Patent Publication Number: US-2006000706-A1

Title: Soil-resistant coating for glass surfaces

Description:
RELATED APPLICATIONS  
      This is a continuation application of U.S. application Ser. No. 09/868,542, filed Feb. 2, 1999, which in turn is a national stage application based on PCT US99/02208, filed Feb. 2, 1999, which in turn claims priority to U.S. application No. 60/113,259, filed Dec. 21, 1998, each of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention provides a coating for glass substrates and the like which resists accumulation of dirt and water stains. Coated glass substrates of the invention can be used in insulated glass units wherein the coating of the invention is carried on an exterior surface of one pane of glass while a reflective coating is applied on the opposite side of the same pane of glass.  
     BACKGROUND OF THE INVENTION  
      Keeping windows and other glass surfaces clean is a relatively expensive, time-consuming process. While cleaning any individual window is not terribly troublesome, keeping a larger number of windows clean can be a significant burden. For example, with modern glass office towers, it takes significant time and expense to have window washers regularly clean the exterior surfaces of the windows.  
      Windows and other glass surfaces can become “dirty” or “soiled” in a variety of ways. Two of the primary manners in which windows can collect dirt involve the action of water on the glass surface. First, the water itself can deposit or collect dirt, minerals or the like onto the surface of the glass. Obviously, dirty water landing on the glass will leave the entrained or dissolved dirt on the glass upon drying. Even if relatively clean water lands on the exterior surface of a window, each water droplet sitting on the window will tend to collect dust and other airborne particles as it dries. These particles and any other chemicals which become dissolved in the water will become more concentrated over time, leaving a characteristic spot or drying ring on the glass surface.  
      The second way in which water tends to give a window or other glass surface a soiled or less attractive appearance is tied to an attack on the glass surface itself. As a droplet of even relatively clean water sits on a glass surface, it will begin to leach alkaline components from the glass. For a typical soda lime glass, the soda and lime will be leached out of the glass, increasing the pH of the droplet. As the pH increases, the attack on the glass surface will become more aggressive. As a result, the glass which underlies a drying water droplet will become a little bit rougher by the time the water droplet completely dries. In addition, the alkaline components which were leached out of the glass will be redeposited on the glass surface as a drying ring. This dried alkaline material not only detracts from the appearance of the glass; it will also tend to go back into solution when the glass surface is wetted again, rapidly increasing the pH of the next water droplet to coalesce on the glass surface.  
      In storing and shipping plate glass, the presence of water on the surfaces between adjacent glass sheets is a chronic problem. One can take steps to shield the glass from direct contact with water. However, if the glass is stored in a humid environment, water can condense on the glass surface from the atmosphere.  
      This becomes more problematic when larger stacks of glass are collected. Large stacks of glass have a fairly large thermal mass and will take a long time to warm up. As a consequence, they will often be cooler than the ambient air when ambient temperature increases (e.g., in the morning), causing moisture in the air to condense on the surface of the glass. Due to limited air circulation, any moisture which does condense between the sheets of glass will take quite a while to dry. This gives the condensed moisture a chance to leach the alkaline components out of the glass and adversely affect the glass surface. The rate of attack can be slowed down somewhat by applying an acid to the surface of the glass. This is commonly done by including a mild acid, e.g., adipic acid, in the separating agent used to keep glass sheets from sticking to and scratching one another.  
      A number of attempts have been made to enable a glass sheet to keep a clean appearance longer. One avenue of current investigation is a “self-cleaning” surface for glass and other ceramics. Research in this area is founded on the ability of certain metal oxides to absorb ultraviolet light and photocatalytically break down biological materials such as oil, plant matter, fats and greases, etc. The most powerful of these photocatalytic metal oxides appears to be titanium dioxide, though other metal oxides which appear to have this photocatalytic effect include oxides of iron, silver, copper, tungsten, aluminum, zinc, strontium, palladium, gold, platinum, nickel and cobalt.  
      While such photocatalytic coatings may have some benefit in removing materials of biological origin, their direct impact on other materials is unclear and appears to vary with exposure to ultraviolet light. As a consequence, the above-noted problems associated with water on the surface of such coated glasses would not be directly addressed by such photocatalytic coatings.  
      A number of attempts have been made to minimize the effect of water on glass surfaces by causing the water to bead into small droplets. For example, U.S. Pat. No. 5,424,130 (Nakanishi, et al., the teachings of which are incorporated herein by reference) suggests coating a glass surface with a silica-based coating which incorporates fluoroalkyl groups. The reference teaches applying a silicone alkoxide paint onto the surface of the glass, drying the paint and then burning the dried paint in air. Nakanishi, et al. stress the importance of substituting part of the non-metalic atoms, i.e., oxygen in a layer of SiO 2 , with a fluoroalkyl group. Up to 1.5% of the oxygen atoms should be so substituted. Nakanishi, et al. state that if less than 0.1% of the oxygen atoms are substituted with a fluoroalkyl group, the glass won&#39;t repel water properly because the contact angle of water on the glass surface will be less than 80°.  
      Such “water repellent” coatings do tend to cause water on the surface of the glass to bead up. If the coating is applied to an automobile windshield or the like where a constant flow of high velocity air is blowing over the surface, this water beading effect can help remove water from the glass surface by allowing the droplets to blow off the surface. However, in more quiescent applications, these droplets will tend to sit on the surface of the glass and slowly evaporate. As a consequence, this supposed “water repellent” coating will not solve the water-related staining problems noted above. To the contrary, by causing the water to bead up more readily, it may actually exacerbate the problem.  
      Other silica coatings have been applied to the surface of glass in various fashions. For example, U.S. Pat. No. 5,394,269 (Takamatsu, et al.) proposes a “minutely rough” silica layer on the surface of glass to reduce reflection. This roughened surface is achieved by treating the surface with a supersaturated silica solution in hydrosilicofluoric acid to apply a porous layer of silica on the glass sheet. By using a multi-component of sol gel solution, they claim to achieve a surface which has small pits interspersed with small “islet-like land regions” which are said to range from about 50-200 nm in size. While this roughened surface may help reduce reflection at the air/glass interface, it appears unlikely to reduce the water-related staining problems discussed above. If anything, the porous nature of this coating appears more likely to retain water on the surface of the glass. In so doing, it seems probable that the problems associated with the long-term residence of water on the glass surface would be increased.  
     SUMMARY OF THE INVENTION  
      In one aspect, the present invention provides a glass article which has a water-sheeting coating and a method of applying such a coating. In accordance with a first embodiment of this invention, a glass article has at least one coated surface bearing a water-sheeting coating. This water-sheeting coating comprises silica sputtered directly onto an exterior surface of the glass. The water-sheeting coating has an exterior face which is substantially non-porous, but which has an irregular surface. This water-sheeting coating desirably reduces the wetting angle of water on the coated surface of the glass article below about 25° and causes water applied to the coated surface of the glass article to sheet.  
