Silver mirror for use in a solar reflector

Technical Field

The invention relates to advanced ultraviolet-resistant specular silver mirrors for use in solar reflectors, and is a continuation-in-part of U.S. patent application. 09/762,719 filed February 9.2001. More particularly, (he invention relates to advanced, ultraviolet-resistant specular silver mirrors for use in solar reflectors, said mirrors being characterized by substantially improved optical durability and superior resistance to tunneling or delamination failure under outdoor environmental conditions.

Background Art

Insufficient weather protection and ultraviolet degradation are problems encountered when using solar reflectors made of a flexible specular silver mirror. When used outdoors, these mirrors must be durable and ultraviolet light (UV) resistant in order to retain their dimensional stability, aesthetic appearance, and specular-reflectance in the visible, ultraviolet, and near infrared wavelengths.

Specular-reflectance is provided in a flexible silver mirror through a silvered composite lamina, having a thin layer of silver vacuum-deposited on the surface of a flexible polymer substrate. Silver is the preferred metal because its reflectivity is substantially higher than that of other metals, such as aluminum. To retain specular reflectance over time, the prior art has focused on the application of advanced adhesives and protective layers, coated over the polymer substrate and silver layer, to protect the mirrors from abrasion, and ultraviolet degradation.

Early techniques used to protect solar mirrors from abrasion, weathering, and ultraviolet degradation were developed with aluminum mirrors. For example, in U.S. Pat. No.

,150, a «4«. reflector is disclosed wherein an opaque aluminum surface, vacuumdeposited an a flexible polyester support-sheet, is protected fam corrosion and weathering with an mter-polymer layer of acrylate or methacrylate copolymers. The support sheet consists of a biaxialiy oriented polyethylene terephthalate lamina having conventional slip agents, to facilitate winding, and a second polyethylene terephthalate lamina which contains no slip agent

Silver is higher in specular reflectionthan aluminum. Thus, the logical assumption had been 10 substitute silver for aluminum in the solar reflector described above. However, this approach has been reported. in U.S. Pat No. 4,645,714, to result in two undesirable phenomena. First, silver is susceptible to corrosion either through the development of pin holes in the acrylate coating ar along peripheral portions of foe silver-coated film. Second, a thin layer of silver, unlike a firin l^er of aluminum, has a spectral window through which ultraviolet (“UV”) fight readily passes. The peak transmission of tins tight is at 320 tun, and «infight contains ultraviolet light in this wavelength. The transmission of ultraviolet light through the silvs־ layer degrades foe underlying polyester substrate causing bubbles in the adhesives, commonly׳ used to adhere the substrate to a rigid support. This degradation and bubbling reduces foe aesthetic and specular functional properties of the solar mirror.

Corrosion inhibitors and UV absorbers, incorporated into foe adhesives or protective film coatings overlaying ^& polyester and silver mirror substrate, have been used to retain these 20 functional properties. However, while corrosion inhibitors do reduce corrosion, they frequently impart an unacceptable color to foe minor, overtime, and do not block foe ultraviolet light In contrast, when ultraviolet light absorbers are incorporated into a protective polymer overlay, foe rate of polyester support degradation is lessened, but silver corrasion is aggravated. Thus, attempts have been made to isolate the corrasion inhibitor and 25 ultraviolet absorber dements from the minor’s reactive components in order to eliminate these undesirable effects.

In Rocfoe, U.S. Pat No. 4,645,714, acorrosion resistant silver nrirror is disclosed wherein a corrosion inhibitor, and an ultraviolet absorber are each incorporated into separate thin overlays of an acrylate inter-polymer paint. The specular reflective minors are formed by 30 vacuum-depositing silver over a polyester support film. UltrapioJet degradation of lhe polyester support, and consequent bubbling of the underlying adhesive, is reduced by incorporating UV absorbers in a second polymer coating that is applied over a first polymer coating, which incorporates a corrosion inhibitor. The first polymer coating is applied direct^׳ over the silver reflective surface. The polymeric substrate, a coextruded biaxially oriented polyester foil, comprises: (I) a polyethylene terephthalate lamina containing conventional slip agents to facilitate roll-winding; and (2) a polyethylene terephthalate lamina containing no *Hp ag”״*, which results in an opticaDy-smooto exposed surface. Ths silver ep^darT^atarpe layer ovedies the smooth surface, of the coexiruded film, and is bonded thereto. Layered over the silver is a first acrylate or methacrylate inter-poJymer coating, having a 03 to 23% glycol ctimercaptoacetate dispersant, which serves as a coupling agent, primer; ?nd corrosion inhibitor. This coating weighs 1-4 g/m². Overlying the first aotylate coating, is a second acrylate coating containing an ultraviolet absorber effective throughout the 300-400 nanometer range. The weight of the second coating is 4-8 g/m². Inclusion of the corrosion inhibitor and the UV absorber into separate layers is designed to keep the UV absorber out-of-contact with the silver, and to avoid any corroding effect On the opposite 15 side of toe coextruded polyester support is aunifann coating, weighing about 10-15 gfa?, of a tacky and pressure-sensitive adhesive (95:5 isooctyl acrylate:ac1yiam1de copolymer). A conventional release Im«., such as a silicone-coated polyester film, may be used to protect the adhesive prior to use. The disclosure of U.S. Pat. No. 4,645,714 is incorporated by reference as though fully set forth herein.

