The Science of Coated Glass and Its Maintenance
Written by Henry Grover Jr.
Member of the IWCA Glass Committee
Passive low-e coatings keep buildings warm during the colder months. Solar control low-e coatings help to keep buildings cool when the weather is very hot. Both are created by either the pyrolytic method or chemical vapor deposition (CVD)which is known as a "hard" coat; or a magnetically sputtered vacuum deposition (MSVD) which is called a "soft" coat. Hard coats are fused to the glass ribbon while it is still on the float line. Whereas soft coats are applied after the glass comes off the ribbon and has attained room temperature. Soft coats are created "offline" in a vacuum. Hard coats were historically first, and are still more durable then soft coats. Although the latter is much less soft these days, the name has stuck. Many different formulas have recently been used. So sputtered depositions (soft coats) are much more advanced then when they were first developed.
What are coatings made of? Essentially different metal compounds. In particular metal oxides and nitrides. Sputtered soft coats are made of many different layers. As many as thirteen. Silver is one such metal used as a layer. It is used for reflecting heat (infrared) radiation. But it is a rather soft metal and hence is vulnerable to physical abrasion along with high relative humidity (RH). For this reason it is protected by other layers below and above it. The complete soft coat is only about five nanometers thick. This is exceptionally thin. Only about 25 atoms deep. The average thickness of a human hair is 50,000 nanometers. A quarter of a million or 250,000 atoms across. For all of these reasons soft coats are usually applied to the second or third surfaces of insulating glass units. Although the more durable soft coats can be found on occasion on the second surface of single plate windows. Regardless of whether we are cleaning soft coats or hard coats we should use extreme caution. Razor blades should never be used. Also certain abrasives that are safe on glass surfaces will scratch soft and hard coats. The only way to know if a commercial product is safe on coated glass is to test it. It will be necessary to buy a sample of Sungate, Solarban, or whatever coating and try out the product in question using the exact same application technique as you will be using on the job.
Pyrolytic hard coats are applied using a chemical vapor deposition of tin oxide while the glass ribbon is still very hot. Aproximately 1100F. At this temperature the float ribbon has already assumed its final thickness & width. But still hot enough to fuse with the coating. These coats are much harder, and thicker than soft coats. In fact they are up to 20 times thicker. So between 50 to 100 nanometers. Or 500 atoms deep instead of only 25. They are still very easy to scratch. So we must use extreme caution. It is also true that metals are vulnerable to certain acids. Tin oxide is typically vulnerable to hydrofluoric, hydrochloric, and sulfuric acid. I have personally witnessed the damage that only a one percent solution of hydrofluoric acid (HF) can do to a Solorcool Silver reflective hard coat. It can weaken this hard coat when used the first time and then literally strip it off the second time around. It can also weaken this old hard coat so that a light cerium oxide polishing compound will strip off the reflective coating in patches the next go around. HF might even strip it off the very first time. The Solar Cool Silver pyrolytic coating was not a Low e coat. It was from the early seventies. Either way the acids listed here can affect hard coats in ways that can cause permanant damage. These are acids that have been used in commercial "glass restoration" stain removal products. Hard coats are not always applied to the weather (first) surface of insulating glass (IG) units. Although they have been used here quite frequently. They are also quite frequently used on the inner surfaces of insulating glass (IG) units. Either 2 or 3. Counting from the weather side in there are four different surfaces;...1,2,3, and 4. Hard coats are quite often applied to the weather surface of IG units and monolythic (single plate) windows. They are also applied to the inner surface of monolythics. So very simply put, we can find them anywhere. The very first step in maintaining and especially restoration of any window is to correctly identify the surface. Caution should always be used when cleaning and most especially when removing any type of pollution or substance that has strongly adhered to any coating. Whether it be Low e or not. Whether it be a hard or a soft coat. Pyrolytic or Sputtered.
Maintenance is easily accomplished with just plain soapy water, a soft T bar applicator, and a squeegee with a sharp rubber edge. Routine cleaning is strongly advised so that any exposed coatings are not contaminated. Which would require much stronger "restoration" techniques. I remember a building in NYC that had PPGs Solar Cool Silver on the first surface. The Solar Cool Silver pyrolytic hard coat as stated already was not a Low e coat. It was from the early seventies. So this building had collected much pollution likely over about fifteen or twenty years. Which turned it an ugly dark brown color. I was called in as a consultant to analyse the condition and find a solution. The simple "fix" was to polish at a very slow rate using a rotary machine, a soft felt pad, and a cerium oxide slurry. Particle size was 3.5 microns, with a purity rating of 99.9 percent. The demo came out perfect turning the surface back to a beautiful silver mirror. It took about thirty seconds per square foot. Cost was mimimal then because cerium oxide powder was very inexpensive compared to today.
This picture was taken roughly 25 years ago. Our industry has advanced a great deal in the technology of window glass restoration. But there is still much advancement to make in the maintenance, restoration, and protection of Low e soft and hard coats. If only because we are now seeing a great increase in the development of newer more technical glass coatings that will have to be properly cared for. Glass coatings are actually driving the glass industry these days. Personally I see specialized chemicals coupled with compositional super-abrasives as one wave of attack. I also see the use of slow release polishing pads based on both mineral and organic compositionals which can be used with pure water. However;... we need the help of manufacturers to accomplish this. It is simply the order of things. This will be the subject of the next article on glass coatings. The title for Part Three will be, "Developing Products for Maintaining, Restoring, and Protecting Glass Coatings". You will not want to miss the last article in this series, special for the American Window Cleaner!