identify film cap material

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Sorting through my boxes, I have quite a few foil-film caps. The size is within range of them being polystyrene, but I suspect mylar. Markings are black. ( clear/silver) My known newer polystyrene happen to have red markings.

Other than tossing one in acetone, any way to evaluate between polystyrene, poltpropo and polyester? The 1u and above that aren't bigger than a AA battery have to be polyester. It is the smaller ones. I suspect they are old enough they are not polypro. 80's vintage.
 
Stupid me. Heat gun. Mylar is tough, good stuff is not. Thay are tough. Darn.
If the foil winding is exposed, you just need to sample a tiny chip from one end.
Burn it, or place it on a soldering iron set at 450°C: if the smell is "waxy", it is PP, if it's neutral it's PC, less neutral PETP (mylar), and if it has a strong smell, it is PS.
 
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