I worry that they don't tell you what the bag is made of. Mylar (or PET, or polyester) is the outer coating of a three layer film. The middle layer is aluminum (or some other metal). The inside layer is generally either LDPE or Polypropylene. The metal layer is a gas barrier, and comes in varying thicknesses, the thicker, the better barrier, but the less pliable. The mylar is also a gas barrier, and it protects the metal layer. LDPE melts at a lower temp, so you don't need such a robust heat sealer to seal them. Polypro has a higher melting temp, so it needs better equipment. Also, because it needs a higher temp, you can put it in boiling water to sterilize the food inside. This is why MRE's use Polypro bags.
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