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Message:: [QUOTE="900fine, post: 377000, member: 6036"]Yep. Start with lots of Vinyl Chloride (C2H3Cl) molecules, string thousands of 'em together, and you get Poly Vinyl Chloride - a reaction known as "polymerization". PVC (aka vinyl) comes in a lot of flavors, depending on how many VCs you strung together and whether or not you did it to produce a long chain or crosslinked (connected) chains. PVC pipe, vinyl for records, and soft plastic bags like they use in hospital IV bags... all PVC.  The additive is known as a "plasticizer", and is basically a weak solvent for PVC. There are different ones depending on what you want to do. Small amounts won't dissolve the PVC; it will soften it up, so the end result is rigid, flexible, or anywhere in between. More plasticizer = more flexible. As Carl said, plasticizer leaks out (or evaporates). That's what makes the famous "new car smell". Organic solvents. And here's the problem for numismatists - the leachate (organic solvent and whatever it brings with it) is nasty for coins. So the bottom line is this - the reason flips are soft and flexible is plasticizer, which is bad for coins. No soft, flexible flips. Being rigid isn't entirely good - they crack - but at least they're safe for coins, because they lack plasticizer.[/QUOTE]