waals forces, etc.) to polymeric layer 103. In an embodiment, for example, polymeric layer 103 is in physical contact, and optionally directly bonded to (e.g., via covalent bonding, intermolecular forces, Van Der Waals forces, etc.) to abrasion resistant layer 104.

[0061] FIG. 2 illustrates a roll-to-roll processing method for making film 100. Initially, a roll 203 of polymer film is provided. As the polymer film is unrolled from roll 203, a metal layer is deposited 202 onto one side of the polymer film during the processing method. An abrasion resistant layer is then applied to the other side of the of the polymer film by applying 204A a layer of uncured abrasion resistant material, followed by UV curing 204B. Finally, an adhesive is applied 201 beneath the metal layer. To facilitate winding the assembled film onto a second roll 205, a release liner from roll 206 is applied beneath the adhesive. Alternative routes for making the films include, but are not limited to, vacuum deposition processes such as sputtering and thermal evaporation and physical and chemical vapor deposition.

[0062] FIG. 3A shows a cross section of an exemplary multilayer reflective film 300A. This film is similar to film 100 shown in FIG. 1, except film 300A further comprises a backside metal protective layer 305 below metal layer 302 and above adhesive layer 301. FIG. 3B shows a cross section of an exemplary multilayer reflective film 300B. This film is similar to film 100 shown in FIG. 1, except film 300B further comprises an adhesion-promoting interlayer 306 below abrasion resistant layer 304 and above polymeric layer 303. FIG. 3C shows a cross section of an exemplary multilayer reflective film 300C. This film is similar to film 100 shown in FIG. 1, except film 300C further comprises both a backside metal protective layer 305 and an adhesion-promoting interlayer 306.

[0063] FIG. 4 shows a cross section of another exemplary multilayer reflective film 400. In this embodiment, the lowest layer comprises a metal layer 402, the layer immediately above the metal layer 402 comprises a polymeric layer 403 and the topmost layer comprises an abrasion resistant layer 404. Optionally, an adhesive layer is applied beneath the polymeric layer, as in film 100. Optionally, a release liner is applied beneath the adhesive layer. In an embodiment, for example, metal layer 402 is in physical contact, and optionally directly bonded to (e.g., via covalent bonding, intermolecular forces, Van Der Waals forces, etc.) to polymeric layer 403. In an embodiment, for example, polymeric layer 403 is in physical contact, and optionally directly bonded to (e.g., via covalent bonding, intermolecular forces, Van Der Waals forces, etc.) to abrasion resistant layer 404.

[0064] FIG. 5A shows a cross section of an exemplary multilayer reflective film 500A. This film is similar to film 400 shown in FIG. 4, except film 500A further comprises a backside metal protective layer 505 below metal layer 502. FIG. 5B shows a cross section of an exemplary multilayer reflective film 500B. This film is similar to film 400 shown in FIG. 4, except film 500B further comprises an adhesion-promoting interlayer 506 below abrasion resistant layer 504 and above polymeric layer 503. FIG. 5C shows a cross section of an exemplary multilayer reflective film 500C. This film is similar to film 400 shown in FIG. 4, except film 500C further comprises both a backside metal protective layer 505 and an adhesion-promoting interlayer 506.

[0065] FIG. 6 shows a cross section of another exemplary multilayer reflective film 600. In this embodiment, the lowest layer 603 comprises a polymeric layer, the layer immediately above the polymeric layer 603 comprises a metal layer 602 and the topmost layer comprises an abrasion resistant layer 604. Optionally, an adhesive layer is applied beneath the polymeric layer. Optionally, a release liner is applied beneath the adhesive layer. In an embodiment, for example, polymeric layer 603 is in physical contact, and optionally directly bonded to (e.g., via covalent bonding, intermolecular forces, Van Der Waals forces, etc.) to metal layer 602. In an embodiment, for example, metal layer 602 is in physical contact, and optionally directly bonded to (e.g., via covalent bonding, intermolecular forces, Van Der Waals forces, etc.) to abrasion resistant layer 604.

[0066] FIG. 7A shows a cross section of an exemplary multilayer reflective film 700A. This film is similar to film 600 shown in FIG. 6, except film 700A further comprises a backside metal protective layer 705 below metal layer 702 and above polymeric layer 703. FIG. 7B shows a cross section of an exemplary multilayer reflective film 700B. This film is similar to film 600 shown in FIG. 6, except film 700B further comprises an adhesion-promoting interlayer 706 below abrasion resistant layer 704. FIG. 7C shows a cross section of an exemplary multilayer reflective film 700C. This film is similar to film 600 shown in FIG. 6, except film 700C further comprises both a backside metal protective layer 705 and an adhesion-promoting interlayer 706.