      In accordance with a second embodiment of the invention, a window is provided having at least one pane of glass having an exterior surface exposed to periodic contact with water. The exterior surface of this pane of glass has a water-sheeting coating comprising silica sputtered directly on the glass surface to a mean thickness of between about 15 Å and about 350 Å. This water-sheeting coating has an exterior face which is substantially non-porous, but which has an irregular surface. The water-sheeting coating causes water applied to the coated surface of the pane of glass to sheet.  
      In a further embodiment of the invention, a sheet of glass has an interior surface bearing a reflective coating thereon and an exterior surface bearing a water-sheeting coating thereon. The reflective coating may comprise a reflective metal layer and at least one dielectric layer. The water-sheeting coating again comprises silica sputtered directly onto the exterior surface of the sheet of glass and this water-sheeting coating has an exterior face which is substantially non-porous, but which has an irregular surface. This water-sheeting coating desirably reduces the contact angle of water on the coated surface of the sheet of glass below about 25° and causes water applied to the coated exterior surface of the pane to sheet.  
      As noted above, the present invention also contemplates a method of rendering a glass surface resistant to soiling and staining. In one embodiment, the method comprises first providing a sheet of glass having an interior surface and an exterior surface. The interior and exterior surfaces of the glass are cleaned. Thereafter, the interior surface of the sheet of glass is coated with a reflective coating by sputtering, in sequence, at least one first dielectric layer, at least one metal layer, and at least one second dielectric layer. The exterior surface of the glass is coated with a water-sheeting coating by sputtering silica directly onto the exterior surface of the sheet of glass. If so desired, the water-sheeting coating can be applied on the same sputter coating apparatus used to create the reflective coating. With appropriate material selection, the water-sheeting coating and one of the dielectric layers of the reflective coating may even be applied in the same sputtering chamber in an oxidizing atmosphere. If so desired, the pane of glass can be coated on both the interior surface and the exterior surface while maintaining the glass in a constant orientation wherein the interior surface is positioned above the exterior surface.  
      In accordance with an alternative method of the invention, a sheet of glass having an interior surface and an exterior surface is provided. A sputtering line is also provided, the sputtering line comprising a series of sputtering chambers, each having a support for a sheet of glass therein. At least one of the sputtering chambers comprises a dual direction sputtering chamber having an upper target position above the support and a lower target position below the support. The interior and exterior surface of the glass are cleaned and, thereafter, the sheet of glass is positioned on the support in the dual direction supporting chamber such that the interior surface is oriented toward the upper target and the exterior surface is oriented toward the lower target. The upper target is sputtered to deposit a dielectric layer. This dielectric layer may be deposited directly on the interior surface of the glass or on a film stack layer previously deposited on the interior surface of the glass. While the sheet of glass remains in the dual direction sputtering chamber, the lower target is sputtered to deposit a water-sheeting coating on the exterior surface of the glass. In one possible preferred embodiment, both the upper target and the lower target are sputtered in an oxidizing atmosphere within the same sputtering chamber.  
      In yet another embodiment, the invention provides a method of coating two sides of a single pane of glass or other substrate in a single pass through a coating apparatus, regardless of the nature of the coating being applied to either side of the glass. In this method, a sheet of glass (or other substrate) having a clean interior surface and a clean exterior surface is provided. A sputtering line is also provided, this line comprising a series of sputtering chambers each having a support for a sheet of glass therein, at least one of the sputtering chambers comprising a downward sputtering chamber having an upper target positioned above the support. A second of the sputtering chambers comprises an upward sputtering chamber having a lower target positioned below the support. The sheet of glass or other substrate is positioned on the support in the downward sputtering chamber such that the interior surface is oriented toward the upper target. The upper target is sputtered to deposit a coating directly on one of the interior surface of the glass or a film stack layer previously deposited on the interior surface of the glass. The sheet of glass is also positioned on the support in the upward sputtering chamber such that the exterior surface is oriented toward the lower target. The lower target is sputtered to deposit a coating on one of the exterior surface of the glass or a film stack layer previously deposited on the exterior surface of the glass. The glass is coated on both the interior surface and the exterior surface while maintaining a constant orientation wherein the interior surface is positioned above the exterior surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic cross-sectional view of a sheet of glass bearing a coating in accordance with the invention;  
       FIG. 2  is a schematic cross-sectional illustration of a multi-pane insulated glass unit incorporating a water-sheeting coating of the intervention;  
       FIG. 3  is a schematic cross-sectional view of a laminated window structure of the type commonly used in automobile windshields bearing a water-sheeting coating of the invention;  
       FIG. 4  is a schematic illustration of a dual direction sputtering chamber for use in accordance with the intervention;  
       FIG. 5  is a schematic illustration of a multiple-zone dual direction sputtering chamber for use in accordance with another embodiment of the invention;  
       FIG. 6  is an atomic force micrograph of a plain, uncoated surface of a sheet of conventional float glass;  
       FIG. 7  is a graph showing a height profile across a short length of the surface of the sheet of glass shown in  FIG. 6 ;  
       FIG. 8  is a atomic force micrograph of a surface of a sheet of float glass bearing a water-sheeting coating in accordance with the invention;  
       FIG. 9  is a three-dimensional representation of an area of the same sheet of float glass illustrated in  FIG. 8 ; and  
       FIG. 10  is a graph similar to  FIG. 7 , but showing a height profile across a short length of the surface of the water-sheeting coating shown in  FIGS. 8 and 9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  schematically illustrates a sheet of glass bearing a pair of coatings in accordance with one useful embodiment of the invention. The sheet of glass  10  includes an exterior face  12  and an interior face  14 . (The designation of “interior” and “exterior” face in the ensuing discussion is somewhat arbitrary. It is assumed, though, that in most circumstances the exterior face will be exposed to an ambient environment wherein it may come into contact with dirt, water and the like. The interior face may also be oriented toward the same kind of ambient environment. In the embodiments illustrated in  FIGS. 2 and 3 , though, this “interior” face is actually protected and a second pane of glass stands between this interior face and the ambient environment.)  
      The interior face  14  of the glass  10  bears a reflective coating  30 . As those skilled in the art will readily recognize, this reflective coating may take any desired form depending on the desired properties. A wide variety of such films are known in the art and the precise nature of the reflective coating  30  is beyond the scope of the present invention.  
      If, for example, the glass article is to be used as a mirror, the coating  30  may simply comprise a relative thick layer of a reflective metal. If so desired, a protective coating of a dielectric material may be applied over the surface of the metal opposite the surfacing contact with the glass. As is known in the art, this will help protect the metal layer from chemical and physical attack. One could also employ any of a variety of mirror coatings known in the art which comprise a layer of a dielectric on either side of a reflective metal layer; many dichroic mirrors known in the art employ such a coating.  