It is believed, however, by Hutchinson, U.S. Pat No. 5,118340, that the reflective films described in U.S. Pat Nos. 4,307,150, and 4,645,714 are generally unsuitable for solar energy applications. Under outdoor conditions, the thin acrylate flood coat, of these films, tends to weather pooriy and to quickly erode. These coatings firns offer an insufficient protective barn« to abrasion and moisture Where toe solar tmxrcr comprises a substrate 25 having a polyester support sheet and als^er of silver, as toe outer acrylate flood coat containing UV absorbers, ultraviolet light erodes and degrades toe polyester support, and toe mirror’s fle^haric. appearance and optical efficiency fail. In order to mitigate this problem, Hutchinson discloses the use of corrosion inhibitors and ultraviolet absorbers in an adhesive, whidt is used to band an abrasion and moisture resistart fluorocarbon protective coating over a polyester and silver mirror substrate. The relevant embodiment therein, describes a corrosion and ultraviolet light resistant flexible reflective film, where toe respective inhibitors and absorbers ate mcorpcrrted into separate coatings of an adhesive. A firin laya־ of silver is vacuum-deposited an a flexible polyester support sheet producing fee specular silver surface.

The adhesive is used to bond a fluorocarbon film, having an abrasion and weather resistant function, to toe surfiace of toe silver. The adhesive is applied in two separate layets. The first adhesive ka׳er is adjacent to the silver deposit and contains a corrosion inhibitor. The second adhesive layer contains a UV absorber and overlays (he first adhesive layer for use in banding the fluorocarbon protective film to the silver surface. The use of adhesives to bond the fluorocarbon film to toe silver surface is a required element of this construction because fluorocarbon films do not bond to metal surfaces. However, this construction is not without its deficiencies when teed, overtime, as a solar mirror. Under ultraviolet fight, the application ofadvanced adhesives ((hose incorporating UV absorbers and corrosion inhibitors) directiy onto a sitver substrate has resulted in degradation of toe silver/adhesive interface. When silver is adhered directly to acrylic films, d^aminatian failures have also occurred. Moreover, fluorocarbon protective films, without UV absorbers often provide an insuffiaeto weather resistant shield. For these reasons, it is believed that these silver mirrors *fam applied as solar reflectors remain lacking in long-term durability, which results in a loss of optical effiaatcy and esthetic appearance.

A need fa an advanced ultraviolet-resistant silver mirror, which effectively screens ultraviolet light, retains its specular optical efficiency and aesthetic appearance, demonstrates 20 substantially improved optical durability compared to prior polymeric reflector mirrors, and

MhThite extraordinary resistance to delamination failure.

pigflnCTire rfInvention

One object of the invention is to provide a construction of an advanced solar reflector 25 nwtarrei having improved optical durability compared to prior art polymeric reflector constructions.

Another object of the invention is to provide a construction of an advanced solar reflector material having improved resistance to tunneling or delamination failure.

A forth!». object of the invention is to provide a construction of an advanced solar reflector 30 mirror in .which (he giver layer is located beneath a polyester film layer to keep the top side of toe silver layer sway from contact with adhesive to allow extensive UV-screeoiQg by the top

־5־ laya, so as to protect both the polyester and the silver/prfymer interface from UV degradation.

