Powder Coating Clinic: Discoloration of Metal-Flake in Powder

Q. We have a metal-flake silver powder color that we use regularly. If the surface is abraded with a Taber wheel—or even a pencil eraser—it darkens significantly in the abraded area. We even have this problem when our parts scrape against cardboard packaging. Our supplier suggested we switch from an unbonded leafing aluminum to a more expensive bonded product. We tried the bonded leafing aluminum, but we still have the same problem. Certain types of friction create this effect, seemingly those that perhaps have a higher coefficient of friction. Do they remove the aluminum flake from the surface? Can you explain this problem and tell us why it happens with some abrasives and not with others? What can we do to stop this?

A. What is happening is that the abrasive is removing any resin that may be laying over the aluminum flake and abrading the surface of the aluminum so that it does not reflect light. The flake shows a flat, gray color where it is abraded. Some abrasives will polish the surface and not cause the darkened look. The rubber eraser probably contains some high-silica pumice or similar abrasive product that scuffs the soft aluminum surface and creates the darker color. The Taber abrader cuts the aluminum surface and breaks up light reflection. A soft substance that lacks sharper edges will not cut and may even polish the aluminum flake. The issue is that you do not have enough hard resin over the flake to protect it for the abrasion. Chances are, the silver color has a fairly high amount of aluminum content. Bonding may help a little, but it will not provide reliable resin thickness over all of the aluminum flakes. I would try some different colors, possibly from different suppliers and with less aluminum content. If changing color is not an option, you will definitely have to apply a clear coat over the silver finish to protect it from abrasion.

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Originally published in the November 2016 issue.