Patent Application
20100266801
The invention relates to solar window films for controlling the influx of solar radiation through windows, and more particularly relates to an improved window film having low emissivity and excellent durability.
Solar control window films are known for reflecting incident solar radiation away from windows and associated structures. As used herein, the terms “solar control window film,” “solar window film,” “window film” and “film” are used interchangeably unless indicated otherwise by their context or usage. Solar window films typically include a thin metallized polymeric film which can be adhered to an interior surface of a glass window with a suitable adhesive material. Typically, a thin layer of reflective metal such as gold, silver, copper, aluminum, or the like is applied to one face of a polymeric substrate by vacuum or vapor deposition, for example. Obtaining good adhesion of the deposited metal to the polymeric substrate can be problematic. In order to protect the metal layer from scratching and from attack by cleaning agents and the like, a second polymeric film can be laminated over the metal layer using a suitable adhesive material.
Solar window films are typically characterized by several characteristics or parameters. For example, a solar window film is often characterized by the percentage of total incident solar radiation (including infrared, visible, and ultraviolet solar radiation) which is reflected by the film when the film is applied to a transparent glass window. This attribute is known as “solar reflectance.” Solar window films also are characterized by the percentage of total incident solar radiation that is absorbed (“solar absorptance”) and by the percentage of total incident solar radiation which passes through the glass and film (“solar transmittance”). The solar reflectance value, solar absorptance value, and solar transmittance value of a solar window film necessarily add to 100 percent (1.0). Solar window films are also characterized by the percentage of incident visible light which passes through the glass and film, which is known as “visible light transmittance.”
One problem with laminated solar window films is that the adhesive layers which bond the laminated layers together can increase the solar absorptance of the film, and thereby promote unwanted heating of the solar film. In an attempt to alleviate this problem, others have attempted to protect the metal layers of solar films with protective coatings rather than laminated protective films. Unfortunately, the protective coatings of known non-laminated solar films may not adequately adhere to the metal layers which they are intended to protect, and the coatings can be prone to scratching, hazing and/or separation from the metal layers to which they are applied when the films are repeatedly cleaned with common window cleaning equipment and cleaning agents. Accordingly, such films must be handled, installed and cleaned with extreme care.
Another characteristic of solar window films is how they respond to far-infrared radiation. Far-infrared radiation naturally radiates from a surface of a warmer object to a surface of a colder object. Accordingly, far-infrared radiation can naturally radiate from a warmer object to a colder exterior window, for example. In particular, when an interior space is heated, thermal energy can be lost when a colder exterior window absorbs far-infrared radiation from warmer interior objects rather than reflecting such energy back into the interior space. In addition, when an interior space is being cooled, unwanted gains in thermal energy can occur when an exterior window heats up due to absorption of far-infrared radiation from warmer exterior objects.
In order to alleviate this problem, low-emissivity solar window films have been developed. “Emissivity” refers to a surface's ability to absorb far-infrared radiation. Accordingly, the term “low-emissivity” is used to describe surfaces that are capable of reflecting rather than absorbing a substantial portion of incident far-infrared radiation. Though known low-emissivity window films can reduce thermal losses or gains which are attributable to unwanted absorption of far-infrared radiation, known low-emissivity window films are prone to the same problems described above for solar window films in general. More specifically, laminated low-emissivity window films include one or more adhesive layers which can cause unwanted absorption of thermal energy by the films. In addition, the protective coatings of known non-laminated low-emissivity window films are prone to scratching, hazing and/or separation from the metal layers to which they are applied when the films are repeatedly cleaned with common window cleaning equipment and cleaning agents, and must be handled, installed and cleaned with extreme care.
In one embodiment, a non-laminated low-emissivity window film according to the invention includes a flexible polymeric substrate having a first surface and an opposed second surface. A metal-adhesion promoting layer is disposed on the first surface, a reflective metal layer is disposed on the metal adhesion-promoting layer, and a transparent polycarbonate coating is disposed on the metal layer. The window film can have an emissivity of about 0.27 to about 0.33 and a visible light transmission of at least about 17 percent.
In another embodiment, the invention includes a method of producing a low-emissivity window film. The method includes providing a flexible polymeric substrate, and depositing a metal layer on one surface of the polymeric substrate to form a metallized surface. The method further includes applying a thin first coat of a UV-curable polycarbonate coating over the metallized surface, drying the first coat, and exposing the first coat to ultraviolet light until the first coat is partially cured. The method also includes applying a thin second coat of a UV-curable polycarbonate coating over the first coat, drying the second coat, and exposing the first coat and the second coat to ultraviolet light until both the first coat and the second coat are fully cured.
These and other aspects and features of the invention will be understood from a reading of the following detailed description together with the drawings.
The reflective metal layer 14 includes a highly reflective metal or metal oxide. In a preferred embodiment, the reflective metal layer 14 is aluminum. Other metals which can be used for the reflective layer are gold, silver, chrome, titanium, nickel-chromium, and the like. The metal used for the reflective metal layer 14 can be selected in order to provide the film 10 with a desired color, for example. The metal layer 14 can be applied to one surface of the polymeric substrate 12 by resistive vapor deposition in a vacuum. Alternatively, the metal layer 14 can be applied by any other suitable process. The density of the metal layer 14 can be varied in order to produce a film 10 having a desired balance between overall visible light transmission and emissivity. Generally, the greater the density of the metal layer 14, the lower the visible light transmission and the lower the emissivity of the film 10. Preferably, the density of the metal layer 14 is selected such that the film 10 has an emissivity of about 0.27 to about 0.33, and a visible light transmission of about 17 percent to about 22 percent. Lower emissivities for the film 10 can also be achieved at lower levels of visible light transmission, and higher levels of visible light transmission can be achieved at higher levels of emissivity.
