De-yellowing

[Tothwolf]

On Sat, 22 Aug 2015, Jules Richardson wrote:
> On 08/22/2015 07:48 AM, Tothwolf wrote:
>> On Sat, 22 Aug 2015, Jules Richardson wrote:
>>> On 08/21/2015 05:28 PM, Evan Koblentz wrote:
>>> 
>>>> Google for "Retr0brite".
>>> 
>>> I've never tried the stuff - but is it a permanent fix, or does the 
>>> yellowing gradually come back?
>> 
>> It almost always seems to comes back to some extent. De-yellowing 
>> treatments only work on the outermost surface of the plastic, and there 
>> are still plenty of free bromides just under the treated surface which 
>> will migrate outward. The re-yellowing (which may or may not be much 
>> less than the original yellowing) also occurs much faster than the 
>> original yellowing. Unless you can somehow eliminate all the free 
>> bromides from the plastic, de-yellowing treatments such as Retr0brite 
>> are just a temporary cosmetic fix.
>
> I thought that might be the case, but thought I might be mistaken given 
> how many people seem to rave about how wonderful this stuff is.

It does give something a nice temporary cosmetic fix, but in most cases it 
just doesn't last. As little as a year later something can be completely 
yellow again. It probably has a lot to do with the specific amount of 
bromide and other stuff in the plastic.

> Within a museum context, where showing original factory condition is 
> probably a desirable thing, have there been any studies to ensure that 
> retrobright (or other treatments) don't cause any damage to the plastics 
> (e.g. making the surface less resistant to scratches, or the plastic 
> more brittle, say)?

Long term UV exposure isn't exactly good for plastics, but I can't see 
lower concentrations of Hydrogen Peroxide doing any damage. Heavy UV 
exposure tends to make a lot of thermoplastics very brittle.

> Personally I quite like the patina of a yellowed machine anyway - being 
> able to see how it sat in a room, or where stickers used to be and such; 
> it's a minor thing, but sometimes it does tell a little bit of a story. 
> Maybe in 20 years if I ever sell any of my collection I might consider 
> it, but for now they may as well stay yellow :)

I had my yellowed TRS-80 Model 100 out yesterday to finally remove its 
internal NiCd battery (I caught it just in time, there was a tiny amount 
of discoloration of the solder mask at one end, which is probably the 
copper trace tarnishing, but the solder mask was still firmly adhered to 
the copper). It would be nice if the machine weren't yellowed, but the 
yellowing doesn't stop me from using it.

My model 100 has mostly lived in a box since I bought it (keeps the dust 
out of the keyboard when I'm not tinkering with it) and it has a very even 
yellowing to the light colored plastic (I don't think it has worsened any 
since I got it). I just haven't been able to justify the time and the cost 
of the materials to "retr0brite" it since I know the yellowing would 
reoccur, even with it stored in a box.

Maybe someone will eventually come up with a way to bring those free 
bromides out to the surface where the Hydrogen Peroxide will take care of 
them.

I wouldn't think it would make much of a difference, but has anyone tried 
putting a plastic part under vacuum to see if that helps?