Author Topic: Plastic color, discoloration & restoration: Chemical treatment vs Layer Removal  (Read 1255 times)

Offline TelePlay

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Saw this spectacular example of chromophore damage on eBay today. The plastic exposed to UV light over time caused the 1974 molded plastic, which originally absorbed orange to create blue, began to absorb more red and changed the color of the plastic surface from light blue to moss green.



This could be a sharp looking phone if the green surface layer were sanded off, chemical treatments would not do anything to help here.

Offline TelePlay

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I have the opportunity to work with colored plastic that was treated with peroxide and then bleach ending with poor color restoration results. This makes me even more sure of the fact that chemical treatment of discolored plastic should never be tried. Chemical oxidants just damage the damaged plastic more and may not work, of may sometime make the plastic look "better" for an unknown length of time (reversion to discoloration in months).

What I discovered was very interesting affects that the oxidizing chemicals had on the plastic, besides changing the chromophores producing a color change in the plastic for the better or worse.

It seems that the application of peroxide and then bleach does something to the surface of the plastic that makes it very hard and impervious to pure acetone, along with changing the color in some way. This is similar to what I discovered with the ivory soft plastic 302 housing that was toasted dark brown from one side as shown above in this topic reply.

http://www.classicrotaryphones.com/forum/index.php?topic=23386.msg234376#msg234376

The toasted chips were placed in pure acetone and only the back, not-toasted, plastic dissolved. The dark brown plastic did not dissolve in now over 2 years.

The plastic I am working on now has the same properties. I can put a drop of pure acetone on the chemically treated surface and it does nothing. Does not dissolve the ABS plastic as would happen on untreated ABS and the acetone drop does not even leave a mark, a ring, showing the ABS is now chemically resistant to acetone. This makes chemical sanding impossible. Only sand paper will work.

Another thing I discovered is the surface is so very hard, that only 180 or 220 grit wet sandpaper will cut through it and when it does, the surface has to be sanded deeper than if the discolored plastic had not first been chemically treated. It seems those two oxidizing chemicals also removed the surface plasticizers making the surface hard as glass. This means, the damage done to the discolored plastic has gone deeper into the surface than plastic discolored by UV radiation normally does. Much more plastic has to be removed to get down to the original, true plastic color.

And this extreme hardness can be seen when sanding with 180 wet grit sandpaper. When first starting, the water stays clear until the grit cuts through the hard surface and the water slowly takes on the color of the plastic, first the discolored plastic and then the original, true color. As the true color begins to appear in the water, it does so more quickly showing that the sanding is getting down to the original plastic which is still has its original plasticizers and is softer, can be sanded more easily and can be affected by acetone.

After sanding off the hard layer to get to the original true color, acetone begins to dissolve the plastic below the removed layer but not as aggressively as if the plastic were not first chemically treated. I used 75% acetone in 25% isopropyl alcohol with Wypall cloth to melt down or out the deep sanding marks left by the rough grit papers. Normally, the Wypall would feel a bit sticky on original plastic indicating that the acetone was cutting off a thin layer of plastic and the color would appear on the Wypall. The damaged plastic I am working on does not respond that way once the true color is reached leading me to believe that the chemical treatment also affected the first thin layer of the true color once the damaged color layer was removed. The test of when to stop sanding is when 75% acetone produces a slight resistance to the Wypall cloth and even then, all of the discoloration may not have been removed.

Taking the "thought to be" restored plastic backup to 2000 grit and 2 polishings with Novus after the first sanding procedure resulted in a nice shine but some of the damaged color, a lighter color, could be seen in the plastic. The sanding procedure was repeated and that resulted in wispy streaks in what was to be the restored plastic (image below). That means back to 400 grit paper to sand the streaks out and work it back up to 2000 and Novus and repeat until a uniform color is obtained.

This also indicates that the chemicals do not uniformly penetrate the plastic (to the same depth) which has always been my observation when removing plastic discolored by UV radiation over time. Might be that UV exposure was uniform or of a slower chemical change resulting in any color change being the same surface depth across the plastic. Liquid oxidants work faster and without agitation can lead to hot reaction spots.

Since it is known that sanding lightens the plastic's surface, working with a light colored plastic makes it impossible to see if any discoloration exists, in this case a faint white discoloration, until the sanded plastic is taken back up to a high shine (a 180 to 1000 grit sanded surface does not show minor imperfections in color). It's only when sanded to 2000 grit and polished with Novus that the imperfections can be seen. That means the sanding process might have to be done 3, 4 or as  many times as it takes to get all of the color damaged plastic off of the surface, to get the plastic back to its original color.

Moral of the story is, if you want to treat a discolored plastic with peroxide or bleach to improve the color and it doesn't work out, physical sanding will be much more difficult and time consuming to repair the damaged plastic. What I've been able to do to restore original plastic color with plastic not treated with oxidants in 10 minutes has now taken me about 2 hours and that was for one portion of one side of one part of all the plastic parts of the telephone. Once I get the "best" procedure down for glass hard plastic, I hope to shorten that time requirement per side/part.

This image shows the wisps which are readily seen in daylight - a bit hard to capture in artificial lighting. The 3rd sanding will be done tomorrow to get rid of the wisps.
« Last Edit: June 06, 2020, 04:36:12 PM by TelePlay »