It's a very interesting subject for many reasons including purely cosmetic, the fact that premiums will b paid for MOCs with crystal clear bubbles and even being a factor in identifying a fake MOC ie perhaps a POTF MOC with a crystal clear bubble
Saying that there are still MOC POTF out there with v clear looking bubbles
Another interesting study regarding yellow bubbles is Kenner v Palitoy
Now it's a general acceptance that there are far more Kenner MOCs out there with yellow bubbles than Palitoy particularly with later MOCs-the most blatant being POTF v Tri-logo. I think it needs to b noted that in terms of numbers far more Kenner MOCs exist compared to Palitoy period
A great fact for us collectors is the 12, 20&21 backs seem to b least affected, I personally have only seen v few and we are talking a bubble tinge not full blown yellow/brown
Again overall the SW cards are prob least in number compared to all Empire and Jedi and again looking at Empire and Jedi I have seen far more yellow bubble Kenner MOCs compared to Palitoy so being a Palitoy collector am happy
So as to diff types of plastic again the biggest diff comparing Kenner to Palitoy is the POTF v Tri-logo. The Tri bubble is so much thinner compared to most other bubble types and some Palitoy Jedi MOCs also have the Tri bubble type.
Maybe environment is a factor and diff counties have diff kinds of weather.
Sunlight can defo affect a MOC horribly ie the awful fading that can happen and indeed bubble and figure yellowing. Then again I know one ex big time collector who had his POTF packed away and they were packed away back in the day mint with crystal clear bubbles only to b unpacked years later showing yellow and even brown bubbles
Looking for mint sealed or unused Palitoy Empire.