Spectrally-selective polymer window films: New 3M technology in sun protection

    The need for rational use of energy has become increasingly significant over the past few decades. This article is devoted to an alternative method of protection against thermal energy of the sun coming through the windows - energy-saving glazing with optically transparent window films.





    Terminology
    Some of the terms in abbreviation format are given to simplify the perception of the material:

    • Light-to-solar-gain (LSG) ratio -
    Visible light transmission coefficient with thermal energy cut-off • Visible light transmission (VLT) - Visible light transmission coefficient
    • Solar heat gain coefficient (SHGC) Solar heat transfer coefficient
    • Near infrared rejection (NIR) Long wavelength
    • Water vapor transmission rates (WVTR) - Water vapor transmission coefficient
    • Metal-free solar reflecting films (SRF) - Metal-free sun-protection films
    • Antimony tin oxide (ATO) - antimony oxide and tin
    • Indium tin oxide (ITO), yn oxide Ia and tin
    • Multilayer optical film (MOF) The multilayer optical film

    Starting the conversation about optically transparent sun-protection films, we recall that white light, ultraviolet (UV) and infrared (IR) are all ranges of the spectrum of electromagnetic radiation, along with Gamma and X-ray radiation.



    In our article, we will talk about the solar spectrum in the range of lengths from 0 to 2.5 Nano Meters (nm), about how Dr. Raghu Padiyat, a 3M researcher, made a unique invention of multilayer optically transparent window films that allow white light to freely penetrate into the room and at the same time block UV and IR radiation. There are no sharp transitions between the ranges, but if we consider the length of the solar spectrum, then 3% is ultraviolet, 44% is visible, 53% is infrared.



    The thermal transmittance of visible light with a thermal cutoff (LSG), referred to as the ratio of the visible light transmission coefficient (VLT) and the solar heat transfer coefficient (SHGC), is often used to determine the effectiveness of a window film. This indicator is suitable for applications in which, in addition to reducing the proportion of passive solar energy entering through the glazing of buildings, an increased level of internal illumination is also required.

    Up to now, all window films with a high LSG coefficient on the market have been manufactured on the basis of the dielectric – silver – dielectric structure [3, 4]. In general, up to three silver layers are used in the performance of these films, which leads to a high reflectance of infrared rays of the long-wavelength NIR and VLT spectra of approximately 70%.



    Silver is selected due to its unique properties [5,6]. One of the disadvantages of using silver is its susceptibility to oxidation. Poor sealing of the edges of the film can lead to darkening and corrosion around the perimeter.



    The problem of corrosion can be solved by using silver alloys instead of pure silver, as well as by carefully sealing the edges of the film. These films also have shielding properties and can interfere with the operation of electronic systems of intra-district communication, GPS, mobile phones, etc.

    It is well known that the industrial production of films based on silver resonator reflectors is difficult, since a very small variation in the thickness of the silver layer results in a significant discoloration, especially when viewed in reflected light. In addition, the presence of silver in coatings requires compaction of the edges of the applied film.

    Another disadvantage of the used window films based on silver / dielectric deposition technology is that these films have a very low water vapor permeability rate (WVTR). Water is used for the installation of films and the removal of its residues between the adhesive layer and the film is extremely important.

    Polymer films reflecting infrared rays



    3M reflective infrared rays polymer multilayer films were developed by 3M for use in automotive windshields and other applications [7,8].

    Anticipating the description of the technology, we offer you a short video illustrating the principle of the film:



    Earlier, Alfrey and others showed that a polymer film comprising hundreds of layers of two materials with different refractive indices can be obtained by coextrusion with the formation of a fluorescent film [9]. The use of polymer multilayer films with birefringent optical systems was further developed by 3M [10, 11]. The use of birefringent materials in these structures leads to several unique properties that cannot be obtained using thin-film optics coated with a spraying method [12].