A still further object of the invention is to provide a construction of an advanced solar roflActer־ tmrmr in which die silver layer is located beneath a polyester film layer to keep toe top side of die silver layer away from contact wife adhesive to allow extensive UV-screening by the top fryer, so as to protect both the polyester and sSve^polymer interface from UV degradation; and wherein. 8. copper layer is added below the silver fryer to allow separation of toe hftgfcnda of toe «Aver from toe pressure sensitive adhesive.

Briefly, the invention provides a construction of an advanced solar reSector mirror in 10 which toe silver layer is located beneath a polyester film layer to keep toe top side of toe silver l^׳er away from contact with adhesive to allow extensive UV-screening by toe top layer, to protect both toe polyester and toe silvar/polymer interface from UV degradation.

Tha foregoing specific objects and advantages of toe invention are illustrative of those which can be achieved by toe present invention and are not intended to be exhaustive or 15 limiting of tote possible advantages which can be realized. Thus, those and other objects and advantages of toe invention will be apparent from toe description herein or can be learned from practicing the invention, both as embodied herein or as modified in view of any varigtinrat which may be apparent to those skilled in the art

Unless specifically defined otherwise, aS technical or scientific terms used herein have toe 20 same meaning as commonly understood by one of ordinary skill in toe ait to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used, in toe practice ar testing of toe present invention, die preferred methods and materials are now described.

Brief Description of Drawings

FIG. 1 is a cross-sectional view showing the advanced solar reflector in which the silver lays is located beneath & polyester film fryer in accordance with the invention construction.

FIG. 2 is a cross-sectional view showing an alternative embodiment of the advanced solar reflector in which the silver layer is located beneath a polyester film layer, and a copper layer 30 is disposed beneath toe silver layer.

FIQ 3 is a gn^fc showing Hemispherical Reflectance (%) versus Equivalent Tone

Exposure in yean; employing “Accelerated Outdoor Weathering ofNonmetaffic Materials

Using Concentrated Sunlight” on the advanced UV-resistant silver mirror construction of the invention and the SS-95 mirror construction of 3M as disclosed in U.S. Patent No. 4,645,714.

FIG. 4 is a graph lowing the percent of area that has not undergone delamination failure as a function of time of immersion in water an the advanced UV-resistant silver nrirrar construction of the invention and samples of silvered thick acrylic rmrrors.

RgstMode for Carrying Out thelnverifioa

The present invention provides advanced uhraviolet resistant specular silver mirrors, for use in solar reflectors, wherein the mirrors are characterized by substantially improved optical durability and superior resistance to tunneling or delamination failure under outdoor environmental conditions.

Reference is now made to foe drawing figures, in which tike numerals represent tike 15 elements, and where there is generally׳ shown foe silver mirror 10, in FIG. 1, in its most basic configuration. The silver mirror 10 is comprised of aUV screening acrylic film ll disposed on an adhesive layer 12 deposited an a polyester film 13. The silver overt®? 14 is located beneath foe polyester film layer, thereby keeping foe top ifluminated side of foe stiver layer from contact wifo the adhesive, which is amateriai thal can cause d^radation of foe 20 silver layer. A pressure sensitive adhesive layer 15 is positioned beneafo foe silver overlay 14.

In foe canstraction foown in FIG. 1, foe silver layer is located beneafo a polyester film hyer, and this results m a sHghtiy lower reflectance compared to a construction in which tire silver layer is above foe polyester layer, this construction keeps foe top side of foe silver layer (foe ifluminated side), and therefore the side most subject to UV-activated 25 degradation (away from contact with the adhesive), which is a material *at can promote silver degradation. The adhesive is highly optically transmissible to visible, ultraviolet, and near infrared light The key to durability and outdoor weathenfoitity of this construction is foe extensive UV-sareaung by foe top lay®־ which protects both the polyester (from UV degradation) and foe sflver/polyinar interface (which is also subject to UV degradation).

The invention builds upon tire previously disclosed novel w׳^׳ of constructing an advanced giver mirror in which commercially available materials, that are individually inadequate as

-Ίsalar minors, are combinedinaumq® way toirc^ reflector. ־The earlier construction describes a solar reflector construction in which SS-95, a low-cosLndlective material foat was (Moped by the 3M Company, is combined (through various bondmg means) with a UV-screening film. SS-95 tn׳ itself does not posses sufficient outdoor weafoerability to be a viable material for use on solar devices. The outdoor weafoerability of SS-95 is inherently very limited because the very thin overcoats feat are used to manufacture the product cannot provide sufficient protection to the underlying alveted reflective lq׳er. However, when protected with a much thicker UV-screening film, outdoor weatherability is achieve!