      In the embodiment of  FIG. 1 , the reflective coating  30  is typified as an infrared reflective coating of the type commonly used in low emissivity solar control films. Typically, such films will comprise a metal layer sandwiched between a pair of dielectric layers. This structure may be repeated to further enhance the infra-reflective properties of the film stack. One example of a useful infrared reflective film stack is disclosed in U.S. Pat. No. 5,302,449 (Eby, et al.), the teachings of which are incorporated herein by reference.  
      The illustrative film stack  30  of  FIG. 1  includes a base coat  32  which may comprise one or more layers of dielectric materials. For example, this base coat  32  may comprise zinc oxide applied at a thickness of about 150-275 Å. A first metal layer  34  may be applied directly on top of this base coat  32 . This metal may be, for example, silver applied at a thickness of between about 100 Å and about 150 Å. A second dielectric layer  38  may be applied over the first metal layer  34 . The thickness of this dielectric layer  38  will depend, at least in part, on whether a second metal layer  40  will be included in the film stack. In a film stack having two metal layers, as shown, this second dielectric layer  38  may typically comprise a relatively thick layer of a metal oxide, such as 700-750 Å of zinc oxide. If so desired, a relatively thin sacrificial layer  36  may be applied between the metal layer  34  and the dielectric layer  38 . This will help protect the metal layer  34  during the sputter deposition of the dielectric layer  38 . The sacrificial layer  36  may, for example, comprise a layer of titanium metal applied at a thickness of 25 Å or less. This titanium metal will oxidize sacrificially during the application of a metal oxide dielectric  38 , limiting any damage to underlying silver layer  34 .  
      In the illustrated film stack, a second metal layer  40  is applied over the second dielectric layer  38 . The second metal layer  40  will usually be made of the same material as is the first metal layer  34 . For example, this second metal layer  40  may comprise about 125-175 Å of silver. Again, a sacrificial layer  42  of titanium or the like may be applied over the metal layer  40  to protect the metal layer during subsequent deposition of the overlying dielectrics  44  and  46 . A third dielectric layer  44  is applied over the sacrificial layer  42 . This dielectric layer  44  can also be a metal oxide, e.g., zinc oxide applied at about 250-300 Å. If so desired, a protective overcoat  46  of another dielectric material can be applied over the dielectric layer  44 . In one preferred embodiment, this overcoat  46  may comprise a 50-60 Å layer of Si 3 N 4 .  
      The water-sheeting coating  20  was applied to the inner surface  12  of the glass. It is preferred that this coating be applied directly on the surface of the glass sheet  12 . As the glass, which will typically be a soda/lime glass, is largely formed of silica and the water-sheeting coating is also desirably formed of silica, this is believed to provide a strong bond between these two layers and may enhance the water-sheeting performance of the coating  20 .  
      The water-sheeting coating  20  of the invention desirably comprises silica deposited directly on the exterior surface  12  of the glass  10 . As will be discussed below in connection with  FIGS. 8-10 , the exterior face  22  of this coating  20  has an irregular surface. (This is schematically shown as a series of irregularly-spaced and -sized spikes on the exterior face  22  of the coating  31 ). Accordingly, attributing any specific thickness to this coating  20  will be inherently somewhat inaccurate. However, the coating  20  desirably has a median thickness of between about 15 Å and about 350 Å, with a range of between about 15 Å and about 150 Å being preferred. The major benefit of this coating at the least cost is believed to be evidenced at a range of about 20 Å to about 120 Å. One preferred manner in which this coating  20  may be applied to the exterior surface  12  of the glass  10  will be discussed in more detail below.  
       FIG. 2  is a schematic illustration of a multi-pane insulated glass unit in accordance with a further embodiment of the invention. Insulated glass units are well known in the art and may not be discussed in any significant detail here. Briefly, though, such an insulated glass unit would generally comprise two panes of glass  10 ,  100  held in a spaced-apart relationship by a spacer  110 . In this embodiment, the water-sheeting coating  20  carried by the exterior surface of the glass  10  is oriented away from the second pane of glass  100  while the reflective coating  30  carried by the interior face of the glass  10  is oriented toward the second pane of glass  100 . The spacer  110  is bonded on one side to the interior surface  102  of the second glass pane  100  and on the other side to the first glass pane  10 . As is known in the art, the spacer may be bonded directly to the interior surface  14  of the glass  10  or the reflective coating  30  may extend out to the margins of the glass  10  and the spacer may be attached directly to that coating  30 .  
      Typically, the spacer will be formed of metal or the like and will have a desiccant  112  retained therein. This desiccant will be allowed to communicate with the gas in the interpane space  115  to remove any moisture which may seep between the panes of glass. An exterior seal  114  may be carried around the external periphery of the spacer  110  to form a reliable gas and moisture barrier.  
       FIG. 3  illustrates another application for a coated glass article of the invention. In this embodiment, the glass sheet  10  is bonded to a second sheet of glass  100  by an intermediate tear-resistant plastic film  130  to form a laminated structure. Such laminated window structures are well known in the field of automobile windows. Typically, this plastic layer  130  will take the form of a relatively thick layer of polyvinylbutyral or the like which is heat-fused to the other two sheets of glass. If so desired, the coating  30  may be omitted. More preferably, though, the reflective film  30  will comprise a heat-temperable infrared reflective film. A variety of such films are known in the art and the precise nature of this film is beyond the scope of the present invention, but any suitable heat-temperable coating  30  may be used.  
      As noted above, the water-sheeting coating is desirably applied by sputtering, as is the reflective coating  30 , if present. These separate coatings can be applied using conventional sputtering equipment by applying the two coatings in separate passes through a sputtering line. For example, before the reflective coating is applied, the water-sheeting coating  20  of the invention can be applied to the exterior surface of the glass by positioning this surface of the glass beneath a silicon target in an oxidizing sputtering atmosphere. Thereafter, a multiple-layer reflective coating can be applied using a series of sputtering chambers in a conventional manner, with each chamber being adapted to sputter one or more specific layers of the desired film stack.  
       FIG. 4  schematically illustrates a dual direction sputtering chamber in accordance with one embodiment of the present invention. Magnetron sputtering chambers are well known in the art and are commercially available from a variety of sources. While a thorough discussion of such magnetron sputtering chambers is beyond the scope of the present disclosure, one relatively useful structure for such a device is disclosed in U.S. Pat. No. 5,645,699 (Sieck), the teachings of which are incorporated herein by reference.  
