Why do certain types of thermoplastics or rubber (i.e. used for soft buttons of a tv remote or the insulation of cheap headphone cables) turn sticky or even gooey after they haven’t been used in a long while?

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Why do certain types of thermoplastics or rubber (i.e. used for soft buttons of a tv remote or the insulation of cheap headphone cables) turn sticky or even gooey after they haven’t been used in a long while?

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They use cheap injection-moldable plastics that tend to break down in the presence of the oils that your skin produces. When you use the thing every day, you’re rubbing off the tiny amount on the surface that’s reacted, so you don’t notice it. When you don’t use it for a while, though, more of it can build up.

Usually this is a filler oil or antioxidant oozing out.

Oxygen and ozone will obliterate many types of polymers very quickly if you don’t add protective chemicals into the mixture. Adding oils to the mix makes the plastics more flexible.

Over time they break down and work their way out.

The have a plasticiser in them which given time and environmental conditions leaches out.

Not a perfect analogy, but think of it like an oily rag where the oil will over time flow out. The rag will end up dry and there will be a pool of oil.

There are three mutually exclusive but very plausible answers here. Are different reasons true for different plastics or are some (or all) of the answers incorrect?

I think it’s a combination of oils from your hand and oxidation.

If you ever have the chance to dig around in an old toolbox, smell the handles of screwdrivers. If any of them smell like vomit, it’s partly made with butyl rubber that’s degraded into butyric acid which is one chemical that gives vomit its unpleasant smell.