1. Field of the Invention
This invention relates to transparent heated windows having an outer surface of hard plastic suitable for use in lightweight aircraft. In the past, laminated windows for aircraft have been provided with internally located heating circuits that are energized to remove fog, mist or ice that deposits on the outer surface of the window in flight and electroconductive anti-static coatings on the outer surface connected to ground to dissipate any charge of static electricity that develops during operation of an airplane before the static charge is dissipated through the heating circuit by forcing an electrical path through the window thickness from the outer surface where the static charge develops to the heating circuit.
One type of such window is composed of laminated glass comprising alternate plies of rigid transparent dielectric material, such as glass or a well known substitute for glass such as polycarbonates, acrylic plastics, polyester resins and certain hard polyurethanes, with plies of relatively flexible interlayer material, such as plasticized polyvinyl butyral and polyurethanes, bonding the plies of rigid dielectric material together to form a laminated window. In laminated glass windows, the anti-static coatings are usually continuous transparent metal or metal oxide coatings of low or moderate electroconductivity bonded to the outer glass sheet of the window. Laminated windows containing one or more glass sheets or other dielectric material layers in lieu of glass are provided with a transparent electroconductive heating circuit.
In one type of heated laminated window, the heating circuit is carried by the relatively flexible plastic interlayer sheet laminated between rigid transparent dielectric plies to form an electrically powerable transparent laminated window. The electroconductive heating circuit of this type is either in the form of a transparent film carried on a carrier film embedded within an interlayer or a series of wires embedded directly within the interlayer. Transparent electroconductive heating circuits and heating circuits composed of wire have also been applied on the inner surface of outer plies of rigid transparent dielectric materials such as glass facing an adjacent flexible interlayer. Heating circuits containing electroconductive wire as the heating element either embedded in the flexible interlayer or adjacent to an interlayer surface could conduct only a limited amount of current before optical distortion resulted from a steep thermal gradient between the heating wire and the space between adjacent wires. Therefore, wire embedded heating circuits of the prior art had limited usefulness in aircraft windows.
In light planes where weight of windows is important, it is preferred to substitute, in place of glass, dielectric materials having less density than glass, such as plastics, for example, acrylics and polycarbonates, as the relatively rigid transparent dielectric layers of the laminated window. However, transparent electroconductive coatings of metal or metal oxide materials of suitable electroconductivity consistent with optical transparency require relatively high temperatures for their application to a substrate. Therefore, while it is suitable to apply such coatings on glass substrates by pyrolysis of metal salt compositions at high temperatures or by cathode sputtering, which is also accomplished at elevated temperatures slightly less than those required for pyrolysis, the high substrate temperatures required for pyrolysis or cathode sputtering make it impossible to obtain satisfactory transparent electroconductive coatings on plastic substrates such as acrylics and polycarbonates without degrading the composition of the plastic substrate during the application of the transparent electroconductive film. Furthermore, when the film is applied at a temperature below which the plastic substrate tends to deteriorate, the film that forms has insufficient electroconductivity to provide the heat needed for the purposes intended.
When an electroconductive heating circuit is applied to the inner major surface of the outer layer of a laminated window in the absence of a so-called anti-static circuit on the outward facing surface thereof, the circuit provides a portion of an electrical path for grounding a discharge of static electricity that develops on the outer surface of the window when the latter is installed in a plane in flight. Since the current path of the discharge is through the thickness of the outer layer, a current discharge causes a hole through said entire thickness of the outer layer, thus weakening the latter and eventually causing the panel to fail in service.
In order to avoid damage to laminated windows resulting from the discharge of static electricity through the thickness of the outer layer, transparent electroconductive films produced by pyrolysis have been coated onto the outwardly facing surface of the outer glass layer of the laminated aircraft windows containing an outer glass layer. While films of suitable durability and electroconductivity can be formed on the outer surface of glass sheets to enable the latter to dissipate static electricity before it discharges through the heating circuit for a reasonable period of service, the requirements for a low temperature of application onto a plastic surface has made it difficult, if not impossible, to obtain films to sufficient durability to serve as anti-static circuits when the latter are applied to the outwardly facing surface of an outer layer of a laminated aircraft window when the outer layer is composed of a plastic, such as acrylic plastic or polycarbonate.