      Generally speaking, though, magnetron sputtering involves providing a target formed of a metal or dielectric which is to be deposited on the substrate. This target is provided with a negative charge and a relatively positively charged anode is positioned adjacent the target. By introducing a relatively small amount of a desired gas into the chamber adjacent the target, a plasma of that gas can be established. Atoms in this plasma will collide with the target, knocking the target material off of the target and sputtering it onto the substrate to be coated. It is also known in the art to include a magnet behind the target to help shape the plasma and focus the plasma in an area adjacent the surface of the target.  
      In  FIG. 4 , the sheet of glass  10  to be coated is positioned on a plurality of support rollers  210  which are spaced along the length of the sputtering chamber  200 . While the precise spacing of these rollers  210  can be varied, for reasons explained more fully below, it is desired that these rollers are spaced a little bit farther apart along at least a interim length of the chamber  200  to increase the effective coating area from the lower target  260 .  
      In the illustrated embodiment, the sheet of glass  10  is oriented to travel horizontally across these rollers, e.g., from left to right. The interior surface  14  of the glass is oriented upwardly while the exterior surface  12  of the glass is oriented downwardly to rest on the rollers  210 . (While this is probably the most typical configuration, it should be understood that the relative orientation of the glass within the sputtering chamber  200  can be switched so long as the relative positions of the upper targets  200  and the lower target  260  are also reversed. As a consequence, it should be noted that designating these targets as “upper” and “lower” targets is simply for purposes of convenience and the relative orientation of these elements within the sputtering chamber can easily be reversed if so desired.)  
      The sputtering chamber  200  shown in  FIG. 4  includes two spaced-apart upper sputtering targets  220   a  and  220   b . While these targets can be planar targets, they are illustrated as being so-called rotary or cylindrical targets. These targets are arranged generally parallel to one another with a plurality of anodes  230  extending horizontally and generally parallel to these targets. As suggested in U.S. Pat. No. 5,645,699, an intermediate anode  230  may also be positioned between these two targets.  
      A gas distribution system is used to supply the sputtering gas to the chamber adjacent the targets  220   a  and  220   b . While a variety of gas distribution systems are known in the art, this distribution system may simply comprise a pair of pipes  235  with a plurality of spaced-apart openings or nozzles oriented generally toward the target.  
      The use of multiple targets positioned above a glass substrate in a magnetron sputtering chamber is fairly conventional in the field. The unique aspect of the sputtering chamber  200   FIG. 4 , though, is the presence of the “lower” target  260 . This target is the target used to sputter the water-sheeting coating  20  of the invention directly on the exterior surface  12  of the glass. As with the upper targets  220   a  and  220   b , the lower target  260  is provided with at least one, and preferably two, anodes  270  in sufficient proximity to establish a stable plasma. The gas distribution pipes  235  shown adjacent the upper targets  220   a  and  220   b  are undesirably far from the lower target  260  and the intermittent presence of the glass  10  will effectively divide the sputtering chamber  200  into two separate functional areas. Accordingly, it is preferred to have separate gas distribution pipes  275  positioned beneath the gas adjacent the lower target  260  to ensure a consistent supply of gas for the plasma adjacent the target. If so desired, the lower pipes  275  and the upper pipes  235  may be a part of the same gas distribution system, i.e., both sets of pipes can be connected to a single gas supply.  
      The nature of the gas supplied by the lower pipes  275  will depend at least in part on the nature of the sputtering target  260 . In conventional magnetron sputtering, the target must serve as a cathode. Due to the dielectric nature of SiO 2 , it can be exceedingly difficult to reliably sputter using a silica target. As a consequence, it is preferred that the target comprise silicon metal rather than silica. The material actually deposited on the exterior surface  12  of the glass can be converted to silica by including oxygen in the gas supplied through the lower gas distribution pipes  275 .  
      While the successive sheets of glass  10  will effectively divide the sputtering chamber, this does not preclude gas introduced in one area of the chamber from travelling elsewhere in the chamber. As it is preferred that the lower target  260  comprise silicon metal sputtered in an oxidizing atmosphere, it is important that the sputtering of the upper targets  220   a  and  220   b  not be adversely affected by the presence of any excess oxygen which may be introduced through the lower pipes  275 . This may effectively preclude the use of this dual direction sputtering chamber  200  to deposit a water-sheeting coating  20  on one side of the glass sheet and an oxygen-sensitive metal on the other surface.  
      More advantageously, the dual direction sputtering chamber of  FIG. 4  can be used to deposit a dielectric layer on the interior surface  14  of the glass and the silica water-sheeting coating  20  on the exterior surface  12  of the glass in a single chamber. The sputtered dielectric may be a nitride or the like so long as the introduction of some metal oxide into the nitride being deposited will not adversely affect the coating being applied. Ideally, though, the dielectric being applied to the interior surface  14  is an oxide (or at least a partial oxide) so that any commingling of the gases introduced through the two sets of pipes  235  and  275  will not adversely affect either the dielectric layer or the water-sheeting coating. For example, one or both of the targets  220   a  and  220   b  may be made of titanium metal or TiO x  (where 1&lt;X&lt;2) and the gas introduced through both sets of gas distribution pipes  235  and  275  may comprise an appropriately balanced mixture of argon and oxygen.  
      In conventional magnetron sputtering chambers, the spacing of the rollers  210  used to support the glass is kept fairly small to permit smaller glass substrates to be processed on the line without any significant risk of having the glass fall between the rollers. In order to minimize the interference of the rollers in applying the water-sheeting coating on the exterior surface  12  of the glass, though, this spacing may be increased. The maximum safe spacing will need to be determined on a case-by-case basis for a given range of anticipated glass sizes. However, the larger the spacing between the rollers disposed in the path from the lower target  260  to the exterior surface  12  of the glass, the greater the percentage of the sputtered silica which will be deposited on the glass. Of course, the rollers in other areas of the sputtering apparatus can be maintained at their normal spacing. It may be desirable to make a few of the rollers in the dual direction sputtering chamber  200  easily removed so the chamber can be converted from the illustrated configuration to a more conventionally operated chamber coating only one side of the glass and having rollers spaced more closely together.  
      Instead of changing the spacing between the rollers, the rollers could instead be made smaller in diameter. Conventional rollers are hollow metal tubes. If so desired, the smaller diameter rollers can be stiffened, e.g., by filling them with a rigid foam. In order to maintain the same transport speed of the glass along the support, these smaller-diameter rollers would have to be turned more rapidly, e.g., by means of a pair of gears having the desired gear ratio.  
      The rollers  210  can be of any conventional structure. It has been found that good results can be obtained by employing cylindrical aluminum rollers about which a rope of Kevlar™ is spirally wound, with the Kevlar™ providing the surface with which the glass is in direct contact.  
