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I've always wondered what it is, as it makes a lot of noise on being deformed. No other common plastic that I've come across has quite the same properties.

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    $\begingroup$ In the old days it was cellophane, and may still be (or just generically called cellophane whether it is or is not). $\endgroup$ – Jon Custer Jun 1 '16 at 13:14
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It's more commonly referred to as 'envelope window film'. I've read somewhere that about 90% of all envelope window films are made from polystyrene, but sadly I cannot find the source at the moment. The remainder is made from polyester, polypropylene or glassine.

For examples see this manufacturer or this one. I'm not affiliated with either company, these are just the only ones I could find that provide specifications and proof my point.

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It usually is BOPS: Biaxially oriented polystyrene.

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    $\begingroup$ Yes I just took a look at an envelope... with IR spec. It's atactic polystyrene. $\endgroup$ – Ben Welborn Aug 5 '16 at 21:42

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