reversing case yellowing

[Göran Axelsson]

Teo Zenios wrote:
>
> ----- Original Message ----- From: "Brian Lanning" 
> <brianlanning at gmail.com>
> To: "General Discussion: On-Topic and Off-Topic Posts" 
> <cctalk at classiccmp.org>
> Sent: Wednesday, February 11, 2009 3:38 PM
> Subject: Re: reversing case yellowing
>
>
>> There was a lot of talk about this on the amiga forums.  Apparently it
>> works great.  The main problem seems to be getting a
>> concentrated-enough batch of hydrogen peroxide, though it's not a big
>> deal.  The other thing is that the ultra-violet light is a key
>> ingredient.  From what I understand, the ultra-violet light
>> destabilizes the oxygen bond so the peroxide can pick it up.
>>
>> brian
>>
>
> Peroxide is just H2O2 (water being H2O). You have an unstable molecule 
> with an extra Oxygen compared to water (which is stable). Any heat, 
> major vibration, or even UV light will decompose peroxide into Water 
> and Free Oxygen, the Oxygen combines with whatever discolors the 
> plastic and then goes into solution. Heat isn't used because people 
> are worried about plastic warping I guess (or because the reaction is 
> too fast). If you look on the English Amiga Board (EAB) they have a 
> more detailed recipe including some other chemicals added to speed 
> things up. Contact with metal also decomposes hydrogen peroxide (as 
> the Russians found out with leaky torpedoes on the Kursk submarine a 
> while back), but adding free oxygen to mild steel will just rust the 
> hell out of it in a hurry (so if you have plastic+ mild steel parts 
> you will have issues).

I don't know if this is going to work but adding lye (sodium hydroxide) 
to raise the Ph in the solution might help passivating the iron surface 
enough to protect it against oxidation.
Archaeologists uses this trick when treating iron against oxidation when 
they conserve their findings.

> The problems I have seen with this method is some pieces do not whiten 
> the same and you have blotches, also some mold marks (swirling caused 
> when the hot plastic is injected into the mold) show up where they 
> were not noticable before.
>
> I generaly just wash/scrub yellowed parts with dishwashing detergent 
> with "oxy" on the label, the scrubbing releases some oxygen from the 
> soap and you slightly whiten whatever you are cleaning (and remove 
> dirt as well). It will not make anything yellowed look new, but I just 
> want  the stuff clean and a shade whiter.
>
> Anything that had yellowed is mostly from a bad mix of UV stabilizer 
> in the plastic (to keep the plastic from turning into dust after years 
> of exposure to sunlight), and the odds it will yellow again down the 
> road are probably good (you just cleaned up the surface and not deep 
> into the plastic). So you will need a coating of UV protectand to keep 
> anything you cleaned looking good.
>
> P.S. If you use concentrated peroxide much above the 3% solution you 
> use on cuts make sure you have a face shield, chemical gloves, and 
> some kind of chemical apron unless you want to go blind, ruin your 
> clothes, and mess up your skin for a while.
>

I'll second that suggestion. Safety first!

/Göran