12 Feb
2009
12 Feb
'09
6:26 a.m.
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
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.
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). 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!
/G?ran