The protective coating or layer 16 is applied over the metal layer 14. Preferably, the protective coating 16 is extremely thin and substantially invisible to infrared radiation such that the coating will not adversely affect the low emissivity of the film 10. In other words, the protective coating 16 is preferably extremely thin and transparent such that infrared radiation passes through the protective coating 16 with little or no absorption by the coating 16. In one embodiment, the protective coating 16 is a hard and highly transparent UV-curable polycarbonate coating, and has an extremely thin thickness of about 1 micron to about 3 microns. Such a polycarbonate coating can include a mixture of pentaerythritol tetraacrylate, pentaerythritol triacrylate, an acrylic ester, a diluent, a photo initiator, and a surface modifier. The polycarbonate coating can also include a trifunctional acid ester to promote adhesion of the protective coating 16 to the metal layer 14. One such trifunctional acid ester is CD9053, which is available from Sartomer Company, Inc., of Exton, Pa. In one embodiment, a mixture is formed which includes about 75-85 percent pentaerythritol triacrylate, about 8-9 percent diluent, about 6-8 percent photo initiator, about 0.1-0.2 percent surface modifier, and about 3-5 percent trifunctional acid ester. This mixture can be combined with a solvent at a ratio of about 1 part mixture to about 4 parts solvent. A thin coat of the resulting solution can be applied to the surface of the metal layer 14 and dried to evaporate the solvent and leave behind the remainder as solids. The remaining solids can be cured by exposure to ultra violet light, resulting in a hard and scratch-resistant low-emissivity protective coating 16.
A polycarbonate protective coating 16 as described above is superior to the protective coatings of known low-emissivity window films due to its high transparency, invisibility to infrared radiation, hardness, and high resistance to hazing, scratching, other mechanical damage, and exposure to common window cleaning agents. In one embodiment, the protective coating 16 has a maximum haze increase of less than about 3 percent when tested according to ASIM D 1044. One process for forming a protective coating layer 16 on the metal layer 14 is described in detail below.
Another embodiment of a non-laminated low-emissivity window film 20 according to the invention is shown in
The adhesion-promoting layer 23 facilitates the adhesion of the metal layer 24 to the coated substrate 22, and substantially reduces the possibility that the metal layer 24 might peel away or otherwise separate from the base substrate 22. In one embodiment, the adhesion-promoting layer 23 can be like the adhesion-promoting layers described in U.S. Pat. No. 6,114,021 to E.I. du Pont de Nemours and Company, for example, the disclosure of which is hereby incorporated by reference in its entirety. One example of a substrate 22 having an adhesion-promoting layer 23 which can be used to produce a film 20 according to the invention is Melinex® X6560, which is available from DuPont Teijin Films™, Hopewell, Va. Melinex® X6560 is a polyester film having excellent optical characteristics and a pretreatment 23 on one surface which promotes metal adhesion to the polyester film. As shown in
Another embodiment of a non-laminated low-emissivity window film 30 according to the invention is shown in
An additional embodiment of a film 40 according to the invention is shown in
As discussed above, the polycarbonate protective coatings 16, 26, 36, 46 of the films 10, 20, 30 and 40 are superior to the protective coatings of known low-emissivity window films due to their high optical transparency, their greater hardness, their greater resistance to hazing, scratching, other mechanical damage, and their greater resistance to damage from exposure to common window cleaning agents. Unfortunately, applying an extremely thin and uniform protective polycarbonate coating to a metallized surface of a window film can be fraught with difficulties. First, when a thin coat of polycarbonate coating is applied to a metallized surface, the wet coating tends to lay on the wetted surface in a non-uniform manner such that thick areas and thin areas are formed. Once cured, such a non-uniformly applied polycarbonate coating may have unacceptably irregular optical qualities, and thinner areas of the coating may not adequately protect the underlying metal layer from damage. In order to solve these problems, the inventors of the present invention have developed a process for applying an extremely thin, highly transparent and substantially uniform polycarbonate coating to a metallized surface of a low-emissivity window film. One embodiment of such a process is illustrated in
As shown in
In one embodiment, at least two coats 36a, 36b of a protective coating are applied over the metal layer 34. As shown in
As shown in
The inventors have discovered that the two-coat process described above yields a protective polycarbonate coating 36 which has a substantially uniform thickness, has exceptional transparency, strongly adheres to the metal layer 34, and is exceptionally hard and highly resistant to hazing, scratching, other mechanical damage, and damage due to chemical attack by cleaning solvents. For example, the maximum haze increase of the coating 36 when tested according to ASTM D 1044 can be less than about 3 percent. Accordingly, a low-emissivity film 30 having a protective polycarbonate coating 36 applied by such a method is believed to be substantially more durable than the protective coatings of other known non-laminated window films having low emissivities.
The above descriptions of various embodiments of the invention are intended to describe and illustrate various aspects and features of the invention without limiting the invention thereto. Persons of ordinary skill in the art will understand that various changes and modifications can be made to the described embodiments without departing from the scope of the invention. All such changes and modifications are intended to be within the scope of the appended claims.
