OK, I'm sure we all would love to restore our yellowed electronics back to their former glory in the original colour, and there's numerous retro electronics fans across the world that do just that using various "retrobright" techniques. Well, I, and a number of others, think that is a bad thing to do, and I'm here to expound my thoughts on why that should be.
Now first off, no body really knows how the retrobright process works. They all involve some form of oxidation of the plastic using hydrogen peroxide and energy, though. Typically the plastic in question is submersed in a bath of hydrogen peroxide solution and exposed to UV light from the sun. The result is a reduction in the yellowing of the plastic.
But how does that work and why? No one really has the answer (as far as I have found, anyway), so first to validate the rest of this post I have to come up with a theory of how it works at a molecular level. And here's my theory:
Plastics are generally a mishmash of long-chain polymers of different products. For example Acrylonitrile Butadiene Styrene (ABS) is long chains of bitadiene criss-crossed with shorter chains of Styrene all in a substrate of Acrylonitrile. The natural colour is a sort of greyish, yellowish, white. That colour is created by the scattering of light by the polymer chains. Those chains are all different lengths, and so the wavelengths of light being scattered are of all different lengths. Which makes sense, at least to my untrained mind.
Now the yellowing (by my thinking) happens when the longer polymer chains break due to light and heat energising them, and the new ends of the chains then bonding with oxygen in the air. The longer chains are the ones more likely to break and be "attacked" by longer wavelengths (sunlight, infra-red, heat, etc), because they're, er, longer? And in doing so you end up with a reduced percentage of longer chains, and a higher percentage of shorter chains, but not of very short chains. So where before you had a reasonably even distribution of lengths now you get a decrease in longer lengths, and an increase in medium lengths, some more shorter lengths, and maybe a handful of extra really short lengths.
Now if the light scattering theory is correct that means that light will be being scattered in different amounts according to the wavelength and the quantity of polymer chains that coincide with that wavelength. Thus, a change in the overall colour to yellowish.
So that's my theory of how plastics yellow over time: light breaks
the polymer chains which bind to oxygen stopping them linking together
again. (Of course there may be other mechanisms as well, such as fire
retardants, but they only compound the problem on top of the polymer
breakdown.)
And that is pretty much a one-way process. Once the chains are broken and the new ends bound to oxygen atoms there's no connecting them back together again unless you can strip those oxygen atoms off and encourage them to bind back together again. And that's certainly not what UV and hydrogen peroxide is doing. UV... that's light - didn't we talk about that before? Can you see where my train of thought is going yet?
So now we get to why I think retrobright is bad. You're doing the same thing that nature does over many years in a matter of hours. On an industrial scale. All that UV and increased oxygen from the hydrogen peroxide hammers those polymer chains into little bits restoring that randomness of length. Yes, it smashes those yellow-causing medium-long chains to smithereens so that they no longer cause the yellowing, but at the same time you're also hammering all the other polymer chains, as UV has a much shorter wavelength, making everything smaller. Like you've taken spaghetti and stamped on it.
All those lovely long polymer chains that bind everything together and give the plastic its strength gone.
Of course we're really only working at the surface level on most of
it, but even so that means that the surface is now degraded and loses
its structural integrity. Worse though is any fine detail like screw
pillars and supports will become brittle.
And that's why I think you should never retrobright your electronics, or at least nothing that isn't replaceable and basically of little value.