Originally Posted by
Grand Bois
I had a friend, now long deceased, that was a pioneer in the plastics industry. He used to say that plastics become brittle because they "lose their polymers" over time.
It's called "outgassing" The plasticizers pretty much get emitted out of the plastic as a gas or vapor or simetimes even ooze out as an oily substance, usually accelerated when the plastic item is exposed to heat and sunlight (UV).
You mgiht have noticed this on your car, if you park it outdoors for long periods of time under the sun and not regularly clean the inner surface of the windshield, an oily substance would build up on the glass. That's the plasticizers from you plastic dashboard parts and finishes.
Chombi