⚠️ Caution about using sun to restore yellow plastics

[AndyDiags]

I've had an interesting case which I haven't seen anyone else document yet.

I have a couple Mac Plus keyboards at various stages of yellowing. For most, only top part was yellowed, bottom part was mostly ok. I decided that for the bottom part, leaving it out in the sun for a couple days would be sufficient and no hydroperoxide retrobright would be needed.

After a couple days in the sun, I put the cases into a closet. Six months later I took them out and found a couple to be completely yellowed!

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One thing to note is that I put a piece of white paper over the label to keep it from safe from the sun, and I even just so happen to take a picture (below). You can see that only the plastic which was exposed to the sun turned yellow after 6 months, the area covered by the white paper reminded unaffected by the yellowing.

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This yellowing only occurred on two of the keyboard cases, others did not yellow after storage. So after 30 years it stayed close to it's original color, but them they were put in the sun for a couple days, and six months later they were completely yellowed.

I should also mention that I've never encountered this occur on a Mac Plus case.

I guess I'd just like to forewarn folks that not all plastics may react the same to various methods of retrobrighting. On some it may work excellent, while on others may not.
I've had the worst luck with the Mac Plus keyboards. I've had the opposite reaction with hydroperoxide for a couple keyboards, where after only 30 minutes in the sun the plastic went well beyond it's original color and got bleached.

 

[3lectr1c]

Yeah, I've seen people warn against this in other places too, citing that it will cause the plastic to go even more brittle. The best method seems to be the one that keeps the plastic away from stress as much as possible, and while this one is easy, it doesn't seem to last long-term.

 

[Paralel]

Sunlight, without peroxide of any kind, is part of what causes yellowing. The UV in sunlight provide enough energy for the bromide in the fire retardant to break the bond to its carrier molecule. Once the bromide is free it attaches itself to another molecule and acts as a chromophore, which is represented by a yellow color. The peroxide destroys the bromide chromophores by breaking the bond between bromide and the molecule its attached to which makes the color disappear. However, the free bromide is still present, that's typically why things re-yellow over time, the bromide rebonds to another molecule to re-create the chromophore. The best thing that could be done after peroxide brightening would be to put a chemical on the plastic that would strongly bind the free bromide into a non-chromophoric molecule. That would guarantee that the bromide wouldn't be able to bind to anything else and cause further yellowing in the future. As far as I am aware, no one has found a safe, effective chemical that can be applied to the plastic which can permanently bond the free bromide.

 

[joshc]

There is a reason I don't like retrobrighting and don't advocate it. It changes the properties of the plastic, often weakening it and the yellowing will always return. I've got a few things retrobrighted by other people and they have all yellowed again since I got them. It's also very easy to make the plastic look worse than before - streaking and other damage can cause if retrobrighting is done a certain way.

Everything in my collection is yellowed to different degrees, some worse than others, but I just accept this as a fact of collecting this old gear - consider it a natural patina of antique Macs, why try to change it?

Another point to make is that if you do have something that isn't very yellowed, you can protect it from UV with Aerospace 303 Protectant spray - I've used this on a very pristine AEK II and it has kept its looks so far. The only problem is that plastic attracts more dust when it is applied.