I believe that is what happens. The plasticizers are what binds the little tiny bits of the plastic together. Sort of like a lube.
Lose that and the plastic doesn't stay together anymore, it cracks.
Kinda...not so much binding, but yes like a lube. Plasticizers (up until last year) were part of my professional life.
A plasticizer lowers the "glass-transition temperature" -- the transition between "glass" (stiff/rigid) and "rubber" (soft/flexible) -- of a polymer (aka "plastic"). The plasticizer gets between the polymer molecules and allows them to wiggle and flex and loosen up rather than to be stiff and rigidly aligned with one another. (It's kinda like the effect that booze has on a party of otherwise-uptight humans.
) At its most extreme, a plasticizer can even act like a solvent and the plastic becomes runny. (Kinda like when a human becomes too boozed up, too relaxed, and collapses in a pool of "liquid" on the floor.
)
So, with the right plasticizer mixed in, a plastic that would otherwise be stiff at ordinary temperatures becomes flexible even at lower temperatures -- because the "glass transition temperature" has been lowered. Lose that plasticizer and the plastic becomes brittle and cracks -- like glass does, rather than flex -- like rubber does. That's why it cracks.
UV damage is a different effect that I'm less-sure about...it breaks bonds in the polymer I think... (I'm not a polymer chemist -- just a chemical engineer
).
