As far as plasticizers go, I was always told that they kept the coverstock "supple" and prevented it from becoming too brittle.
And as for the ball "death", I think they die from several different reasons related to oil absorption.
Small, microscopic particles of dirt are picked up right along with the oil, and clog the pores. This can also cause a cover to "die" prematurely if not cleaned promptly and properly. Couple this with the fact that coverstocks have become chemically sensitive ( meaning the pores themselves can be effected by the chemicals in some cleaning solutions, causing them to either shrink signifcantly or close all together, losing their absorption properties ) and you have the recipe for dying balls.
I think that the presence of residual plasticizers in coverstocks MAY be beneficial to the coverstocks lifespan and, with regular proper cleaning, detract little from a balls performance.
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"The difference between stupidity and genius is that genius has its limits."
Albert Einstein