      In some specific applications, the dual direction sputtering chamber  200  of  FIG. 4  may be sufficient to apply the entire desired coating to both the interior and exterior surfaces of the glass. More often, though, the sputtering chamber  200  would be part of a sputtering line comprising a series of sputtering chambers. Each sputtering chamber in the line could include both an upper target and a lower target, but in most conventional applications the film stack applied to the upper surface of the glass will be more complex (i.e. will comprise a series of distinct layers of varying composition) and thicker than is the water-sheeting coating of the invention. As a consequence, a majority of the sputtering chambers can comprise conventional, downward sputtering chambers having only an upper target, with no target positioned beneath the supports.  
      If the sputtering line comprises a combination of downward sputtering chambers and dual direction sputtering chambers  200 , the position of the dual direction chambers along the sputtering line can be varied. If the water-sheeting coating of the invention is applied by sputtering a silicon-containing target (e.g., one formed primarily of silicon or formed of silicon doped with aluminum) in an oxidizing atmosphere, one should not attempt to deposit an oxidizable metal layer (e.g., an infrared reflective silver layer of the type conventionally used in low emissivity film stacks) on the upper surface of the glass in the same chamber. Accordingly, at least those chambers used to sputter a metal layer may be operated as a downward sputtering chamber by omitting the lower target. It would be possible, though, to deposit a metal oxide (e.g., SiO 2 , ZnO or SnO 2 ) on the upper surface of the glass in the same chamber.  
      Conventional wisdom would suggest to one skilled in the art that the water-sheeting coating of the invention be applied in the first sputtering chamber or, if necessary, the first several sputtering chambers to make sure that the water-sheeting coating is applied before the glass surface is damaged or soiled by contact with the rollers supporting the glass within the chambers. Quite surprisingly, it has been found that the opposite is true—the water-sheeting coating of the invention is optimally applied in the last sputtering chamber. If more than one dual direction sputtering chamber  200  is necessary to deposit a sufficiently thick water-sheeting coating without unduly slowing down glass speed through the sputtering line, the water-sheeting coating is optimally applied in the last few sputtering chambers.  
      If the water-sheeting coating of the invention is applied at the beginning of the sputtering line, the majority of the exterior surface of the glass will exhibit the desired water-sheeting properties. However, the margins of the glass may not exhibit these improved properties on a consistent basis. This is believed to be due to a slight overspray of the coating applied to the upper surface of the glass after deposition of the water-sheeting coating, wherein a very small amount of the material being applied to the upper surface will drift down to the lower surface and overlie the water-sheeting coating adjacent the edges of the glass sheet. While this oversprayed coating is thin enough as to have no readily discernable effect on the optical properties of the glass, this virtually invisible coating compromised the benefits of the water-sheeting coating around the edges of the glass. By applying the silica to the exterior surface of the glass toward the end of the sputtering line, the amount of overspray deposited on top of the silica coating can be minimized and the beneficial water-sheeting effects of this coating can be preserved.  
      A dual direction sputtering chamber  200  such as that shown in  FIG. 4  is believed to minimize the cost and maximize production efficiency in applying coatings to both sides of the sheet of glass. Less desirably, a water-sheeting coating of the invention could be applied in one pass while the reflective coating is applied to the other side of the glass in a second pass, flipping the glass between the passes to permit all of the targets to be positioned on the same side of the supports in the chamber(s). This is much less efficient than the process outlined above, though, and is not believed to be suitable for low-cost commercial glass production.  
      As the glass substrate moves through the chamber, there will be times when the glass does not effectively shield the upper targets  200   a  and  200   b  from the lower target  260  or vice versa. As a consequence, material from the upper targets will be deposited on the lower target and material from the lower target can be deposited on one or both of the upper targets. The sputtering chamber  200  of  FIG. 4  is ideal if the upper targets  220   a ,  220   b  and the lower target  260  have substantially the same composition. If the upper targets have a different composition from the lower target, though, the cross-contamination of the different targets can lead to problems in sputtering or in maintaining consistent product quality.  
      At least in theory, this problem may be overcome by independently controlling the power supplied to each of the sputtering targets to ensure that each target is sputtering only when the glass is positioned to shield the upper and lower targets from one another. Current commercially available power supply controllers are not configured in this fashion, however. Furthermore, the control logic for such an arrangement can be unduly difficult if the sputtering line is used to coat glass substrates of varying sizes rather than a consistent size.  
       FIG. 5  illustrates one possible sputtering chamber  300  which can be used to coat both the interior surface  14  and the exterior surface  12  of the substrate in a single pass without significant cross contamination of the sputtering targets. Elements serving an analogous function to elements shown in  FIG. 4  bear like reference numbers, but indexed by 100, e.g., the upper gas distribution pipes  335  of  FIG. 5  are functionally analogous to the upper gas distribution pipes  235  of  FIG. 4 .  
      The sputtering chamber  300  of  FIG. 5  is effectively divided into three coating zones  300   a ,  300   b  and  300   c  by a pair of barriers  340 . Some fraction of the gas in one coating zone may flow into another coating zone, so it is best to use a similar atmosphere in all three zones. However, the barriers  340  serve to effectively limit the amount of material sputtered in one coating zone which lands on a target in another coating zone.  
      In the embodiment of  FIG. 5 , each of the three coating zones  300   a - 300   c  is adapted to hold up to four targets, with two targets positioned above the substrate and two positioned below the substrate. Hence, there are six upper target mounts  321 - 326  positioned above the path of the glass and six lower target mounts  361 - 366  positioned beneath the path of the glass. This allows maximum flexibility in using this single multi-zone sputtering chamber  300  to manufacture products having different properties.  FIG. 5  schematically illustrates each of the upper target mounts  321 - 326  vertically aligned with one of the lower target mounts  361 - 366 , respectively. It should be understood, however, that the targets need not be vertically aligned in this fashion and may be more advantageously positioned in a horizontally staggered arrangement.  
      In the configuration shown in  FIG. 5 , the first coating zone  300   a  has two upper targets ( 320   a  and  320   b ), but no lower targets on the lower target mounts  361  or  362 . While a sputtering gas should be supplied to the upper gas distribution pipes  335  and power should be supplied to the upper anodes  330  in the first coating zone, there is no need to deliver any gas to the lower gas distribution pipes  375  or any power to the lower anodes  370 . The second coating zone  300   b  has two lower targets  360   c  and  360   d , but neither of the upper target mounts  323  and  324  carry sputtering targets. Similarly, the third coating zone  300   c  has two lower targets  360   e  and  360   f , but neither of the upper target mounts  325  and  326  carry sputtering targets. Optimally (as discussed above), the first coating zone  300   a  is used to apply the outermost layer of the reflective film stack carried by the interior surface  14  of the substrates while the last two coating zones  300   b  and  300   c  are used to sputter the water-sheeting coating  20  on the exterior surface  12  of the substrates.  
