Stock material or miscellaneous articles – Light transmissive sheets – with gas space therebetween and...
Reexamination Certificate
1998-04-08
2002-05-21
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Light transmissive sheets, with gas space therebetween and...
C428S332000, C428S426000, C428S430000, C428S212000, C359S359000, C052S786110, C052S786120, C296S084100, C296S096140
Reexamination Certificate
active
06391400
ABSTRACT:
TECHNICAL FIELD
This invention pertains generally to the field of thermal control films. More particularly, the present invention pertains to visibly transparent heat reflective thermal control films which comprise an optical coating and which are suitable for use in glazing applications. More specifically, the present invention pertains to coated polymer sheets which comprise a thin flexible polymeric sheet (which serves as the substrate), and at least one multilayer coating coated thereon, said coating comprising two or more contiguous alternating layers of high and low index of refraction inorganic dielectric material. The coated polymer sheets are characterized by a high transmission of visible radiation (i.e., visibly transparent) and a high reflectance at one or more near infrared radiation center wavelengths (i.e., heat reflective), and rely primarily on the interference effects of the dielectric layers to achieve these results.
BACKGROUND
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification immediately preceding the claims. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
The term “glazing” is used herein in its conventional sense and relates to the use of transparent materials (e.g., glass) to fill apertures as in, for example, windows, viewports, and the like. Vehicular glazing generally refers to the use of transparent materials such as windows, windshields, windscreens, canopies, panes, and the like, in vehicles such as automobiles, trains, boats, aircraft, and spacecraft. Architectural glazing generally refers to the use of transparent materials as windows, viewports, skylights, panes, and the like in buildings, such as domestic buildings and commercial buildings.
Conventional window glass, which has been used since medieval times as the glazing material of choice, is highly transparent to visible radiation. Visible light is transmitted through the glass and is absorbed or reflected by materials on the opposite side (e.g., floor, walls, flirniture, plants, and other objects in the interior of a building). These absorbing materials re-emit some of the absorbed radiation according to their temperature. Near ambient temperatures (i.e., ~5 to 40° C.), these objects have a blackbody peak emission in the infrared region at approximately 7-10 microns (see below). Although conventional glass is largely transparent between ~0.2 microns and ~3.5 microns, it is substantially opaque at infrared wavelengths greater than ~4 microns, and the re-radiated heat (e.g., at 7-10 microns and longer wavelengths) is either absorbed by the glass or reflected back (e.g., into the interior of the building). This is the fundamental basis for glass-covered greenhouses.
While windows often enhance the aesthetics and futnctionality of buildings and vehicles, they can also cause undesirable gain or loss of heat. In warm climates, exterior heat may enter through windows, thereby increasing air conditioning loads. In cold climates, interior heat is lost through windows, thereby increasing heating demands. Increases in the size of windows used in automobiles coupled with reductions in the size of vehicular air conditioners have increased the need for vehicle windows which reduce heat load. See, for example, Chiou, 1986; Nyman, 1990; Huber, 1988; Hymore etal., 1991; Lynam, 1990.
Heat loss through a window may arise from a convective/conductive/emissive process, for example, where interior hot air raises the temperature of the glass, by convection, the thermal energy is distributed throughout the glass, by conduction, and some of the thermal energy is emitted or radiated, by emission, to the exterior. Heat loss by emission can be ameliorated by reducing the emissivity of the window glass, for example, by introducing a low emittance or “low E” (for infrared) coating (which is typically a thin metal film). Emissivity or emittance refers to the propensity of a surface to emit or radiate radiation of a specified wavelength, and is quantified as the ratio of radiant flux per unit area emitted by body to that of a blackbody radiator at the same temperature and under the same conditions. Thus, a perfect blackbody has an emissivity of 1.0. Ordinary window glass has an infrared emissivity of about 0.84. Window glass with a “low E” coating has a much lower infrared emissivity, often as low as 0.15, and heat loss through such a window is greatly reduced.
Optical coatings have found widespread application in the field of glazing, particularly as a means to control heat loss and/or heat gain. In many applications, optical coatings are used to “block” the transmission of electromagnetic radiation (e.g., infrared radiation, visible radiation, ultraviolet radiation) to some degree. In some applications, it is desirable to block some or all of the electromagnetic radiation of a particular wavelength band while transmitting some or all of the electromagnetic radiation of another particular wavelength band.
Thus, in one conunon application, an optical coating is employed to substantially block infrared electromagnetic radiation while substantially transmitting visible electromagnetic radiation. Such optical coatings are often referred to as “heat mirrors,” “hot mirrors,” or “thermal control films.” For glazing applications, it is usually desirable that these optical coatings also be substantially visibly transparent.
An important application for optical coatings is as thermal control films for solar radiation. The sun, which is the source of solar radiation, is a modest yellow star with a diameter of about 1.4 million kilometers at an average distance from the earth of about 150 billion kilometers. The sun has interior temperatures on the order of 8 to 40 million K and a surface temperature of about 5800 K. The rate of energy emission from the sun is about 3.8×10
23
kW, of which 1.7×10
14
kW is intercepted by the earth. Of this amount, 30% is reflected, 47% is converted into low temperature heat and re-radiated into space, and about 23% powers the evaporation and precipitation cycle of the earth's biosphere. The extraterrestrial solar irradiance at normal incidence is about 1373 W/m
2
. At an air mass of one (see below), the irradiance is about 925 W/m
2
, the bulk of which falls in the wavelength band from about 200 nm (in the ultraviolet) to about 2000 nm (in the near infrared).
To accurately predict solar intensity in the visible region, the sun may be characterized as a blackbody with a temperature e of approximately 5800 K. To accurately predict solar intensity in the infrared region, the sun may be characterized as a blackbody with a temperature of approximately 5900 K. Blackbody radiation may be modeled using a Planck distribution, according to which the energy density in the range &lgr; to &lgr;+d&lgr;, denoted dU(&lgr;), is given by:
dU(&lgr;)=[(8&eegr;hc/&lgr;
5
)(e
−hc/&lgr;kT
)/(1-e
−hc/&lgr;kT
)]d&lgr;
wherein T is the blackbody temperature, &lgr; is the wavelength, h is the Planck constant, c is the speed of light, and k is the Boltzmann constant. The wavelength of peak emission, &lgr;
max
, as a function of temperature, may be determined for the Planck distribution as:
T&lgr;
max
=hc/5k=2.878×10
−3
m K
Using this model, and a temperature of 5800 K, the sun's peak emission occurs at approximately 0.50 microns (i.e., 500 nm), near the middle of the visible region. By comparison, a human body with a surface temperature of about 25° C. (~300 K) has peak emission at approximately 9.6 microns, well into the far infrared region. An object which is hot to the touch at about 100° C. (~375 K) has peak emission at approximately 7.7 microns.
Solar radiation is attenua
Fulton Michael L.
Russell Thomas A.
Stout, Uxa Buyan & Mullins, LLP
Thibodeau Paul
Zacharia Ramsey
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