[0067] An example trajectory of incident solar electromagnetic radiation is schematically represented in FIGS. 1, 3A-3C, 4, 5A-5C, 6 and 7A-7C via a dotted arrow, although it will be readily appreciated that a wide range of incident trajectories are useful with the present reflective films and methods. As shown by the example trajectory in these figures, incident solar electromagnetic radiation first interacts with the abrasion resistant layer prior to interaction with subsequent layers in the stack. Accordingly, use of abrasion resistant layers having a selected cut-off wavelength in the disclosed geometries allows for protection of the underlying layer (e.g., metal, polymer and/or adhesive layers) by decreasing, or preventing, transmission of incident electromagnetic radiation capable of substantially degrading the underlying layers. In addition, use of abrasion resistant layers having a selected cut-off wavelength in the disclosed geometries allows for enhanced overall efficiency of reflection by enhancing transmission of wavelengths that are effectively reflected by the underlying layers providing reflection (e.g., the metal layer) without substantially degrading the underlying layers. Therefore, selection of the cut off wavelength of the abrasion resistant layer in the present invention provides significant benefits for solar concentrating power applications

[0068] The invention may be further understood by the following non-limiting examples.

Example 1: Increasing Solar-Weighted Reflectance of Polymer Film Reflectors by Modifying the Screening Profile of UV Absorbing Additives

[0069] Central to the mirror film concept is the incorporation of ultraviolet (UV) screening layers to protect underlayers from photodegradation. This results in blocking a significant part (over 2.5% of the available terrestrial resource) of the solar spectrum that could otherwise be reflected and thereby increase the solar-weighted reflectance value. Realistic modification of the UV screening functionality can achieve an improvement in reflectance by 1 to 1.5%. The challenge in such unscreening of part of the UV spectrum is to assure that wavebands needed to provide adequate protection of chemical bonds present in the various layers of the reflector construction are not removed so that requisite weatherability of the overall reflector is maintained.

[0070] To illustrate this aspect of the invention, FIG. 14 provides an overlay plot showing: (i) the solar irradiance (W/m.sup.2/nm) as a function of wavelength and (ii) spectral hemispherical reflectance (%) as a function of wavelength for a conventional reflective film incorporating a broadband ultraviolet absorber (UVA) package to provide UV resistance to the reflective film. As shown in FIG. 14, a significant amount of the recoverable solar irradiance is not reflected due to the presence of the broadband ultraviolet absorber (UVA) package of conventional reflective films of the art, thereby, decreasing the overall efficiency of such films for concentrating solar power applications. The present invention, therefore, provides reflective films that enhance the recoverable solar irradiance while at the same time maintaining protection of the reflective film from UV initiated degradation.

[0071] Ultraviolet Absorbers.

[0072] Commercial mirror film constructions use broadband ultraviolet absorber (UVA) packages to provide UV resistance of the entire stack. In one example, the ultraviolet absorbing additives incorporated into an acrylic film are specifically designed for UV screening and are not required to protect the acrylic itself which is inherently UV stable. In one example, the abrasion resistant coating (ARC) requires ultraviolet absorbers to give itself the long term weatherability required for use with concentrating solar power collector applications. An added benefit of these ultraviolet absorbers is that some abrasion resistant coatings exhibit nearly identical optical screening properties as the acrylic film. FIG. 8 shows the spectral hemispherical absorptance of two reflective film constructions. One case (red line) shows the amount of light absorbed by a laminated UV-screening acrylic film. The other case (blue line) indicates the amount of light absorbed by a UVA-containing ARC coating. The difference in absorptance provides a measure of the UV screening functionality of the respective screening layers. At wavelengths below 365 nm the two curves overlay almost exactly, indicating that the ultraviolet absorber package used in the abrasion resistant coating provides UV screening nearly identical to the acrylic film over those wavelengths. Between 365-400 nm the spectral difference between the ARC vs. acrylic film is shifted by about 2.5 nm; such a small shift at these higher UV wavelengths will generally have a minimal effect on weatherability between these two screening layers.

[0073] A plot of spectral transmittance of an ARC coated onto quartz is shown in FIG. 9. FIG. 9 shows the spectral hemispherical transmittance with a cut-off wavelength of about 385 nm. Very little structure is exhibited throughout the wavelength region of interest (>300 nm) except for a fairly steep shoulder feature that separates a region that is highly absorbing vs. a region that is highly transmitting. The wavelength at which transmittance equals 50% is generally defined as the cut-off wavelength (.sub.CutOff), below which transmittance rapidly drops to a near zero value. FIG. 9 indicates that the cut-off wavelength is about 385 nm.