Prior to the present invention, none of the alternate transparent electroconductive coatings available for use as anti-static coatings on plastic layers could provide the necessary combination of optical transparency, electroconductivity and durability in service for even a single flight. Accordingly, attempts were made to develop a coating of sufficient durability to last for a single flight so that each time a plane landed, the anti-static coating could be replenished. However, the latter solution is not desirable because it requires additional operations each time a plane lands and, furthermore, no suitable anti-static coating has been developed which can be applied to a plastic substrate at temperatures normally encountered at airports.
2. Description of the Prior Art
From the foregoing, it is obvious that there is a need to overcome the several disadvantages of the laminated aircraft windows so as to provide a durable static electricity dissipating circuit on the outer surface of the window, a heating circuit that does not develop poor optical properties, and suitable bonding between adjacent layers of the laminated window. Also, a convenient method of producing a laminated window with these improved characteristics is needed.
U.S. Pat. No. 2,813,960 to Egle et al shows a laminated heated window in which heating wires are sewn or embedded in an organic interlayer material such as cellulose derivatives, polyvinyl, polyamides or silicones or in ceramic materials as well as glass so that these materials can be generally used for area heating either in transparent or opaque bodies. The heating element is completely embedded in the insulating heated body material.
Other patents that use metal filaments or wire to carry electric current in an electroconductive transparent panel include U.S. Pat. Nos. 2,526,327 to Carlson; 2,932,710 to Coale et al; 3,484,583 and 3,484,584 to Shaw; 3,729,616; 3,795,472; 3,745,309 and 3,895,433 to Gruss; 3,414,713 to Reifeiss; 3,409,759 to Boicey et al; 3,223,829 to Davy et al; and 3,888,711 to Breitner. These latter references are directed to glass-plastic laminates in which electroconductive wires are embedded within the interlayer of the laminate so that the heating wires are supported within the plastic interlayer material.
Other laminated heated glass windows having electroconductive elements applied as transparent coatings to an inner surface of an outer glass sheet are found in many patents. U.S. Pat. No. 3,261,739 to Porter is typical.
In lightweight airplanes, the mass of laminated window units is an important factor. Hence, it would be desirable to obtain heated windows using materials lighter than glass.
U.S. Pat. No. 3,310,458 to Mattimoe and Hofmann discloses a laminate containing an outer ply of a stretched acrylic resin, a vinyl butyral resin interlayer, and another layer of plastic having a continuous electroconductive film facing the interlayer. The electroconductive film is applied to the plastic layer other than the outer ply because of the difficulty of applying a continuous electroconductive film on stretched acrylic without harming its resistance to crack propagation. Such a unit is subject to destruction due to the build-up of static electricity on its exterior surface, which periodically discharges through the film applied to the other plastic layer. If a surface coating is applied to the outer surface of an outer acrylic sheet at a substrate temperature below that which would cause substrate damage, it must be applied by vacuum evaporation. However, such coatings are not permanent and their anti-static properties are soon lost as the coating wears out.
Other heated laminated windows for aircraft used prior to the present invention found in many patents, of which U.S. Pat. No. 3,816,201 to Armstrong and Hoover is typical, comprise a pair of glass sheets laminated together by a composite interlayer consisting essentially of a carrier layer of polyethylene terephthalate polyester film ("MYLAR") containing a gold electroconductive coating on one surface thereof and adhered by layers of polyvinyl butyral to outer sheets of glass. This type of system has several disadvantages. The laminated unit has a tendency to delaminate at the surface between each interlayer and the gold film and also between the interlayer and the carrier layer. Furthermore, it is necessary to use in these units bus bar materials, which cure at low temperature and which are not very durable, in combination with the gold film that forms an electroconductive component of the heating circuit.
U.S. Pat. No. 2,470,509 to Marini discloses the application of a pair of heating wires disposed near the opposite surfaces of an interlayer disposed between two glass sheets. The interlayer is press polished before it is laminated to the glass sheets and the heating wires are disposed at or near the interface of a relatively rigid transparent sheet of glass and a relatively flexible sheet of interlayer material.
U.S. Pat. No. 3,629,040 to Hinton et al discloses a method of applying heating wires and feed conductors or bus bars directly to the surface of a glass sheet, using an adhesive to maintain the wires in position and subsequently applying electric current to burn off the adhesive. A soldering iron is used to apply the bus bars to the heating wires.