Yellowed plastic from age/UV damage?

[RickRollerLT1]

On some older white molded kits kits I bought sealed in the past, they would be severely yellowed or almost entirely tan.  What's even weirder is I've gotten opened kits with parts out of their baggies (and some complete/started kits in ziploc baggies) that were still pure white inside.

I heard this phenomenon primarily occurs with certain styrene/ABS materials that were left outside or in warm temperatures out of their packaging, but then I've seen it in sealed stuff too. Google-fu can't get me a real clear answer due to all the variables and factors, but just curious on your experiences and opinions.

Edited November 23, 2020 by RickRollerLT1

[Modelbuilder Mark]

Here is a good technical answer, https://www.polymersolutions.com/blog/what-makes-plastic-turn-yellow/

My guess is the kit was exposed to wither high heat, or sun at some point. I have kits that are 60 years old, and not noticeably yellow to my eyes, but others that spent some time in an attic or garage, that are a bit yellow despite being in a box. One other option, exposure to various chemicals, again if in garage for example. I know chrome that is stored too near bleach, like kits being kept in the laundry room, could cause chrome the "vanish" on kits over time.