      The arrangement of targets in the multiple-zone sputtering chamber  300  of  FIG. 5  is merely illustrative and it should be understood that the target arrangement can be varied to maximize production efficiency for different products. For example, if a thicker water-sheeting coating is desired at the same glass speed, a silicon-containing target can be mounted on each of the lower target mounts  361 - 366  while none of the upper target mounts  321 - 326  carry a target. If a thinner coating will suffice (or if glass speed through the coating chamber is suitably reduced), only the last two lower target mounts  325  and  326  can be provided with targets while each of the first four upper target mounts  321 - 324  carry sputtering targets. Of course, any one or more of the coating zones  300   a - 300   c  can be operated much like the dual-direction sputtering chamber  200  of  FIG. 4  by mounting targets in the upper and lower target mounts of the same zone.  
      The apparatus of  FIGS. 4 and 5  and the method of depositing coatings using such coating systems is discussed in the present application primarily in the context of applying a reflective film stack on one side of the glass and a water-sheeting coating on the other side of the glass. It is to be understood, however, that this apparatus and method can be used to apply coatings to both sides of a pane of glass regardless of the nature of the coatings applied thereto. For example, the apparatus can be used to apply an anti-reflective coating on both sides of a pane of glass, to apply infrared reflective coatings to both sides of a transparent or translucent organic substrate, or to apply a water-sheeting coating to each side of the same substrate.  
      The advantage of the systems illustrated in  FIGS. 4 and 5  is that a substrate can be provided with a sputtered coating (regardless of composition) on both sides in a single pass through the coating apparatus while the glass is maintained in a constant orientation, i.e. wherein it does not need to be flipped, turned or otherwise manipulated. This enables the use of a simple set of standard transport rollers to move the glass along the production line. In the absence of the present invention, one typically would have to either manually handle the glass to flip it and send it back through the coating apparatus in a separate run, or use a complex glass handling system which must hold the substrate and flip it at some point during the production process. This enables glass having coatings on both sides to be produced particularly economically without any loss in coating quality.  
      In the past, it was assumed that even if one were to coat the bottom side of the glass, contact with the rollers would mar that coating and/or damage the bottom surface of the glass prior to application of the coating. Surprisingly, however, the present invention demonstrates that both sides of the glass can be coated in a single pass with excellent results.  
      The precise operating conditions (e.g. target composition, plasma composition, etc.) under which the water-sheeting coating of the invention is applied can be varied as necessary to optimize the deposition of a coating of the desired thickness. Given the present teaching as a guide, one of ordinary skill in the art should be able to select suitable operating conditions to apply a coating of the invention without undue experimentation.  
      A layer of SiO 2  in accordance with the invention may be sputter deposited using a silicon dioxide target in an inert atmosphere, but silica is a poor conductor and it can be difficult to sputter such dielectric materials in a DC sputtering apparatus. One could instead use a pure silicon target in an oxidizing atmosphere, but such targets are difficult to sputter in a consistent, controlled fashion because silicon is a semiconductor. To improve sputtering and reduce arcing, it is preferred that a target comprising silicon with about 5% aluminum be sputtered in an oxidizing atmosphere.  
      Even if an aluminum-doped silicon target is employed, the atmosphere in the sputtering chamber can be varied to achieve the optimum sputtering rate. While the sputtering atmosphere should be oxidizing, it need not be pure oxygen. To the contrary, a mixture of oxygen and an inert gas will enhance the sputtering rate. It is believed that a sputtering gas comprising oxygen and up to about 40% argon (preferably 0-20% argon) maintained at about 3×10 −3  mbar will suffice. The power applied to the sputtering target should be optimized to reduce arcing yet maximize sputtering rate. A power of up to about 80 kW should yield acceptable results.  
      One manufacturing arrangement which has been found to work well utilizes three rotary sputtering targets of silicon doped with about 5% aluminum, with a power of about 42 kW being applied to each target. The atmosphere in the sputtering chamber comprises 100% O 2  at a pressure of about 2.5-4.5 mTorr. The glass substrate is moved past these sputtering targets at about 225-500 inches per minute.  
      In manufacturing float glass, molten glass is floated on a bath of molten tin and the glass is referred to as having an upper side and a lower, or “tin” side. Most commonly, when float glass is provided with a reflective coating, the coating is applied to the upper side of the glass due to some minor surface imperfections in the tin side of the glass which can arise due to contact with support rollers in the annealing lehr. If a sheet of float glass  10  is to be provided with both a water-sheeting coating  20  and a reflective layer  30 , it is preferred that the upper surface of the sheet glass be used as the interior surface  14  of the glass to receive the reflective coating  30  while the tin side of the glass is used as the exterior surface to receive the water-sheeting coating  20 .  
       FIG. 6  is an atomic force micrograph of one square micron (μm) of the surface of the tin side of an untreated sheet of float glass.  FIG. 7  is a graph representing a profile of the same side of the sheet of glass along about a 1 μm line on that surface. Both of these images were obtained by atomic force microscopy using a Digital Instruments Nanoscope III using a standard silicon tip.  
       FIGS. 6 and 7  illustrate a relatively smooth surface. While this surface is not perfectly smooth and it appears to have a slightly rough appearance in  FIG. 6 , it is important to note that the scale of these images is quite small. To place these images in perspective, two peaks in the profile of  FIG. 7  are highlighted by a pair of arrows. The two darker arrows toward the left in  FIG. 7  (at about 0.25 μm along the abscissa) mark the beginning and the apex of a first peak A; the two lighter arrows toward the right in  FIG. 7  (at about 0.9 μm along the abscissa) mark the apex and end of a second peak B. The first peak A is less than 0.7 nm in height while the second, taller peak B is only about 1.7 nm tall.  
       FIGS. 8-10  are analogous representations of a sheet of float glass on the tin side of which a water-sheeting coating of the invention has been applied.  FIG. 8  is a micrograph much like  FIG. 6 , also representing 1 μm 2  of the surface.  FIG. 10  is a graph much like  FIG. 7 , but wherein the ordinate axis represents a range of 20 nm rather than the smaller 5 nm range of  FIG. 7 .  FIG. 9  is a perspective view which highlights the surface features of the water-sheeting coating. The smaller vertical bar to the right of the primary image is a legend representing the gray scale associated with different heights from the base surface.  