[0074] Materials, such as polymers having organic bonds that would otherwise be susceptible to photolytic damage, are thereby afforded some level of UV protection. The cut-off wavelength of the acrylic film used in some reflective films is very close to 385 nm. FIG. 10 provides a list of typical organic bonds and the associated wavelength required to break those bonds. Blanksby and Ellison present a more extensive discussion that includes bond dissociation energies of more than 100 representative organic molecules. The molecular bond strength energy (E) and photon wavelength are related by:

=hc/E,(1)

where h is Planck's constant and c is the speed of light. FIG. 10 illustrates a number of organic bond strengths and corresponding photon wavelengths required to break them.

[0075] The degree of UV screening protection provided is a function of the optical density of the film or coating. This property is controlled by Beer's Law:

()=().Math.L.Math.C,(2)

where () is the spectral absorbance or optical density, is wavelength, () is spectral molar absorptivity or extinction coefficient, L is the thickness of the film or coating and C is the concentration of ultraviolet absorbing additives. The amount of transmitted light, , is related to the absorbance by:

()=10.sup.()=10.sup.().Math.L.Math.C(3)

[0076] The extent to which UV photons are blocked or transmitted can therefore be tailored or controlled in three ways. First, () is an inherent property of the type of UVA that is used. A wide variety of ultraviolet absorber packages exist. For example, the spectral properties of different types of commercial ultraviolet absorber products available from Ciba Specialty Chemicals, a major international supplier of UVA additives, are shown in FIG. 11. As can be seen, the cut-off wavelength can be shifted by choosing different types of ultraviolet absorber products.

[0077] Increasing the thickness of the coating or film will exponentially decrease transmittance at all wavelengths. This property can be achieved, for example by using increasingly thick acrylic films. Increasing thickness also increases the longevity of the screening functionality. Successive layers of ultraviolet absorbers provide protection for underlying absorber molecules, thereby allowing downstream ultraviolet absorbers to survive longer.

[0078] Finally, greater loading results in higher concentration of ultraviolet absorbers and, consequently, lower transmittance. This can also effectively shift the cut-off wavelength. For a given ultraviolet absorber package, higher concentration shifts the cut-off wavelength to higher wavelengths. For example, FIG. 12 shows the spectral transmittance properties associated with four ultraviolet absorber packages. FIG. 12a shows the transmittance for abrasion resistant coatings using several ultraviolet absorbers at 2% loading, and FIG. 12b presents data for 4% loading. The cut-off wavelength of the A0111481 UVA package shifts .sub.CutOff from 390 nm at 2% to 400 nm at 4%.

[0079] Increased UV Reflectance.

[0080] The solar-weighted hemispherical reflectance (SWHR) can be increased by using a modified ultraviolet absorber package that shifts .sub.CutOff to lower wavelengths. ASTM G173 provides a typical/standard terrestrial solar spectrum as shown in FIG. 14.

[0081] The percent of the terrestrial solar spectrum resource that can be regained by shifting the cut-off wavelength to lower wavelengths (P.sub.SR) is the total amount of sunlight available below 385 nm (2.62%) times the power density between .sub.CutOff and the cut-off wavelength of previous mirror film constructions (385 nm) divided by the total UV power density between 300 and 385 nm (23.4 W/m.sup.2):

[00001]$\begin{array}{cc}{P}_{\mathrm{SR}}=\frac{2.62.Math.\%.Math.{}_{{}_{\mathrm{CutOff}}}^{385.Math..Math.n.Math..Math.m}.Math.I\left(\right).Math.d.Math..Math.}{{}_{300.Math..Math.n.Math..Math.m}^{385.Math..Math.n.Math..Math.m}.Math.I\left(\right).Math.d.Math..Math.}& \left(4\right)\end{array}$

To obtain the percentage point increase in SWHR requires an inclusion of the spectral reflectance, (), as part of the integrand:

[00002]$\begin{array}{cc}{P}_{\mathrm{SWHR}}=\frac{2.62.Math.\%.Math.{}_{{}_{\mathrm{CutOff}}}^{385.Math..Math.n.Math..Math.m}.Math.I\left(\right).Math.\left(\right).Math.d.Math..Math.}{{}_{300.Math..Math.n.Math..Math.m}^{385.Math..Math.n.Math..Math.m}.Math.I\left(\right).Math.d.Math..Math.}& \left(5\right)\end{array}$

[0082] Results are plotted in FIG. 13, showing the percentage of the terrestrial solar spectrum regainable as a function of ultraviolet absorber cut-off wavelength, and the corresponding increase in solar-weighted hemispherical reflectance for the spectral reflectance of Cu/Ag/PET. As an example, .sub.CutOff375 nm for the A0111261 and Cyasorb UV1164L UVA packages shown in FIG. 12. If one of these ultraviolet absorber packages is used, an additional 0.5% reflectance is achieved.

