Shponglefan wrote on 2024-03-31, 02:58:Is heat really the cause? […]
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Matchstick wrote on 2024-03-30, 22:17:
Heat is the cause. Cause even stored in a box in an attic, plastic will yellow...
The only factor is the ambient temp in an attic is very high.
Is heat really the cause?
I've noticed that with old cases, areas that were shielded from oxidation tended to not yellow versus areas that were exposed. This is obviously not a heat related issue, since the case would have likely been a relatively uniform temp.
I also have a retrobrighted case that was stored in my basement in cool temperatures (below 20 C), and it has gradually re-yellowed.
There have been some ... not well controlled... experiments concerning this. Since what is mostly out there is idle (if somewhat informed) speculation on the matter, I will throw my own in as well.
Heat has been demonstrated to be a common factor in most yellowing. even with devices that are in the sunlight, you will find that the areas MOST yellowed (or yellowed the fastest), are areas where heat was radiated from within the device.
Other experiments have had yellowing happen with just the heat, such as the afore mentioned "In a box, in the attic" anecdote.
What all of these have in common is heat, plus exposure to atmosphere.
I would propose that what is happening, is a breakdown and release of polymer's plasticisers and hardeners, which migrate to the surface, oxidize, and turn brown. Heat facilitates this migration, and upon exposure to atmosphere, these compounds break down. Exposure to UV light further catalyzes this reaction.
This would explain why "Sunlight made it more noticeable", "Stickers seem to stop it", "It still happens in the darkness in the attic", etc are all True.
I would say that the Retr0brite process does not "Reverse" the process, so much as it further, and more aggressively, breaks the more complex oxygenated complexes that form in the top few microns of the plastic down into simpler, smaller oxygenated compounds, which removes the yellowness-- but still leaves the plastic with less plasticizer in it, (which would be consistent with the retrobrited plastics being more brittle, and prone to scuffing)
Compare: Linseed oil hardens into a complex chaotic polymer film on exposure to oxygen in the atmosphere. Catching it on fire is ALSO oxidation, but does NOT produce said film-- it produces smaller molecules.
Depending on the plastic, and its characteristics (concerning thermal stability), heat alone (in conjunction with the hydrogen peroxide) is sufficient to get very good results. (Some plastics are not tolerant of the temperatures required, and will deform!! Others, are perfectly OK with some pretty warm solutions, and can be brightened very effectively indoors with just a covered saucepan on simmer full of pharmacy grade peroxide.) For the less therm0stable plastics, you need the UV light.
In both cases, all that is really happening is that the reactivity of the oxygen is given a boost by an outside energy source.
I'd like to see some more structured experiments performed, with various kinds of applied energy added, such as low-level electrolysis of cool solutions, exposure to more ionized oxygen species (like ozone, or other peroxides besides hydrogen), etc, with careful notes taken about temperature and energy supplied, to determine where the "magic threshold" is.
