In the world of plastic playing cards, polyvinyl chloride (PVC) is the cheap crap that lesser cards are made of, like the clear plastic Bicycle decks, and the uber-cheap dollar store plastic decks.
QUALITY playing cards are made of cellulose acetate - it's the same stuff used by KEM/USPC. I believe it's also the same as what was used in the Bicycle Prestige decks as well as the Karnival Death Heads.
Sidebar: It's a common misconception that the Prestige decks were made by USPC's KEM division. The boxes are marked "made in Spain" and were produced, like the Death Heads, by Fournier out of Vitoria, Spain.