CA2130810A1

CA2130810A1 - All-polymeric ultraviolet reflecting film

Info

Publication number: CA2130810A1
Application number: CA 2130810
Authority: CA
Inventors: Walter J. Schrenk
Original assignee: Individual
Current assignee: Dow Chemical Co
Priority date: 1992-02-25
Filing date: 1992-11-25
Publication date: 1993-09-02
Also published as: JPH07507152A; MX9301005A; TW242604B; AU3224293A; US5540978A; EP0627991A1; WO1993016878A1

Abstract

An all-polymeric ultraviolet light reflective film which is lower in cost than previously used reflector materials, is weather resistant, and which does not absorb significant amounts of solar ultraviolet energy is provided. The film includes a sufficient number of alternating layers of at least first and second diverse polymeric materials which have an average percent transmission of greater than about 50 percent between wavelengths of 300 to 400 nm.
A substantial majority of the individual layers of the film have optical thicknesses in the range where the sum of the optical thicknesses in a repeating unit of the polymeric materials is between 0.15 µm to 0.228 µm, and the first and second polymeric materials differ from each other in refractive index by at least about 0.03 in the wavelength range of from 300 to 400 nm. The reflective film is useful as a reflective material in solar detoxification systems, as a protective material in indoor and outdoor lighting systems, as a UV mirror in the fields of medical imaging, astronomical telescopes, and microscopy or in chemical reactions using UV radiation.

Description

WO 93/16878 PCT/US92/1016~

ALL-POLYMERIC ULTRAVIOLET REFLECTING FILM

This invention relates to an all-polymeric ultraviolet reflecting film, and moreparticularly to a reflector which is substantially transparent to visible and near infrared 5 wavelengths while reflecting a substantial portion of solar ultravioiet wavelengths.
The present invention provides an all polymeric ultraviolet light reflective fiim which is lower in costthan previousiy used reflector materials, is weather resistant, and aoes notabsorbsignificantamountsofsolarultravioletenergy. Whilea preferred useforthefilm of the present invention is as a reflective material in solar detoxification systems, the film aiso is 10 useful in other applications where ultraviolet light reflectivity, but visible light transparency Is required.
The terms "reflective", "reflectivity", "reflection", and "reflectance" as used herein refer to total reflectance (that is, ratio of reflected wave energy to incident wave energy) of a sufficiently specular nature. The use of these terms is i ntended to encom~oass sem i-15 specular or diffuse reflection as well. In general, reflectance measurement refers to reflectanceof light rays into an emergent cone with a vertex angle of 15 degrees centered around the specular angle. By the term "diverse" is meant that the polymeric materials need not differ in any respect except in terms of refractive index Thus, while adjacent layers may be chemlcally diverse, if such materials have the same refractive index, then for purposes of the presen-20 invention they are not "diverse".
A specific intensity of reflectance, when used herein, is the intensity of reflectionwhich occurs at a wavelength where no substantial absorption occurs. For example, the films of the present invention are designed to reflect ultraviolet light having wavelengths in tne range of from about 300-400 nm. Light of other wavelengths, such as in the visible range, pass 25 through (that is, are transmitted by) the films. It is at these ultraviolet wavelengths to which the inlensity of reflection is referring.
Fig. 1 is a schematic cross section of a two component multilayer ultraviole-refle~g film of the present invention, the film including protecti~/e skin layers on both exterior surfaces thereof;

