Specifically what causes white plastic to yellow with age?
White plastic is a polymer. Over time, the polymer chain breaks down and re-arranges its chemical bonds due to things like oxidation or damage from ultraviolet light. Stabilizers are added to the polymer to slow down this degradation. Over time, the degradation of the polymer chain plus degradation of the stabilizers cause the color change.
However, there’s a more specific reason that a particular (and popular) plastic called ABS yellows, related to bromine. The good news is that it’s reversible, though it’s a bit of a pain. See http://retr0bright.wikispaces.com/ .
My addition: Most plastics seem to turn white.
So seconding that its the ABS used for electrical/electronic products that turns yellow… its got the bromine based flame retardant in it. UV may make it degrade, but the earlier compositions may age without UV. I’m using a 20 year old keyboard that is only slightly yellow.
It’s not just electronics though. I have one of those kitchen bag sealer doohickeys that’s approx 15 years old that I think was a pale almond color that’s been in a dark cabinet, air conditioned, unplugged, and it’s now an ugly yellow. Not that I care much. But it just makes it look dirty. Also, part of the door handles on our 12 year old white fridge are yellowed. And that’s not dirt either. It is exposed to light though, both indirect from windows, and direct from fluorescent. That’s mainly what I’ve noticed it on. But I do know what you all are referring to on electronics too. I just gave Goodwill an 18ish year old set of PC speakers that turned yellow, and found a box with a gazillion yellowed plugs, cords, and adapters that I need to donate too.
ultraviolet light speeds things up. oxygen is in most places that we are and that is sufficient.