    In these films, the width of the spectrum and the location of the strip boundary are determined by the thickness of each pair of layers. The thickness of these layers is selected so that a single reflection band appears in the infrared part of the spectrum of electromagnetic waves. With the appropriate choice of the left and right border of the strip and precise control of the thickness of the pair of layers, highly efficient infrared reflectors of the long wavelength region of the spectrum with high transmittance of visible light can be created. The optical properties of long-wavelength infrared reflectors created from polymeric materials are advantageous due to low optical absorption, insignificant optical dispersion, and optical constants of birefringent systems. These films may have a high visible light transmission, sharp reflective band boundaries and low out-of-band unevenness. In a structure with an ABAB layer of a simple ¼ wave, in which A and B are two polymeric materials with different refractive indices, for structural reasons, they limit the reflection band in the range from 800 nm to 1200 nm. A further increase in the width of the spectrum will result in secondary reflection bands, giving color to the film. Since the spectrum of incident solar radiation extends far beyond the value of 1200 nm, it is necessary to provide means to reduce the fraction of solar energy entering through the glazing and exceeding the value of 1200 nm. for structural reasons, limit the reflection band in the range from 800 nm to 1200 nm. A further increase in the width of the spectrum will result in secondary reflection bands, giving color to the film. Since the spectrum of incident solar radiation extends far beyond the value of 1200 nm, it is necessary to provide means to reduce the fraction of solar energy entering through the glazing and exceeding the value of 1200 nm. for structural reasons, limit the reflection band in the range from 800 nm to 1200 nm. A further increase in the width of the spectrum will result in secondary reflection bands, giving color to the film. Since the spectrum of incident solar radiation extends far beyond the value of 1200 nm, it is necessary to provide means to reduce the fraction of solar energy entering through the glazing and exceeding the value of 1200 nm.





    Infrared absorbing nanofilters were studied for use in glazing [13,14]. These materials have a fairly high transmission of visible light, as well as significant absorption in the long-wave infrared region of the spectrum. Such materials can be applied to polymer films that absorb infrared rays to further increase the solar heat transfer coefficient of the glazing system. Tin-antimony oxide (ATO) coatings are especially interesting since their absorption band extends beyond the long-wave infrared region of the spectrum.

    RESULTS AND DISCUSSION



    The simulated and measured light transmission spectra of a multilayer polymer film consisting of 224 layers, made using PET and PMMA, are shown in Figure 1a. As can be seen from Figure 1a, almost all light in the range 850 nm - 1200 nm is reflected in the absence of transmission loss (except for the loss of Fresnel zones) in the visible part of the spectrum and in the infrared region of the spectrum beyond 1200 nm. When using ATO coating on the inside of this film, the transmission in the visible part of the spectrum can be adjusted to about 70%, while almost the entire long-wavelength IR region in the range 850–2500 nm can be blocked (Figure 1b) while maintaining a high multilayer reflection coefficient material. The thickness or amount of ATO in the polymer layer can be increased or decreased as desired to control the visible light transmission coefficient. Particles such as carbon black, which have the ability to absorb in the visible part of the spectrum, are used to produce window films with an excellent transmittance of visible light [15]. In addition, these particles can be included to drastically reduce the transmission coefficient in the visible part of the spectrum without significantly changing the transmission coefficient of infrared radiation or the concentration of ATO in the coating.



    Figure 1a and 1b: Simulated and measured light transmission spectra of an uncoated polymer multilayer film (Figure 1a) and an ATO coated film (Figure 1b).

    In contrast to sprayed films with a silver / dielectric structure, all reflection bands based on dielectric components have a transition to shorter strip lengths with an increase in the angle of incidence (far from normal incidence). This angle shift is caused by the dependence of the cosine of the phase angle between the rays reflected from adjacent contact surfaces.

    Due to the increase in the angle of incidence, the centers of the reflection bands with polarization perpendicular to the plane of incidence of the beam and with polarization parallel to the plane of incidence of the beam pass to shorter wavelengths taking into account the effective phase thickness of the layers. A high birefringent polymer can be used to create dielectric reflectors that maintain or increase their reflection coefficient with increasing angle of incidence. In addition, for non-normal incidence, the polarization effects in isotropic materials limit the steepness of the border of the natural light strip, which can have a significant effect on the color purity.

    Birefringent polymers can be used to create a reflector that has a consistent small-wavelength band boundary at all angles for both light with polarization parallel to the plane of incidence of the beam and light with polarization perpendicular to the plane of incidence of the beam, eliminating these difficulties.

    Since the reflection band of the multilayer polymer reflector goes over to shorter wavelengths that contain more solar energy (Figure 3a), the solar heat transfer coefficient rapidly decreases at large incidence angles. As can be seen from Figs. 2a and 2b, this transition is significantly higher in multilayer polymer structures compared to window films based on the dielectric / silver structure. The optical properties of these two types of film when falling along the normal and at 60 from the normal (set as 0 in the table and in the figures) are presented in Table 1.