In the present invention there are improvements and additions to this concept An acrylicbased UV-screening film (named Karad⁰*) is a commercially available product This product possesses properties that make it a good top layer for foe Advanced Solar Reflector. While other sfiver reflector films (different than SS-95) were found to be commercially available, these other commeraafly avafiable silver reflector films do not possess good outdoor weafoerability because of inherent limitations in these materials and their constructions. These commerciany available silvered films are made formdoor use, such as enhancing the performance of office light fixtures. Wife foe addition of atop layer of UV-screening film (e.g. Korad™), a suitable solar reflector results, which demonstrates much improved outdoor weafoerability.

Referring to FIG. 2, there is shown a further embodiment of the advanced uftravioletretistant silver ת«.»™. for use in solar reflectors, fo fois embodiment, a copper layer 16 is situated below foe silver layer 14 and above pressure sensitive adhesive layer 15. This construction produces an advanced ultraviolet-resistant silver mirror, vrftidi when used in solar reflectors, provides even better outdoor weafoerability because of the separation of foe 25 backside of foe 52 ver layer from foe pressure sensitive adhesive layer 15.

Accelerated exposure testing performed on foe constructions of FIGS. 1 and 2, Tor example, using accelerated laboratory-controlled and accelerated outdoor environment testing demonstrates substantially improved optica! durability compared with priorp^mrac reflector mirrors, in that prior art polymer minors were especially susceptible to figure mode 30 known as tunneling wherein the reflective layer catastrophically delaminates from foe polymer layer in foe presence of moisture. Further, accelerated weather exposure tests of this new construction (which does not include thin acrylic flood coat !were) shows fiiatfois construction exhibits extraordinary optical durability compared to 3M’s commercial material (SS-95) of Roche PaL No. 4,465,714.

Tmtnstrial Applicability

Figure 3 show's the dramatic improvement in optical durability of the new reSector construction of the invention (that does not include thin acrylic flood coat l^ess) compared with 3M’s commercial material (SS-95) based on Roche’s patent The unfilled triangle symbols show the change in reflectance for the SS-95 reflector as a function of real-time 10 exposure in Phoenix, AZ. The filled symbols depictresuits for a preferred embodiment of improved laminate reflector construction. These materials were subjected to accelerated outdoor exposure 81 about -5 times natural sunlight These exposures ware made using an ACUVEX* (Accurate Controlled Ultra Violet Exposure) weathering device, in accordance wife foe testing standard ASTM G 90, Accelerated Outdoor Weathering of Nonmetallic IS Materials Using Concentrated Natural Sunlight״ The filled square symbols are for our improved reflector haring fire construction 2,4 mil thick acrylic UV screen / PET / Ag / Cu / ! PSA, where PET is polyethylene terephthalate (apolyester) and PSA is «!pressure sensitive adhesive The equivalent time of exposure for foe ACUVEX* exposed materials is based 00 cumulative dose of ultraviolet sunlight.

As can be seen, while the initial reflectance of foe SS-95 material is slightly greater than fixe improved reflector, foe reflectance of the SS-95 drops very rapidly after about -2 years of outdoor exposure. The improved invention mirror, however, retains higb reflectance for over 9 years equivalent outdoor exposure.

Another deficiency' of the prior art was !dated to constructions in which silvered thick 25 acrylic film« were specified. The inherent adhesion of vacuum-deposited silver wife an acrylic film is poor, □pon exposure to moisture during outdoor sendee, solar mirrors that incorporate a alv«/aaylic construction were found to he particularly susceptible to a failure mode known as frmnetmg wherein foe silver layer catastrophically delaminates from the base acrylic film. The thick acrylic film absorbs moisture and swells, producing mechanical 30 Mroses that induce ddammation with the silver layer. This effect is shown explicitly in FIG

4. These results are for tests in .which candidate silvered polymer solar mirror constructions are immersed פו water andihe % of deaminated area is measured 8s a function of length of immanann time. For samples of silvered thick acrylic films as specified in the cited prior ait, 60% of the τπγγτπγ surfaces had (Manfmatert after only 4 days. This constrtntes complete and unacceptable failure.

While the invention has he® described with reference to specific embodiments, it is to be understood that various modifications and aheraiions of the invention will become apparent to those <+11«־«^ in the art and such mo&Bcafions and alterations can be made without depardng from the scope and spirit of the invention which is defined by the claims hereafter.