Correcting the retrobrite "marble" effect

[KeplerOfMars]

TL;DR Summary

Correcting the marble discoloration after retrobrite goes wrong.

Hi all,

For those of you are unfamiliar with the concept of "retrobriting" plastic, it involves exposing yellowed plastic to UV light whilst being submerged in some variant of peroxide. This has proven to be effective in reversing the yellowing effect on old plastics.

However, certain plastics do not react well to this process and can sometimes become discolored or what is know as the "marble effect". This has happened to some plastics that I have exposed to peroxide and the sun. In my attempt to correct the marble effect, I have experimented with a number of methods, none of which have worked.

Nevertheless, I have found that if I submerge the marbled plastic in water for over 10 minutes, the marble discoloration seems to disappear and the plastic becomes a uniform color again. However, once dried, the marble reappears.

My question therefore is, how do I correct the marble effect? Is there something that I can apply to the plastic that mimics the effect that water has on it but will be permanent?

Thanks!

 

[BillTre]

What kind of plastics do you speak of?

• ones that marble
• ones that don't

Do you have any references on this?Water can be obsobed into different plastics to varying degrees which might explain some of this.

 

[KeplerOfMars]

I am not really sure what type of plastic they are but they were typically used for computer cases, keyboards and fascias during the 80s and 90s.

 

[BillTre]

Well, its going to be all chemistry, so knowlefge of involved materials would be helpful.

 

[KeplerOfMars]

What kind of plastics do you speak of?

• ones that marble
• ones that don't

Do you have any references on this?Water can be obsobed into different plastics to varying degrees which might explain some of this.

Assuming water is being absorbed, is there anything that can be applied on top to retain it? I apologize if that sounds ridiculous.

 

[BillTre]

The water molecules can diffuse into various plastics to different degrees based on their differing chemistries.
However, it might just be a surface problem, like etching the surface.
I am guessing the combination of H₂O₂ plus UV (in the sunlight) could do some chemical modification to just the surface molecules of the plastic. This would not be surprising. UV blockers are put into plastics for a reason.

I have seen something like this in aquariums. They often get dirty with dried or deposited on chemicals of various kinds which can be either scraped off or removed with chemicals like plastic.
However, there are cases where certain chemicals can leave a visible mark by etching away some of the surface of the glass. This often goes away when it is wetted with water (making an optically smooth surface), but comes back when dried.

On aquarium glass you can feel this with your fingers (because it is a relatively large change in the surface) and you can't scrap it off with a razor blade. Your changes (if this is the case) are probably at a molecular scale. Instead of removing plastic molecules, the molecules could have just been changed.

If the plastic is affected by solvents (not all are), you could try wiping the surface with some solvent an see the effect on the marbling. This might briefly make soluble a few plastic molecules to fill the holes or more around the altered molecules, or wipe them off.

Maybe you could paint it to retain the water in the plastic, but that would take care of the marbling in a different way and not need the water.

 

[TeethWhitener]

Here’s my guess. Computer case plastic is probably ABS. It’s a nice cheap impact resistant polymer used all over the place for these types of applications (e.g., Lego bricks are ABS). However, the “B” in ABS is butadiene, which is probably irreversibly oxidized upon exposure to peroxide. This will alter the chemical and physical properties of the plastic, making it 1) less tough, and 2) more hydrophilic. So my guess is when you get it wet, a thin layer of water is retained long enough to make the surface appear homogeneous (due to refractive effects). After a few minutes, the water evaporates completely, revealing the discoloration caused by the chemical changes induced by the peroxide.

Just my $0.02.

 

[Tom.G]

As a "long-term temporary" fix try some Carnauba car wax (polish). Be sure to get the wax WITHOUT any abrasives in it, you may have to call the manufacturer to find out which of their products meet those requirements.

Cheers,
Tom

p.s. A quick Google search for Carnauba wax shows it is also available as sticks and flakes.