WO 93/16878 f~ PCI /US92/10162 Fig. 2 is a persF~ective and somewhat schernatic reDrffentation of a solar detoxification reflector system; and Fig. 3 is a graph of wavelength versus predicted reflectance for multilayer films fabricated in accordance with the present invention.
The present invention provides improved multilayer all-polymeric uitraviolet reflecting films with a number of desirable properties including substantial ultraviolet reflectivity over the wavelength range of 300 to 400 nm while not absorbing any substantiai amounts of ultraviolet radiation, substantial transparency to visible and near infrared light, and the capability of being laminated to substrates to form a number of useful articles. The 10 optical theory of multiple reflections from layers having differing refranive indices demonstrates the dependency of the effect on both individual layer thickness and refractive index of the material. See, Radford et al, " Reflectivity of Iridescent Coextruded Multi layered Plastic Films", Polymer Engineering and Science 13, 3, pg. 216 (1973). The primary or first order reflected wavelength for a two component multilayer film for normal incidence is given by the 15 equation below .~1 = 2(n,dl+n2d2) where, Al is the wavelength of first order reflenion in nanometers, and spans the range of 300-400 nm, n1 and n2 are the refractive indices of the two polymers, and d1 and d2 are the layer thicknesses of the two polymers, also in nanometers.
20For a three or more component film, the above equation may be generalized to:
: m AI = 2 ~: nidi i-l where Al, n, and d are as defined above and m is an integer greater than 1. Thus, for example, 25 forathreecomponentfilmhavingapolymerrepeatingunitofABCB,theequationis:
~4 = 2(nAdA + ngd~ + ncdc + n~dB) If dA = d9 = dc, then the sum of the optical thicknesses in a repeat unit varies within the range of from 0.15 ~m to about 0.20 ym. Preferably, the optical thickness range for each indlvidual layer in the ABCB repeating unit to span 300 to 400 nm is from 0.025 ~lm to 0.036 llm.
As can be seen, the first order reflected wavelength is proportional to the sum of the optical thicknesses of the two polymers (where optica! thickness, n,dl, is the product of layer thickness times refractive index). In addition to first order reflections, higher order reflections occur at integer fractions of the first order. The relative intensity of these hlgher order reflections depends on the ratio of the optical thickness of the polymer components To produce a film which reflects a broad bandwidth of wavelengths in the range of from about 300 to 400 nm, a layer thickness gradient may be introduced across the thickness of the film. Thus, in one embodiment of the invention, the layer thicknesses wili Increase : ~ -2-WO g3/16878 ~ O PCI~/US92/10162 ,. ~
monotonically across the thickness of the film. By monotonicaily, it is meant that the layer thicknesses increase at a predetermined rate across the thickness of the fiim. See, Schren~, U.S
Patent No. 3,687,589. As can be seen from the above equations, variations in individual layer thickness, d, have a direct effect on the optical properties of the film.
The layer optical thicknesses needed for reflecting in the 30û to 400 nm range described above have all been described for reflectance of light at normal incidence (that is, 0) on the film. The reflected wavelength varies with the angle of incidence of the solar energy.
As the angle of incidence varies from 0 (normal incidence) to 45, the shif I is about 55 nm .
To accommodate the wavelength shift and the probabilitythat not all light will 10 strike the ultraviolet reflecting film at normal incidence, the layer optical thicknesses in the film may be designed to accommodate this somewhat larger range of 300 nm to 455 nm. While the film would reflect some visibie light at normal incidence, it would be better able to reflect ultraviolét light at a range of angles of incidence. The maximum optical thicknesses of the layers in this design would increase about 15 percent, so that the sum of optical thicknesses i n a repeating unit are in the range of from 0.15 ym to 0.228 ,um. Such a design would insure that substantially all ultraviolet light impinging upon the film was reflected, even if the light were incident at an angle other than normal to the film.
Fig. 1 schematically ;llustrates a twf~component ultraviolet reflective film 10 having a repeating unit AB in accordance with the present invention. The film 10 includes 20 alternating layers of first polymer 12 having a refractive index, nl, and a second polymer 14 having a refractive index, n2. Fig. 1 shows a preferred form of the invention where substantially all of the layers of the film have optical thitknesses where the sum of the optical thicknesses of the repeat unit varies between 0.15 llm to 0.20 ym. Preferably, each individual layer has an optical thickness of between 0.07 llm and 0.11 llm. Fig 1 also depicts skin layers of 25 a polymer ~C) 18 positioned on both major exterior surfaces of the reflective body to protect the other layers from scratches or weathering or to provide support for the other iayers Preferably, the polymers chosen have a refractive index mismatch of at least 0.03 at the wavelengths 300 to 400 nm. Typically, refractive indices of materials, including polymers, are measured at a convenien~ wavelength in the visible range such as 589 nm sodium vapo- It is known that refractive indices of polymers can i ncrease at shorter wavelengths. However, it is difficult to measure refractive indices at ultraviolet wavelengths. We have found, however, thatthe refractive index mismatch of two diverse polymers, chosen based on published refractive indices at visible wavelengths, remains at least as large at ultraviolet wavelengths.
Accordingly, choosing a refractive index mismatch of at least 0.03 will be a conservative 35 estimate of the actual mismatch which occurs at ultraviolet wavelengths.
Preferably, for a three or more component system, the polymeric material havlng the highest refractive index differs from the polymeric material with the lowest refractive index WO 93/16878 ~ ~ J ~ ~3 1 0 PCr/US92/10162 ~,, ,.~