    It should be noted that there are no standards for off-axis characteristics. Industry Standards Methods (see Council for the Assessment of Translucent Structures, www.nfrc.org) and software (Window 5, available for download from windows.lbl.gov/software/window/window.html) are intended for off-center calculations taking into account the type of materials, based on the algorithm described by Furler [15], these calculations lead to insufficient approximation for birefringent materials. As a result, calculations of annual energy requirements provide an approximate forecast for the savings achieved by using multilayer polymer window films. In addition, since the incident solar energy varies from place to place and depends on a large number of factors, including water vapor that can condense and precipitate, the surface albedo, recharge and concentration of atmospheric pollutants, among other things, the solar heat transfer coefficients vary depending on the shape spectrum of incident solar radiation.

    Table 1: Solar transmission properties of a polymer window film for after-sales application and a window film with a dielectric / silver structure.


    A type VLT (%) VLR (%) SHGC UV Reflection (%)
     0 60 0 60 0 60  
    Multilayer polymer with ATO 69 60 8.5 thirteen 0.51 0.42 99.9
    7-layer ITO / Ag 69 62 8.0 12 0.47 0.44 99.9




    Figure 2a and 2b: Transmission coefficient of an ATO-coated multilayer polymer film (Figure 2a) and a 7-layer ITO / Ag / ITO film (Figure 2b) at normal incidence and at 60 from normal incidence.



    CONCLUSION


    Infrared reflective polymer multilayer films were coated with ATO infrared absorbing nanoparticles to create window films for aftermarket application with high light transmittance and high heat transfer coefficient. It was shown that these films have a higher coefficient of heat removal at increased elevation angles of the sun. Since these films do not contain any sprayable layers, they have high water vapor permeability rates and are easier to install. A comparison of these films and spray films with a silver / dielectric structure is presented.

    Literature
    1. www.v-kool-usa.com
    2. www.vista-films.com
    3. PH Berning and AF Turner, “Forced Transmission in Absorbent Films Used in the Design of a Band-Pass Filter,” 47 (3), 230, J . Opt. Soc. Am., 1957.
    4. PH Berning, “Principles for the Design of Architectural Cladding,” 22 (24), 4127, Appl. Opt., 1983
    5. J. Boettcher, M. Scott, B. Koster, and M. Kominami, “Solar-reflective and metal-free film results in multi-layer performance,” p. 513, Glass Processing Days, June 2001 year
    6. J. Boettcher, M. Kominami, and M. Scott, “Results of multilayer performance using a metal-free and neutral color reflecting solar rays”, p. 538, Glass Processing Days, 2003.
    7. T. Alfrey Jr ., EF Gurnee and WJ Schrenk, “Physical Optics of Fluorescent Multilayer Plastic Film”, 9 (6), 400, Poly. Eng. Sci, 1969.
    8. JM Jonza, MF Weber, AJ Ouderkirk and CA Sover, “Optical Film”, US Patent 5,882,774, March 16, 1999.
    9. JA Wheatley, MF Weber, AJ Ouderkirk, “Optical Film with sharp border ", US patent 6,157,490, December 5, 2000
    10. MF Weber, CA Stover, LR Gilbert, TJ Nevitt, and AJ Ouderkirk, “Large birefringent optics in multilayer polymer reflectors”, Science, 287, 2451, 2000.
    11. S. Schlem and GB Smith, “Weak LaB6 nanoparticles in polymers as optimized sun glazing ”, 82 (24), 4346, Appl. Phys. Lett., 2003.
    12. GB Smith, MJ Ford, C. Masens and J. Muir, “Energy-Saving Coatings in the NanohouseTM Project,” 4, 381, Current Applied Physics, 2004.
    14. DJ McGurran, RL Brott, JA Olson, US Patent No. 6,811,867, November 2, 2007.
    15. RA Furler, “Angular Dependence of the Optical Properties of Homogeneous Glasses,” 97 (2), 1129, ASHRAE Trans., 1991.

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    UPD: Friends, thanks for the comments and your feedback!
    We want to offer some more articles on this topic:

    Publications of Dr. Raghu Padiyath, inventor of optically transparent films

    Energy efficient IGUs with polymeric nearinfrared reflecting films
    Spectrally Selective Window Films

    Also the results of testing the Composit laboratory of our films: http: // www.svetoplast.su/prestige.html

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