[0083] Another target for .sub.CutOff is 355 nm. As shown by its light absorption profile, polyethylene terephthalate (PET) absorbs in the range from 290 to 350 nm and, as it does, it degrades photolytically [Wypych]. It is hypothesized that the most critical region to protect via the UV coatings is 300 to 345 nm. Thus, a cut-off wavelength of 355 nm is feasible in terms of protecting both the underlying PET film and the abrasion resistant coating as well. From FIG. 13, pushing the UV cut-off wavelength down to 355 nm allows recapture of 1.5% of the available solar resource, which translates into an increase in SWHR of 1.3% (based on the spectral reflectance of typical Cu/Ag/PET reflective film constructions).

REFERENCES

[0084] Kanouni, M., Degradation and Stabilization of Organic Coatings, Ciba Specialty Chemicals presentation at NREL, Apr. 8, 2004. [0085] Blanksby, S. J., and Ellison, G. B., Bond Dissociation Energies of Organic Molecules, Acc. Chem. Res., Vol. 36, 2003, pp. 255-263. [0086] Wypych, G., Handbook of Material Weathering, 2nd Edition, Chem Tech Publishing, 1995, pp. 357-363. [0087] U.S. Pat. No. 4,307,150 for Weatherable Solar Reflector, issued on Dec. 21, 1981. [0088] U.S. Pat. No. 4,645,714 for Corrosion-resistant Silver Mirror, issued on Feb. 24, 1987. [0089] U.S. Patent Application Publication US 2012/0011850 for Broadband Reflectors, Concentrated Solar Power Systems, and Methods of Using the Same, published on Jan. 19, 2012. [0090] ASTM Standard D4060, Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser. [0091] ASTM G173, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 Tilted Surface.

STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS

[0092] All references throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).

[0093] The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the present invention and it will be apparent to one skilled in the art that the present invention may be carried out using a large number of variations of the devices, device components, methods steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.

[0094] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art, in some cases as of their filing date, and it is intended that this information can be employed herein, if needed, to exclude (for example, to disclaim) specific embodiments that are in the prior art. For example, when a compound is claimed, it should be understood that compounds known in the prior art, including certain compounds disclosed in the references disclosed herein (particularly in referenced patent documents), are not intended to be included in the claim.

[0095] When a group of substituents is disclosed herein, it is understood that all individual members of those groups and all subgroups and classes that can be formed using the substituents are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. As used herein, and/or means that one, all, or any combination of items in a list separated by and/or are included in the list; for example 1, 2 and/or 3 is equivalent to 1 or 2 or 3 or 1 and 2 or 1 and 3 or 2 and 3 or 1, 2 and 3.

[0096] Every formulation or combination of components described or exemplified can be used to practice the invention, unless otherwise stated. Specific names of materials are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same material differently. One of ordinary skill in the art will appreciate that methods, device elements, starting materials, and synthetic methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such methods, device elements, starting materials, and synthetic methods are intended to be included in this invention. Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.

[0097] As used herein, comprising is synonymous with including, containing, or characterized by, and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, consisting of excludes any element, step, or ingredient not specified in the claim element. As used herein, consisting essentially of does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term comprising, particularly in a description of components of a composition or in a description of elements of a device, is understood to encompass those compositions and methods consisting essentially of and consisting of the recited components or elements. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

[0098] It must be noted that as used herein and in the appended claims, the singular forms a, an, and the include plural reference unless the context clearly dictates otherwise. Thus, for example, reference to a layer includes a plurality of such layers and equivalents thereof known to those skilled in the art, and so forth. As well, the terms a (or an), one or more and at least one can be used interchangeably herein. It is also to be noted that the terms comprising, including, and having can be used interchangeably. The expression of any of claims XX-YY (wherein XX and YY refer to claim numbers) is intended to provide a multiple dependent claim in the alternative form, and in some embodiments is interchangeable with the expression as in any one of claims XX-YY.

[0099] The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.