by at ieast about 0.03. The refractive indices of other components may be intermediate that of the components having the highest and lowest refractive index.
The polymeric materials utili2ed in the practice of the present invention are unique in the combination of properties that they must possess. The poiymeric materials do 5 not absorb any substantial amounts of u ltraviolet radiation and inherently resist degradation by ultraviolet light without the addition of ultraviolet light absorbers. By this it is meant that the polymeric materials used in the practice of the present invention maintain an average percent transmission of greater than about S0 percent between 300 to 400 nm. As solar ultraviolet makes up only 3 to 4 percent of the total energy from the sun, absorption of 10 significant amounts of the ultraviolet portion of the spectrum by a polymer severely detracts from its ability to find use in the present invention.
Generally, the individual polymers must be also substantially transparent to visible light, and preferably are also substantially transparent to wavelengths in the near infrared spectrum. As discussed above, the polymers must be resistant to degradation by 15 ultraviolet light. Many thermoplastic polymers such as polystyrene and polyvinyl chloride are not resistant to degradation by ultraviolet radiation and must have incorporated therein UV
absorbingcompoundstoimprovestability. However, UVabsorbingstabilizerswill notfunction in the context of the present invention where non-absorption of ultraviolet radiation is a requirement.
Polymeric materials useful in the present invention include polymethyl methacrylate such as Cyro H15-012 ~trademark) available from Cyro Industries (refractive index = 1.49), polyvinyiidene fluoride such as Kynar (trademark) available from Atochem North America, Inc. (refractive index = 1.42), polychlorotrifluoroethylene such as Aclar 22A
(trademark) available from Allied Signal Corporation (refractive index = 1.41), and 25 polymethylpentene-1 such as TPX (trademark) available from Mitsui Chemicals (refractive index 1.46 ). A preferred multilayer ultraviolet reflective film includes polyvinylidene fluoride as the first polymeric material and polymethyl methacrylate as the second polymeric material.
Both polyvinylidene fluoride and polymethyl methacrylate have excellent stability and resistance to degradation in ultraviolet light as well as being nonabsorbers of uitraviolet light.
30 In a preferred form, the reflective film includes relatively thick protective skin layers of polyvinylidene fl uoride on each exterior surface and optical Iy active alternating layers of polyvinylidene fluoride and polymethyl methacrylate in the interior.
It is preferred that the polymers selected have compatible rheologies for coextrusion. That is, as a preferred method of forming the multilayer films is the use of 35 coextrusion techniques, the melt viscosities of the polymers must be reasonably matched to prevent layer instability or nonuniformity. The polymers used also should have sufficient interfacial adhesion so that the films will not delaminate. Alternatively, a thlrd polymer may be used as an adhesive or glue layer to secure the first and second polymer layers ~ogethe WO93/16878 ~ a PCI/lJS92/10~62 The multilayer ultraviolet reflective films of the present invention possess majoradvantages over prior art processes which use expensive metal and dielectric or chemicai vapor deposition techniques. The films of the present invention can be tailored to reflect ultravlolet light c,ver the 300 to 400 nm bandwidth; they can be readily coextruded and can have large 5 surface areas; and they can be iaminated to substrates which are shaped in a variety of useful articles such as a parabolic reflector.
Multilayer films in accordance with the present invention are most advantageously prepared by empioying a multilayered coextrusion device as described in U.S
Patent Nos. 3,773,882 and 3,884,606 the disclosures of which are incorporated herein by 10 reference. Such a device provides a method for preparing multilayered, simultaneously extruded thermoplastic materials, each of which are of a substantially uniform layer thitkness Prefetably, a series of layer multiplying means as are described in U.S. Patent No. 3,759,647 the disclosure of which is incorporated herein by reference may be empioyed.
The feedblock of the coextrusion device receives streams of the diverse 15 thermoplastic polymeric materiais from a source such as a heat plastifying extruder. The streams of resinous materials are passed to a mechanical manipulating section within the feedblock. This section seNes to rearrange the original streams into a multilayered stream having the number of layers desired in the final film. Optionally, this multilayered stream may be subsequently passed through a series of layer multi plyi n~ means i n order to further i ncrease 20 the number of layers in the final film.
The multilayered stream is then passed into an extrusion die which is so constructed and arranged that streamlined flow is maintained therein. Such an extrusion device is described in U.S. Patent No 3,557,265, the disclosure of which is in~orporated by reference herein. The resultant product is extruded to form a multilayered film in which each 25 layer is generally parallel to ~he maior surface of adjacent layers.
The configuration of the extrusion die can vary and can be such as to reduce thethickness and dimensions of each of the layers. The precise degree of reduction in thickness of the layers delivered from the mechanical orienting section, the configuration of the die, and the amount of mechanical working of the film after extrusion are all factors which affect the 30 thickness of the individual layers in the final film.