      By comparing these two sets of figures, it appears that the water-sheeting coating of the invention has a significantly more irregular surface than does the uncoated surface shown in  FIGS. 6 and 7 . In  FIG. 8 , there appear to be a series of spaced-apart projections rising from the surface of the glass, but it is difficult to determine in this view the height of these projections.  FIGS. 9 and 10  give a better indication of the height and shapes of these projections. In  FIG. 10 , the two darker arrows highlight the apex and end of one peak A while the two lighter arrows point to the apex and end of a second peak B. In contrast to the rather small peaks in  FIG. 7 , the second, smaller peak B in  FIG. 10  is about 4.3 nm tall while the first peak A is nearly 10 nm tall. This is over five times as tall as the peaks illustrated in  FIG. 7 .  
      It is also worth noting that the surface of the coating shown in  FIGS. 8-10  is uneven, but appears to be relatively non-porous. This is in sharp contrast to the photomicrographs in Takamatsu et al.&#39;s U.S. Pat. No. 5,394,269, which show a porous sol gel-derived coating having pores on the order of 50-200 nm penetrating the coating.  
      For reasons which are not currently understood, these images suggest that sputter depositing silica on the surface of the glass yields a coating with a surface having a series of fairly sharp, distinct peaks. No meaningful statistical analysis of coated surfaces have been performed, so it is not known if  FIGS. 6-10  are representative of their respective surfaces. As a matter of fact, it is acknowledged that these images could be atypical of the overall surfaces of the samples in question, so it may not be appropriate to attach too much significance to the apparent differences in the surface structure of these two glasses. However, this data does suggest that the surface of the water-sheeting coating  20  of the invention is relatively non-porous and differs from an untreated float glass surface in that it is significantly more uneven and irregular, having a number of discrete, spaced-apart peaks rising significantly above the rest of the surface.  
      The behavior of a sheet of glass coated with a water-sheeting coating of the invention is visibly different from that of a similar sheet of glass not bearing the present coating. A glass surface bearing a water-sheeting coating  20  tends to sheet water more readily and is noticeably easier to clean without any visible streaks or defects than is a comparable sheet of glass under the same conditions.  
      To provide an accurate comparison of a coating of the invention to a directly comparable sheet of glass not bearing the coating, a comparative sample was prepared. A plain, untreated pane of glass was thoroughly cleaned and laid horizontally on a set of rollers. A small, square piece of glass was laid on the upper surface of the pane of glass to serve as a template covering part of the surface of the pane. The pane and overlying template were passed into a magnetron sputtering chamber and a coating of about 35 Å of SiO 2  was deposited. The template was then removed, leaving a pane of glass with a water-sheeting coating  20  of the invention over most of its surface, but having an uncoated area which was beneath the template during the sputtering operation. The opposite side of the glass, i.e., the side of the glass facing away from the side provided with the SiO 2  coating, was coated with a low emissivity, infrared-reflective film stack having two silver layers spaced apart from one another and from the glass using a plurality of dielectric layers.  
      The partially coated surface of the glass pane was visibly inspected. When completely clean, the boundaries of the uncoated area which underlied the template during sputtering was essentially undetectable to the unaided eye, indicating that the water-sheeting coating had a minimal impact on the basic optical properties of the glass. A fine spray of atomized water droplets was sprayed on the surface using a simple, hand-operated spray bottle of the type conventionally used to spray household cleaning products. Once the spray was applied, the boundaries of the uncoated area were readily visible. The water on the area bearing the coating  20  sheeted to an apparently uniform film of water, but the area without the coating had a less uniform appearance.  
      A conventional cleaning solution commercially available under the trademark Windex® was sprayed on the surface of the glass pane and the surface was wiped with a paper towel until the area bearing the coating  20  appeared dry and no longer showed any visible streaks. When wiping ceased, the uncoated area still had visible streaks of moisture. While these visible streaks on the uncoated area eventually dried without leaving any substantial residual streaking on the glass, it is believed that the average person would tend to continue to wipe this area until all visible streaks disappeared, meaning that the person would expend less time and effort cleaning a glass article bearing a water-sheeting coating  20  than a glass article without such a coating.  
      The change in surface properties brought about by the present invention are readily discernable on a qualitative level, but it can be more difficult to quantify these differences in a meaningful manner. Nonetheless, the following examples are believed to illustrate the difference between an uncoated sheet of glass and a sheet of glass bearing a water-sheeting coating  20  of the invention. In each of the following Experimental Examples 1-3, two test samples, Sample A and Sample B, were provided. Sample A comprised a plain sheet of soda-lime glass and Sample B was a similar sheet of soda-lime glass bearing a water-sheeting coating  20  of the invention. The water sheeting coating was applied using three 95% silicon/5% aluminum rotary targets at a power level of 42 kW in an oxygen atmosphere of about 3.5 mT with the glass moving at a rate of about 500 inches per minute.  
     EXPERIMENTAL EXAMPLE 1  
      Both of the samples were subjected to a salt spray test in accordance with ASTM B117 using a 5% salt solution for 250 hours. Briefly, the samples were cleaned and placed in a Singleton SCCH #20 Corrosion Cabinet at an angle of about 15-30° from vertical, with Sample B being oriented such that the surface bearing the water-sheeting coating  20  was oriented to face downwardly. A 5% salt solution (5 wt % sodium chloride, 95 wt % distilled water) was atomized in the cabinet at about 35° C. for 250 hours, with the salt solution being collected at a rate of about 1.8 ml per 80 cm per hour in the collection cylinders in the cabinet. Thereafter, the samples were removed from the cabinet, rinsed, allowed to dry and visually inspected. Sample A had more numerous water spots than did Sample B and the water spots on Sample A were more visible than the light streaks on Sample B.  
      Each sample was then cleaned using paper towels and Windex®. The haze of each sample was then measured using a BVK-Gardner Haze-Gard Plus according to ASTM D-1003 and ASTM D-1044, employing an integrating sphere integrating light over the spectral range associated with the CIE-C standard. Sample A, the standard glass sheet, had a haze measurement of about 0.15% while the haze measurement on Sample B, the sample bearing a water-sheeting coating  20 , was about 0.10%.  
      The contact angle of the water on the surface of the glass sheet was then measured using a commercially available measuring device, with the contact angle for Sample B being measured on the surface bearing the coating  20 . The contact angle for Sample A was about 32 degrees; the contact angle for Sample B was about 12 degrees.  
     EXPERIMENTAL EXAMPLE 2  
      Handling the samples with tongs, each sample was first dipped in a beaker of boiling tap water maintained at about 100° C. and held there for about 5 seconds, after which it was deposited in a beaker of ice water maintained at about 0° C. and held there for about 5 seconds. This process was repeated 25 times. The samples were then placed in a Singleton Model SL23 humidity test chamber maintained at about 90% relative humidity at about 120° F. (about 49° C.) for about 500 hours. Each sample was then visually inspected. As in Experimental Example 1, it was determined that Sample A exhibited more numerous and more visible water spots than did Sample B.  
