Take a generic bag of crisps, or a pack of ice cream: it takes some effort to tear open, you may want to use scissors, but then once you've opened it it's torn so effortlessly that you have to be careful to not tear it so much the contents fall out. Same for a heavy duty 250mic PET film: virtually impossible to tear with your bare hands initially, but if you cut it with something sharp just a little then all of it is torn with next to no effort, along a relatively tidy line (almost looking like it was cut with scissors all along, although unevenly). Same goes for a Mylar emergency blanket. But then this doesn't seem to be the case for e.g. Fedex package wrapping LDPE film: upon being cut it's torn easily for just some time, but then it kind of just stretches and you have to apply more force yet, and the tear line isn't tidy (looking like it was chewed or something). So I'm wondering what properties of the material determine this behavior? What plastics will expose the former and what plastics will expose the latter behavior?
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