The ultraviolet reflective films of the present invention find a number of uses As a primary example, they may be used as reflectors in a solar detoxification system. Such a system 20 is depicted schematically in Fig. 2. As shown, groundwater containing organic contaminants is pumped through line 22 by pump 24 to a series of solar reflector arrays 26 35 While only two such arrays are shown for simplicity, it will be understood that multiple arrays may be arranged in series or parallel to treat large quantities of contaminated water The reflector arrays 26 include parabolic-shaped solar ultraviolet reflectmg film 28 laminated to a transparent substrate which reflects the ultraviolet portion of the sun's -S-WO 93/16878 ;~ 81 ~ PCr/US92/10162 energy back toward transparent tubing 30 through which the contaminated l iquid flows As previously explained, a ~atalyst, such as titanium dioxide, may either be mixed with the flowi ng liquid stream or mounted on a porous matrix withjn the tubing 30. The remainder of the solar energy (that is, wavelengths between 400to 2100 nm) passesthrough reflecting film 28, 5 including preferably both the visible and near infrared portions of the spectrum. This prevents undue heating of the liquid stream which would otherwise occur if all of the solar energy was concentrated on the tubing. Alternatively, the visible and infrared solar energy transmitted through the reflector arrays may be collected or concentrated separately for other processes requiring solar heating.
However, in some instances, it may be dffirable to provide some heating to the liquid to enhance a particular catalyzed degradation reaction. ~n those instances, the refiecti ng film of the present invention may be designed to reflect some or all of the solar visible and/or infrared energy. This may be accomplished by laminating the all polymeric ultraviolet reflective film of the present invention to a broadband visible and near infrared (400 to 2100 nm) all polymeric reflecting film.
Alternatively, a sufficient number of layers having optical thicknesses i-n the range needed to reflectthe desired amount of visible and/orinfrared energy may be coextruded with the ultraviolet reflecting layers. As taught in th~ above-referenced applications, such layers may be optically thick ( >~.45 llm) or a combination of optically thick and optically very thi n 20 (<0-09 ~lm), with a portion of the layers being opticallythin (<0.09 ~lrn and <0.45 ~lm), where optical thickness is defined as the product of actual layer thickness and refractive index of the polymer making up the layer.
After catalytic treatment and exposure to the solar ultraviolet energy, the treated liquid may be sent, for example, to a holding basin 33. From there, it may be pumped, via 25 pump 32 and line 34, either back underground or to a commercial or industrial plant for use Other uses of the ultraviolet reflecti ng film of the present i nvention include UV
mirrors which are used in the fields of medical imaging, astronomical telescopes, and microscopy. Chemical reactions also use ultraviolet light as a curing mechanism The reflecti ng film of the present invention may be used to reflect or concentrate such radiation onto 30 chemical reactants. Ultraviolet radiation is also used as a tool for sterilization. The UV
reflective films of the present invention may find use in directing ultraviolet radiation onto articles to be sterilized. Other outdoor uses for the films of the present invention include .
Iaminating the film to windows or skylights to protect interior furnishings from the degradative effects of ultraviolet light. For example, the ultraviolet reflecting films of the 35 present invention could be laminated to or included in automotive window glass to protect the interior upholstery and dashboard.
There is also a need for ultraviolet reflective films in indoor lighting to protect persons, foods, clothing or furniture from the harmful, degradative effects of ultraviolet light g 1 t;
WO 93/16878 . PCI~US92J10162 The reflective films of the present invention could be fabricated into protective tubes around fluorescent light sources. As the film is transparent to visible light, no loss of visible light would occur. Rather, only the undesired ultraviolet radiation v~ould be ~ontained Ultraviolet light Is known to be harmful tothe human eye. The UV reflective film of the present invention may 5 find use in sunglasses or welders~ goggles. The film could be used in umbrellas to prevent ultraviolet radiation from reaching the skin of a person. Further, the film coul~ be used to block UV light from reaching and causing fading and degradation to clothing and furniture Microlithography, industrial micro-machining, and ultraviolet laser reflection are other fields in which the ultraviolet reflecting films of the present inv~ntion may find use.
In order that the invention may be more readily unde-stood, reference is made tothe following examples, which are intended to be illustrative of the invention, but are not intended to be limiting in scope.
Exam~
To demonstrate the ultraviolet reflecting capabilities of the film of the present 15 invention, a computer simulation was run to predict the reflectance characteristics of a two-component polymethyl methacrylate/polyvinylidene fluoride multilayer film. The simulation used a software program entitled "Macleod Thin Film Optics" available from Kidger Optics, Sussex, England. The sum of the optical thicknesses of the layers in the AB repeat unit of the film were assumed to be in the range of from 0.15 ~lm to 0.20 ~lm, and the individual layers 20 were assumed to have optical thicknesses in the range of 0.07 ~lm to 0.11 llm. A refractive index mismatch of 0.07 was assumed based on the actual mismatch of the two polymers when measured atvisible wavelengths.
Fig. 3 depicts the predicted reflectance results for films havi ng 100, 200, 400, 650, and 1300 alternating layers, respectively. As can be seen, as the number of layers in the 25 multilayer film increases, the predicted reflectance of the film approaches 100 percent reflectance in the wavelength range of 300 to 400 nm. This demonstrates the strong ultraviolet light reflecting capabilities of the multilayer films of the present invention.

