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Building A Super-abrasive Compound For Restoring Coated Glass

Written by Henry Grover Jr.

What are Super Abrasive Compounds?

SACs are Polishing Compounds which are based on a homogenous mixture of certain super-abrasive dry powders and a thick liquid carrier. SACs have the same proportion of their components throughout any given sample. They can be made with a rather low powder density or a high powder density.

What Carrier Liquids Can Be Used?

My favorite is glycerin. It is very simple to use. It also goes quickly into water so it is easy to work with. Or we could use polyethylene glycol which also goes easily into water. Another alternative is to first thicken some water with guar gum, locust bean gum, or xanthan gum and then mix this with either glycerin and or polyethylene glycol (PEG). It is easiest to add the super abrasive powder to the thickened water first, then add the glycerin or PEG after. Use a blending machine. I also like to dissolve some citric acid crystals in the water before thickening to adjust the pH so as to prevent bacteria from forming if the end product will be doing some time on a shelf.

What Exactly Are Super-abrasives?

These come as powders. Which are based on microscopic or even nanoscopic particles. I am most familiar with cerium oxide, microcrystalline silica, aluminum oxide, diamond, and zirconium oxide. Most Window Cleaners use cerium oxide. The highest quality is white. Other colors such as pale orange are due to impurities. Cerium oxide is great for polishing glass because it has mechanochemical properties when used on this surface. Here we are talking about coated glass which surface is usually titanium dioxide. As for optical grade microcrystalline silica, this can be exceptionally ineffective or very good depending on the product. It definitely has it's place because it is half the price of cerium oxide. Even now since the cost of cerium has come back down somewhat. Aluminum oxide is rather ineffective most of the time but definitely has it's place for different technical reasons which I won't get into here. Diamond is a tricky item. It is difficult to control with a machine. Easily causing scratches. But when used correctly it will do things that all the other super-abrasives cannot. It is also a great powder to add to cerium or microcrystalline silica just to kick either of these up a notch! Zirconium oxide is a good powder to add with cerium and will work by itself just fine.

How Important is Particle Size When Restoring Coated Glass?

Size is very important in my opinion. My preference is 3.5 microns average particle size. However this should not be your only consideration. When looking at the tech sheet for any product/powder you need to pay attention to what is called the Particle Size Distribution or PSD. Very simply put this is the percentage of particles that fall within the average size range. It can be graphed. You want the tightest or most narrow curve. PSD affects the quality of the finished surface. Also how fast you can remove stock. Because a very narrow PSD allows for many more contact points which results in a much more uniform and smooth finish/surface. If using a machine with a felt pad on a coated surface I suggest moving slow. Most guys spin at 1200 rpms when on glass but coated surfaces are much more sensitive to scratching. I have even heard of a company using a nano-scopic aluminum oxide on a damaged silver coating, but at a high rpm. This was an exceptional example of what a "lesser effective" aluminum oxide can accomplish. The coating had been "weakened" with hydrofluoric acid, such that cerium was literally stripping it off the window. Typically stains are much easier to remove from coated surfaces than clear glass or dark tinted glass. So heavily aggressive polishing techniques are not required.

What of Particle Hardness and Shape?

If the particle is softer than the surface you are working on you will not accomplish much. At least if you are attempting to remove mineral deposits from sprinklers or concrete efflorescence. Both glass and coated glass (hard coats) have a hardness of about 7 months. The soft coats are called that for a reason. But are becoming more resistant to abrasion these days. Particles that are harder than coated surfaces can more easily scratch if they bunch up (agglomerate). Shape is important because particles that are amorphous or cubic can have rather sharp edges. These are rather effective at cutting but can also more easily cause scratching. There are other particles out there that are shaped like a ball or flat flakes. Balls roll and flakes slide. So either shape tends to rub rather that cut. The shape or the morphology of the particle is listed on the tech sheet for the superabrasive powder/product you are buying.

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What are Functionalyzed Particles?

These are super-abrasive particles which are treated with a variety of special chemical groups. This changes the chemical properties of the surface of the particle. Not the hardness or shape of the particle. The reason why this is done is to change how the powders blend with the carrier fluid. Functionalizing also changes how particles react with the surfaces that are being polished. Cerium just by its natural makeup chemically reacts with glass and at the same time mechanically polishes glass. Chemical engineers are able to do similar things by chemically treating super-abrasive particles.

How Does one Apply A SAC?

The easiest way is to load a large syringe with the compound. Then clean the window. Then wet the window with some clean soapy water. Next put several drops of the compound on the polishing pad. Now just polish the window. You can soap again and squeegee at anytime to see if you have removed all of the stain. This works for plain clear glass and coated glass too. It is always a good practise to breath on the polished surface to see if all of the spots have been removed. If so the condensation will appear even. There will not be any "ghost spots" that appear in the condensate and disappear after it evaporates.

What of Machines?

There are three basic types of machines. You will find rotaries, random rotaries, and random oscillators. We measure these by both rpms or opms. The most effective at removing stains from any surface are the rotaries. Again speed or rpms is critical to the surface you are working on. Coated surfaces must be treated very special. Always use exceptional care and start very slow. They will always release any type of hard water spot easier than clear or dark glass so it is not necessary to be very aggressive.

What of Pads?

My personal preference for coated glass is soft. Here is a link to a good graph on the hardness ratings of felt. You can choose.

http://www.usfelt.com/sae_felt_specs.html

But there are many other types of pads out there such as polyurethane. Along with the different materials that have been used technologists have experimented with different designs. As Marc Tanner says however, "We are the scientists". This technology is a technology of the field. They don't bring the field into the lab. This is why we are developing it. Not them.

Why Keep the Pad Totally Flat All The Time?

Simply that it works best this way. I have developed a special wheel shown here in two sizes. Three inch and six inch. This wheel has a universal joint positioned between the motor and the metal disk/wheel. You could also use a double gimbal if desired. Either way it gives you a 12 degree angle of variance. Which makes it very easy to work with. It also improves the effectiveness of the polishing technique. Causing the wheel to more easily "grab" the surface. This dramatically reduces abrasion haze too. Some workers will actually turn the felt wheel on edge ripping into the glass or coated surface. Which causes zillions of very minor scratches. They usually can only be seen on a sunny day. Further they are most noticeable on the second story when the sun is just so high above the horizon. But on a cloudy day, or when the sun is on the opposing side of the building such scratches can be completely invisible.

My Warning To Every Window Cleaner

Do not ever treat coated windows like ordinary glass. Your product might be safe on glass but highly destructive on a coated surface. Testing is very important. I always suggest sourcing whatever coating you are looking to work on first, and experiment on that using whatever product you want to use on the building. Always know too that someone or several people might have been there before you. They might have done serious damage with acids or crude abrasives. Such damage might be hiding underneath pollution or newly formed hard water spots. So that when you perform your restoration you could either strip off the chemically damaged factory coating, or just simply reveal the badly scratched surface. Which could be major scratches easily seen even on a dark night, or a virtually invisible abrasion haze only visible in just the right light at just the right angle. If you can first adequately educate your customer to the complications in working on surfaces like this you should be able to win their respect, ward off any dangers to your company, and get the money you truly need and deserve to do the job. That is what this article is all about. Use it in the Field. I wish you all my very best!

Super-Abrasive Compound Based on a Microcrystalline Silica

Super-Abrasive Compound Based on a Microcrystalline Silica



Written by Henry Grover Jr.
A Member of the IWCA Glass Committee

More information can be obtained by going to my blog at www.glass-smart.blogspot.com and henrygroverjr@gmail.com




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