      Each sample was then cleaned and the haze and contact angle measurements were taken in much the same manner outlined above in Experimental Example 1. The haze measurement for Sample A was 0.34% while that for Sample B was 0.14%. Sample A exhibited a contact angle of about 20° while the contact angle for Sample B was about 12°.  
     EXPERIMENTAL EXAMPLE 3  
      Two samples of uncoated glass (Samples A1 and A2) and two samples of coated glass (Samples B1 and B2) were cleaned and their haze measurements were taken. Each of the uncoated samples had haze measurements of about 0.09% while the haze measurement for the glass with a water-sheeting coating  20  was about 0.08%.  
      A cement mixture was prepared by mixing 4 ounces (about 11.5 g) of portland cement to 1000 ml of water. Two samples of uncoated glass (Samples A1 and A2) and two samples of coated glass (Samples B1 and B2) were held in this solution for about ten minutes then removed. Samples A1 and B1 were then rinsed liberally with water (but without any rubbing) and allowed to dry; Samples A2 and B2 were allowed to air dry without rinsing.  
      All four samples were hand cleaned using Windex® and paper towels. The residual soiling on Samples A1 and A2 from the cement test smeared during this cleaning, making it more difficult to clean the glass. In contrast, neither Samples B1 nor Sample B2 smeared and both of these samples dried noticeably quicker than Sample A1 or Sample A2, respectively.  
      Once the samples had been thoroughly hand cleaned, haze and contact angle measurements were made. After the cement treatment, the haze for Samples A1 and B1 remained unchanged at 0.09% and 0.08%, respectively. The haze measurement for Sample B2 likewise remained unchanged at about 0.08%, but the haze measurement for Sample A2 increased slightly from about 0.09% to about 0.10%. The contact angle for Samples A1 and A2 were measured prior to the cement treatment at about 26°; Samples B1 and B2 had contact angles of about 11° at the same stage. After the cement treatment, the contact angle for Sample A1 was about 32° while the contact angle for Sample B1, the other rinsed sample, was about 10°. The contact angle for Sample A2 was about 33° while the contact angle for Sample B2, the other air-dried sample, was about 14°.  
     EXPERIMENTAL EXAMPLE 4  
      The performance of glass bearing a water-sheeting coating  20  of the invention was compared to plain, uncoated glass and to other glass coatings which claim to make the surface easier to clean. Each sample started with a sheet of float glass and, aside from the uncoated glass sample, had a coating applied to a surface thereof; the sample ID assigned to each sample type and the coating applied thereto is set forth in the following table:  
                                   Sample ID   Coating applied                  168   35 Å SiO 2  sputtered using 100% O 2         169   50 Å SiO 2  sputtered using 100% O 2         170   50 Å SiO 2  sputtered using 80/20 mixture of O 2 /Ar       171   100 Å SiO 2  sputtered using 80/20 mixture of O 2 /Ar       173   Window Maid ™ coating, commercially available from          ,           applied in accordance with manufacturer&#39;s instructions       174   Glass Shield ™ coating, commercially available from          ,           applied in accordance with manufacturer&#39;s instructions       175   Clear Shield ™ coating, commercially available from          ,           applied in accordance with manufacturer&#39;s instructions       176   uncoated glass                  
 
      A set of these samples were subjected to an accelerated weathering test and the contact single and ease of cleaning was checked on a periodic basis. In the weathering test, the samples were placed in a stainless steel enclosure maintained at a temperature of about 160° F. (about 71° C.). A 300 W ultraviolet light source (sold by Osram under the trade made Ultra-Vitalux) was positioned toward the bottom of the enclosure and samples were positioned at an angle of about 45° with respect to horizontal with the bottom edges of the sample spaced about  10  inches (about 25 cm) from the bulb. Periodically, the samples were removed from the enclosure and the contact angle was measured in much the same manner as that outlined above. The contact angles were as follows:  
                                              Contact Angle, by number of days in weathering test                                             Sample ID   0   1   2   3   4   5   10                                                     168   13           20                   169   11   14.3   17.7   21   27   33       170   11           17   25.5   34       171   6   26.5   26.5   32   33   34       173   41           50   51.5   53   42       174   23           48   48.5   49   46       175   74           62   66   70   66       176   35   35               31   35                  
 
      In addition, the ease of cleaning the sample was tested by spraying Windex® on the coated surface of the sample or, in the case of the uncoated sample, on the surface which was in contact with the tin bath during the float manufacturing process. That surface was manually wiped with a paper towel until the surface appeared to be clean and essentially streak-free. The ease of cleaning was determined on scale of 1-5, with the ease of cleaning normal, uncoated glass prior to any environmental exposure being defined as 3, a very easy to clean glass surface being rated 1 and a sample which is substantially more difficult to clean being rated 5. (While this rating system is somewhat subjective, it does give a rough qualitative indication of the ease with which the glass can be cleaned.) The results of this testing were as follows:  
                                              Contact Angle, by number of days in weathering test                                             Sample ID   0   1   2   3   4   5   10                                                     168   1   1.5   1.5   2   3               169   1   1.5   1.5   2   3   4       170   1   1   1   1.5   3   4       171   1   3   3   3   3   4       173   4   4   4   5   5   4   4       174   5   5   5   5   5   5   5       175   5   5   5   5   5   5   5       176   3   3                   3                  
 
      These results indicate that a water-sheeting coating  20  of the invention makes the glass surface significantly easier to clean than either the standard, uncoated glass or glass coated with any one of several commercially available coatings designed to make glass easier to clean. As a matter of fact, these commercially available coatings actually made the glass seem more difficult to clean. (While these coatings may be effective in some applications, it is believed that the “ease of cleaning” standards employed in this Experimental Example are fairly representative of how an average home owner would perceive ease of cleaning. For example, even if streaks of the cleaning fluid on the pane of glass might dry without leaving any permanent streaks, an average person is likely to keep wiping the area until the glass appears clean to avoid any residual streaking.)  
      The advantageous effects of the water-sheeting coating of the invention did appear to drop off over time in this accelerated weathering test. In particular, after 5 days or so in this test, coatings of the invention yielded results comparable to those achieved with uncoated glass samples. Even after such degradation, the samples bearing a water-sheeting coating  20  had a lower contact angle and remained easier to clean than did the commercially available coatings evaluated in these tests.  
      It is unclear what correlation there may be between time of ordinary exposure to the elements and time in the accelerated weathering test used in this example. It is believed, however, that a coating  20  of the invention will continue to show enhanced cleanability for an extended period of time. As a matter of fact, preliminary tests indicate that much of the benefit of the coating  20  may be restored with appropriate cleaning even after degradation in accelerated weathering testing, suggesting that the benefits of the coating can be restored relatively simply even after they have diminished due to exposure to the elements.  
      While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptations and modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.