Claims

1. An ultraviolet light reflective all-polymeric film of at least first and second diverse polymeric materials which do not absorb significant amounts of ultraviolet radiation, the film comprising a sufficient number of alternating layers of said first and second polymeric materials such that at least 30 percent of ultraviolet light of a wavelength of between 300 to 400 nm incident on said film is reflected, said first and second polymeric materials having an average percent transmission of greater than 50 percent between wavelengths of 300 to 400 nm, a substantial majority of the individual layers of said film having optical thicknesses in the range where the sum of the optical thicknesses in a repeating unit of said polymeric materials is between 0.15 µm to 0.228 µm, and wherein said first and second polymeric materials differ from each other in refractive index by at least about 0.03 in the wavelength range of from 300 to 400 nm.

2 The ultraviolet light reflective polymeric film of claim 1 in which said individual layers have optical thicknesses of between 0.07µ m to 0.11 µm.

3. The ultraviolet light reflective polymeric film of claim 1 in which said first polymeric material is polyvinylidene fluoride and said second polymeric material is polymethyl methacrylate.

4. The ultraviolet light reflective polymeric film of claim 1 in which said first polymeric and second polymeric materials are selected from the group consisting of polymethyl methacrylate, polyvinylidene fluoride, polychlorotrifluoroethylene, and polymethylpentene- 1

5. The ultraviolet light reflective polymeric film of claim 1 in which said filmcomprises at least 200 layers.

6. The ultraviolet light reflective polymeric film of claim 1 in which at least 80 percent of ultraviolet light of a wavelength of between 300 to 400 nm incident on said film is reflected.

7. The ultraviolet light reflective polymeric film of claim 1 in which said polymeric film includes first, second, and third diverse polymeric materials of alternating layers in a repeating unit ABCB.

8. The ultraviolet light reflective polymeric film of claim 1 in which said polymeric film includes first, second, and third diverse polymeric materials of alternating layers in a repeating unit ABC.

9. The ultraviolet light reflective polymeric film of claim 1 in which said polymeric film has been laminated or coextruded with a transparent substrate material or with a visible and/or infrared light reflecting body.

10. The ultraviolet light reflective polymeric film of claim 1 in which the filmincludes protective skin layers on either or both major exterior surfaces of said film.